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MEASUREMENT OF FOREST ECOSYSTEM-ATMOSPHERE EXCHANGE OF
δ13C – CO2 USING FOURIER TRANSFORM INFRARED SPECTROSCOPY
AND DISJUNCT EDDY COVARIANCE
By
MARIA OBIMINDA L. CAMBALIZA
A dissertation submitted in partial fulfillment of therequirements for the degree of
DOCTOR OF PHILOSOPHY
WASHINGTON STATE UNIVERSITYDepartment of Civil and Environmental Engineering
MAY 2010
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To the Faculty of Washington State University:
The members of the Committee appointed to examine the dissertation of MARIA
OBIMINDA CAMBALIZA find it satisfactory and recommend that it be accepted.
___________________________________George H. Mount, Ph.D., Chair
____________________________________Brian K. Lamb, Ph.D.
____________________________________Halvor H. Westberg, Ph.D.
____________________________________John D. Marshall, Ph.D.
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DEDICATION
To Alfredo and Lagrimas Cambaliza
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ACKNOWLEDGMENT
I first learned about the environmental engineering program at Washington State
University through Fr. Daniel McNamara, SJ, a priest and a scientist. He was chair of the
Physics department of the Ateneo de Manila University where I was working as an
instructor. When I was seriously considering graduate school back in 2001, Fr. Dan
recommended that I look into the research of Dr. George Mount, a long-time friend from
graduate school days. Since then, my life has changed and the next seven years saw me
working on a very exciting, inter-disciplinary biosphere-atmosphere research that always
kept me on my toes. I would like to thank Fr. Dan for giving me that gentle push that
started this wonderful experience for me.
I would like to thank my faculty adviser, Dr. George Mount, and his family. I feel
very lucky and blessed that I ended up working with Dr. Mount. He is very passionate
with both teaching and research and his enthusiasm is contagious. I encountered many
challenges in my research, sometimes seemingly insurmountable problems, but he
believed in me and encouraged me to charge on even when I doubted myself. I cannot
thank him enough for the once-in-a-lifetime, very positive, enriching experience, as well
as for opening opportunities for me after graduate school. It is very rare to have an
adviser who goes out of his way to make sure that his graduate students are successful
during and after graduate school.
I also thank my committee members, Drs. Brian Lamb, Hal Westberg, and John
Marshall. Together with my advisor, they all helped me achieve my goals and made sure
that I receive the critical guidance I needed along the way. I thank Dr. John Marshall for
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guiding me with the biological aspect of my research and for coming down to Boardman,
Oregon in 2005. I enjoyed the many long discussions on carbon isotopes and ecosystem
processes as we looked closely at my FTIR results. Many thanks also go to Dr. Brian
Lamb for the very helpful discussions especially when I was having difficulties with my
flux analysis. I would also like to thank him for the RA support in the spring semester of
2007. I also thank Dr. Hal Westberg, who despite being retired, remained interested in
my work and never failed to encourage me to continue on especially when I encountered
an unforeseen problem with calibration.
To my friends who helped me with my fieldwork: Robert Gibson, Steve Nelson,
Brian Rumburg Jenny Filipy and Jacob McCoskey, thank you all! I am very thankful to
my friend Rob who spent two summers with me in Boardman, Oregon. He made our
fieldwork experience much more interesting. We shared some unforgettable, hilarious
experiences in Boardman and Hermiston that must be documented someday. Many
thanks also go to Brian and Jenny not only for working with me in the field but most
especially for their friendship, support, and encouragement. I have gotten to know them
quite well and vice versa, and their friendship helped me through some very challenging
times.
I would also like to thank Mr. Richard Winkler for working long hours with me in
the laboratory. Wink helped me with the software control of the instrument. Without his
generosity and expertise, it would probably take me longer to complete my degree. Wink
is one of the smartest, most focused persons I have had the pleasure to work with. I am
also thankful for his friendship.
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I also thank Dr. Dave Griffith of the School of Chemistry, University of
Wollongong, Australia, for the many helpful discussions on data analysis and FTIR
instrumentation, as well as for the use of the MALT program. I also thank him for his
very helpful recommendation on how to clean the FTIR gas cell.
Many thanks also go to Gary Held and Kurt Hutchinson of the WSU Engineering
Instrument Shop for helping me build the DEC instrument, and all other auxiliary
instruments needed for mounting various sensors in the field. Both Gary and Kurt are
very competent, capable, and skillful, and the College of Engineering is very fortunate to
have them.
I would also like to thank Ben Harlow and Dr. Dave Evans of the WSU Stable
Isotopes Core Laboratory for the exciting, collaborative work I have done in their
laboratory. I also thank the Idaho Stable Isotope Laboratory for the analysis of bulk leaf
and soil respiration samples. Many thanks also go to my friend Dr. Nerea Ubierna for the
many helpful and fun conversations over the phone, over lunch, and over an ice cream
sundae. My discussions with her clarified many isotope concepts and helped me
understand the implications of my measurements. I thoroughly enjoyed working with her
and Ben Harlow in the laboratory figuring out how to make gas measurements with the
CF-IRMS even more precise and accurate.
I also thank Dr. Shelley Pressley for all her help with the flux data analysis and
for the many insightful exchange of ideas. She was always available for consultation. I
thank her for her generosity with her time and expertise and for being a great role model
for other female scientists in LAR. I would also like to thank Lee Bamesberger, LAR’s
hard working retired technician. I cannot thank him enough for helping me prepare the
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dilutions of my calibration tanks, for helping choose the right vacuum pumps, and for all
the many laboratory preparations that go into a successful field campaign. Many thanks
also go to past and present graduate students of LAR: Erik Velasco, Jack Chen, Tara
Strand, Steve Edburg, Doris Montecastro, Rasa Grivicke, Jacob McCoskey and Kara
Yedinak for their friendship and support. Thank you also to the CEE staff: Vicki
Ruddick, Maureen Clausen, Lola Gillespie, Cyndi Whitmore, Glenda Rogers and Tom
Webber for their assistance and for making sure that everything runs like a well-oiled
machine so that the rest of us can focus on research. I would also like to thank my best
friend Dr. Gemma Narisma for the many phone conversations while she was still in the
United States.
Many thanks also go to my Filipino friends: Nelly and Cesar Zamora, Rhoda
Rimando, Ed and Julie Cenzon, Suzette and Greg Galinato, Jave Pascual, and Dan and
Medy Villamor, who all became my surrogate family in Pullman. Thank you for your
friendship and support.
I also thank Greenwood Resources for the use of the Boardman poplar site, with
special thanks to Mr. Mike Berk, caretaker of the plantation.
Finally, I thank my family: mom, dad, my sister Genalin and nanay Lucing for
their love, prayers, and encouragement. I am thankful to my parents for teaching me the
importance of education and for instilling in me the values of patience, hard work, and
perseverance. I would not be able to go this far without my family’s unwavering support.
This research was supported by the National Science Foundation.
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ATTRIBUTION
The research presented in this dissertation is a compilation of results from field
measurements during the summers of 2004 to 2006 in a managed poplar forest near
Boardman, Oregon, as well as laboratory measurements at the WSU Stable Isotope Core
Laboratory and Idaho Stable Isotope Laboratory, University of Idaho. This dissertation
has three important chapters: a description of the development of Fourier Transform
Infrared Spectroscopy – disjunct eddy covariance (FTIR – DEC) instrument for the
measurement of the vertical and temporal distribution of CO2 – δ13C in a forest ecosystem
(chapter 2), isotopic CO2 flux measurements using the FTIR – DEC approach (chapter 3),
and improvement of analysis of low-concentration gas samples with continuous flow
isotope ratio mass spectrometry (CF – IRMS) (chapter 4). Although Maria Obiminda
Cambaliza is the first author of these manuscripts, the results presented in this
dissertation are products of the ideas and efforts from many collaborators. M. O.
Cambaliza was responsible for all data analysis (except for those reported in chapter 4),
and for organizing and facilitating field measurements during the summers of 2004 to
2006.
Drs. Brian Lamb, Hal Westberg, and George Mount were the co-principal
investigators in a biosphere-atmosphere research proposal that was funded by the
National Science Foundation (NSF). They received funding for the development of a
suite of instruments that quantify the biosphere-atmosphere interaction in a forest. One
of these instruments was the FTIR – DEC, which M. O. Cambaliza helped to develop
together with G. Mount. The DEC sampling strategy was suggested by Drs. Brian Lamb
and Hal Westberg. The FTIR – DEC instrument underwent several modifications, as M.
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O. Cambaliza and G. Mount learned more about how to optimize isotopic CO2
measurements. Many of these modifications were the result of sensitivity analyses and
many brainstorming discussions. Modifications were carried out by Gary Held and Kurt
Hutchinson at the WSU Engineering Instrument shop. Mr. Richard Winkler helped with
the automation and control of the FTIR – DEC instrument. Dr. John Marshall provided
invaluable guidance with the interpretation of FTIR – DEC isotope measurements and the
design of the soil respiration chambers. He also helped with the field campaign in 2005
where he measured very important supporting data such as the carbon isotope ratios of
bulk leaf and phloem sap samples. Drs. Brian Lamb and George Mount helped with the
design of the experimental observations in the field (vertical profile and fixed height
measurements). Dr. Brian Lamb also provided invaluable guidance with the analysis of
the flux data as well as the vertical profile measurements.
Benjamin Harlow, and Drs. Nerea Ubierna, Dave Evans, John Marshall and
George Mount are the co-authors of the manuscript presented in chapter 4. Ben Harlow
and Nerea Ubierna helped with the laboratory measurements, which were conducted at
the WSU Stable Isotope Core Laboratory. They also helped with the data analysis,
organization, and writing of the manuscript. Dr. Dave Evans provided guidance with the
design of the experiments. Dr. George Mount provided initial helpful comments on the
first version of the paper. Both Drs. John Marshall and Dave Evans gave detailed
insightful comments that helped improve and polish the final version of the paper.
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MEASUREMENT OF FOREST ECOSYSTEM-ATMOSPHERE EXCHANGE OF
δ13C – CO2 USING FOURIER TRANSFORM INFRARED SPECTROSCOPY
AND DISJUNCT EDDY COVARIANCE
Abstract
By Maria Obiminda L. Cambaliza, Ph.D.Washington State University
May 2010
Chair: George H. Mount
The measurement of the stable isotopic content and isotopic flux of atmospheric
carbon dioxide is important for understanding the carbon budget on ecosystem, regional,
and global spatial scales. Conventional measurements of the isotopic composition of
atmospheric CO2 involve laboratory mass spectrometry analysis of grab samples from the
field, which limits the location, collection frequency and throughput of samples. More
technologically advanced methods (e.g. tunable diode laser spectroscopy) suffer from
interferences with other chemical species. We have developed a new measurement
method based on Fourier-transform infrared spectroscopy (FTIR) and disjunct eddy
covariance (DEC) for fast, continuous, real-time measurement of the carbon isotopic
composition of atmospheric CO2. Molecular absorption is measured in the 2100 to 2500
cm-1 spectral region of the 13CO2 and 12CO2 vibration-rotation bands with concentrations
of both isotopologues used to determine δ13C. We demonstrate the capability of this new
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technique in a managed poplar forest near Boardman, Oregon with measurements during
the summers of 2005 and 2006 from a 22-meter tower in a 16-m forest canopy. Long-
term calibration using reference gas cylinders yielded field accuracy and precision for the
forest measurements of 0.5‰ and 0.8‰, respectively, for the 45-second cycle time
between samples. The signature of ecosystem respiration derived from the nighttime
vertical profile measurements of CO2 - δ13C was –26.6‰, about 2‰ more enriched than
the isotopic composition of measured bulk leaf samples from the forest. Ecosystem
respired CO2 was ~1.6‰ more enriched than soil-respired CO2.
A comparison of the FTIR – DEC total CO2 fluxes against standard eddy
covariance measurements showed excellent (10%) agreement. FTIR – DEC
measurement of the CO2 isoflux enabled the estimation of the mean carbon isotope ratio
of the photosynthetic flux (δP). The average δP (-24.9‰) was 13C – enriched compared
with the isotopic composition of bulk leaf samples. These results are consistent with
observations using conventional methods. The FTIR – DEC technique can
simultaneously measure several important trace gases, which will make it a powerful tool
for application in forest, agricultural and urban ecosystems.
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TABLE OF CONTENTS
DEDICATION...............................................................................................................iii
ACKNOWLEDGMENT ................................................................................................ iv
ATTRIBUTION...........................................................................................................viii
Abstract .......................................................................................................................... x
LIST OF TABLES........................................................................................................ xv
LIST OF FIGURES ..................................................................................................... xvi
CHAPTER 1: INTRODUCTION AND LITERATURE REVIEW ................................. 1
1.1. Overview and Objectives ....................................................................................... 1
1.2. Background on the carbon isotope ratio.................................................................. 5
1.2.1. Definitions and isotopic composition of common sources ................................. 5
1.2.2. Stable carbon isotopes and ecosystem processes ............................................... 7
1.2.3. Determination of the carbon isotope ratio of ecosystem respired CO2............... 8
1.2.4. Determination of canopy – scale photosynthetic discrimination ..................... 11
1.3. Measurement of Carbon isotope ratio ................................................................... 13
1.4. Measurement of biosphere-atmosphere exchange of isotopic CO2 ........................ 17
1.5. Summary ............................................................................................................. 23
1.6. References ........................................................................................................... 24
CHAPTER 2: A NEW FOURIER-TRANSFORM INFRARED INSTRUMENT FOR
MEASUREMENT OF TEMPORAL AND VERTICAL DISTRIBUTION OF δ13C – CO2
1. AN OVERVIEW OF RESULTS FROM MEASUREMENTS IN A MANAGED
POPLAR FOREST IN NORTHERN OREGON............................................................ 34
2.1. Abstract ............................................................................................................... 34
xiii
2.2. Introduction ......................................................................................................... 36
2.3. Methodology........................................................................................................ 40
2.3.1. Site Description .............................................................................................. 40
2.3.2. Description of experiment............................................................................... 41
2.3.3. Leaf sampling, soil respiration measurement and isotope analysis................... 44
2.3.4. Analysis of Measured Spectra ......................................................................... 45
2.3.5. Calibration...................................................................................................... 47
2.4. Results and discussion.......................................................................................... 47
2.5. Conclusions ......................................................................................................... 52
2.6. References ........................................................................................................... 54
CHAPTER 3: A NEW FOURIER TRANSFORM INFRARED INSTRUMENT FOR
MEASUREMENT OF TEMPORAL AND VERTICAL DISTRIBUTION OF δ13C – CO2
2. MEASUREMENT OF ECOSYSTEM-ATMOSPHERE EXCHANGE OF ISOTOPIC
CARBON DIOXIDE USING DISJUNCT EDDY COVARIANCE............................... 69
3.1. Abstract ............................................................................................................... 69
3.2. Introduction ......................................................................................................... 71
3.3. Theory and Methods ............................................................................................ 75
3.3.1. Canopy-scale photosynthetic discrimination.................................................... 75
3.3.2. Site description and meteorological conditions................................................ 77
3.3.3. Flux measurement and spectroscopic analysis of air samples .......................... 78
3.3.4. Leaf sampling and isotopic analyses ............................................................... 81
3.4. RESULTS AND DISCUSSION........................................................................... 82
3.4.1. FTIR – DEC and IRGA – EC Measurements of CO2 and H2O fluxes .............. 82
xiv
3.4.2. Isoflux estimates, isotopic flux partitioning, and discrimination estimates ....... 83
3.5. Conclusions ......................................................................................................... 86
3.6. References ........................................................................................................... 88
CHAPTER 4: ANALYSIS OF LOW-CONCENTRATION GAS SAMPLES WITH
CONTINUOUS-FLOW IRMS: ELIMINATING SOURCES OF CONTAMINATION
TO ACHIEVE HIGH PRECISION ............................................................................. 103
4.1. Abstract ............................................................................................................. 103
4.2. Introduction ....................................................................................................... 104
4.3. Methods ............................................................................................................. 106
4.3.1. Isotopic analysis ........................................................................................... 106
4.3.2. Description of gas sample preparation methods............................................. 109
4.3.3. Experiment 1: Identification of sources of contamination............................. 111
4.3.4. Experiment 2: Application to gas samples of near-ambient CO2 concentrations
............................................................................................................................... 112
4.4. Results .............................................................................................................. 113
4.4.1. Identification of sources of contamination..................................................... 113
4.4.2. Application to gas samples of near-ambient CO2 concentrations ................... 115
4.5. Discussion.......................................................................................................... 115
4.6. References ......................................................................................................... 119
CHAPTER 5 ............................................................................................................... 127
SUMMARY AND CONCLUSIONS .......................................................................... 127
5.1. Summary ........................................................................................................... 127
5.2. Personal thoughts and recommendations for future measurements...................... 132
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LIST OF TABLES
CHAPTER 1
Table 1.1. Isotopic composition of various carbon sources. .......................................... 31
Table 1.2. Summary of capabilities of various spectroscopic techniques........................ 32
CHAPTER 2
Table 2. 1. Concentrations and carbon isotope ratios of four reference tanks used in the
field. ...................................................................................................................... 59
Table 2. 2. Estimate of the isotopic composition of respired carbon as a function of length
of averaging time. .................................................................................................. 60
Table 2. 3. CO2 concentrations and carbon isotope composition of soil pool measured
from the headspace of the static closed chambers.. ................................................. 61
CHAPTER 3
Table 3. 1. Concentrations and carbon isotope ratios of four reference cylinders used in
the field for calibration of FTIR derived CO2 and δ13C........................................... 93
xvi
LIST OF FIGURES
CHAPTER 1
Figure 1.1. Carbon isotopic discrimination by C3 photosynthesis.. ................................. 33
CHAPTER 2
Figure 2. 1. Schematic diagram of field experiment set-up. ........................................... 62
Figure 2. 2. Examples of typical measured, fit and residual spectrum ........................... 63
Figure 2. 3. Long-term on-line calibration measurements taken in the field from August
11 to 17, 2006........................................................................................................ 64
Figure 2. 4. Nighttime vertical and temporal distributions of a) CO2 and b) δ13C. .......... 65
Figure 2. 5 . Daytime vertical and temporal distribution of a) CO2 and b) δ13C. ............ 66
Figure 2. 6. Estimate of the signature of ecosystem respiration, δ13CR........................... 67
Figure 2. 7. Carbon isotopic composition of leaves as a function of height. ................... 68
CHAPTER 3
Figure 3. 1. Diurnal averages of (a) temperature, (b) photosynthetically active radiation,
(c) friction velocity. ............................................................................................... 94
Figure 3. 2. Wind speeds and wind directions observed during the duration of field
experiments.. ......................................................................................................... 95
Figure 3. 3. Schematic diagram of the field experiment set-up....................................... 96
Figure 3. 4. Comparison of FTIR – DEC and IRGA – EC measurements of CO2 fluxes.97
xvii
Figure 3. 5. Comparison of FTIR – DEC Latent Heat flux measurements against IRGA –
EC estimates.......................................................................................................... 98
Figure 3. 6. The isoflux of 13CO2. .................................................................................. 99
Figure 3. 7. Temperature dependence of the nighttime CO2 flux. ................................ 100
Figure 3. 8. The total CO2 flux and its photosynthetic assimilation (FA) and respiratory
(FR) components .................................................................................................. 101
Figure 3. 9. Half hour averages of the canopy-scale carbon isotope discrimination ..... 102
CHAPTER 4
Figure 4. 1. Schematic diagram of the vacuum backfill manifold................................ 121
Figure 4. 2. Maximum contamination on low-concentration CO2 gas samples as a
function of the various gas filling treatments.. ..................................................... 122
Figure 4. 3. Carbon isotope ratio as a function of the measured area for all Helium-filled
replicates of all treatments.. ................................................................................. 123
Figure 4. 4. Residual contamination as a function of the ten treatments.. .................... 124
Figure 4. 5. Two-source mixing model calculation of the uncertainty in the carbon
isotope ratio of a low-concentration sample.. ....................................................... 125
Figure 4. 6. Effect of three sample preparation techniques in air samples of identical CO2
concentrations but distinctly different carbon isotope ratios. ................................ 126
1
CHAPTER 1
INTRODUCTION AND LITERATURE REVIEW
1.1. Overview and Objectives
Unrestrained use of fossil fuels and widespread land cover changes since the
industrial revolution have added significant amounts of CO2 in the atmosphere with
mixing ratios rising from 280 ppmv prior to the industrial revolution to ~385 ppmv today.
This excessive anthropogenic contribution has overwhelmed the natural carbon absorbing
capacity of oceans and terrestrial ecosystems. For the years between 2000 and 2005,
carbon emissions due to fossil fuel burning and cement production were estimated to be
7.2 ± 0.3 GtC/yr (Denman, et al., IPCC AR4, 2007). Approximately 60% of these
emissions, 4.1 ± 0.1 GtC/yr, remain airborne and the remaining 40% is partitioned
between the ocean and land reservoirs. There has been negligible change in the capacity
of the oceans (2.2 ± 0.5 GtC/yr) to absorb carbon between the 1990’s and 2005.
However, there is an inter-annual and inter-decadal variability in the growth rate of
atmospheric CO2, which tracks the changing magnitude of the biospheric sink. Although
the global carbon cycle is reasonably well understood, it still remains difficult to predict
the future response of the terrestrial ecosystem to changing environmental drivers such as
increased temperature, precipitation, and nutrient fertilization. Therefore, there is a
strong need to characterize ecosystem response to different environmental conditions
during various seasons to understand the complex flow of carbon between the biosphere
and atmosphere.
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The net ecosystem exchange (NEE) of carbon dioxide is now continuously
monitored in over 400 ecosystems via FLUXNET, an international network of
micrometerological tower sites that make use of eddy covariance techniques to measure
the exchange of CO2, H2O and energy between the terrestrial ecosystem and the
atmosphere (FLUXNET, http://daac.ornl.gov/FLUXNET/, accessed August 19, 2009).
However, net flux measurements are not able to separate the two opposing contributions,
photosynthesis and respiration, which are differentially influenced by changing
environmental conditions (Yakir and Sternberg, 2000).
The measurement of the isotopic composition of atmospheric CO2 has emerged as
a very important tool for understanding the flow of carbon between the biosphere and
atmosphere. The measurement of stable carbon isotopes enabled the investigations of
complex interactions that are not possible with the analysis of partial pressures alone.
Keeling’s defining research in the 1950’s (Keeling 1959, 1961) clearly showed the
influence of ecosystem processes on the isotopic composition of atmospheric carbon
dioxide. It has been recognized that photosynthesis and respiration impart unique
isotopic signatures in the atmosphere. The process of photosynthesis discriminates
against the 13CO2 isotopologue of carbon dioxide and preferentially fixes the lighter 12CO2
into plant biomass, leaving the air in the canopy isotopically enriched. On the other hand,
ecosystem respiration returns to the atmosphere isotopically depleted carbon dioxide.
In recent years, there has been an increase in the application of stable carbon
isotope measurements to understand ecosystem carbon exchange. Variations in the
carbon isotopic signature of ecosystem respiration, δ13CR, have been linked to changing
environmental drivers such as precipitation, vapor pressure deficit, and soil moisture
3
content (e.g., Bowling et al., 2002, McDowell et al., 2004, Knohl et al., 2005, Lai et al.,
2005, Pataki et al., 2003). Information from the carbon isotopic composition of net
ecosystem exchange (NEE) has provided an additional constraint to facilitate the
partitioning of NEE into its photosynthetic and respiratory components (Yakir and Wang,
1996, Bowling et al., 2001, Knohl and Buchmann, 2005, Ogee et al., 2003 and 2004,
Zhang, 2006). Isotopic flux partitioning is important, as it will help in understanding the
sensitivity of these isolated fluxes to climatic changes in moisture, temperature and
precipitation. On a global scale, flux partitioning has been also used to partition CO2 into
its land and ocean uptake components (Ciais, et al., 1995a &b, Keeling, et al., 1995,
Battle, et al., 2000).
The main objective of this work is to develop a Fourier transform infrared
spectroscopy (FTIR) instrument for fast, real-time, in-situ measurement of isotopic CO2
exchange (concentration, carbon isotope ratio, and isotopic CO2 flux) between the
terrestrial ecosystem and atmosphere. This dissertation is divided into five chapters. The
first chapter provides (1) background on the concept of carbon isotope ratio and its
application to ecosystem scale processes such as photosynthesis and respiration, (2)
discussion of the conventional and spectroscopic techniques for δ13C – CO2
measurements, (3) description of the development of flux methods for isotopic CO2 flux
measurements, and (4) recent advances in the application of stable isotope measurements.
The next two chapters are manuscripts that will be submitted for publication in Journal of
Agricultural and Forest Meteorology. The first manuscript is entitled “A new Fourier
transform infrared instrument for measurement of temporal and vertical distribution of
δ13C – CO2 1. An overview of results from measurements in a managed poplar forest in
4
northern Oregon”. It describes in detail the development of an FTIR system for
continuous, real-time measurement of isotopic CO2 in forest ecosystem. The third
chapter is entitled “A new Fourier transform infrared instrument for measurement of
temporal and vertical distribution of δ13C – CO2 2. Measurement of ecosystem-
atmosphere exchange of isotopic carbon dioxide using disjunct eddy covariance”. It
describes the development of a new approach combining FTIR spectroscopy with
disjunct eddy covariance, a direct flux measurement technique, for simultaneous
measurement of NEE of CO2, latent heat flux, and isotopic CO2 fluxes. The fourth
chapter is entitled “Analysis of low-concentration gas samples with continuous-flow
IRMS: eliminating sources of contamination to achieve high precision” (Cambaliza et al.,
2009, copyright Wiley-Blackwell). This manuscript resulted from the observation that
there is still no standard procedure/protocol amongst isotope laboratories in analyzing gas
standards using continuous flow isotope ratio mass spectrometry (CF-IRMS). The lack
of a protocol presented a time consuming challenge in analyzing the working gas
standards that were used to calibrate our field measurements. Thus, we first developed a
procedure for preparing and analyzing gas samples on a CF – IRMS. The optimized
procedure was used to ascertain the isotopic composition of our reference cylinders that
were used during field measurements. The fourth chapter discusses (1) sources of
contamination in CF-IRMS analysis of near-ambient CO2 concentration gas samples, (2)
analytical procedures that reduce the contamination, and (3) the identification of a
precise, efficient method for introducing gas samples into vials. The last chapter of this
thesis is a summary of insights gained from this research work and outlines in detail
5
recommendations to improve the instrumentation for future measurements of isotopic
CO2 in forest ecosystems.
1.2. Background on the carbon isotope ratio
This section is focused on providing a theoretical framework for the concept of
stable carbon isotopes as it applies to ecosystem processes, beginning with a discussion
of typical isotopic compositions of various carbon sources. A theoretical framework on
isotopic flux partitioning is given followed by a discussion on photosynthetic
discrimination (Δ) and isotopic composition of ecosystem respiration (δR), their
importance to the science of global change and their measurement.
1.2.1. Definitions and isotopic composition of common sources
Carbon isotope measurements are usually expressed in terms of the “delta
notation” (in per mil or ‰), which gives the ratio of the heavier to the lighter
isotopologue relative to the Vienna Pee Dee Belemnite (VPDB) standard:
�
δ13C =R13sampleR13standard
−1⎡
⎣ ⎢
⎤
⎦ ⎥ ×1000 (1)
where R13 = 13C16O2/12C16O2 is the ratio of the abundances of 13C16O2 and 12C16O2 in the
sample and in the standard, respectively.
The natural variations in the carbon isotopic content of different compounds
(called the isotope effect) are observed whenever chemical bonds are created or destroyed
6
during an enzymatic reaction (termed kinetic fractionation) or during molecular diffusion
where the heavier isotopologue diffuses much slower than the lighter isotopic form
(termed diffusion fractionation). Photosynthesis, as will be discussed later for the C3
photosynthetic pathway, is a clear example where both diffusion and kinetic
fractionations are observed.
Table 1.1 shows the isotopic composition of common carbon sources such as air,
fossil fuels and plant biomass. Emissions from fossil fuel combustion (coal and natural
gas burning as well as flaring) and cement production range from about –24 to –44 ‰
(Ciais et al., 1997) and slowly change the isotopic composition of the air we breathe. The
mean carbon isotope ratio of fossil fuels used by Ciais et al., (1997) in their inverse
model to partition atmospheric CO2 between the land and ocean reservoirs was –28.4 ‰.
They observed a small influence on the inferred ocean/land partitioning when they varied
the isotopic signature of fossil fuels by about 1‰, which corresponded to a change in the
northern hemisphere terrestrial uptake of 0.3 GTC.
Depending on the photosynthetic pathway, plant biomass has an isotopic
signature ranging from either –12 to –15 ‰ (for plants following the C4 photosynthetic
pathway, e.g., grass) or –21 to –35 ‰ (for plants utilizing the C3 pathway, e.g., trees and
shrubs). C3 and C4 photosynthesis are the two most important photosynthetic pathways in
the study of global environmental change with the C3 pathway more widely distributed
globally than C4.
Background CO2 such as found in the unperturbed region above a forest canopy
currently has a molar concentration and carbon isotope ratio of ~385 ppmv (Carbon
dioxide information analysis center, http://cdiac.ornl.gov/trends/co2/sio-mlo.html,
7
accessed August 17, 2009), and ~ -8‰ (Bowling, et al., 2008), respectively. The carbon
isotopic composition of atmospheric CO2 was –6.4 ‰ before the industrial revolution
(Fung et al., 1997) and has since continually decreased to ~-8‰. The dilution of the
heavier isotopologue of carbon dioxide in atmospheric CO2 is due to the continued
uncontrolled use of fossil fuels, which are isotopically lighter than background CO2
(Table 1).
1.2.2. Stable carbon isotopes and ecosystem processes
This section discusses the carbon isotopic composition of ecosystem processes:
photosynthesis and respiration. During daytime, the carbon isotope ratio of air within the
canopy is determined by the combined effects of photosynthetic discrimination against
13CO2 as well as the turbulent mixing of background air with CO2 respired by the
ecosystem. From conservation of mass, the net ecosystem exchange of total CO2 is given
as
�
F = Fe + ρdCa
dt= FR + FA (2)
where
�
Fe = ρw 'Ca' is the eddy CO2 flux, ρ is the density of air, Ca is the ambient carbon
dioxide concentration,
�
ρdCa dt is the storage term, FR is the total ecosystem respiration,
and FA is the net photosynthetic assimilation term (we follow here the notations of
Bowling et al., 2003b). Multiplying each term in equation (2) by its respective isotope
ratio and converting into isotopic notation, we obtain the equation for the isoflux of 13CO2
(Bowling et al., 2001, 2003b, see also Zobitz, et al., 2008)
8
�
δNEEF = δeFe + ρdδaCa
dt= δRFR + δPFA (3)
�
Fδ = δRFR + δ pFA (4)
where
�
Fδ = δNEEF is the isoflux of 13CO2, defined as the product of the net ecosystem
exchange and its isotopic composition (δNEE). We note that the isoflux is analogous to, but
mathematically distinct from, the net ecosystem exchange of 13CO2. δR is the carbon
isotope ratio of ecosystem respiration derived from the nighttime Keeling plot regression
of δ13C versus [CO2]-1 (Keeling, 1958, 1961), δeFe is the eddy isoflux term, and
�
ρdδaCa
dt
is the isotopic storage flux. The carbon isotope ratio of photosynthetic assimilation, δP, is
expressed as
�
δP = δ13Ca − Δcanopy (5)
where δ13Ca is the carbon isotope ratio of ambient CO2, and Δcanopy is the canopy-
integrated photosynthetic discrimination (discussed below). When the ecosystem is in
isotopic disequilibrium, i.e., δR ≠ δP, new information can be obtained from equation (3)
and equations (2), (4), and (5) form the basis for either partitioning the net ecosystem
exchange of total CO2 into FR and FA, or for estimating the canopy-scale photosynthetic
discrimination, Δcanopy. Determination of canopy scale photosynthetic discrimination and
its importance in the science of global change is discussed below.
1.2.3. Determination of the carbon isotope ratio of ecosystem respired CO2
9
The carbon isotope ratio of ecosystem respiration, δR, is determined using the
Keeling plot approach (Keeling, 1958, 1961) or the flux ratio method developed by
Griffis et al. (2004). Here we discuss the classic Keeling plot regression, as it is still as
widely used today. The relationship between variation in the mixing ratio and
corresponding change in the carbon isotopic composition of atmospheric carbon dioxide
was first presented in Charles Keeling’s seminal works in the 1950’s (1958, 1961).
Using results from mass spectrometric analysis of air samples collected in 5-L flasks
from the Pacific coast of North America, he determined a systematic linear relationship
between the carbon isotope ratio, δ13C, and the reciprocal of the CO2 partial pressure of
ambient air:
�
δ13C = i + m 1CO2[ ]
⎛
⎝ ⎜ ⎞
⎠ ⎟ (6)
Keeling used equation (6) to explain changes in the carbon isotopic content of
atmospheric air and to identify the sources that bring about the gradients on a regional
scale. Equation (6) is derived using a two-source mixing model that is based on the
assumption that the atmospheric concentration of species in the ecosystem (CC) is a mass
balance combination of a background concentration (CB) and a contribution from sources
or sinks in the ecosystem (CS) (Pataki et al., 2003, Yakir and Sternberg, 2000):
�
CC = CB + CS (7)
Multiplying both sides of the equation with the carbon isotope ratio of each component,
we obtain
�
δ13CC( )CC = δ13CB( )CB + δ13CS( )CS (8)
Combining equations (7) and (8) and rearranging terms
10
�
δ13CC =CB δ13CB − δ
13CS( )CC
+ δ13CS (9)
Equation (9) is applied in the field by taking simultaneous measurements of the carbon
isotope ratios and partial pressures of carbon dioxide at varying heights in the canopy.
Each sample represents the combined contribution from a background CO2 component
and an additional source component. The Keeling plot regression assumes that the
carbon isotope ratio of the background air and the additional source are constant at the
time of measurement. Pataki et al., (2003) recommends that the Keeling plot approach be
applied only at night when the conditions are stable, photosynthesis has ceased, and the
only additional CO2 source that modifies the concentration of canopy CO2 is from
ecosystem respiration. Application of the Keeling plot during daytime may be difficult,
as the background CO2 may continually change as the ground heats up and the convective
boundary layer develops. Recycling of respired CO2 inside the canopy may also occur
during daytime (i.e., re-fixation of respired CO2, see Sternberg et al., 1997). Under stable
nighttime conditions, the CO2 mixing ratio builds up inside the canopy due to ecosystem
respiration. When all the CO2 is mathematically exhausted from the ecosystem ([CC] →
∝ in equation 9), the Keeling plot intercept represents the integrated carbon isotopic
composition of respiration from all of the aboveground and belowground respiring
sources in the ecosystem, i.e. i = δ13CS = δR.
The main difficulty in applying the Keeling plot approach is that the intercept is
far removed from the measurement data. Small errors in the CO2 and δ13C measurements
may result in significant uncertainty in the intercept. Thus, this difficulty imposes high
precision requirements for the measurement of CO2 and δ13C. Except for the quantum
11
cascade laser absorption spectroscopy (QCLAS), most current spectroscopic techniques
for in situ stable isotope measurements have poorer precisions (on the order of 0.2 to 0.8
‰) relative to conventional isotope ratio mass spectrometry. However, because of the
abundance of data obtained by spectroscopic techniques, averaging of δ13C and CO2
measurements is possible, which significantly improves the precision and accuracy of
determination of the Keeling plot intercept. The ability of spectroscopic techniques to
measure the isotopologues of CO2 independently also enables the use of the concentration
ratio method (Griffis et al., 2004), which relies on the slope of the 13CO2 versus 12CO2
regression line (and not on the intercept) to determine the carbon isotope ratio of
ecosystem respiration, and this will be discussed later.
1.2.4. Determination of canopy – scale photosynthetic discrimination
This section briefly discusses the process of photosynthesis with particular focus
on the C3 photosynthetic pathway. As will be discussed in chapter 2, field experiments
were conducted in a managed poplar forest, a C3 ecosystem. The term C3 photosynthesis
originated from the observation that the first product of this photosynthetic pathway is a
3-carbon molecule. The process of photosynthesis discriminates against the heavier
13CO2 isotopologue of carbon dioxide and preferentially fixes the lighter 12CO2 in the
process of carboxylation. Figure 1 shows a diagram of the C3 photosynthetic pathway.
In this metabolic pathway, isotope effects or fractionations are most strongly expressed in
two major steps: first, during the diffusion of CO2 through the stomatal pathway, and
second, during the enzymatic reaction of CO2 with the enzyme Rubisco (Ribulose
12
Bisphosphate Carboxylase Oxygenase). The discrimination by C3 photosynthesis
(Farquhar et al., 1989) is expressed as:
�
Δ = a + b − a( ) pipa
(10)
where a = 4.4‰ is the fractionation due to the differential diffusivity of 13CO2 and 12CO2
through the stomates, b = ~27.5‰ (Fung, et al., 1997) is the fractionation due to the
discrimination by Rubisco, pi and pa are the intercellular and ambient partial pressures of
CO2, respectively. The magnitude of the discrimination against 13CO2 largely depends on
the intercellular partial pressure of CO2 (pi), which in turn is controlled by the rate of
photosynthesis and stomatal conductance. The stomates are like apertures that are
sensitive to environmental parameters such as light, and moisture. Hence, the
discrimination is an effective indicator of plant response to changing environmental
conditions. Farquhar et al. (1989) provides a comprehensive discussion on the theory of
and environmental effects on carbon isotope discrimination during photosynthesis.
Direct measurement of canopy-scale discrimination (Δcanopy) is not trivial, as it
involves the measurement of isotopic CO2 flux exchanged between the biosphere and
atmosphere. The canopy-integrated discrimination has been determined in the past using
an indirect approach that involves the estimation of the canopy-scale conductance using
the Penman-Montieth equation (Bowling et al., 2001). It is only in recent years that
advances in instrumentation have made it possible to continuously measure the isotopic
composition of carbon dioxide in situ at relatively high time resolution (Bowling et al.,
2003b, Griffis et al., 2008). The ecosystem carbon isotope discrimination is an important
parameter that constrains global estimates of the carbon budget. An overestimation in the
mean global magnitude of Δ by about 3‰ translates to about a 20% underestimation in
13
the strength of the biospheric sink (Fung et al., 1997). We note that from the canopy
scale discrimination, the carbon isotope ratio (δA) of photosynthetic assimilation (FA) can
be estimated using equation (5).
1.3. Measurement of Carbon isotope ratio
The C-stable isotopes of atmospheric CO2 have been traditionally determined by
collecting whole air samples in 2-L flasks followed by off-line cryogenic CO2 extraction
and subsequent analysis on a dual-inlet isotope ratio mass spectrometer (IRMS). While
the use of large sample size has made it possible to achieve very high measurement
precision, it also severely limits the number and location of samples that can be collected
in the field. Cryogenic extraction is also not able to separate contaminant N2O from CO2
(same mass), and a mass balance correction for N2O must be applied to dual-inlet
measurements of CO2 isotopes (Ferretti et al., 2000, Ghosh and Brand, 2003).
The development of continuous flow IRMS in the 1980s and its commercial
appearance in the 1990s made possible the high-precision compound-specific isotope
measurement of a large number of small organic and inorganic samples. Brenna, et al.,
(1997) provided a detailed review of the various continuous flow techniques interfaced to
the IRMS. In 1998, Ehleringer and Cook described the use of the PreCon, an automated
pre-GC concentration device that was originally designed for the analysis of N2O and
CH4 trace gases (Brand, 1995) but was demonstrated to work well for separating and
condensing small volumes of CO2. The PreCon was interfaced to a continuous flow
IRMS to analyze CO2 in air samples collected in small 100-ml vials. In Ehleringer and
14
Cook’s method, a 250-µL sub-sample was extracted with a gas-tight syringe from the
septum-fitted 100-ml vial and injected into the helium carrier stream for analysis.
Because of the small volume requirement, it was possible to sub-sample the flasks for
repeated CO2 analysis leading to reported precisions of < 0.05 ‰. The automated pre-
concentration process and the use of smaller flasks made possible the collection and
analysis of more samples, and the presence of a GC column separated N2O from the CO2,
eliminating the contamination issues described above.
More than a decade ago, Prosser, et al., (1991) described an automated method
for rapid isotopic analysis of high-concentration CO2 from breath samples collected in
13-ml septum-capped vials. The technique involved the use of an autosampler where the
headspace gas was transferred to the helium carrier stream using a double-holed needle
probe. More recently, Tu et al. (2001) described an analogous method, but focused on
the isotopic analysis of near-ambient CO2 samples from 10-ml septum-capped vials for
Keeling plot applications. The concentration of ambient CO2 was sufficiently high that a
direct headspace measurement with the autosampler interfaced to a continuous-flow
IRMS (CF – IRMS) was possible without cryogenic pre-concentration. They found that
this relatively inexpensive method achieves a precision of 0.08 ‰ δ13CVPDB (standard
deviation) and allows for higher collection throughput from the field. While the
continuous-flow method increased the throughput from the field relative to the
conventional dual-inlet IRMS, it was still based on grab samples rather than continuous
measurement of the temporal and spatial distribution of the CO2 concentration and carbon
isotope ratio.
15
A number of spectroscopic techniques for field use in-situ stable isotope
measurements of atmospheric CO2 have recently been developed or evaluated (Bowling,
et al., 2003a, Esler, et al., 2000a & b, Tuzson, et al., 2008, Mohn, et al., 2007, 2008,
Griffith, et al., 1997 & 2006). Table 1.2 shows a summary of these techniques and their
reported precision for δ13C. To date, the quantum cascade laser based absorption
spectroscopic (QCLAS) technique (Tuzson, et al., 2008) has the highest precision and
approaches that of the traditional IRMS method. The tunable diode laser absorption
spectroscopic (TDLAS) technique has been recently applied to investigate ecosystem-
atmosphere interactions (Bowling, et al., 2005, Griffis et al., 2004, 2005, 2008) and to
identify urban CO2 sources (Pataki, et al., 2006). Both the TDLAS and QCLAS make
use of single spectral line pairs for quantifying 12CO2 and 13CO2. Thus, both methods are
limited only to the measurement of single chemical species and are susceptible to
interferent absorbances, as there are numerous uncharacterized trace gases absorbing in
the mid-infrared region in the atmosphere.
In this work, we have considered an alternative optical technique, Fourier
transform infrared spectroscopy (FTIR). FTIR spectroscopy simultaneously measures the
molecular absorption of many atmospheric trace gases using their unique rotational-
vibrational absorption bands in the mid-infrared region, and so is not limited to the
measurement of single species. The large wavelength coverage allows simultaneous
measurement of several important trace gases such as CO2, N2O, CH4, CO, and H2O in a
single air sample. Isotopic substitution changes the distribution of the vibrational and
rotational energy states of a molecule so that independent measurement of a number of
isotopomers, including the 12C16O2 and 13C16O2 isotopomers is possible (Esler, et al.,
16
2000b). Absorption from other interfering species has little effect on the result, as their
spectra merely form part of the residual spectrum after data reduction, thus making the
FTIR spectroscopy approach much less susceptible to interferent gases.
Esler et al. (2000a and b) first described the development of an in situ method for
high-precision stable carbon isotope measurement that is based on Fourier transform
infrared spectrometry. They reported a precision of the order of 0.1 ‰ δ13C with 15-
minute integration times for ambient CO2 samples (Table 2). Recent application of their
system in the field (Griffith et al., 2006) achieved a precision of 0.2 to 0.5 ‰ for
acquisition times of several minutes. More recently, Mohn et al. (2007) developed and
laboratory-tested a calibration strategy for an FTIR system that yielded a precision of
0.15 ‰ obtained at spectral averaging times of 12 (256 scans) to 24 minutes (512 scans).
They tested their system in the field (Mohn et al., 2008) and measured continuously at 1-
minute cycle time (16 scans) where the uncertainty was about 5 times more (~0.75 ‰)
than those obtained at 512 scans. An inherent trade-off exists between precision and
speed of spectral acquisition.
We have developed a new instrument for fast, continuous real-time stable carbon
isotope measurement based on FTIR spectroscopy. Short cycle times between samples
were necessary since the system was developed not only to measure the spatial and
temporal distribution of CO2 – δ13C but also the rapid exchange of isotopic CO2 between
the atmosphere and the terrestrial ecosystem. This was made possible by combining the
FTIR approach with a direct flux measurement method called disjunct eddy covariance
(DEC). The DEC technique is briefly discussed in the next section. Fast field
17
measurements were conducted using four co-added scans corresponding to 15-second
acquisition time per sample.
1.4. Measurement of biosphere-atmosphere exchange of isotopic CO2
Information obtained from isotopic CO2 flux measurements has been used to
partition the net ecosystem exchange (NEE) of CO2 into its photosynthetic (Fa) and
respiratory (Fr) components (Yakir and Wang, 1996, Bowling et al., 2001, Knohl et al.,
2005, Ogee et al., 2003 and 2004, Zhang, 2006). The ability to partition NEE is
important, as it will help facilitate understanding of the sensitivity of these opposing
components to changes in moisture, temperature and nutrient fertilization. Measurement
of the isotopic CO2 fluxes can also constrain to inverse models that estimate the flow of
carbon into various reservoirs (Ciais et al., 1995, Fung et al., 1997). Recently, isotopic
CO2 flux measurements have enabled the development of a flux ratio method that
estimates the signature of ecosystem respiration (δ13CR) directly from the slope of the
nighttime 13CO2 versus 12CO2 fluxes. Thus, the δ13CR calculated from this approach is
directly related to the flux footprint (Griffis et al., 2004).
The application of stable isotope measurements to biosphere – atmosphere
exchange has been limited mainly because of the lack of instrumentation for fast, direct
and continuous measurement of the isotopic CO2 concentrations. In the past, the net
ecosystem exchange of 13CO2 has been estimated using indirect flux measurement
approaches such as the gradient technique (Yakir and Wang, 1996, Bowling, et al.,
2003b, Griffis, 2004), hyperbolic relaxed eddy accumulation (Bowling, et al., 1999a &b,
18
2003b), and the eddy covariance/flask method (Bowling, et al., 1999, 2001). Griffis, et
al. (2008) and Bowling, et al. (2003b) provide detailed reviews of the advantages and
disadvantages of these approaches specifically when applied to isotopic CO2 flux
measurements. The three methods are briefly described here. Notations in equations
(11) to (18) follow Bowling et al. (2003b).
In the flux gradient approach, the flux of some atmospheric constituent (e.g. CO2)
is estimated from the vertical gradient of its concentrations
�
Fe = ρK C1 − C2
z1 − z2(11)
where ρ is the density of air (mol m-3), C1 and C2 are the concentrations (µmol mol-1) at
the two measurement heights (m), z1 and z2, and K is the eddy diffusivity (m2 s-1), which
can be empirically determined from the Monin-Obukhov similarity theory. This
approach assumes similarity in the eddy diffusivities for momentum, energy, and scalars
such as CO2 and its isotopomers.
The isotopic flux is determined by multiplying the concentration terms by the
carbon isotope ratio at each height. Thus, for the gradient technique, the eddy isoflux
term is given as
�
δeFe = ρKδ13C1( )C1 − δ13C2( )C2
z1 − z2(12)
Prior to the application of spectroscopic techniques to isotopic flux gradient
measurements, the isotopic flux was estimated by taking flask measurements at two
heights above the canopy (Yakir and Wang, 1996, Bowling et al., 2003b). The flasks
were taken to the laboratory for analysis at a later time, making the method labor
intensive and with limited the throughput from the field. More recently, Griffis et al.
19
(2004) combined the flux gradient approach with TDLAS, which allowed for more
convenient and continuous measurement of the isotopic CO2 fluxes. However, the
gradient technique still has a number of concerns: (1) it requires empirical corrections to
account for atmospheric stability (Moncrieff, et al., 2000), (2) there is a possible disparity
in the flux footprint associated with the two inlets (Griffis et al., 2008), (3) it remains
difficult to discern small gradients in daytime concentrations (Griffis et al., 2008), and (4)
the approach assumes that the vertical profiles of the concentration are well-defined,
which requires that the inlets must be positioned much higher than the roughness
sublayer. This requirement limits the application of the technique mostly to short
canopies such as those of grasslands and crops (Bowling, et al., 2003b).
The hyperbolic relaxed eddy accumulation (HREA) method is a modification of
the conditional sampling concept of the standard REA technique (Bowling et al., 1999a
and b). While the standard REA method conditionally sampled updrafts/downdrafts
based on vertical wind velocity, the HREA sampling threshold is based on both w and
CO2. Just as in the REA technique, the CO2 flux is estimated from the difference in the
concentration between the updrafts (Cup) and downdrafts (Cdown)
�
Fe = ρbσ w CUP − CDOWN( ) (13)
where σw is the standard deviation in the vertical wind speed (m s-1), and b is an empirical
coefficient. Updraft and downdraft samples are initially collected in flexible bags and
then transferred to flasks for later analysis. Equation 13 can be modified to quantify the
biosphere-atmosphere exchange of 13CO2
�
δeFe = ρbσ w δ13CUP( )CUP − δ13CDOWN( )CDOWN( ) (14)
20
where δ13CUP and δ13CDOWN are the corresponding carbon isotope ratios of the updraft and
downdraft samples. The main concern with the REA method is the ability to detect small
isotopic differences in the updrafts and downdraft samples. Modifying the standard REA
method enabled the detection of as much as 8 – 10 ppmv differences in the CO2
concentrations of the updraft/downdrafts samples. However, to enable detection of small
isotopic differences in the updrafts/ downdrafts, a large fraction of air is discarded (as
much as 80%) because of the relatively high threshold (Bowling, 2003b). Thus, most of
the sampling is limited only to the larger flux containing eddies (Griffis et al., 2008).
In the EC/flask method, fast 10-Hz measurement of the CO2 concentration is
combined with flask measurements to quantify the isoflux (Bowling et al., 1999b, 2001,
2003). In the standard eddy covariance approach, the flux of CO2 and the eddy isoflux
are given as
�
Fe = ρ ′ w Ca′ (15)
�
δeFe = ρ ′ w δ13Ca( )Ca[ ]′ (16)
where the primes represent fluctuations from the mean, and the overbars denote Reynolds
averaging. The carbon isotope ratio of ambient air, δ13Ca, is estimated from flask
measurements taken at various heights in the canopy. For a small range of CO2, a linear
relationship can be established between the CO2 concentration and the carbon isotope
ratio
�
δ13Ca = mCa + a (17)
where m and a are the slope and y-intercept of the regression line. Thus, equation (9) is
now expressed as
21
�
δeFe = ρ ′ w mCa + a( )Ca[ ]′ (18)
This method is attractive mainly because of its simplicity. However, the approach
depends on a limited number of flasks measurements to estimate the fluctuations in 13CO2
and it also implicitly assumes that the relationship between δ13Ca and Ca (equation 17) is
valid at all turbulent time scales.
Recently, Griffis et al. (2008) evaluated the use of the eddy covariance (EC)
technique in combination with tunable diode laser absorption spectroscopy (TDLAS) for
direct and continuous measurement of the isotopic CO2 fluxes. Their long-term
measurements showed very good agreement between the net CO2 flux measured with the
EC-IRGA and the EC-TDLAS systems. The EC approach is the most direct flux
measurement technique and has the fewest theoretical assumptions (for a detailed review
of the EC approach, see Aubinet, et al., 2000). However, its application is limited to a
few atmospheric compounds because it requires instrumentation that has a very fast
response (usually 10 Hz). The TDLAS approach easily satisfies this criterion, as it is
able to scan the single absorption lines of the CO2 isotopomers at a very high rate (500
Hz).
In this dissertation, we combine FTIR spectroscopy with disjunct eddy covariance
(DEC), a direct flux measurement approach to quantify the isoflux in forest ecosystems.
FTIR spectroscopy is a scanning measurement technique that simultaneously measures
the absorption of several infrared-active species. Although the FTIR system cannot scan
at very high speed, it is an attractive alternative, as it is able to detect multiple
atmospheric molecules at the same time. Thus, the fluxes of CO2 and its 12C16O2 and
22
13C16O2 isotopomers, as well as other greenhouse gases (e.g. H2O, N2O, CH4) can be
simultaneously quantified in each air sample.
The DEC approach is a direct flux measurement method but it relaxes the demand
for very fast instrumentation and allows use of a subset of the continuous time series to
accurately measure the fluxes of species of interest (Rinne et al., 2000, 2001, 2008,
Lenschow et al., 1994). In the DEC method, grab samples of air are taken very quickly
(within tenths of a second) but analyzed within tens of seconds to accommodate the use
of only moderately fast instrumentation. Grab sampling occurs at a constant time interval
Δt so that s' and w' are discrete functions of
�
ti = to + iΔt . The flux is determined by
�
Fs = ′ w ′ s =1N
′ w to + iΔt( ) ′ s to + iΔt( )i=1
N
∑ (12)
where the brackets denote averaging within the disjunct time series. Rinne, et al., (2008)
tested the robustness of this new approach by conducting the first field inter-comparison
measurement between the DEC method and the classic eddy covariance technique. They
found very good agreement of the results with slight overestimation (<10%) of daytime
fluxes measured by the DEC technique and underestimation of the nighttime fluxes
relative to the EC results. In recent years, the DEC method has been successfully applied
to measure fluxes of biogenic volatile organic compounds (Rinne, et al., 2001, Karl, et
al., 2003, Karl, et al., 2004, Grabmer, et al., 2004). An up-to-date detailed resource on
the theory and application of disjunct sampling is found in Turnipseed et al., (2009).
23
1.5. Summary
A general overview of stable carbon isotope measurement and its importance to the
science of global environmental change has been given. Because of the importance of
stable isotope measurements in ecosystem and global-scale processes, we can foresee
continued development of spectroscopic instrumentation for fast, continuous, real-time
measurement of ecosystem-atmosphere exchange of isotopic carbon dioxide. Long-term
measurement of δ13C – CO2 in many different types of ecosystems will provide key
information on photosynthetic discrimination and ecosystem respiration carbon isotope
ratio, which were both shown to be sensitive to climatic drivers and hence, can be useful
indicators of ecosystem response to changing environmental conditions. Such long-term,
wide-range measurements will provide important constraints to global inverse models
that partition atmospheric CO2 into its land and ocean uptake components. This
dissertation describes in detail the development of instrumentation for fast, continuous,
real-time measurement of δ13C – CO2 in forest ecosystems that is based on Fourier
transform infrared spectroscopy and disjunct eddy covariance. The results are
encouraging and future long-term measurements will help elucidate the complex
exchange of carbon between the biosphere and atmosphere.
24
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Badeck, F.W., Tcherkez, G., Nogues, S., Piel, C., Ghashghaie, J., 2005. Post-photosynthetic fractionation of stable carbon isotopes between plant organs – awidespread phenomenon. Rapid Communications in Mass Spectrometry, 19, pp.1381 – 1391.
Battle, M., Bender, M.L., Tans, P. P., White, J. W. C., Ellis, J. T., Conway, T., Francey,R. J., 2000. Global carbon sinks and their variability inferred from atmospheric O2
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Bowling, D.R., Delaney, A. C., Turnipseed, A. A., Baldocchi, D. D., Monson, R. K.,1999a. Modification of the relaxed eddy accumulation technique to maximizemeasured scalar mixing ratio differences in updrafts and downdrafts. J. Geophys.Res., 104, pp. 9121 – 9133.
Bowling, D.R., Baldocchi, D. D., Monson, R. K., 1999b. Dynamics of isotopic exchangeof carbon dioxide in a Tennessee deciduous forest. Global Biogeochem. Cycles,13, pp. 903 – 922.
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Bowling, D. R., Sargent, S. D., Tanner, B. D., Ehleringer, J. R., 2003a. Tunable diodelaser absorption spectroscopy for stable isotope studies of ecosystem-atmosphereCO2 exchange. Agricultural and Forest Meteorology, 118, 1 – 19.
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Brand, W. A., 1995. PreCon: A fully automated interface for the Pre-GC concentrationof trace gases on air for isotopic analysis. Isotopes in Environmental and HealthStudies, 31, pp. 277 – 284, doi: 10.1080/10256019508036271.
Brenna, J. T., Corso,T. N., Tobias, H. J., Caimi, R. J., 1997. High – precision continuous-flow isotope ratio mass spectrometry. Mass Spectrometry Reviews, 16, 227–258.
Cambaliza, M. O. L., Harlow, B. A., Ubierna, N., Mount, G. H., Marshall, J. D., Evans,R. D., 2009. Analysis of low-concentration gas samples with continuous-flowisotope ratio mass spectrometry: eliminating sources of contamination to achievehigh precision. Rapid Commun. Mass Spectrom. 23, 3868–3874.
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Table 1.1. Isotopic composition of various carbon sources.
Source δ13C (‰)
Air ~ -8
C3 Plant Biomass (leaves, roots, stem, etc.) ~ -21 to –35
(typically between –25 to –28)
C4 Plant Biomass ~ -12 to –15
Fossil Fuels (oil, coal, natural gas, flaring
and cement production)
~ -24 to –44 (typical mean magnitude used
in global inverse models is –28.4)
32
Table 1.2. Summary of capabilities of various spectroscopic techniques recently
developed or evaluated for continuous, in-situ stable carbon isotope measurements.
Research Group System Precision Remarks
Bowling et al.(2003)
Tunable DiodeLaser Absorptionspectroscopy
0.25 ‰ δ13C
Tuzson et al.(2008)
Quantum CascadeLaser BasedAbsorptionspectroscopy(QCLAS)
0.03 ‰ δ13C, 0.05‰ δ18O
Esler et al.(2000b)
Fourier TransformInfraredSpectroscopy(FTIR)
0.15 ‰ δ13C 15-min integration time(256-coadded scans)
Griffith et al.(2006)
FTIR 0.2 to 0.5 ‰ δ13C Recent field application ofthe same FTIR systemdescribed by Esler et al.(2000b)
Mohn et al.(2007)
FTIR 0.15 ‰ δ13C 12 to 24-min integrationtime (256 to 512 co-addedscans)
Mohn et al.(2008)
FTIR ~0.7 ‰ δ13C; 24-min averages of 1-min measurementswere done to obtain0.15 ‰ δ13Cprecision similar tolaboratory results
1-min spectral averagingtime (16 scans); Fieldtesting results of systemdescribed by Mohn et al.(2007);
33
Figure 1.1. Carbon isotopic discrimination by C3 photosynthesis. This image was taken
from the Biosphere-atmosphere stable isotope network website (www.basinisotopes.org).
34
CHAPTER 2
A NEW FOURIER-TRANSFORM INFRARED INSTRUMENT FORMEASUREMENT OF TEMPORAL AND VERTICAL DISTRIBUTION
OF δ13C – CO2 1. AN OVERVIEW OF RESULTS FROM MEASUREMENTSIN A MANAGED POPLAR FOREST IN NORTHERN OREGON.
2.1. Abstract
Stable isotopic analysis of atmospheric carbon dioxide is essential for
understanding the complex carbon exchange between the biosphere and the atmosphere.
Isotopic abundances have traditionally been measured by laboratory mass spectrometric
analysis of grab samples taken from field sites. This approach has limited the throughput
and frequency of measurements. We describe an alternative approach that is based on
Fourier-transform infrared spectroscopy (FTIR) for fast, real-time, in-situ measurement
of the stable carbon isotope ratio of carbon dioxide in ambient air. Molecular absorption
was measured in a 4-pass 1-meter length cell in the 2100 to 2500 cm-1 region of the 13CO2
and 12CO2 isotopic vibration-rotation bands. From the absorption data, concentrations of
both isotopomers were measured; from the ratio of their concentrations, δ13C was
determined. We demonstrate the capabilities of this technique using measurements in a
poplar forest plantation near Boardman, Oregon. Measurements were made over several
summers at six levels on a 22-meter tower erected in a 16-m canopy. Long-term
measurement of our reference gas in the field yielded accuracy for the forest
measurements of 0.5 ‰ and precision of 0.8 ‰, respectively, at 45-second cycle time
between samples. Nighttime and daytime vertical profiles of δ13C and CO2 were
35
measured. The signature of ecosystem respiration, δ13CR, derived from the Keeling-plot
and concentration-ratio methods, was -26.6‰ and -25.8‰, respectively, about 2‰ more
enriched than the carbon isotope ratio of bulk leaf samples. CO2 respired from leaves has
been previously reported to be enriched relative to bulk tissue δ13C. Comparison of δ13CR
with the isotopic signature of soil respiration, δ13CSR, showed that ecosystem respired CO2
was also more enriched than soil-respired CO2. These results are consistent with recent
findings using conventional methods. Because δ13CR represents the overall contribution
from all respiring sources, we can expect from mass balance calculations that the isotopic
composition of ecosystem respiration will have a value that is intermediate between the
isotopic ratios of respired CO2 from above and below ground components. We describe
here (1) the details of the experiment, (2) concentration determination, (3) isotopic ratio
determination, and (4) initial results from the Boardman poplar forest.
36
2.2. Introduction
The stable isotopic composition of carbon dioxide exchanged between the
biosphere and atmosphere provides useful information about the physiological and
ecological processes controlling the carbon cycle. The measurement of stable carbon
isotopes (e.g., Keeling, 1958) enabled the investigations of complex interactions between
the biosphere and atmosphere that are not possible from the analysis of mixing ratios
alone. Since Keeling’s seminal work fifty years ago (Keeling, 1958, 1961), stable carbon
isotope measurements have been used to study the response of ecosystems to changing
environmental drivers (e.g., Bowling et al., 2002, McDowell et al., 2004, Knohl et al.,
2005, Lai et al., 2005, Pataki et al., 2003) and to investigate the effects of land use change
on ecosystem respiration (Griffis, et al., 2005). Information from the carbon isotopic
composition of net ecosystem exchange (NEE) has provided an additional constraint to
facilitate the partitioning of NEE into its photosynthetic and respiratory components
(Yakir and Wang, 1996, Bowling et al., 2001, Knohl et al., 2005, Ogee et al., 2003 and
2004, Zhang et al., 2006). On a global scale, stable carbon isotope measurements have
been used to partition CO2 into its land and ocean uptake components (Ciais et al., 1995a
&b, Keeling et al., 1995, Fung et al., 1997). All these investigations were possible
because photosynthesis discriminates against the heavier 13CO2 isotopologue and
preferentially fixes the lighter 12CO2 into plant biomass. The stable carbon isotope ratio
can therefore be used as a natural tracer for ecosystem processes such as photosynthesis
37
and respiration. A comprehensive resource on the theory of carbon isotope
discrimination is found in Farquhar et al. (1989).
The stable isotopes of CO2 in air have conventionally been measured by taking
grab samples from the field followed by analysis in the laboratory with an isotope ratio
mass spectrometer (IRMS). Traditionally, the samples were collected in 2-L flasks
followed by cryogenic CO2 extraction and subsequent analysis on a dual-inlet IRMS.
This method provides the best precision for isotope measurements (< 0.1 ‰). However,
the cryogenic pre-concentration process is tedious, time-consuming, and requires an
experienced operator, as fractionation and contamination may occur at any step (Brenna,
et al, 1997). The entire process limits the number of samples that can be collected from
the field. Cryogenic pre-concentration is also unable to separate N2O from CO2 because
of the similar vapor pressure versus temperature relationships of these two species
(Ferretti, et al., 2000). The N2O must be separated on a column or eliminated with a mass
balance correction.
In recent years, Tu et al. (2003) and Knohl et al. (2004) demonstrated that it is
possible to use continuous-flow isotope ratio mass spectrometry (CF-IRMS) for the
analysis of small grab samples collected in inexpensive 10 or 12-ml septum-capped vials
without compromising precision. While this method increased the sample throughput
from the field relative to the traditional method, it still does not allow for continuous
measurement of the spatial and temporal distributions of the carbon isotope ratio.
A number of spectroscopic techniques for in-situ stable isotope measurements of
atmospheric CO2 have recently been developed. Bowling et al. (2003) evaluated a
tunable diode laser absorption spectrometer (TDLAS) for measuring the carbon isotopic
38
composition of air at ambient CO2 mixing ratios of 350 - 700 ppmv and δ13C abundance
of –6 to – 16 ‰. They reported a precision of 0.25 ‰ δ13C and 0.4% for CO2 mole
fraction (1.6 ppmv at 400 ppmv). The TDLAS method has since then been applied to
investigate ecosystem-atmosphere interactions (Bowling et al., 2005, Griffis et al., 2004,
2005, 2008). More recently, Tuzson, et al. (2008) described a quantum cascade laser
based absorption spectrometer (QCLAS) designed for high-precision, in-situ monitoring
of atmospheric CO2 with a precision of 0.03 ‰ δ13C and 0.05 ‰ δ18O, respectively. To
date, the QCLAS technique has the highest precision and approaches that of the
traditional IRMS method. Both the TDLAS and QCLAS make use of single spectral line
pairs for quantifying 12CO2 and 13CO2. Thus, both methods are limited only to the
measurement of carbon dioxide, and single line methods are always susceptible to
interferent absorbances from the real atmosphere.
We have considered an alternative optical technique, Fourier transform infrared
spectroscopy (FTIR), which measures simultaneously the molecular absorption of
atmospheric trace gases using their unique rotational-vibrational absorption bands in the
mid-infrared region. FTIR spectroscopy is an attractive method as it is not limited to the
measurement of carbon dioxide; the large wavelength coverage allows simultaneous
measurement of important trace gases such as CO2, N2O, CH4, CO, and H2O in a single
air sample. Because isotopic substitution alters the distribution of the vibrational and
rotational energy states of a molecule, the 12C16O2 and 13C16O2 isotopomers of carbon
dioxide can be independently and simultaneously measured (Esler, et al., 2000b).
Absorption from other interfering species does not affect the result, as their spectra
39
merely form part of the residual spectrum after data reduction. Thus, the reduction of
FTIR spectra is much less susceptible to interferent gases.
Esler, et al. (2000a and b) first described the development of an in situ method for
high-precision carbon isotope measurement that is based on Fourier transform infrared
spectrometry. They reported a precision of the order of 0.1 ‰ δ13C with 15-minute
integration times for ambient CO2 samples. A recent application of their system in the
field (Griffith et al., 2006) achieved a precision of 0.2 to 0.5 ‰ for spectral acquisitions
of several minutes. More recently, Mohn, et al. (2007) developed and laboratory-tested a
calibration strategy for an FTIR system and yielded a precision of 0.15 ‰ obtained at
spectral averaging times of 12 (256 scans) to 24 minutes (512 scans). They tested their
system in the field (Mohn et al., 2008) and measured continuously at 1-minute cycle time
(16 scans) where the uncertainty was about five times more (~0.75 ‰) than those
obtained using 24-min periods (512 scans). An inherent trade-off exists between
precision and speed of spectral acquisition. To address this, they took 24-minute
averages of their 1-minute measurements to improve their precision similar to that
obtained in the laboratory.
In this work, we describe in detail the development of an FTIR system for
continuous real-time measurement of isotopic CO2 in forest ecosystems for determination
of [CO2], δ13C, and isotopic CO2 fluxes. Fast measurements were done at four co-added
scans equivalent to short spectral acquisition time of 15 seconds, yielding an accuracy
and precision of 0.5 ‰ and 0.8 ‰, respectively. Fast measurements and short cycle
times between samples were necessary, as the system was developed not only to measure
the temporal and spatial variation of δ13C – CO2 but also to capture the rapid exchange of
40
isotopic CO2 by combining FTIR with disjunct eddy covariance (DEC), a direct flux
measurement method (Rinne and Guenther, 2001, Rinne et al., 2008, Turnipseed, et al.,
2009). Detailed results of our isotopic CO2 flux measurements will be reported in a
companion paper. In this paper, we demonstrate the capability of our system to
simultaneously measure the temporal and spatial distributions of carbon dioxide and δ13C
in real time in a forest ecosystem.
2.3. Methodology
2.3.1. Site Description
Measurements were made from a 22-meter tower erected in a poplar forest
plantation located near Boardman, Oregon (45.8°N, 119.7°W). Experiments were
conducted for 4 weeks in each of the summers of 2005 and 2006. The poplar forest,
managed by Greenwood Resources (http://www.greenwoodresources.com/),
encompassed an area of about 100 km2 in flat terrain with an elevation 202 meters a.s.l.
The site was drip irrigated six days a week during the growing season. The poplar trees
were genetically identical and grew as much as 3 m per year for at least 7 years before
they were harvested and chipped for pulp. The average canopy height was 16 m in 2006.
The soil was sandy with little or no organic matter. There were almost no understory
plants growing under the canopy and there was little accumulation of litter.
On sampling days, the daylight hours were sunny, clear, and warm with
maximum temperatures generally greater than 30°C. Prevailing winds showed a diurnal
41
pattern. During days characterized by slow wind speeds (< 4 m/s), the winds came from
the southeast and southwest at night (between 165 and 240°) and shifted to northeast and
northwest during the day (between 350 and 60°). During days characterized by high
wind speeds (> 4 m/s with wind gusts as much as 12 m/s), the nighttime prevailing wind
direction was consistently from the southwest between 210 and 240° while daytime wind
direction was mainly from the west between 255 and 285°.
2.3.2. Description of experiment
Air samples were analyzed at 0.5 cm-1 spectral resolution using a Bruker Optics
V22 FTIR spectrometer (Bruker Optik GmbH, Ettlingen, Germany) with a globar light
source and a liquid nitrogen-cooled InSb detector (Teledyne Judson Technologies,
Montgomeryville, PA). The InSb detector is sensitive in the mid-infrared region from
1800 – 4000 cm-1 (5.5 µm – 2.5 µm) with specific peak detectivity of 2.11x1011 cm Hz1/2
W-1. The FTIR system was enclosed in a plexiglass housing that was temperature-
controlled at 27 OC ± 0.1 OC. The 135-ml sample gas cell was enclosed within the FTIR
itself with a heating sleeve that was controlled to a temperature of 33°C ± 0.1 OC.
Two steps were taken to increase the precision of our spectrometric data. First, it
was necessary to eliminate CO2 absorption in that part of the optical path outside the
sample gas cell. This was accomplished by continuously purging the interferometer and
detector compartments with nitrogen gas at 6L min-1. Second, sample gas pressure and
gas cell temperature were monitored using a solid-state pressure transducer (Scientific
42
Technologies, Inc., Logan, UT.) and a Pt RTD temperature sensor (Omega Engineering,
Inc., Stamford, CT.). Analyses were conducted at pressures of about 700 mbar.
Using molecular absorption in a 4-pass 1-meter length cell in the 2060 to 2310
cm-1 region of the 12CO2 and 13CO2 isotopic vibration-rotation bands, concentrations of
both isotopomers were derived from the measured spectra. From the concentration ratio
of the two isotopomers, the carbon isotope ratio (δ13C) was determined. This region was
used to minimize nonlinearity and deviations from the Beer-Lambert law. In the middle
of this window (2310 cm-1), maximum peak absorbance for 12CO2 was ~ 0.15. Esler et al.
(2000a) experimentally determined for a similar FTIR set-up that the degree of
nonlinearity was not a source of systematic error as long as the analyzed spectra included
only rotational-vibrational lines with measured absorbances less than 0.4. The spectral
window we used for our analysis has rotational-vibrational lines well within this
absorbance limit.
A schematic diagram of the field experiment set-up is shown in Figure 2.1. Air
samples from the canopy were introduced into the instrument by pulling air from inlet
valves at various heights on a 22-meter tower so that vertical profiles or fixed-height
measurements could be performed. Inlet valves were installed in the tower sampling line
(2.5-cm i.d. PVC pipe system) and positioned at 0.3, 3.9, 7.3, 10.7, 13.7, and 20.4 m. A
blower running at 190 l/min flow pulled air from the tower toward the system at all times.
The measurement system was composed of the disjunct eddy covariance (DEC)
sampler and the FTIR spectrometer. The DEC sampler consisted of a 1-L stainless steel
intermediate sampling reservoir (ISR) and a set of fast valves to control and direct the
airflow. The DEC sampler took fast grab samples either for isotopic CO2 flux
43
measurements or for vertical profile and fixed-height δ13C – CO2 measurements. From a
continuous air flow of 190 L min-1 in the 2.5-cm PVC sampling line, a fast time section
of rapidly moving air was trapped into the 1-L ISR using fast valves with ~15 ms
response times (Parker Hannifin Corp, New Britain, CT and Leybold Vakuum,
Germany). When the valves were engaged (closed), the fast moving air was diverted into
a by-pass line to prevent overheating of the high-volume pump (The Spencer Turbine
Company, Windsor, CT). The isolated air sample in the ISR was then introduced into the
evacuated FTIR gas cell. Analysis relied on four co-added FTIR scans corresponding to
a 15-second measurement integration time. While the air sample in the gas cell was
analyzed, the fast valves at each end of the reservoir were activated (opened) to allow the
rapidly moving air to flow continuously through the ISR. After analysis in the FTIR, the
gas cell was flushed with zero air and evacuated in preparation for the next cycle. The
total cycle time between samples was about 45 seconds. RMS noise in the system was ~3
parts in 105 for a 45-second cycle time (4 co-added scans, 15-second measurement
integration time). Sample acquisition and recording of data were computer-controlled
using a program written in the LabView language (National Instruments, Austin, TX).
During vertical profile measurements, two quick, “snap shot” samples were taken
from each height, cycling progressively from the lowest inlet to the highest intake valve.
One vertical profile was completed every 15 minutes for the six-valve system utilized.
Two background lamp spectra and a calibration spectrum from a calibrated gas bottle of
CO2 were analyzed approximately every 50 minutes. In addition, a background spectrum
was taken after completing one vertical profile. When analyzing background or
calibration samples, the gas cell was initially purged by rinsing with either instrument
44
grade zero air or reference tank air before filling to approximately 700-mb pressure with
the analyte gas.
Environmental parameters such as air temperature, relative humidity,
photosynthetically active radiation (PAR), net radiation, soil temperature and volumetric
water content were also monitored at the site. All environmental parameters were stored
at 1-min temporal averages in the computer using two Campbell data loggers (Campbell
Scientific, Inc., UT).
2.3.3. Leaf sampling, soil respiration measurement and isotope analysis
Leaves from various heights in the canopy were collected and transported to the
laboratory, dried at 65°C for 48 hours and subsequently analyzed for δ13C. Eighteen static
closed chambers (Mora and Raich, 2007) were installed on the ground in a radial pattern,
extending out from the flux tower in clusters of three chambers. The CO2 in the chamber
headspace was allowed to equilibrate for a period of 24 hours. Following the method of
Mora and Raich (2007), a gas sample was withdrawn from the chamber headspace and
analyzed within 48 hours. At equilibrium, the carbon isotopic composition of the
chamber headspace is assumed to be equal to the isotopic content of the CO2 pool in the
soil. The CO2 concentration and isotopic content of the soil pool was then used to
calculate for the isotopic composition of the soil respiration flux using the theory
developed by Ubierna et al., (2009) and Cernusak et al., (2004):
�
δ13CSR = (δ p − a) ×Cp
Cp − Catm
⎡
⎣ ⎢
⎤
⎦ ⎥ − δatm − a( ) × Catm
Cp − Catm
⎡
⎣ ⎢
⎤
⎦ ⎥ (1)
45
where δ13CSR is the isotopic composition of soil respiration, δp and Cp are the carbon
isotopic composition and CO2 concentrations of the soil pool, respectively, (measured
from the chamber head space), a is the kinetic fractionation term due to molecular
diffusion (equal to 4.4‰), and δatm and Catm are the carbon isotope ratio and CO2
concentration of atmospheric CO2, respectively (assumed to be –8‰ and 380 ppmv,
respectively).
All isotopic analyses of leaf and soil respiration samples were performed at the
Idaho Stable Isotopes Laboratory (ISIL) in the Department of Forest Resources,
University of Idaho. Gas samples were analyzed for δ13C using a Finnigan GasBench II
interfaced to a Delta XP IRMS (Finnigan MAT, Bremem, Germany). Leaf samples were
analyzed for δ13C on a NC 2500 EA (Carlo Erba Instrument, Milan, Italy) interfaced to a
Delta+ IRMS (Finnigan, MAT, Bremen, Germany). Carbon isotope measurements are
usually expressed in terms of the delta notation (in per mil or ‰) relative to the Vienna
Pee Dee Belemnite (VPDB) standard:
�
δ13C =R13sampleR13standard
−1⎡
⎣ ⎢
⎤
⎦ ⎥ ×1000 (2)
where R13 = 13C/12C is the ratio of the abundances of 13C and 12C in the sample and in the
standard.
2.3.4. Analysis of Measured Spectra
The mixing ratios of 13CO2 and 12CO2 and other IR active species (H2O, N2O, and
CO) were determined from the measured single beam spectrum using an iterative least
46
squares fitting technique (MALT: Multiple Atmospheric Layer Transmission developed
by Griffith at the University of Woolongong, Australia, see Griffith, 1996, Esler, et al.,
2000a and b). The multi-component FTIR spectrum was analyzed by first calculating a
set of reference spectra for all species based on their HITRAN (Rothman, et al., 2008)
molecular line parameters at the measured temperature and pressure for the observation.
MALT incorporates the FTIR instrument line shape parameters such as the apodization,
field of view, and resolution as well as the environmental parameters such as gas
temperature and pressure so that the reference spectrum closely simulates the actual
spectrum measured by the instrument. The reference spectrum was then fitted to the
measured spectrum using non-linear least squares quantitative analysis with MALT
(Griffith, et al., 2003 conference, Griffith, et al., 2006) to separately determine the
concentrations of the species by minimizing the residual spectrum in an iterative fitting
process. This quantitative analysis provides molecular mixing ratios with accuracies
better than ± 5% depending on the accuracy of the molecular line parameters in the
HITRAN database (Griffith, et al., 2006) and accurate characterization of the instrument
line shape function (Griffith, et al., 2003). Higher accuracy is achieved by regular
calibration of the FTIR system using reference gases. Figure 2.2 shows a typical example
of the measured, fit and residual spectra where we observe a small average peak-to-peak
variation of about 5 parts in 104 (RMS level ~ 0.02%) in the residual spectrum after
iteratively fitting the measured spectrum with the reference spectrum using the non-linear
least squares technique. This exhibits a very high quality fit to the observational data for
both [CO2] and δ13C.
47
2.3.5. Calibration
Four calibration cylinders were used in the field. Table 2.1 shows the
concentration and carbon isotope ratios of the standards. One reference tank was
dedicated for online calibration measurements. When performing fixed-height
experiments, online calibration measurements were as frequent as every half-hour. As
stated above, online calibration was conducted every 50 minutes during vertical profile
measurements. In addition, offline calibrations were also carried out in the field with all
four tanks analyzed on a daily basis.
2.4. Results and discussion
Analytical precision in the field was determined from the results of the online
calibration runs. Figure 2.3 shows the carbon isotope ratio of calibration tank #1 over a
period of approximately five days from July 23 to 27, 2006. The average carbon isotope
ratio was determined to be –9.6 ‰ with a standard deviation of ± 0.8 ‰ (n = 182). The
actual carbon isotope ratio of tank #1 was determined from isotope ratio mass
spectrometry measurements to be –10.14 ± 0.04 ‰. Based on the results of these long-
term measurements, we determined our analytical precision and accuracy to be 0.8 and
0.5 ‰, respectively, at 45-second measurement cycle time. The corresponding precision
for CO2 concentrations was determined to be 1.3 ppmv at 380 ppmv. On the ecosystem
scale, the simultaneous measurement of δ13C and CO2 is particularly critical for
partitioning the net ecosystem exchange (NEE) into its photosynthetic assimilation (FA)
48
and ecosystem respiration (FR) components. To determine the importance of
uncertainties of the order of 0.8‰, a sensitivity analysis was conducted on the net
photosynthetic assimilation flux, FA. Using the theory developed by Bowling, et al.
(2001), and their noontime values for the NEE, isoflux, and stomatal conductance, we
determined that a change of 0.8 ‰ in the carbon isotope ratio of ecosystem respiration
(δ13CR) results in a change in the assimilation flux of the order of 1%. On the global
scale, Badeck et al. (2005) determined that a change in the signature of respiration of the
order of 0.7‰ results in a change in the biospheric sink by about 5%. These are small
changes.
The nighttime vertical profiles of CO2 and δ13C were measured at the Boardman,
OR site to determine the carbon isotope ratio of ecosystem respiration. Figures 2.4a) and
b) show the simultaneous nighttime vertical and temporal profile measurements of CO2
and δ13C on July 22, 2006 from 12:00 midnight to 5:30 AM. Winds generally came from
the south, with magnitudes ranging from 1 – 3 ms-1 at the time of measurement. The
vertical profiles of CO2 and δ13C were the result of taking one-hour running averages of
the “snap shot” measurements taken at each height. A one-hour average is equivalent to
the average of eight spectra at each height (equivalent to 32 co-added scans). The dashed
line in the contour plots marks the canopy height (~16 m). High CO2 concentrations were
observed within the canopy because of the combined effects of vegetation and soil
respiration, stable nighttime conditions, and slow winds. The ecosystem becomes a
source of CO2 at night and the slow winds and the shallow boundary layer allow for the
accumulation of the respired CO2 within the canopy. The highest CO2 concentrations
were observed closest to the ground where the corresponding carbon isotope ratios were
49
most depleted (more negative). The CO2 concentrations progressively decreased with
increasing height and approached background values above the canopy. Conversely, the
corresponding carbon isotope ratios increased with height. A clear inverse relationship
was observed between the CO2 concentrations and δ13C. Figures 2.4c) and d) show the
contour plots of the standard deviations resulting from the running averages in Figures
2.4a) and b).
Because of photosynthesis, daytime contour plots of CO2 and δ13C (Figure 2.5a)
and b), respectively) show that the air within and above the canopy was isotopically
enriched (δ13C was less negative) and that the carbon dioxide concentrations were lower
relative to background values. The CO2 concentrations and the corresponding δ13C values
were also uniformly distributed within and above the canopy indicating that there was
good air mixing due to the unstable conditions. At the time of measurement, the winds
generally came from the north and northeast direction ranging from 1 - 3 ms-1 and high
air temperature rising from 29ºC at noon to 32ºC at 14h PST. Contour plots of the
corresponding standard deviations of the one-hour running averages are shown in Figures
2.5c) and d). Because of air turbulence, the natural variation in daytime CO2 was small
and the standard deviation in δ13C was essentially determined by the instrument precision.
Following the approach of Keeling (1958, 1961), the carbon isotopic composition
of ecosystem respiration, δ13CR, was obtained from the y-intercept (the source term) of
the nighttime δ13C versus [CO2]-1 plot (Figure 2.6a). The y-intercept represents the
integrated carbon isotope ratio of the respired CO2 from all sources in the ecosystem. To
investigate the effect of data averaging on the isotopic composition of ecosystem
50
respiration, we applied various averaging times on our nighttime data set (i.e., no
averaging, 30-min, 1-hr, 2-hr, and 3-hr running average) and determined δ13CR using the
Keeling plot approach. The result of the calculation is shown in Table 2.2. As expected,
the Pearson correlation coefficient increased with averaging time but the isotopic
signature of respired carbon was essentially stable and insensitive to the smoothing
process. For further analysis, we have chosen the intermediate averaging period of one
hour to smooth the noise without losing the details and natural variations in the data.
The Keeling plot has been traditionally used to determine δ13CR because
conventional mass spectrometry directly compares the isotopic ratio of samples to that of
the standards. Because techniques such as FTIR spectroscopy directly measure the
concentrations of the isotopologues of carbon dioxide, the signature of ecosystem
respiration can be alternatively calculated from the slope of the linear fit of the 13CO2
versus 12CO2 concentration plot (Figure 2.6b) using the following relationship derived
from equation (1) (Griffis, et al., 2004)
�
13CO2 = 1+δ13CR
1000⎛ ⎝ ⎜
⎞ ⎠ ⎟ × Rstandard ×
12CO2 (3)
Both methods yielded similar values for δ13CR within the limits of their standard
deviations, which were –26.6 ± 0.5 ‰ and –25.8 ± 0.5 ‰ for the Keeling plot and
concentration ratio methods, respectively.
We compared the signature of ecosystem respiration to the carbon isotopic
composition of leaves collected from various heights in the canopy (Figure 2.7). As
expected, a strong vertical gradient in the carbon isotopic composition of leaves was
observed. Leaf δ13C was about 3 ‰ more enriched (less negative) close to the top (at 12
51
m) relative to the bottom (~ 2 m) of the canopy. This result is consistent with the findings
of other groups (Duursma and Marshall, 2006, Ometto et al., 2006, Berry et al., 1997,
Broadmeadow et al., 1992) and has been attributed to gradients in light intensity and
nitrogen concentration within the canopy. The controls over Ci/Ca and leaf δ13C have
been discussed in detail in the literature (Farquhar, et al., 1989) where Ci/Ca is the ratio
of the intercellular and ambient partial pressures of carbon dioxide. Greater Ci/Ca was
observed for leaf samples deeper into the canopy where light intensities were weaker and
N concentrations lower relative to the top. Higher Ci/Ca in turn results in more depleted
leaf carbon isotope ratios. The dashed line in Figure 2.7 marks the average leaf carbon
isotope ratio (-27.6 ‰) in the canopy. We find that the respired ecosystem carbon
dioxide (δ13CR = -26.6 ± 0.5 ‰) was enriched relative to the bulk leaf carbon isotopic
composition by as much as ~ 2 ‰. Although the carbon isotope ratio of ecosystem
respiration represents the integrated contributions from various components (above and
below ground compartments), our result is consistent with recent findings that respired
CO2 is enriched relative to its substrates (Ghashghaie et al., 2001, Ocheltree and
Marshall, 2004, Klump et al., 2005).
We also compared the signature of ecosystem respiration against the carbon
isotope ratio of soil respiration. Table 2.3 shows the average concentration and isotopic
composition of CO2 in the soil pool for the clusters of three static closed chambers and
the corresponding isotopic signature of the soil respiration flux. The resulting average
isotopic composition of soil respiration flux was δ13CSR = –28.2 ± 0.5 ‰. We find that
the respired ecosystem carbon dioxide (δ13CR = -26.6 ± 0.5 ‰) was also enriched by
about 1.6 ‰ relative to soil respired CO2. This result was consistent with the findings of
52
Klump, et al., (2005). They found that above ground respiration (from shoots) was
isotopically enriched relative to below ground respiration (from roots) by about 4 ‰ on
average. Because the carbon isotope composition of ecosystem respiration represents the
combined contributions from all sources, we expect from mass balance calculations that
δ13CR will have a value that is intermediate between the respired fluxes from above- and
below-ground compartments.
2.5. Conclusions
We describe in detail a new in-situ, real-time measurement technique for stable
isotope measurements that is based on Fourier Transform Infrared spectroscopy (FTIR).
Based on long-term measurement of our gas standard in the field, we report an accuracy
and precision for isotopic ratios of 0.5 and 0.8 ‰, respectively, corresponding to a 15-
second spectral acquisition time. The instrument can run virtually unattended for
24h/day taking data at various heights in a canopy or at a single height on time centers
less than one minute. This time resolution is sufficient for application of disjunct eddy
correlation to determine the biosphere-atmosphere exchange of isotopic CO2 fluxes. We
deployed the FTIR system in a managed poplar forest for three summers where temporal
and spatial distributions of CO2 and δ13C were simultaneous measured. The signature of
ecosystem respiration, δ13CR, derived from the Keeling plot and concentration ratio
method was more enriched than the carbon isotope ratio of bulk leaf samples by as much
as 2 ‰. This result was consistent with current findings that the respired CO2 is more
enriched than its organic substrates. Comparison of δ13CR with the isotopic signature of
53
soil respiration, δ13CSR, showed that ecosystem respired CO2 was also more enriched than
soil respired CO2. Recent findings have shown that above ground respiration is
isotopically enriched relative to below ground respiration. Because δ13CR represents the
overall contribution from all respiring sources, we can expect from mass balance
calculations that the isotopic composition of ecosystem respiration will have a value that
is intermediate between the isotopic ratios of respired CO2 from above- and below-
ground components. The ability of the system to measure the temporal and spatial
distribution of CO2 and δ13C with a temporal resolution of less than one minute
continuously 24h/day is encouraging, and future long-term measurements will allow us to
investigate the complex exchange of carbon between the atmosphere and the terrestrial
ecosystem.
The main advantage of FTIR spectroscopy relative to other existing spectroscopic
techniques is its ability to simultaneously measure CO2 and its isotopomers, as well as
other important greenhouse gases. This contrasts, e.g., with a TDLAS and QCLAS
systems, which measure only a single absorption feature of a single isotopomer (a
minimum of two absorption lines is required to obtain a ratio). With the many poorly
understood trace gases present in the real atmosphere, as opposed to laboratory
conditions, the possibility of interfering spectral absorbance from those other species at
the wavelengths chosen for the measurement is very real and will change the isotopic
ratio determined. By measuring many spectral absorption features of the different
isotopomers simultaneously, it is easy to discriminate against any interferent species.
When combined with the disjunct eddy covariance flux measurement technique, the
FTIR method becomes a powerful tool for quantifying not only the concentrations of
54
molecular isotopes and their ratios, but also the rapid exchange of these important trace
gases between terrestrial ecosystems and the atmosphere. This approach offers the
exciting possibility of applying FTIR spectroscopy not only to forest ecosystems but also
to agricultural and urban environments.
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Tu K.P., Brooks P.D., Dawson T.E., 2001. Using septum-capped vials with continuous-flow isotope ratio mass spectrometric analysis of atmospheric CO2 for Keeling plotapplications. Rapid Commun. Mass Spectrom, 15, 952 - 956.
Turnipseed, A. A., Pressley, S. N., Lamb, B., Nemitz, E., Allwine, E., Cooper, W. A.,Shertz, S., Guenther A. B., 2009. The use of disjunct eddy sampling methods for thedetermination of ecosystem level fluxes of trace gases. Atm. Chem. Phys. 9, 981 –994.
Tuzson, B., Mohn, J., Zeeman, M. J., Werner, R. A., Eugster, W., Zahniser, M.S.,Nelson, D. D., Mcmanus, J. B., Emmenegger, L., 2008. High precision andcontinuous field measurements of δ13C and δ18O in carbon dioxide with a cryogen-free QCLAS. Appl. Phys. B, 92, 451–458.
Ubierna, N., Marshall, J.D. and Cernusak, L. (2009) A new method to measure carbonisotope composition of CO2 respired by trees: stem CO2 equilibration. Functionalecology, pp., accepted.
Yakir, D., Wang, X. F., 1996. Fluxes of CO2 and water between terrestrial vegetationand the atmosphere estimated from isotope measurements. Nature, 380, 515 – 517.
Zhang, J., Griffis, T. J., Baker, J. M., 2006. Using continuous stable isotopemeasurements to partition net ecosystem CO2 exchange. Plant, Cell andEnvironment, 29, 483 – 496.
59
Table 2. 1. Concentrations and carbon isotope ratios of four reference tanks used in the
field.
Tank No. Concentration (ppmv, ± 1%) Carbon isotope Ratio (‰)
1 378.7 -9.98 ± 0.08
2 383.7 -39.15 ± 0.03
3 324.7 -39.7 ± 0.12
4 449.5 -39.87 ± 0.06
60
Table 2. 2. Keeling plot estimates of the isotopic composition of respired carbon as a
function of length of averaging time.
Length of RunningAverage
Y-intercept, δ13CR Slope Pearson’s correlation,Pr
No averaging -26.2 ± 1 7172 ± 420 0.73
30-min -26.7 ± 0.5 7426 ± 238 0.89
1-hr -26.6 ± 0.5 7360 ± 199 0.93
2-hr -26.0 ± 0.3 7110 ± 130 0.97
3-hr -25.8 ± 0.2 7062 ± 108 0.98
61
Table 2. 3. CO2 concentrations and carbon isotope composition of soil pool measured
from the headspace of the static closed chambers. The carbon isotope ratio of soil
respiration flux, δ13CSR, is calculated from equation (2). Values are shown as the average
for the clusters of three chambers. Standard deviations are shown in the parenthesis.
Cluster Number Soil pool CO2
(ppmv)
Soil pool δ13C (‰) δ13CSR (‰)
1 6330 (1401) -22.5 (1.1) -27.8 (0.9)
2 3336 (1413) -22.1 (1.2) -28.6 (0.5)
3 4408 (2743) -22.3 (1.0) -28.4 (0.2)
4 4437 (229) -22.8 (0.1) -28.5 (0.1)
5 4683 (336) -22.0 (0.1) -27.7 (0.1)
6 5281 (639) -22.5 (0.6) -28.1 (0.5)
62
Figure 2. 1. Schematic diagram of field experiment set-up.
63
Figure 2. 2. Examples of typical measured, fit and residual spectrum resulting from the
application of the nonlinear least squares analysis to reduce for the 12CO2 and 13CO2
concentrations in the actual data. The RMS fit was ~ 0.02%.
1.1
1.0
0.9
0.8
0.7
0.6
Spec
tral
Inte
nsity
228022402200216021202080
wavenumber (cm-1)
-0.0010
-0.0005
0.0000
0.0005
0.0010
measured fit residual 13CO2 P-branch
13CO2 R-branch mixed with12CO2 P-branch
64
Figure 2. 3. Long-term on-line calibration measurements taken in the field from August
11 to 17, 2006. Analytical precision and accuracy were determined to be 0.8 and 0.5 ‰,
respectively.
-12
-11
-10
-9
-8
-7
δ 13
C (‰
)
12:00 AM7/23/06
12:00 AM7/24/06
12:00 AM7/25/06
12:00 AM7/26/06
12:00 AM7/27/06
PST
FTIR measurements average FTIR value actual IRMS value
65
Figure 2. 4. Nighttime vertical and temporal distributions of a) CO2 and b) δ13C. The corresponding standard deviations to the
one-hour running averages are shown in c) for CO2, and d) for δ13C.
20
15
10
5
Hei
ght
(m)
1:00 AM7/22/06
2:00 AM 3:00 AM 4:00 AM 5:00 AM
PSTA)
460 460 455 455 450
445
445
440 440
440 435 430 425
420 415 410 405
20
15
10
5
Hei
ght
(m)
1:00 AM7/22/06
2:00 AM 3:00 AM 4:00 AM 5:00 AM
PSTB)
-8 -8.2 -8.4 -8.6
-8.8
-8.
8 -
8.8
-8.
8
-8.8 -9 -9.2
-9.4
-9.4
-9.6
-9.8
-10
-10 -10.2
-10.2
-10.2
-10
.2
-10
.2
-10
.4
-10
.4
-10.4
-10.4 -10.6 -10.8
20
15
10
5
Hei
ght
(m)
1:00 AM7/22/06
2:00 AM 3:00 AM 4:00 AM 5:00 AM
PSTC)
22 20
18 16
16 14 14
14 14
12
12
12 12
12
12
12
10 10
10
10
10
8
8
8
8
8
8
6 6
6
4 20
15
10
5
Hei
ght
(m)
1:00 AM7/22/06
2:00 AM 3:00 AM 4:00 AM 5:00 AM
PSTD)
1.4 1.3 1.3
1.3
1.2 1.2
1.2
1.1
1.1
1
1
1
1
0.9
0.9
0.9
0.9
0.9
0.8
0.8
0.8 0.8
0.8
0.8
0.7
0.7
0.7
0.7
0.7
0.7
0.6
0.6
0.6
0.6 0.6
0.6
0.6
0.5
0.5
0.5
0.5
0.4
0.4
0.3
0.3
66
Figure 2. 5 . Daytime vertical and temporal distribution of a) CO2 and b) δ13C. The corresponding standard deviations
to the one-hour running averages are shown in c) for CO2, and d) for δ13C.
20
15
10
5
Hei
ght
(m)
12:30 PM7/20/06
1:00 PM 1:30 PM 2:00 PM
PSTA)
364
363 363
363
362
362
362
362 362
361
361
361
360
360
359
359 359
358
358
358 358
357
357
357
357
356
356
356
356
20
15
10
5
Hei
ght
(m)
12:30 PM7/20/06
1:00 PM 1:30 PM 2:00 PM
PSTB)
-6.8 -6.9
-6.9
-6.9 -6.9
-7
-7
-7
-7
-7
-7
-7
-7.
1
-7.1
-7.1 -7.1
-7.1 -7.2
-7.2
-7.2
-7.
2
-7.2
-7.3
-7.3 -7.3
-7.4 -7.4
-7.4
-7.4
-7.5
-7.6 -7.6
-7.7 -7.7
20
15
10
5
Hei
ght
(m)
12:30 PM7/20/06
1:00 PM 1:30 PM 2:00 PM
PSTC)
8.5 8
7.5
7.5
7
6.5
6
6 5.5
5.5
5.5
5 5
5
4.5 4.5
4.5
4.5
4 4
4
4
3.5 3.5
3.5
3
3
2.5
20
15
10
5
Hei
ght
(m)
12:30 PM7/20/06
1:00 PM 1:30 PM 2:00 PM
PSTD)
0.9
5
0.9
5
0.9 0.9
0.9
0.85
0.8
0.8
0.8
0.8
0.8 0.75
0.75 0.75
0.75
0.75
0.7
0.7
0.7
0.7
0.7
0.65
0.65
0.65
0.6
0.6 0.6
0.6
0.6
0.6
0.6
0.55
0.55
0.5
0.5
0.5
0.5
0.45
0.4
5
0.4
67
Figure 2. 6. Estimate of the signature of ecosystem respiration, δ13CR, calculated from
(a) Keeling plot approach (δ13CR = –26.6 ± 0.5 ‰, y-intercept), and (b) concentration
ratio method (δ13CR = –25.8 ± 0.5 ‰).
5.6
5.4
5.2
5.0
4.8
4.6
13CO
2 (p
pmv)
47046045044043042041012CO2 (ppmv)
B)
y = a + bx, Pr = 0.99a = 0.082927 ± 0.00229b = 0.011348 ± 5.17e-06
δ13CR = -25.8 ± 0.5
July 22, 200612 midnight to 5:30 a.m.
-12
-11
-10
-9
-8
-7
-6δ
13C
(‰)
2.50x10-32.452.402.352.302.252.202.15
CO2-1 (ppmv-1)
A)
y = a + bx, Pr = 0.93a = -26.6 ± 0.5b = 7360 ± 199
From 12 midnight to 5:30 a.m.July 22, 20061-Hr Ave data
68
Figure 2. 7. Carbon isotopic composition of leaves as a function of height. Dashed line
indicates the average leaf δ13C. Error bars represent the precision of the isotope ratio
mass spectrometer (± 0.1‰).
14
12
10
8
6
4
2
0
Heig
ht (
m)
-30 -29 -28 -27 -26 -25 -24
δ13C of leaves (‰)
69
CHAPTER 3
A NEW FOURIER TRANSFORM INFRARED INSTRUMENT FORMEASUREMENT OF TEMPORAL AND VERTICAL DISTRIBUTION OF δ13C –
CO2 2. MEASUREMENT OF ECOSYSTEM-ATMOSPHERE EXCHANGE OFISOTOPIC CARBON DIOXIDE USING DISJUNCT EDDY COVARIANCE
3.1. Abstract
Techniques for ecosystem carbon isotopic flux measurements include the gradient
method, hyperbolic relaxed eddy accumulation, flask/eddy covariance method, and more
recently, a combination of the tunable diode laser absorption spectroscopy with eddy
covariance, a direct flux measurement technique. We describe an alternative approach
that is based on Fourier-transform infrared spectroscopy and disjunct eddy covariance
(FTIR – DEC). FTIR – DEC is a direct flux measurement technique that allows for the
simultaneous determination of the fluxes of several important trace gases such as CO2
and its isotopologues, and H2O, N2O, CO and CH4. Flux measurements of CO2 and its
isotopologues were conducted in a poplar forest plantation near Boardman, Oregon in the
summers of 2005 and 2006. A comparison of the CO2 fluxes derived from FTIR – DEC
derived against conventional eddy covariance measurements using an infrared gas
analysis (IRGA - EC) showed very good agreement between the two approaches (within
10%, Pr = 0.89), which lends credibility to the isotopic fluxes. The main source of
statistical uncertainty in our flux measurements is the slow time resolution of the FTIR –
DEC approach relative to continuous EC measurements. This resolution issue can be
addressed in future measurements by decreasing the cycle time between samples or
70
increasing the averaging interval. The measurement of the biosphere-atmosphere
exchange of isotopic carbon dioxide provides constraints that enable (1) partitioning of
the net ecosystem exchange (NEE) of CO2 into its one-way flux components:
photosynthetic assimilation and ecosystem respiration, or (2) direct determination of the
canopy-integrated photosynthetic discrimination. Simultaneous FTIR measurements of
CO2 and its isotopic composition enabled the direct determination of the isoflux of 13CO2
in the forest. Information from the isoflux and the NEE were used to estimate the canopy
scale discrimination, Δcanopy, and the carbon isotope ratio (δP) of the photosynthetic
assimilation flux. We find that the average δP (-24.6 ± 5.1 ‰) was 13C – enriched
compared with the mean isotopic composition (-27.6 ± 1 ‰) of bulk leaf organic matter,
which agrees with observations that the carbon isotope ratio of recent assimilates are
more enriched then the bulk leaf material from which they were extracted. Initial results
with the FTIR – DEC approach are encouraging and future measurements will find
applications not only in forest ecosystems but also in agricultural and urban
environments.
71
3.2. Introduction
The measurement of the isotopic composition and fluxes of carbon dioxide is an
important tool for understanding the complex cycling of carbon in the biosphere and
atmosphere. On the ecosystem scale, the measurement of the net ecosystem exchange of
isotopic CO2 provides additional constraints to partition the daytime total CO2 flux into
its components: gross photosynthetic assimilation (FA), and ecosystem respiration (FR)
(Yakir and Wang, 1996, Bowling et al., 2001, Knohl and Buchmann, 2005, Ogee et al.,
2003, Zhang et al., 2006). The competing processes of photosynthesis and respiration
impart unique isotopic effects in the atmosphere. Photosynthesis preferentially
assimilates the lighter isotopologue of carbon dioxide, leaving the ambient air
isotopically enriched in the canopy. Respiration, on the other hand, releases isotopically
depleted carbon dioxide. The ability to distinguish these two processes through isotopic
flux partitioning is important, as it allows for the independent investigation of the
sensitivity of photosynthesis and respiration to changing environmental conditions (Yakir
and Sternberg, 2000).
The measurement of isotopic CO2 fluxes also enables the direct determination of
the canopy-scale discrimination (Δcanopy) (Griffis et al., 2005, Bowling et al., 2003a),
which characterizes the isotopic fractionation against 13CO2 during photosynthesis. At
the leaf level, Farquhar et al., (1989) described the discrimination as
�
Δ = a + b − a( ) pipa
(1)
where a is the fractionation associated with gaseous diffusion through the stomates
(4.4‰), b is the enzymatic fractionation during carboxylation (~27‰), pi and pa are the
72
partial pressures of CO2 inside the leaf and in the ambient atmosphere. Quantifying the
canopy-scale discrimination is not trivial, especially for heterogeneous ecosystems. In
the past, it has been indirectly estimated using a big-leaf model, which involves the
estimation of the canopy-scale conductance using the Penman-Montieth equation
(Bowling et al., 2001). The measurement of ecosystem discrimination is important, as it
provides much needed constraint to global inversion models that partition anthropogenic
CO2 emissions into their land and ocean uptake components (Fung et al., 1997, Ciais et
al., 1995, Randerson et al., 2002). Fung et al., (1997) showed that a 3‰ overestimation
of the global- averaged discrimination translates to a 20% underestimation of the strength
of the biospheric sink.
The biosphere-atmosphere interaction of isotopic CO2 has been measured in the
past using indirect flux measurement techniques such as the gradient method (Yakir and
Wang, 1996, Griffis et al. 2004), hyperbolic relaxed eddy accumulation (Bowling et al.,
1999, 2003a), and a combination of flask and eddy covariance (flask/EC) methods
(Bowling et al., 1999, 2001, 2003a, Knohl and Buchmann, 2005). Each of these
approaches has advantages and disadvantages, which are discussed in detail by Bowling
et al (2003a) and Griffis et al. (2008).
More recently, the isoflux (defined as the product of the flux and its isotopic
composition) has been directly measured using the eddy covariance approach combined
with tunable diode laser absorption spectroscopy (TDLAS) (Griffis et al. 2008). The eddy
covariance (EC) method is the most direct and robust approach for measuring vertical
fluxes of atmospheric constituents in the turbulently mixed planetary boundary layer. In
73
the EC method, the flux of a scalar property s is the integral of the covariance of the
fluctuations of s and the vertical wind speed w within the time interval [tA, tB]
�
Fs = ′ w ′ s =1
t2 − t1′ w t( )
t1
t2∫ ′ s t( )dt (2)
where
�
′ w t( ) = w t( ) − w and
�
′ s t( ) = s t( ) − s , and the over bar refers to the time average.
It requires very fast instrumentation to capture the fluctuation in w and s at all significant
frequencies. The TDLAS measures the concentration of CO2 at a very fast rate (2 ms
sampling time) using single absorption lines for each of the isotopologues of carbon
dioxide (Bowling et al., 2003b). Thus, the TDLAS easily satisfies the instrument
requirement of the eddy covariance approach. However, because a single TDLAS uses
only single spectral lines for quantifying 12CO2, 13CO2, 12C18O16O, it is limited only to the
measurement of the isotopic CO2 fluxes.
Here we present an alternative flux measurement approach that is based on
Fourier transform infrared spectroscopy (FTIR) and disjunct eddy covariance (DEC)
(Rinne and Guenther, 2001). The DEC method makes direct eddy covariance
measurements but relaxes the demand for instantaneous concentration measurements. In
the DEC method, grab samples of air are taken very quickly (with tenths of a second time
resolution) but analyzed within tens of seconds to accommodate the use of only
moderately fast instrumentation. Grab sampling occurs at a constant time interval Δt so
that s' and w' are discrete functions of
�
ti = to + iΔt . The flux is determined by
�
Fs = ′ w ′ s =1N
′ w to + iΔt( ) ′ s to + iΔt( )i=1
N
∑ (3)
where the brackets denote averaging within the disjunct time series. Rinne, et al., (2008)
tested the robustness of this new approach by conducting the first field intercomparison
74
measurement between the DEC method and the classic eddy covariance technique and
found very good agreement of the results with slight overestimation (<10%) of daytime
fluxes measured by the DEC technique and underestimation of nighttime fluxes relative
to the EC results. In recent years, the DEC method has been successfully applied to
measure fluxes of biogenic volatile organic compounds (Rinne and Guenther, 2001, Karl,
et al., 2003, 2004, Grabmer, et al., 2004).
FTIR spectroscopy measures the molecular absorption of IR – active species
using multiple rotational – vibrational lines over a wide wavelength region, which allows
for the simultaneous measurement of several species such as CO2 and its isotopologues,
and H2O, N2O, CO, and CH4 (Griffith et al., 2006, Esler et al., 2000a & b, Mohn et al.,
2007, 2008). Spectral integration times may range from seconds to several minutes,
depending on the number of co-added scans chosen for the analysis. For disjunct eddy
covariance flux measurements, there is only a small increase in the statistical error (≤
8%) if the time interval between grab samples is less than the integral time scale of
turbulence (Lenschow et al., 1994), which is usually between 15 to 60 seconds in a forest
(Turnipseed et al., 2009).
In this work, we (1) compare the measured fluxes from the FTIR – DEC approach
with the results from conventional IRGA – EC method for (nonisotopic) carbon dioxide
and water vapor, (2) report the isoflux of 13CO2 measured by the FTIR – DEC technique,
and (3) estimate the canopy scale photosynthetic discrimination, Δcanopy, in a genetically
uniform poplar forest using the measured NEE and isoflux. We discuss the main sources
of errors in our isotopic CO2 measurements and suggest procedures to increase accuracy
in the future.
75
3.3. Theory and Methods
3.3.1. Canopy-scale photosynthetic discrimination
From conservation of mass, the net ecosystem exchange of total CO2 is given as
�
F = Fe +dCa
dt= FR + FA (4)
where
�
Fe = w 'Ca' is the total CO2 flux measured by eddy covariance or disjunct eddy
covariance,
�
dCa dt is the storage term, FR is the total ecosystem respiration, and FA is the
net photosynthetic assimilation term (for convenience, we follow here the notations of
Bowling et al., 2003a). Multiplying each term in equation (4) by its respective isotope
ratio and converting into isotopic notation, we obtain the isoflux of 13CO2 (Bowling et al.,
2001, 2003a, see also Zobitz, et al., 2008)
�
δNEEF = δeFe +dδaCa
dt= δRFR + δPFA (5)
�
Fδ = δRFR + δ pFA (6)
where
�
Fδ = δNEEF is the isoflux of 13CO2, δR is the carbon isotope ratio of ecosystem
respiration derived from the nighttime Keeling plot regression of δ13C versus [CO2]-1
(Keeling, 1958, 1961), δeFe is the eddy isoflux term measured by the FTIR – DEC
approach, and
�
dδaCa
dt is the isotopic storage flux. The carbon isotope ratio of
photosynthetic assimilation, δP, is expressed as
76
�
δP = δ13Ca − Δcanopy (7)
where δ13Ca is the carbon isotope ratio of ambient CO2 (assumed to be -8‰), and Δcanopy is
the canopy-integrated photosynthetic discrimination. When the ecosystem is in isotopic
disequilibrium, i.e., δR ≠ δA, new information can be obtained from equation (5) and
equations (4), (6), and (7) form the basis for either partitioning the net ecosystem
exchange of total CO2 into FR and FA, or for estimating the canopy-scale photosynthetic
discrimination, Δcanopy. In this work, we estimated Δcanopy by first partitioning F into its
components (FR and FA) using conventional regression of nighttime flux with temperature
for well-mixed conditions (u* > 0.1 ms-1) where nighttime
�
F = FR = f T( ) (Goulden et al.,
1996, 1997, Bowling et al., 2003a, Griffis et al., 2005). The daytime photosynthetic
assimilation was then calculated from
�
FA = F − FR . Because of windy and well-mixed
conditions for both daytime and nighttime measurements during our summer
observations, we assume that the storage CO2 flux and the storage isoflux terms are
negligible in the succeeding calculations (Aubinet et al., 2000) and the net ecosystem
exchange of CO2 is equivalent to the measured eddy flux. Under this assumption, the
isoflux of 13CO2 is simply equivalent to the eddy isoflux term
�
Fδ = δeFe = w ' δ13CO2 × CO2( )' . We note that the isoflux is analogous to, but
mathematically distinct from, the net ecosystem exchange of 13CO2 (Bowling et al.,
2003a). The carbon isotope ratio of ecosystem respiration, δ13CR, was taken to be – 26.6 ±
0.5 ‰, which was the intercept of the Keeling plot regression of our nighttime vertical
profile measurements on July 22, 2006.
77
3.3.2. Site description and meteorological conditions
Flux measurements were made from a 22-meter tower installed in a managed
poplar forest near Boardman, Oregon (45.8ºN, 119.7ºW). The site has an elevation of
202 meters a.s.l., and encompassed an area of approximately 100 km2 of genetically
identical poplars in a nearly flat terrain. Trees were drip irrigated six days/week during
the growing season. The soil was sandy and there were almost no understory growing
under the canopy. Viable flux measurements were conducted for a period of
approximately seven days (July 25 – 29, August 15 – 17) in the summer of 2006. The
average canopy height was 16 m. The site was managed by Greenwood Resources
(http:://www.greenwoodresources.com/).
Figures 3.1 and 3.2 show the meteorological conditions at the time of
measurement given in terms of the diurnal averages of the air temperature,
photosynthetically active radiation (PAR), friction velocity (u*), and polar plots of wind
speed and wind direction. Daylight hours were sunny and warm (Figures 3.1a and b),
with maximum average temperature as high as 30ºC, and midday PAR was ~1000 µmol
m-2 s-1. Average diurnal low temperature was ~17ºC. The measurement period was also
characterized by high wind speeds with magnitudes ranging from 3 ms-1 to ≥ 6 ms-1 at
least 60% of the time (Figures 3.2a and b). The prevailing winds also displayed a diurnal
pattern. Daytime wind direction was consistently from the west between 255º and 285º
while nighttime prevailing winds were mainly from the southwest between 210º and
240º. Strong turbulent mixing was also observed at the time of measurement as indicated
by the large magnitudes of the average diurnal friction velocity (u*), which ranged from
0.4 to 0.9 ms-1 (Figure 3.1c).
78
3.3.3. Flux measurement and spectroscopic analysis of air samples
Figure 3.3 shows the schematic diagram of the field experiment set-up
(Cambaliza et al, 2010, to be submitted to Agricultural and Forest Meteorology). The
conventional eddy covariance (EC) system was composed of a 3-D acoustic anemometer
(WindMaster Pro Ultrasonic Anemometer, Gill Instruments Ltd., Hampshire, England)
and an open-path infrared gas analyzer (IRGA) (OP-2 Open-path CO2/H2O Analyser,
ADC BioScientific Ltd., Herts, England). The anemometer and IRGA were installed at
the top of the tower at the end of a 3-meter boom. To avoid flow distortions from the
tower, the boom was mounted towards the west, as the prevailing wind directions were
mainly from the west and southwest. The IRGA was calibrated for CO2 once a week
using two Scott-Marin Inc gas standards (331 and 430 ppmv ± 2%). The water vapor
signal was compared with measurements from a Vaisala HMP45A humidity sensor on
the tower. The comparison was used as an approach for calibration of the IRGA water
vapor response. Both the DEC and EC systems used the data from the sonic
anemometer.
The disjunct eddy covariance (DEC) system was composed of a DEC sampler and
the FTIR spectrometer (Figure 3.3, Cambaliza et al 2010). The DEC sampler consisted
of a 1-L intermediate sampling reservoir (ISR) and a set of fast valves that control and
direct the airflow from the sampler to the spectrometer. Using a high-volume blower (The
Spencer Turbine Company, CT, USA) running continuously at 190 L min-1, air samples
from the canopy were introduced into the DEC system by pulling air from inlet valves
installed at six positions (0.3, 3.9, 7.3, 10.7, 13.7, and 20.4 m) on the tower sampling line
(2.5-cm i.d. PVC pipe system). The set of six intake valves positioned at various heights
79
on the tower and under computer control coordinated with air sampling enabled us to
carry out either vertical profile or fixed height measurements. CO2 flux and isoflux
measurements were performed by sampling air from the highest inlet valve positioned at
20.4 m (1.3 x canopy height). The total length of the sampling line from the inlet close to
the acoustic anemometer to the FTIR spectrometer was 42.5 m. From a continuous air
flow of 190 L min-1 in the PVC sampling line, a time section of the rapidly moving air
was trapped in the ISR using fast valves with ~ 15 ms response times (Parker Hannifin
Corp, New Britain, CT, USA and Leybold Vakuum, Germany). When the valves were
engaged (closed), the fast moving air was diverted into a by-pass line to avoid heating the
blower. The isolated air trapped in the ISR was then transferred to the FTIR gas cell for
analysis using four co-added FTIR spectral scans corresponding to a 15-s measurement
integration time. While the air sample in the gas cell was analyzed, the fast valves
positioned at each end of the ISR were activated (opened) to allow the rapidly moving air
to flow through the reservoir. After analysis in the FTIR, the gas cell was evacuated in
preparation for the next sample. The total cycle time between samples was 45 seconds
due to cell flushing, cell filling, pressure stabilization, and measurement. FTIR
calibrations were performed using standard gases every 30 minutes.
Spectroscopic measurement of air samples was performed at 0.5 cm-1 spectral
resolution using a Bruker Optics V22 FTIR spectrometer (Bruker Optik GmbH,
Ettlingen, Germany) with a globar light source and a liquid nitrogen-cooled Indium
Antimonide detector (Teledyne Judson Technologies, Montgomeryville, PA). The FTIR
spectrometer was enclosed in a plexiglass housing that was temperature controlled at
27ºC. The 135-mL gas cell within the FTIR was enclosed in a heating sleeve and
80
temperature controlled at 33 ± 0.1ºC. CO2 absorption in the optical path outside the gas
cell was eliminated by continuously purging the interferometer and the detector
compartments with nitrogen gas at 6L min-1. Sample pressure and temperature was
monitored and recorded using a solid-state pressure transducer (Scientific Technologies,
Inc., Logan, UT) and a Pt RTD temperature sensor (Omega Engineering, Inc., Stamford,
CT). Concentrations of the 12CO2 and 13CO2 isotopomers and other IR active species such
as H2O, CO, and N2O were derived from the single beam spectrum using molecular
absorption in the 4-pass 1-m length cell in the 2060-2310 cm-1 region. The spectra were
analyzed using a program called MALT (Multiple Atmospheric Layer Transmission)
(developed by Griffith, 1996, 2006, see also Esler et al., 2000a and b), which employs an
iterative non-linear least squares fitting approach. From the individual concentrations of
the isotopomers of CO2, the carbon isotope ratio (δ13C) was determined. Calibration for
CO2 and δ13C was achieved using four calibration cylinders (Table 1). One reference tank
was dedicated for online calibration measurements performed every half-hour. In
addition, offline calibrations were also carried out in the field with all four cylinders
analyzed on a daily basis. Tanks 1 and 2 are standard gas mixtures from Scott – Marin,
Inc (Riverside, CA). Tanks 3 and 4 are reference cylinders prepared from the dilution of
carbon isotope reference gases (Oztech Trading Corp., Safford, AZ) with instrument
grade zero air. The carbon isotope ratios of the reference gases were determined using
continuous-flow isotope ratio mass spectrometry following the method of Cambaliza, et
al (2009).
The lag time between the sonic anemometer and the DEC sampler was
determined by releasing puffs of high-CO2 sample gas at the highest inlet and recording
81
the time it takes for the puffs to be detected by the FTIR system. The recorded lag time
was 7.4 ± 0.5 seconds. DEC and EC fluxes were averaged over 30 minutes and standard
density corrections were applied (WPL corrections, Webb et al., 1980). For details of
the experimental instrumentation, see Cambaliza et al, (2010).
3.3.4. Leaf sampling and isotopic analyses
Leaves from various heights in the canopy were collected and transported to the
laboratory, dried at 65°C for 48 hours and subsequently analyzed for δ13C. Isotopic
analyses for δ13C of leaf samples were performed at the Idaho Stable Isotopes Laboratory
(ISIL) in the Department of Forest Resources, University of Idaho, on a NC 2500 EA
(Carlo Erba Instrument, Milan, Italy) interfaced to a Delta+ IRMS (Finnigan, MAT,
Bremen, Germany). Carbon isotope measurements are usually expressed in terms of the
delta notation (in per mil or ‰) relative to the Vienna Pee Dee Belemnite (VPDB)
standard:
�
δ13C =R13sampleR13standard
−1⎡
⎣ ⎢
⎤
⎦ ⎥ ×1000 (8)
where R13 = 13C/12C is the ratio of the abundances of 13C and 12C in the sample and in the
standard.
82
3.4. RESULTS AND DISCUSSION
3.4.1. FTIR – DEC and IRGA – EC Measurements of CO2 and H2O fluxes
Figure 3.4a shows the half-hour averaged CO2 fluxes measured by the FTIR –
DEC and IRGA – EC systems for a period of approximately seven days. The measured
fluxes from the two systems track each other well; this is clearly observed in the diurnal
averages (Figure 3.4b). The maximum daytime total CO2 flux was about –10 µmols m-2
s-1, which roughly balanced the nighttime respiration. The errors bars shown in Figure
3.4b represent the standard deviations about the half-hour mean values. The FTIR – DEC
measurements displayed larger standard deviations, which are attributed to the reduced
time resolution of the disjunct sampling method compared to the continuous sampling EC
approach (45 second FTIR – DEC cycle time versus 100 ms IRGA – EC time resolution).
Turnipseed et al. (2009) empirically determined the error associated with disjunct
sampling at various sampling time intervals (Δt = 1 to 60s) relative to continuous eddy
covariance measurement. As Δt increased, a larger spread in the data was observed as
expected, since fewer points were used to estimate the covariance. At Δt = 45s, these
authors predicted errors as large as 45% in the DEC – derived fluxes. We plotted the
FTIR – DEC diurnal averages versus the IRGA – EC estimates and found that the
measured fluxes from the two systems agree to within 10% (Figure 3.4c), which is well
within the statistical error predicted by Turnipseed et al. (2009).
Figure 3.5 shows the latent heat flux measured by the two systems. The FTIR –
DEC approach consistently underestimated the IRGA – EC derived fluxes. A
83
comparison of the diurnal averages (Figure 3.5b) from the two systems reveals that the
FTIR – DEC captures about 75% of the IRGA – EC estimates (Figure 3.5c). Our result is
consistent with the observations of other groups that have used long sampling tubes for
the measurement of water flux (Ammann et al., 2006, Goulden et al., 1996). Using a
closed path IRGA – EC system, Goulden et al., (1996) observed the damping of high-
frequency turbulent fluctuations for H2O (but not for CO2 fluxes) and attributed it to
adsorption and desorption of water vapor within the sampling line. Ammann et al. (2006)
applied an empirical ogive approach to correct for the high-frequency losses due to the
adsorption effects and found excellent agreement between their closed-path DEC
measurements with conventional open-path IRGA – EC estimates. The use of a sampling
tube by itself already adds uncertainty in the flux measurements because the radial
gradient in the air velocity dampens the fluctuations in the gas concentrations (Leuning
and Judd, 1996). However, the excellent agreement between the FTIR – DEC and IRGA
– EC estimates for CO2 demonstrates that the low-pass filtering effect of the sampling
tube does not add considerable uncertainty in the DEC derived flux measurements. But
for the case of water vapor, there is always an additional uncertainty due to tube-related
dampening effects.
3.4.2. Isoflux estimates, isotopic flux partitioning, and discrimination estimates
The half-hour diurnal average of the isoflux is shown in Figure 3.6. Maximum
daytime isoflux was ~ 200 µmols m-2 s-1 ‰. We observe a diurnal pattern that mirrors the
total CO2 flux (shown in Figure 3.4). Daytime isoflux was positive, which is consistent
84
with positive discrimination against 13CO2 during photosynthesis. Nighttime isoflux was
negative, which corresponded with the release of depleted (more negative) CO2 during
respiration. As expected, the isoflux also displayed considerable variation about the half-
hour diurnal averages and this observation can be attributed to the lack of a large number
of disjunct sampling points within the 30-minute averaging interval.
Using conventional regression of nighttime CO2 flux against air temperature
(Figure 3.7), the total daytime CO2 flux was partitioned into its photosynthetic and
respiratory components (Figure 3.8). Following the approach of Bowling et al., (2003a),
Figure 3.7 shows the daily nighttime CO2 flux plotted against air temperature (open
circles) as well as the average CO2 flux binned in 1ºC intervals (solid circles). The error
bars represent the standard deviations of the averaging process. In general, we observe a
very weak exponential relationship despite the wide range of temperature values. When
we include the outlier at T = 24ºC, the correlation was 0.77; excluding the outlier
significantly improved the correlation (R2 = 0.94) between FR and air temperature. To
optimize the partitioning of F into FA and FR, we used the exponential relationship
resulting from the exclusion of the outlier in the succeeding calculations.
The total CO2 flux and its components are shown in Figure 3.8. As expected, the
respiration flux, FR, showed the same diurnal pattern as the air temperature in Figure 3.1a.
The maximum respiration flux (~ 16 µmol m-2 s-1) occurred at around 16:00 PST. The
assimilation flux, FA, closely mirrors the diurnal average of the photosynthetically active
radiation (PAR) shown in Figure 3.1b. FA was essentially zero at 19:30h until about
5:00h and peaked at approximately noontime with maximum magnitude of about –25
µmol m-2 s-1. We propagated the errors (for this case, the standard deviation of the
85
diurnal averages of F and FR) using the method of Taylor (1997) to calculate the
corresponding standard deviations of the diurnal average of FA shown in Figure 3.8.
Using equations (4), (6) and (7), we calculated the canopy scale discrimination
(Figure 3.9). We did not observe a trend or a diurnal pattern in the discrimination but
found the hourly daytime values to be randomly scattered about an average daytime
magnitude of 16.6 ± 5.1 ‰. We find the average canopy discrimination to be lower than
the mean discrimination reported by Bowling et al., (2003a) for an irrigated alfalfa field
in Utah by about 1 ‰. It was also lower than the mean C3 discrimination estimated by
Pataki et al., (2003) for 33 sites of coniferous and deciduous forests in North and South
America by about 1.4 ‰. However, it is in closer agreement with the flux-weighted
canopy discrimination estimated by Bowling et al. (2001) using a big-leaf analogue
model for the canopy. Their modeled values ranged from 16.8 to 17.1 ‰ for a
deciduous forest in eastern Tennessee (with uncertainties between 2.7 and 4.7 ‰).
Using equation (7),
�
δP = δ13Ca − Δcanopy , the mean carbon isotope ratio of
photosynthetic assimilation was estimated to be δP = –24.6 ± 5.1 ‰. δP represents the
average isotopic composition of the carbon dioxide that was drawn by photosynthesis and
is likely to reflect the carbon isotope ratio of recent assimilates (i.e. sugars, as they are the
first products of photosynthesis). We compared the carbon isotope ratio of the
assimilation flux,δP, with the isotopic composition of leaves collected from various
heights in the poplar canopy. Leaf δ13C was 3 ‰ more enriched close to the top (at 12 m)
relative to the isotopic composition of leaves deeper in the canopy (~ 2 m), consistent
with the findings of other groups (Duursma and Marshall, 2006, Ometto, et al., 2006,
Berry, et al., 1997, Broadmeadow, et al., 1992). Variation in the isotopic composition of
86
bulk leaf with canopy height has been attributed to gradients in light intensity (Farquhar
et al., 1989) and nitrogen concentration within the canopy (Duursma and Marshall, 2006).
The average bulk leaf isotopic composition was –27.6 ± 1 ‰. We find that the mean
carbon isotope ratio of the assimilation flux (δP = –24.9 ‰) was enriched relative to the
isotopic composition of bulk leaf samples. This result is in agreement with observations
of other groups that the carbon isotope ratio of recent assimilates are more enriched
relative to whole leaf organic matter (Duranceau et al., 1999, Ocheltree and Marshall,
2003, Badeck et al., 2005, see also Bowling et al., 2008 for a detailed review). We find
that the enrichment was about 3.0 ‰, which is within the range (2.2 to 3.0 ‰) reported
by Ocheltree and Marshall (2003) when comparing the carbon isotope ratio of plant
tissue (mature leaves and growing tips) against soluble carbohydrates at various light
treatments.
3.5. Conclusions
We have demonstrated the capability of combining Fourier-transform infrared
spectroscopy (FTIR) with disjunct eddy covariance (DEC) for the simultaneous
determinations of the biosphere-atmosphere exchange of total CO2, H2O, and carbon
dioxide isoflux in a poplar forest. There was very good agreement (~ 10%) between the
total CO2 flux derived from the FTIR – DEC approach and the conventional IRGA – EC
method. This is well within the statistical uncertainty empirically determined by
Turnipseed et al., (2009) for a sampling interval of 45 seconds. Disjunct sampling of the
87
water vapor flux constantly underestimated the IRGA – EC measurements. This result is
consistent with observations by other groups and has been attributed to the damping of
high frequency fluctuations due to the adsorption and desorption of water vapor on the
inner walls of the sampling line.
We observed considerable variations (standard deviations of the diurnal averages)
in the FTIR – DEC flux estimates that are chiefly attributed to the limited temporal
resolution of the disjunct sampling technique relative to the conventional IRGA – EC
approach (45 second time resolution versus 100 ms sampling interval). We consider the
reduced time resolution to be the main source of statistical uncertainty in the DEC –
derived fluxes. Increasing the temporal resolution is therefore a priority for future
measurements with the FTIR – DEC system. The uncertainty can be considerably
reduced either by decreasing the cycle time between samples or increasing the averaging
interval.
Simultaneous measurement of CO2 and its isotopic composition enabled the
measurement of the isoflux using the FTIR - DEC approach. Information from the
isoflux of 13CO2 and the net ecosystem exchange of total CO2 were used to directly
determine the canopy-scale photosynthetic discrimination. We found that the carbon
isotopic composition of the assimilation flux was 13C – enriched relative to whole leaf
samples by about 3.0 ‰. This result agrees with findings that recent assimilates are
enriched relative to the whole organic matter from which they were removed.
The ability of Fourier transform infrared spectroscopy to simultaneously measure
the concentrations of several important trace gases gives it a unique advantage over other
existing spectroscopic techniques. When combined with disjunct eddy covariance, a
88
direct flux measurement method, it becomes a powerful tool for investigating the
complex exchange of various infrared-active species in forest, urban and agricultural
ecosystems.
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Table 3. 1. Concentrations and carbon isotope ratios of four reference cylinders used in
the field for calibration of FTIR derived CO2 and δ13C.
Tank No. Concentration (ppmv, ± 1%) Carbon isotope Ratio (‰)
1 324.7 -39.7 ± 0.12
2 449.5 -39.87 ± 0.06
3 378.7 -9.98 ± 0.08
4 383.7 -39.15 ± 0.03
94
Figure 3. 1. Diurnal averages of important meteorological parameters measured from
July 25 – 29 and August 15 – 17, 2006: (a) temperature, (b) photosynthetically active
radiation, (c) friction velocity.
1.2
0.8
0.4
0.0
u* (
m s
-1)
20151050
hr
1200
800
400
0
PAR
( µm
ol m
-2 s
-1)
35
30
25
20
15
10
Tem
pera
ture
(ºC
)
Diurnal AverageA)
B)
C)
95
Figure 3. 2. Wind speeds and wind directions observed during the duration of field
experiments (July 25 – 29, August 15 – 17, 2006). Wind speed magnitudes ranged from
about 3 to 6 ms-1. Wind direction was consistently from the west (255º to 285º) during
daytime while nighttime prevailing wind direction was southwest (210º to 240º).
0
0.2
0.4
0.6
0.8
0.2 0.4 0.6 0.8
0
45
90
135
180
225
270
315
0 - 1 1 - 2 2 - 3 3 - 4 4 - 5 5 - 6 6+
A) Daytime WS and WD
0
0.2
0.4
0.6
0.8
0.2 0.4 0.6 0.8
0
45
90
135
180
225
270
315
0 - 1 1 - 2 2 - 3 3 - 4 4 - 5 5 - 6 6+
B) Nighttime WS and WD
96
Figure 3. 3. Schematic diagram of the field experiment set-up (Cambaliza et al., 2010).
97
Figure 3. 4. Comparison of FTIR – DEC and IRGA – EC measurements of CO2 fluxes.
(a) Time series data measured by the two approaches. (b) Diurnal average of the data
shown in figure 4(a). (c) Linear regression between the FTIR – DEC and the IRGA – EC
estimate showing that the two approaches agree to within 10%.
-60
-40
-20
0
20
40
60
CO2
Flux
(µm
ol m
-2 s
-1)
7/25/06 7/27/06 7/29/06
PST
8/15/06 8/17/06
FTIR - DEC IRGA - EC
4A)
-30
-20
-10
0
10
20
CO2
Flux
(µm
ol m
-2 s
-1)
20151050Hour
Diurnal Average
IRGA - EC FTIR - DEC
4B)
-20
-15
-10
-5
0
5
10
15
FTIR
- DE
C CO
2 Fl
ux (µm
ol m
-2 s
-1)
1050-5-10IRGA - EC CO2 Flux (µmol m-2 s-1)
y = a + bx, Pr = 0.89a = -4.161 ± 0.615b = 0.897 ± 0.067Diurnal Average
4C)
98
Figure 3. 5. Comparison of FTIR – DEC Latent Heat flux measurements against the
IRGA – EC estimates. (a) Time series data measured by the two approaches. (b) Diurnal
average of the data shown in Figure 5 (a). (c) Linear regression between the two
approaches showing that FTIR – DEC underestimates the IRGA – EC measurement for
latent heat by about 26%, which is attributed to the damping of the high frequency
1000
800
600
400
200
0
-200
Late
nt H
eat
Flux
(W
m-2
)
7/25/06 7/27/06 7/29/06
PST
8/15/06 8/17/06
FTIR - DEC IRGA - EC
5A)
800
600
400
200
0
-200
Late
nt H
eat
Flux
(W
m-2
)
20151050
Hour
IRGA - EC FTIR - DECDiurnal Average
5B)
500
400
300
200
100
0
-100
FTIR
- DE
C La
tent
Hea
t Fl
ux (
W m
-2)
400350300250200150100
IRGA - EC Latent Heat Flux (W m-2)
Diurnal Average
y = a + bx, Pr= 0.86a = -18.7 ± 14.6b = 0.74 ± 0.06
5C)
99
fluctuations due to the adsorption and desorption of water vapor in the inner walls of the
pvc sampling line.
Figure 3. 6. The isoflux of 13CO2 derived from the covariance of the fluctuations of the
vertical wind speed and the product of CO2 and its isotopic composition.
400
200
0
-200
-400
Isof
lux
( µm
ol m
-2 s
-1 ‰
)
20151050hr
100
Figure 3. 7. Temperature dependence of the nighttime CO2 flux (unfilled circles). FR
was averaged and grouped in 1ºC intervals (solid circles). A weak exponential
relationship was obtained. Excluding the outlier at T = 24ºC, the resulting exponential fit
was FR(T) = 5.5014e0.0352T, R2 = 0.94.
20
18
16
14
12
10
8
6
Resp
iratio
n ra
te, F
R ( µ
mol
m-2
s-1
)
2624222018Air temperature (ºC)
including outlier at T = 24 ºCFR(T) = 5.9516e0.0309T
R2 = 0.77
not including outlier at T = 24º CFR(T) = 5.5014e0.0352T
R2 = 0.94
101
Figure 3. 8. The total CO2 flux and its photosynthetic assimilation (FA) and respiratory
(FR) components after applying the conventional regression of the nighttime CO2 flux
against temperature (Figure 3.7).
-30
-20
-10
0
10
20
CO2
Flux
(µm
ol m
-2 s
-1)
20151050hr
1816141210
Diurnal Average
F Fa Fr
102
Figure 3. 9. Half hour averages of the canopy-scale carbon isotope discrimination
calculated from FR and FA in Figure 3.8. Average daytime canopy discrimination was
about 16.6 ± 5.1 ‰.
40
30
20
10
0
Disc
rimin
atio
n ∆
(‰
)
18161412108hr
∆AVE = 16.6 ± 5.1 ‰
103
CHAPTER 4
ANALYSIS OF LOW-CONCENTRATION GAS SAMPLES WITHCONTINUOUS-FLOW IRMS: ELIMINATING SOURCES OF
CONTAMINATION TO ACHIEVE HIGH PRECISION
4.1. Abstract
Developments in continuous-flow isotope ratio mass spectrometry have achieved
rapid analysis of δ13C in CO2 of small-volume gas samples with precisions ≤ 0.1 ‰.
Prior research has validated the integrity of septum-capped vials for collection and short-
term storage of gas samples. However there has been little investigation into the sources
of contamination during preparation and analysis of low concentration gas samples. In
this study we determined (1) sources of contamination on a Gasbench II, (2) developed
an analytical procedure to reduce contamination, and (3) identified an efficient, precise
method for introducing sample gas into vials.
We investigated three vial filling procedures: (1) automated flush-fill (AFF), (2)
vacuum back-fill (VBF), and (3) hand-fill (HF). Treatments were evaluated based on
time required for preparation, observed contamination, and multi-vial precision. Worst-
case observed contamination was 4.5 % of sample volume. Our empirical estimate
showed that this level of contamination results in error of 1.7 ‰ for samples with near-
ambient CO2 concentrations and isotopic values that followed a high-concentration
carbonate reference with an isotope ratio of –47 ‰ (IAEA-CO-9). This carry-over
contamination on the Gasbench can be reduced by placing a helium-filled vial between
the standard and succeeding sample or by ignoring the first two of five sample peaks
104
generated by each analysis. High-precision (SD ≤ 0.1 ‰) results with no detectable
room-air contamination were observed for AFF and VBF treatments. In contrast
precision of HF treatments was lower (SD ≥ 0.2 ‰). VBF was optimal for the
preparation of gas samples, as it yielded faster throughput at similar precision when
compared to AFF.
4.2. Introduction
The development of continuous flow isotope ratio mass spectrometry (CF-IRMS)
in the 1980s and its commercial appearance in the 1990s made possible the high-
precision isotope measurement of large numbers of samples.1 In 1991, Prosser et al.2
described an automated method for rapid isotopic analysis of high-concentration CO2
from breath samples collected in 13-mL septum-capped vials. The technique involved the
use of an autosampler where the headspace gas was transferred to the helium carrier
stream using a double-holed needle probe. More recently, Tu et al.3 described an
analogous method but focused on the isotopic analysis of near-ambient CO2 samples
from 10-mL septum-capped vials for Keeling plot applications. Without cryogenic pre-
concentration, Tu et al.3 achieved precision of 0.08 ‰ δ13C for ambient concentration air
samples using a Gasbench II headspace sampler coupled to an IRMS. This application of
CF-IRMS techniques to low concentration gas samples allows for higher throughput and
reduced sampling costs compared to traditional dual-inlet IRMS.
105
Despite several studies3-5 using continuous flow preparation devices for CF-IRMS
analysis of low concentration gas samples, methods for sample introduction to vials are
not yet standardized. Several methodological developments have focused on improving
vial integrity and storage3-7 but all had different parameters for sample introduction to the
IRMS. Common methods are through a flushing technique or glove bags. Flushing
durations and flow rates vary among laboratories. Tu et al.3 flushed their vials manually
with a double-holed needle for 10 s at a flow rate of ~ 1 L min-1. Midwood et al.5 flushed
their specially designed metal vials for 190 s at a flow rate of 80 mL min-1 while Knohl et
al.4 did not mention the flush-fill duration and flow rate during preparation of their
calibration gases. Standardized methods for vial filling are required not only for
unknown gas samples, but also for true internal standards and quality control samples.
For flush-filling techniques, it is essential that the procedure result in complete
volumetric turnover.8 Residual room air in the vial clearly results in inaccurate
measurement. To date, contamination due to insufficient flushing times has been
discussed in detail only for the analysis of high-concentration carbonate samples8.
Improving precision in the analysis of low concentration gas samples using CF-
IRMS techniques requires optimization of all steps of the analysis; vial preparation and
filling, instrument method, and normalization. We compared two sources of
contamination: memory effects on the Gasbench II and room air contamination for three
sample preparation methods. These vial-filling methods were: (1) automated flush-fill
(AFF), (2) vacuum back (VBF), and (3) hand fill (HF). The specific objectives of this
work were: (1) analyze and eliminate contamination during preparation and analysis, (2)
compare methods for the preparation of gas samples, and (3) determine the optimal
106
sample preparation method evaluated by the absence of contamination, speed of
preparation, and vial-to-vial uncertainty.
4.3. Methods
4.3.1. Isotopic analysis
Isotopic measurements were performed at the Laboratory for Biotechnology and
Bioanalysis II, Stable Isotope Core at Washington State University, Pullman, WA, USA.
The carbon isotope ratio of low concentration CO2 gas samples was measured using a
Gasbench II interface (ThermoFinnigan, Bremen, Germany) with GC-PAL autosampler
(CTC analytics, Zwingen, Switzerland) connected to a Delta plus XP IRMS
(ThermoFinnigan, Bremen, Germany). The autosampler was equipped with a double-
holed needle that samples the headspace from 12-mL borosilicate vials (Labco Limited,
High Wycombe, UK #938W) capped with butyl rubber septa. Total run time for each
sample was 875s. Included in the total run time was a 20-s transfer time to allow the
analyte gas and helium carrier to fill the sampling line. These gasses then passed through
an inline nafion water trap and into a 100-µL sampling loop for 80s. During the load
cycle of the sampling loop, 3 pulses of a pure CO2 tank were introduced into the mass
spectrometer through an open split as an external reference. The 3rd peak of the
monitoring gas was used for all initial calculation of isotope ratios by arbitrarily defining
the monitoring gas as 0‰ δ13CVPDB. After 80s of filling the sampling loop, an eight-port
valve (VICI Valco, Houston, TX) was switched to inject for 60s. This process was
107
repeated five times, to ultimately provide five sample peaks. During the inject phase, gas
samples passed through a Poraplot Q GC column (25m l x 0.32 mm ID, 10 µm thickness,
Varian, Inc., Walnut Creek, CA, USA) held at 40°C followed by another nafion trap.
After the GC column, analyte gas and helium entered the IRMS through a movable
sample open split. Moving the open-split capillary could be achieved through the
software, diluting the sample entering the IRMS. We used the split out/dilution setting to
achieve consistent signal among samples and standards, and to remove earlier eluting
N2O which has isobaric interference for mass-to-charge ratio, m/z 44, 45, 46. For
experiments where the purpose was to observe maximum contamination in the system
and for blank vials, the dilution was turned to off/split in.
Prior to every sample sequence, stability of the instrument was verified to be <
0.06 ‰ δ13CVPDB from ten or more injections of the CO2 monitoring tank, and at least five
conditioning samples from the one blank vial were run through the system. Linearity was
verified to be within the manufacturers recommendations of < 0.06 ‰ per volt for the
range of 500-8000mV m/z 44. Isotopic values were expressed in terms of the
delta notation (in ‰) relative to the Vienna Pee Dee Belemnite (VPDB) standard:
�
δx =Rx
Rstd−1
⎛ ⎝ ⎜
⎞ ⎠ ⎟ (1)
where Rx and Rstd are the ratios of 13C relative to 12C in the CO2 in the sample and in the
VPDB standard, respectively.
Three carbonate standards that covered a range of delta values were used to
reduce uncertainty in isotope measurements.9-11 The span of calibration standards was
–47.32 ‰ (IAEA-CO-9) to +1.95 ‰ (NBS19). NBS18 (-5.01 ‰) was used as a quality
108
control (QC). The carbonate standards were prepared according to Revesz et al.12:
standards were transferred to 12-mL septum-capped exetainers, flush-filled with helium
for 5 minutes, digested with 100 % H3PO4 in a CO2 – free environment, and allowed to
equilibrate for 24 hours. Vials were flush-filled in the gas bench with tank air at 26 mL
min-1 using the double-holed needle. The internal pressures of all vials were equilibrated
with atmospheric pressure to reduce variability of results.3 The flush-fill needle was left
in the vial for 60 seconds after shutting off the supply of the flushing gas to achieve
atmospheric equilibrium. A pressure gauge (model HHP701-2, Omega Engineering,
Stanford CT) fitted with a needle was used to verify internal vial pressure.
All sequences were normalized using the Laboratory Information Management
System (LIMS) for Light Stable Isotopes (USGS, Reston VA) with NBS19 and IAEA
CO-9 as anchor points. LIMS uses ordinary least squares regression (OLS) to normalize
isotopic values and perform hourly drift corrections if needed. An hourly drift correction
was applied to the data only if all references and QC samples showed improvement.
Final isotopic composition of each sample was evaluated from the best three of five
sample peaks, with vial precision typically < 0.1 ‰ (1SD). The first two peaks were
typically ignored because they were most likely to be affected by carryover from the
previous sample. Occasionally the last peak was ignored due to helium dilution of the
signal. In cases where the goal was to measure prior sample carryover, the area and
δ13CVPDB of all peaks were used. We chose 10mV as a minimum peak detection
parameter because it was the smallest peak above typical background fluctuations our
system could reliably detect. Since diagnostic tests encompassed 500-8000mV only, it
109
should be noted that some additional bias might exist for sample peaks <500mV. We
present only the mean values for such peaks, normalized the same way as other samples.
4.3.2. Description of gas sample preparation methods
We compared three filling techniques to prepare gas samples: (1) automated
flush-fill (AFF), (2) vacuum back- fill (VBF), and (3) hand-fill (HF). All vials were
initially open to room air and caps tensioned prior to all treatments. In the AFF method,
vials were flush-filled using the GC-PAL autosampler on the Gasbench II for 5 and 10
minutes at a flushing rate of 26 mL min-1 . The autosampler was programmed to leave the
flushing needle in the vial for 60s after turning off the flush gas. This was determined
previously to keep the vials at only a slightly positive pressure. We combined AFF with
vial pre-cleaning treatments, where individual vials were pre-evacuated and filled with
helium one or three times.
For the VBF technique, vials were evacuated and filled via a vacuum backfill
manifold (Figure 4.1). The line was constructed of 1/4” stainless steel tubing with 8
valved ports designated V1 (1/4 turn Swagelok plug valves, part # SS-4P4T, The
Swagelok Company, Solon, OH, USA) which reduce into 28G needles. Both ends of the
line have separate valves, designated V2 (Swagelok 1/4” integral bonnet needle valve,
part #SS-1RS4) which allows the user to control connection to a vacuum pump (Edwards
RV 1.5, Edwards, Tewksbury, MA, USA) and the flush gas. The flush gas for the VBF
technique was helium in the case of blank preparation or a gas tank filled with actual
sample. The entire VBF manifold and the connection to the regulator of the flushing tank
110
were initially evacuated. Vacuum was measured with a Varian thermocouple type 0531
(Varian, Inc., Walnut Creek, CA, USA) sending unit and model 801 analog gauge.
Pressure was measured with a pressure transducer and digital readout (MKS 122A-1123,
PDR-D-1, MKS Instruments, Andover, MA, USA). After each evacuation, the manifold
was isolated from the vacuum pump and vials were filled with the tank gas to
atmospheric pressure by slowly adjusting the tank regulator. The VBF method was
applied in two treatments: 1 evacuation/backfill, and 3 evacuations/backfills.
In the HF method, vials were evacuated with the VBF line, left under vacuum and
manually filled with the tank gas using a 30-mL syringe. The syringe was used to pierce
an empty septum-fitted Swagelok 1/4” nut that was attached to the tank regulator, set at
138 kPa (20 psi). After three rinses of the syringe, sample vials were initially over-
pressurized by injecting 20 mL of sample gas into the 12-mL exetainers. Excess vial
pressure was released by briefly puncturing the vial septum with a needle, allowing for
the pressure to equilibrate with ambient pressure. The HF method was applied in two
treatments: one evacuation/handfill, and three evacuations/handfills.
In summary, the three sample introduction methods (AFF, VBF, HF) in
combination with vial pre-filling procedures (no evacuation, 1 evacuation, and 3
evacuations) yielded a total of 10 treatments. These treatments were: 5-min AFF with no
evacuation, 5-min AFF with 1 evacuation, 5-min AFF with 3 evacuations, 10-min AFF
with no evacuation, 10-min AFF with 1 evacuation, 10-min AFF with 3 evacuations, HF
with 1 evacuation, HF with 3 evacuations, VBF with 1 evacuation, VBF with 3
evacuations. The HF and VBF filling techniques were not used with the no-evacuation
procedure because these techniques always require an evacuated vial.
111
4.3.3. Experiment 1: Identification of sources of contamination
Contamination was assessed using He-filled vials (blanks) for the 10 treatments
previously described. Three replicates (designated as blanks 1, 2, and 3, respectively)
were prepared for each treatment and successively positioned after an IAEA-CO-9 (-
47.32‰) carbonate standard. The default minimum peak detection parameter was
lowered from 50 to 10 mV. The [CO2] for gas samples can be determined from the area
of the voltage signal peak13-14. Therefore the magnitude of contamination was estimated
by taking the ratio of the maximum sample area of the blanks over the maximum area of
a 380-ppm CO2 sample for this instrument. The source of contamination was identified
using the δ13C values of the blank vials. Memory effects (carry-over contamination) in
the Gasbench were associated with the depleted δ13C values of the IAEA-CO-9 standard;
in contrast, contamination due to laboratory air had a significantly more enriched δ13C (≈
-9‰).
The CF-IRMS sampled each vial and analyzed it five times. The gas in the vial
was diluted as He was injected to transfer the sample into the mass spectrometer.
Therefore, our ability to detect small contaminations in the blanks decreased over the
analysis time required for each sample: room air contamination might have been detected
in the first sample peak but most likely would disappear in the remaining peaks. A
common practice for post-processing isotope data is to discard the first sample peak
because it is subject to larger errors. We decided to use the third peak in our analysis of
112
blanks 2 and 3 to identify the treatments with room air contamination that would still be
present even after post-processing.
4.3.4. Experiment 2: Application to gas samples of near-ambient CO2
concentrations
The results obtained from Experiment 1 (blanks) were used to select among each
category (AFF, HF and VBF) the treatment that (1) produced minimum contamination,
and (2) required the least preparation time. The selected treatments were 5-min AFF with
1 evacuation/He fill, VBF with 3 evacuations/backfills, HF with 3 evacuations/handfills.
Those three treatments were used for further testing with the CO2 tanks to evaluate the
best method for gas standard preparation. Three sample vials were prepared per filling
treatment. To avoid carry-over contamination, a blank (helium-filled) vial was positioned
between the carbonate standard and the CO2-filled sample vials. This experimental set-up
enabled us to attribute any observed residual contamination to the sample preparation
method.
The CO2 tanks were dilutions prepared by mixing carbon isotope reference gases
with instrument grade zero air. Carbon isotope reference gases were: tank 1, –23.5‰
(Cambridge Isotope Laboratories Inc., Andover, MA, USA) and tank 2, –38.9‰ (Oztech
Trading Corporation, Safford, AZ, USA). The CO2 concentrations were 396.4 ppmv (±
1%) and 383.7 ppmv (± 1%) for tanks 1 and 2, respectively.
113
4.4. Results
4.4.1. Identification of sources of contamination
The highest contamination (~ 4.5 % of the area of a 380-ppm sample) occurred in
the 5-min AFF method when no vial pretreatment was done (Figure 4.2). Contamination
was reduced (< 4%) by either flushing for 10 minutes or rinsing the vials with He before
flushing. No improvement was observed by rinsing the vials three times instead of one.
For both VBF and HF methods contamination was about 4% and was not reduced by
applying more evacuations (Figure 4.2). One replicate for the 10-min AFF with 1
evacuation was not included in Figures 4.2 and 4.3, as the observed voltage area for this
treatment was almost twice as large as those observed for the other treatments. An outlier
test using orthogonal distance regression on the δ13C and peak areas of all treatments
confirmed that the result for the 10-min AFF with 1 evacuation was an outlier in the data
set.
To identify the sources of contamination we analyzed the δ13C values of the
blanks. For each filling technique three replicates (blanks 1, 2 and 3) were placed in
sequential order after an IAEA CO-9 standard. Blank 1, positioned immediately after
IAEA CO-9, had more negativeδ13CVPDB and larger voltage areas (Figure 4.3). We found
contamination in blank 1 for nine treatments (data for 10-min AFF with 1 evacuation was
not included in the figure). For blank 2 only four treatments presented contamination and
for blank 3 only three treatments had detectable contamination (Figure 4.3). In cases
where we still observed contamination in blanks 2 or 3, the δ13CVPDB values were similar
114
to laboratory room air (Figure 4.3). These results clearly show that: (1) the carry-over
contribution from IAEA CO-9 was observed in the sample positioned immediately after
the carbonate (blank 1), and (2) the residual room-air contamination in blanks 2 and 3
were only observed for some treatments, i.e., not all treatments were susceptible to room
air contamination. When we analyzed the third peak of blanks 2 and 3, only the two HF
and the 5-min AFF methods exhibited presence of contamination (Figure 4.4). This
residual contamination in the HF and 5-min AFF methods was clearly an artifact of the
sample preparation method. Flush filling for five minutes (no evacuation) was
insufficient to turn over the gas in the vial, while doubling the flush fill time to 10
minutes or rinsing the vial with helium before filling completely removed the residual
room air. Puncturing the HF vials to release excess pressure may have introduced room
air during pressure equilibration.
Simulation using a two-source mixing model predicts that a 4.5% contamination
(maximum contamination measured in Experiment 1) translates to as much as ~1.7 ‰
error. This error would result when the differential isotopic composition of the sample
gas and the contaminant is about –37 ‰ (Figure 4.5), as in the case where the
contamination is carry-over from IAEA CO-9 (-47 ‰) and the sample gas is room air
with an isotopic signature of –9.3 ‰ (Figure 4.5). As the stable isotope composition of
contamination becomes more similar to that of the sample, there is less added
uncertainty. The error decreased when the carbon isotope ratio of the contaminant
approached that of the sample. This result suggests that carry-over contamination can be
masked whenever the carbon isotope ratio of the contaminant is similar to the sample.
For the case where the contamination is isotopically very different (e.g. -47 ‰) and of
115
very low concentration (nominally equal to the minimum peak detection limit of 10 mV),
the contamination contribution is about 0.4% of the area of a 380-ppm sample.
Simulation predicts that this contribution translates to an error of ~ 0.15 ‰, which is of
the same order as the precision of continuous - flow IRMS systems.
4.4.2. Application to gas samples of near-ambient CO2 concentrations
The AFF and VBF techniques yielded similar precisions (≤ 0.1‰) while the
largest variability in results was observed for the HF method (≥ 0.2‰) (Figure 4.6). The
carbon isotope ratio for tank 2 obtained with the HF method was slightly enriched (-
38.3‰) relative to the AFF (-38.9 ‰) and VBF (-38.8‰). This result is consistent with
the introduction of room air (~ -9.3 ‰) into the more depleted HF samples (~-38 ‰)
during pressure equilibration. We did not observe the same isotopic enrichment for tank 1
samples prepared with the HF technique. This observation is consistent with the result of
the two-source mixing model simulation shown in Figure 4.5. Since the carbon isotope
ratio of the sample (tank 1, -23.5‰) was closer to that of the ambient samples, the small
contamination introduced by the HF technique did not significantly alter the δ13C
estimated for the sample.
4.5. Discussion
Continuous flow isotope ratio mass spectrometry is now frequently used to
investigate the carbon isotopic exchange between the biosphere and atmosphere3, 14-16,
116
making use of septum-capped vials to collect gas samples from remote field sites
followed by laboratory analysis. To reduce errors associated with gas collection and
storage, earlier studies focused on improving the reliability of septum-capped vials prior
to application in the field3-4. Previous studies have investigated the flushing times
required for carbonate standards preparation8, but there is no standard protocol for
introducing low-concentration gas samples into vials for analysis on the same sampling
device. Through improved vial preparation and analysis, we found that error can be
reduced in low-concentration samples.
Carry-over contamination associated with the laboratory analysis can add
uncertainty to the results. The carry-over contribution was attributed to the transfer of a
small portion of sample from the previous to the current vial. Significance of carryover
depends on both prior sample concentration and its stable isotope composition. When we
diluted our carbonate standard with helium, carry-over contamination was not observed
in any of the blank vials (data not shown). Therefore, sequence design must consider
sample concentration and expected isotopic composition, and similar samples should be
grouped together in the sequence. We found errors due to carry-over as large as 1.7 ‰.
Errors in the isotopic ratio of collected gas samples of the order of 1.7 ‰ can potentially
introduce significant uncertainty when used in Keeling plot applications to determine
signatures of ecosystem respiration. For example, Badeck et al.17 reported that on the
global scale, an uncertainty in the autotrophic respiration of the order of 0.7 ‰ translates
to as much as 5 % change in the estimate of the biospheric sink.
Carry-over can cause significant errors in the analysis of low-concentration
samples, such as those at ambient [CO2]. Carry-over can be reduced by placing a blank
117
(Helium-filled) vial between a standard and the gas sample or by discarding the first two
sample peaks during post-processing of data. We hypothesize that carry-over is a
potential artifact of the continuous flow system. Unlike a traditional off-line analysis
where pure gases are transferred in controlled amounts via dual inlet, and then evacuated
between samples, the gas bench system relies on a continuous flow of helium to transfer
the analyte gas to the IRMS. In the gas bench itself, we see the sample gas continually
diluted as it is transferred to the IRMS. In a separate test, we compared blanks
immediately following carbonates with the sample open split in the “out” position.
Performing this test reduced the carbonate signal in the IRMS compared to the split “in”
position, but it did not reduce the blank effect of a subsequent vial. Therefore, we think
that the carryover observed is occurring in the gasbench and not in the IRMS. Memory
effects in CF-IRMS systems must therefore be addressed in future efforts to design
systems with high reliability and precision.
Ambient air contamination was shown to be an artifact of the sample preparation
method. We found that a basic (<10 minutes or without vial pretreatment) flush-fill
method can lead to incomplete volumetric turnover of gas in the vial. This result is
consistent with recent findings of Paul and Skrzypek8 who recommended flushing times
of ≥ 600 s to completely remove air in the vials. Results indicated that either the 5-min
AFF with He pre-cleaning or the VBF with 3 sample fills was a satisfactory method for
analyzing low-concentration CO2 gas tanks. Accordingly, either technique can be used to
prepare gas standards for Gasbench applications. However, we found the VBF technique
to be a more efficient method with higher throughput. With the use of the vacuum
backfill manifold, it took approximately the same time to evacuate and fill eight vials as it
118
did to fill one vial on the Gasbench using the AFF method. The variation in the HF data
was associated with less uniformity due to manual fills.
We recommend VBF, or AFF with vial pretreatment, for filling low-concentration
gas samples into vials. The VBF method can be more readily applied to field
measurements. The procedure is a slight modification to the method described by Knohl
et al.15 Vials fitted with Kel-F disks can be simultaneously evacuated using the vacuum
backfill (VBF) manifold and a vacuum pump. The VBF set-up will allow for several
vials to be pre-rinsed with canopy air before filling. The approach can facilitate a larger
throughput from the field, increasing the statistical robustness of analysis of samples.
The vacuum line system could easily be deployed in the field.
All of our procedures sampled gas from pressurized tanks, which may not be
practical for all field samples. Where low pressure, low volume samples are all that is
available (and a VBF system is inaccessible), the HF method is the most suitable outlined
here. Procedures we tested provide insight into the differences between the HF technique
and what we consider the gold standard, the VBF approach. Therefore, techniques
presented here could be used to prepare multiple gas standards in exetainers or to test HF
methods before implementing them in the field. Precision of low-concentration gas
samples with CF-IRMS devices can also be improved by taking steps to reduce carryover
in the device itself. It is recommended to build sequences with similar samples
(concentration and stable isotope composition), incorporate helium blanks, and use post-
run data processing to reduce external sources of variation.
119
4.6. References
1. Brenna JT, Corso TN, Tobias HJ, Caimi RJ. Mass Spectrom. Rev. 1997; 16: 227.
2. Prosser SJ, Brookes ST, Linton A. Biol. Mass Spectrom. 1991; 20: 724.
3. Tu KP, Brooks PD, Dawson TE. Rapid Commun. Mass Spectrom. 2001; 15: 952.
4. Knohl A, Werner RA, Geilmann H, Brand WA. Rapid Commun. Mass Spectrom.
2004; 18: 1663.
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Figure 4. 1. Schematic diagram of the vacuum backfill manifold. The line consists of
1/4” stainless steel tubing with 8 valved ports designated V1 which reduce into 28G
needles. Both ends of the line have separate valves, designated V2 which allows the user
to control connection to a vacuum pump and the flush gas.
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Figure 4. 2. Maximum contamination on low-concentration CO2 gas samples as a
function of the various gas filling treatments. The three sample introduction methods
(AFF = automated flush fill, VBF = vacuum backfill, HF = hand fill) were combined with
vial pre-filling procedures (no evacuation, 1 evacuation, and 3 evacuations) for a total of
ten treatments. The HF and VBF filling techniques were not used with the no-evacuation
procedure because these techniques always require an evacuated vial. Data for the 10-min
AFF with 1 evacuation were not included as the observed voltage area for this treatment
was almost twice as large as those observed for the other treatments. An outlier test
using orthogonal distance regression on the δ13C and peak areas of all treatments
confirmed that the result for the 10-min AFF with 1 evacuation was an outlier in the data
set.
6
5
4
3
2
1
Cont
amin
atio
n (%
)
5-m
in A
FF
10-m
in A
FF
5-m
in A
FF
10-m
in A
FF
VBF HF
5-m
in A
FF
10-m
in A
FF
VBF HF
No evacuation 1 evacuation with 1 He fill 3 evacuations with 3 He fills
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Figure 4. 3. Carbon isotope ratio as a function of the measured area for all Helium-filled
replicates of all treatments. Blank vials 1, 2, and 3 are the helium-filled replicates
sequentially positioned after the IAEA CO-9 standard.
-50
-40
-30
-20
-10
0δ 13
C of
firs
t pe
ak (
x100
0)
0.110.100.090.080.070.060.05
Area of first peak (Vs-1)
vial1 vial2 vial3
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Figure 4. 4. Residual contamination as a function of the ten treatments. Peaks one and
two out of five were omitted. Three treatments (5-min AFF no evacuation, HF with 1
evacuation, and HF with 3 evacuations) still showed contamination following data post
processing.
3.5
3.0
2.5
2.0
1.5
1.0
0.5
Resi
dual
Con
tam
inat
ion
(%)
5-m
in A
FF
10-m
in A
FF
5-m
in A
FF
10-m
in A
FF
VBF HF
5-m
in A
FF
10-m
in A
FF
VBF HF
No evacuation 1 evacuation with 1 He fill 3 evacuations with 3 He fills
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Figure 4. 5. Two-source mixing model calculation of the uncertainty in the carbon
isotope ratio of a low-concentration sample. The model was based on the difference in
the isotopic signatures of the sample gas and the contaminant for the worst-case
contamination of 4.5 %. The isotopic signature of the sample was assumed to be -9.3 ‰,
which was the average measured δ13C of room air in our laboratory.
-2.0
-1.5
-1.0
-0.5
0.0
0.5
Erro
r in
δ 13
C of
sam
ple
(x10
00)
-35 -30 -25 -20 -15 -10 -5
Differential δ13C (x1000)
126
Figure 4. 6. Effect of three sample preparation techniques in air samples of identical
CO2 concentrations but distinctly different carbon isotope ratios (δ13C of tank1 = -23.5
‰, and δ13C of tank2 = -38.9 ‰).
-24.0
-23.5
-23.0
δ 13C
(x10
00)
5-m
in A
FF +
1He
fill
VBF
_3sa
mpl
e fil
ls
HF_3
sam
ple
fills
-39.0
-38.5
-38.0
tank 1 (-23.5 ‰) tank 2 (-38.9 ‰)
127
CHAPTER 5
SUMMARY AND CONCLUSIONS
5.1. Summary
This dissertation described the development of a novel approach to measure the
exchange of isotopic CO2 in a forest ecosystem. The first chapter provided background
on the measurement of the isotopic composition of atmospheric carbon dioxide, its
importance in the study of global environmental change, and a detailed discussion on the
various techniques for concentration, stable isotopes, and isotopic flux measurements.
Results of field and laboratory measurements as well as scientific conclusions were
presented in chapters 2, 3, and 4.
The uncontrolled use of fossil fuels has significantly increased the atmospheric
concentration of carbon dioxide since the industrial revolution. Although the distribution
of CO2 into its land, ocean, and atmospheric reservoirs is relatively well known, there
remain many uncertainties in the global carbon budget. Quantification of the changing
partial pressure of CO2 is not sufficient to understand its flow into and out of the various
carbon reservoirs. The measurement of the stable isotopic composition of CO2 provides
additional constraints that aid in understanding the partitioning of atmospheric carbon
dioxide into its land and ocean components.
The traditional method for isotopic CO2 measurement makes use of laboratory
isotope ratio mass spectrometry (IRMS). While this method provides the best precision,
it does not allow for continuous measurement of the isotopic content of carbon dioxide.
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This approach limits long-term investigations of ecosystem response to changing patterns
in environmental drivers such as temperature, precipitation, and nutrient availability.
Several spectroscopic methods have been proposed in the literature as an alternative to
IRMS for continuous, real-time measurement of isotopic CO2. These spectroscopic
techniques rely on the strong isotopic absorption of carbon dioxide in the infrared. In this
work, we have chosen to develop an approach that is based on Fourier Transform
Infrared spectroscopy (FTIR). FTIR is a scanning method that is not as fast or as precise
as tunable diode laser absorption spectroscopy (TDLAS) or quantum cascade laser
absorption spectroscopy (QCLAS), but its ability to measure without ambiguity and
simultaneously a number of important trace gases (many of which are greenhouse gases)
makes it an attractive and flexible method that will find application in forest, urban, and
agricultural ecosystems. Since QCLAS and TDLAS make use of a single spectral line
feature to measure the concentration of an isotopologue of CO2, they are inherently
susceptible to interferences from the many poorly understood compounds that absorb in
the infrared and are present in a forest ecosystem or urban environment. The use of large
wavelength coverage in FTIR spectroscopy makes it easy to distinguish any interferent
features in the absorption spectra.
The first manuscript (“A new Fourier-transform infrared instrument for
measurement of temporal and vertical distribution of δ13C – CO2 1. An overview of
results from measurements in a managed poplar forest in northern Oregon”) described in
detail a new FTIR approach for in-situ, real-time stable isotope measurement in forest
ecosystems. The instrument can run virtually unattended for 24h/day taking data at
various heights in a canopy or at a single height on time centers less than one minute. We
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deployed the FTIR system in a managed poplar forest in the summers of 2004 to 2006
where temporal and vertical distributions of CO2 and δ13C were measured. Based on our
long-term measurement of a gas standard in the field, we report accuracy for the forest
measurements of 0.5 ‰ and a precision of 0.8 ‰, respectively, corresponding to a 45-
second cycle time between samples. This time resolution was sufficient for application of
disjunct eddy correlation to determine the biosphere-atmosphere exchange of isotopic
CO2 fluxes.
The nighttime vertical profile measurements of δ13C and CO2 contrasted
significantly with the daytime results. The nighttime measurements revealed high
accumulations of CO2 within the canopy because of the combined effects of ecosystem
respiration, stable nighttime conditions, and slow winds. The highest CO2 concentrations
were observed closest to the ground; they progressively decreased with increasing height
and approached background values above the canopy. Conversely, the corresponding
carbon isotope ratios increased with height. A clear inverse relationship was observed
between the CO2 concentrations and δ13C.
Because of photosynthesis, the daytime contour plots of CO2 and δ13C show that the
air within and above the canopy was isotopically enriched and that the CO2
concentrations were lower relative to background values. The CO2 concentrations and
the corresponding carbon isotope ratios were also uniformly distributed within and above
the canopy indicating that there was good air mixing due to unstable conditions.
The signature of ecosystem respiration (δ13CR) was derived from the application of
the Keeling plot regression on nighttime δ13C – CO2 vertical profile measurements. The
resulting estimate was more enriched than the carbon isotope ratio of bulk leaf samples
130
by as much as 2 ‰. This result was consistent with observations that the respired CO2 is
more enriched than its organic substrates. Comparison of δ13CR with the isotopic
signature of soil respiration, δ13CSR, showed that ecosystem respired CO2 was also more
enriched than soil respired CO2. Recent findings have shown that above ground
respiration is isotopically enriched relative to below ground respiration. We can expect
from mass balance calculations that the isotopic composition of ecosystem respiration
should be intermediate between the isotopic ratios of respired CO2 from above and below
ground components.
The second manuscript was entitled “A new Fourier transform infrared instrument
for measurement of temporal and vertical distribution of δ13C – CO2 2. Measurement of
ecosystem-atmosphere exchange of isotopic carbon dioxide using disjunct eddy
covariance”. This chapter focused on the combination of FTIR spectroscopy with
disjunct eddy covariance for the simultaneous determinations of the biosphere-
atmosphere exchange of total CO2, H2O, and isoflux in the poplar forest. We first
compared the total CO2 flux and the Latent Heat flux derived from the FTIR – DEC
approach and the conventional eddy covariance method using an infrared gas analyzer
(IRGA – EC). The FTIR – DEC CO2 flux measurements agreed with the IRGA – EC
estimates to within 10%, which is well within the statistical uncertainty empirically
determined by Turnipseed et al., (2009) for a sampling interval of 45 seconds. Disjunct
sampling of the water vapor flux constantly underestimated the IRGA – EC
measurements. This result is consistent with observations by other groups and has been
attributed to the damping of high frequency fluctuations due to the adsorption and
desorption of water vapor on the inner walls of the sampling line.
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Simultaneous measurement of CO2 and its isotopic composition enabled the
measurement of the isoflux using the FTIR - DEC approach. Information from the
isoflux of 13CO2 and the net ecosystem exchange of total CO2 were used to directly
determine the canopy-scale photosynthetic discrimination. We found that the carbon
isotopic composition of the assimilation flux was 13C – enriched relative to whole leaf
samples by about 2.7‰. This result agrees with findings that recent assimilates are
enriched relative to the whole organic matter from which they were extracted.
We observed considerable variations in the FTIR – DEC flux estimates that are
chiefly attributed to the limited temporal resolution of the disjunct sampling technique
relative to the conventional IRGA – EC approach (45 second time resolution versus 100
ms sampling interval). We consider the reduced time resolution to be the main source of
statistical uncertainty in the DEC – derived fluxes. Increasing the temporal resolution is
therefore a main consideration for future measurements with the FTIR – DEC system.
The uncertainty can be considerably reduced either by decreasing the cycle time between
samples or increasing the averaging interval.
The ability of Fourier transform infrared spectroscopy to simultaneously measure
the concentrations of several important trace gases is an advantage over other existing
spectroscopic techniques. When combined with disjunct eddy covariance, FTIR – DEC
becomes a powerful tool not only for quantifying the concentrations of molecular
isotopologues and their ratios, but also for investigating the rapid exchange of various
infrared-active species in forest, urban and agricultural ecosystems.
The fourth chapter (“Analysis of low-concentration gas samples with continuous-
flow IRMS: eliminating sources of contamination to achieve high precision”) is already
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published in Rapid Communications in Mass Spectrometry. This manuscript resulted
from the fact that there is still no standard procedure/protocol amongst isotope
laboratories in analyzing gas samples using continuous flow isotope ratio mass
spectrometry (CF-IRMS). The lack of a protocol presented a major challenge in
analyzing the working gas standards that were used during our field measurements.
Thus, we first developed a procedure for preparing and analyzing gas samples on a CF –
IRMS. The optimized procedure was then used to ascertain the isotopic composition of
the four reference cylinders that were used during field measurements. The chapter
discussed (1) sources of contamination in CF-IRMS analysis of near-ambient CO2
concentration gas samples, (2) analytical procedures that reduce the contamination, and
(3) the identification of a precise, efficient method for introducing gas samples into vials.
5.2. Personal thoughts and recommendations for future measurements
This research was a multi-disciplinary project and it gave me the opportunity to be
exposed to various disciplines such as spectroscopy, instrument development, plant
physiology, biosphere-atmosphere interactions, and carbon cycling. Apart from finishing
the dissertation, the most challenging aspect of this project was the high precision
measurement of the carbon isotope ratio, a quantity that is inherently small in magnitude
(i.e. detection of changes smaller than 1 part in 104 was not trivial). From 2005 to 2006,
we learned how to modify our experimental methods to increase the sensitivity of the
instrument for δ13C measurements such as purging the spectrometer, detector and sample
compartments with nitrogen gas. When the background CO2 dropped to ~2 ppmv, we
133
saw a considerable improvement in our measurements. Frequent online calibration with
a reference tank as well as continuous measurement of gas cell pressure and temperature
also enabled us to track any experimental irregularities such as instrument drift.
However, there are still several ways to further improve the measurement of the isotopic
composition of carbon dioxide for future measurements. Here is a short list of
recommendations:
1) To reduce the cycle time between samples, and increase the time resolution of the
FTIR – DEC samples, replace the platinum RTD temperature sensor and the
Omega pressure transducer for fast response but accurate sensors that have short
equilibration times. This will effectively reduce the “dead time” in between
samples.
2) To improve the retrievals of δ13C, use two reference cylinders with a range of
carbon isotope ratios from –5 to –12 ‰, with corresponding concentrations of 320
to 450 ppmv respectively, for the online calibration of samples. Cambridge
isotope laboratory carries isotopic CO2 gas standards with the appropriate range
that can be mixed with instrument grade zero air to obtain the corresponding
concentrations.
3) Our sensitivity analysis showed a 1.4‰ uncertainty in the retrieved δ13C for every
0.1ºC change in gas cell temperature. Therefore, a more stringent control of gas
cell temperature and feed lines is also recommended because of the temperature
dependence of δ13C.
4) If funding is available for a new FTIR instrument, then it is best to purchase a
vacuum system. This modification will remove the need to transport around 15
134
T-sized cylinders of nitrogen gas to a distant site to purge the instrument for a
two-week field campaign.
5) To improve the measurement of the lag time between the acoustic anemometer
and the FTIR spectrometer, a flow meter can be installed in the tower sampling
line. As we have done in the field, puffs of high concentration CO2 samples can
be released at the highest intake point in the tower and the time it takes for the
puffs to be detected by the FTIR spectrometer recorded. With the addition of the
flow meter, the dependence of the lag time on the flow rate can be determined.
6) The instrument was computer controlled using a National Instruments FieldPoint
(FP) computer. The FP also recorded data from all the auxiliary sensors (Pt RTD
sensors, pressure transducers, and GPS) aside from instrument control. The
increase in sensors in the summer of 2006 taxed the memory of the FP, and was
probably the reason why the instrument would occasionally stop data acquisition
at random times. As more sensors are added to the system, it would be best to
replace the FP with an updated acquisition system with much larger memory and
higher speed.
7) It would also worthwhile to invest in a detector that does not require cooling with
liquid nitrogen but has comparable or higher sensitivity and signal to noise ratio
(SNR) in the 2100 to 2600 cm-1 region as the InSb detector we used in the field.
The use of an InSb detector with Peltier cooling with excellent SNR that can
operate at room temperature will remove the need to haul liquid nitrogen to
remote field sites.
135
Apart from implementing the suggested recommendations, where do we go from
here? There are many exciting science problems that can be addressed by long-term
measurements of CO2 and δ13C. Here is a short list of some of the possible science
problems that can be investigated with the FTIR – DEC system:
a. How does canopy discrimination change with changing seasons? How does an
ecosystem’s primary productivity and discrimination against 13CO2 vary with
changing environmental conditions such as temperature and moisture?
b. When measurements are conducted in a mix C3 – C4 ecosystem, is it possible to
partition the overall contribution of C3 and C4 photosynthesis to the
ecosystem’s productivity?
c. When there is a land cover change in an ecosystem, e.g. a change from a forest
to an agricultural system or a change from C3 to C4-dominated species, how
much will be the change in the ecosystem’s discrimination against 13CO2 and
its net productivity?
d. As stated previously, the FTIR-DEC system simultaneously measures the rapid
exchange of several important trace gases such as N2O, H2O, and CH4. If there
is an increase of nitrogen fertilization in an ecosystem, is it possible to
correlate N2O emissions with canopy discrimination?
e. Apart from the possible future projects suggested above, the existing data set
obtained from the poplar forest is also an important resource that must be
further explored and can potentially be an exciting project for another graduate
student.
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This project exposed me to the exciting experience of instrument development,
fieldwork, and data analysis. It was interesting to be immersed in various disciplines and
to see how this project brought together seemingly unrelated topics. The magnitude of
the work was often overwhelming and it would not have been possible to complete the
various tasks without the expertise and time of many people who were part of this
project. Every step of the way presented a different challenge that undoubtedly enriched
my research experience as a graduate student. Ever since I came on board the project in
2003, there was never a dull moment.