Picarro Cavity Ring Down Spectrometry (CRDS) for High-Precision Continuous

Monitoring of Atmospheric CO2 and CH4

Dave Lowry, Rebecca Fisher, Mathias Lanoisellé, Sri Sriskantharajah and Euan Nisbet

Department of Earth Sciences, Royal Holloway, University of London, Egham, Surrey TW20 0EX, United Kingdom

Presented at:Presented at:APRIL Meeting, London, 12/09

Funded by NERC since 2008

CRDS Theory• When the laser is on, the cavity quickly fills with circulating laser light. A fast photo-detector senses the small mount of light leaking through one of the mirrors (not 100% reflective) to produce a signal that is directly proportional to the intensity in the cavity.

• When the photodetector signal reaches a threshold level (in a few tens of microseconds), the continuous wave (CW) laser is abruptly turned off. The light already within the cavity continues to bounce between the mirrors (about 100,000 times).

• The light intensity inside the cavity steadily leaks out and decays to zero in an exponential fashion. This decay, or "ring down", is measured in real-time by the photodetector.

http://www.picarro.com/technology/whatiscrds.php

The Model We Use

• Picarro Inc.WS-CRDS Analyzer for CO2/CH4/H2O in air - Model G1301 (Made in California)

• Wavelength-specific

• Simultaneous, precise measurement of CO2, CH4 and H2O

How much does it cost?

At current exchange rates about £40,000 once installedAutomated inlet parts - £7,000Inlet control software development - £5,000Calibration gas suite - £3,000

Tasks

• Instrument Tests– Standardization with NOAA calibration gases– Precision– Water vapour correction– Long-term drift

• Development of Automated Inlet– Software– Automated valves and flow

• Deployment at Remote Sites– South Atlantic islands– RHUL

Picarro G1301

NOAA standardization• Currently 6 NOAA standards in use in lab - 2 ambient air, 4 synthetic• CRDS only measures 12CCO2 and 12CCH4 peaks

• Synthetic are depleted in 13CCO2 and enriched in 13CCH4 so extra correction

necessary if not using ambient air standards

Precision - International Requirements for Background Stations

• Northern Hemisphere - CO2 ± 0.05 ppm, CH4 ± 2.0 ppb• Southern Hemisphere - CO2 ± 0.025 ppm, CH4 ± 1.0 ppb• Residuals for 5 NOAA standards - CO2 ± 0.018 ppm, CH4 ± 0.16 ppb• Achieved on 10 min ave (120 points) - CO2 ± 0.018 ppm, CH4 ± 0.15 ppb

Water Vapour Correction• Factory applied

for CO2 but….• Currently no

consensus for the CH4 correction

• Calculated by having two instruments side by side on the same air line, one of which has a Nafion-dried airstream - reduces water from 1.5% to 0.1%

= raw CH4 (ppb) x (1 + %H2O x constant)

Calculated constant = 0.0101109

Nafion reduces correction from ≈27 to 2 ppb

0.01898 ppm (18.98 ppb) / 1% H2O

5.075 ppm / 1% H2O

Long Term Drift

• CO2 - no obvious drift - up to 0.1 ppm fluctuation

• CH4 drifted by 3-4 ppb in 1 year (less when air is dried) - <0.2%

Automated Inlet Development6 standard bottles:3 calibration1 target1 zero1 long term

Automated Inlet Software• Controls cylinder outlet pressure at 0.5 bar (7 psi)• Controls flow at 300-500 cc/min• Gives real-time data display• Creates calibration file and makes drift correction between calibrations• Remote data transfer capabilities

Deployment in the South Atlantic

• Falkland Islands and Ascension Island

• Close to airports for communications

Deployment at RHUL

• Very local source now affecting all air from W-NW during working hours

25 March 2009

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00:00 02:00 04:00 06:00 08:00 10:00 12:00 14:00 16:00 18:00 20:00 22:00 00:00

CO

2 (ppm)

-40

-30

-20

-10

0

10

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40

P3 - P2 (ppm)

P2_CO2_CORR

P3_CO2_CORR

P3 - P2

Source ?

P2

P3

LiCor