pacific exploratory mission-tropics carbon...

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. D21, PAGES 26,195-26,207, NOVEMBER 20, 1999

Pacific Exploratory Mission-Tropics carbon monoxide measurements in historical context

N. S. Pougatchev, • G. W. Sachse, 2 H. E. Fuelberg, 3 C. P. Rinsland, 2 R. B. Chatfield, 4 V. S. Connors, 2 N. B. Jones? J. Notholt, 6 P. C. Novelli, ? and H. G. Reichle Jr. 8

Abstract. The three-dimensional (3-D) distribution of carbon monoxide (CO) over the southern Pacific during the NASA Global Tropospheric Experiment Pacific Exploratory Mission-Tropics (PEM-T) (August-October 1996) has been analyzed in comparison to other CO measurements. The following data sets have been used in the study: National Oceanic and Atmospheric Administration Climate Monitoring and Diagnostic Laboratory surface level sampling; Commonwealth Scientific and Industrial Research Organization aircraft measurements over Cape Grim, Tasmania; solar spectroscopic measurements at Lauder, New Zealand; and data from two spaceborne Measurement of Air Pollution From Satellite experiments. For the PEM-T mission back trajectories analysis and 3-D modeling of the CO transport have been performed. It has been demonstrated that CO measurements obtained by different in situ and remote techniques can be used to build the picture of the CO climatology over the large geographical area. The structure of the CO distribution over the western part of the southern Pacific during the austral spring is mainly controlled by emission from biomass burning in Australia and Africa and subsequent long-range transport. The prevailing westerly transport occurs in the middle and upper troposphere, whereas the marine boundary layer remains relatively clean and uniform. Barriers in the form of the Intertropical Convergence Zone and South Pacific Convergence Zone protect the equatorial area (equator to 10øS) from direct impact of biomass burning plumes from north and southwest. Consistency between the measurements taken in different years and modeling results indicates that the observed feature is a stable phenomenon. Outside the equatorial area the CO vertical distribution has a broad distinctive maximum at the altitude range 5-8 km and latitudes between 20øS and 30øS. This maximum is a stable feature, and its location indicates the area where the most intensive westerly transport occurs.

1. Introduction

Carbon monoxide (CO) plays an important role in tropospheric chemistry and is a representative indicator of atmospheric pollution because it is emitted primarily by anthropo- genic and natural sources (e.g., fuel combustion, biomass burning) [Logan et al., 1981; Khalil and Rasmussen, 1988, 1990, 1994]. Being responsible for removing ---75% of the hydroxyl radical (OH) from the troposphere [Thompson, 1992], CO is one of the most important factor that determines concentrations of atmospheric oxidants. The importance of CO has resulted in intensive measurements of this trace gas on a regular

•Department of Physics, Computer Science, and Engineering, Chris- topher Newport University, Newport News, Virginia.

2NASA Langley Research Center, Hampton, Virginia. 3Department of Meteorology, Florida State University, Tallahassee. 4NASA Ames Research Center, Earth Science Division, Moffet

Field, California. SAtmospheric Division, National Institute of Water and Atmo-

spheric Research, Lauder, New Zealand. 6Alfred Wegener Institute for Polar and Marine Research, Potsdam,

Germany. 7NOAA Climate Monitoring and Diagnostic Laboratory, Boulder,

Colorado.

8Department of Marine, Earth, and Atmospheric Science, North Carolina State University, Raleigh.

Copyright 1999 by the American Geophysical Union.

Paper number 1999JD900465. 0148-0227/99/1999 JD900465 $09.00

basis and during numerous field campaigns [e.g., Seiler, 1974; Seiler et al., 1984; Fraser et al., 1986a; Sachse et al., 1988; Reichle et al., 1986, 1990; Connors et al., 1998; Novelli et al., 1992, 1994a, b, 1998a, b]. On the basis of an extensive and internally consistent data set of CO concentrations in the lower troposphere (38 locations from 2 to 10 years time series) a global zonally averaged model has been constructed [Novelli et al., 1998b]. However, with tropospheric life time of ---2 months [Hough, 1991] and with variable, nonuniformly distributed sources (e.g., biomass burning, fossil fuel combustion), neither zonal nor vertical uniformity is attained.

This complexity of the CO distribution and its temporal variations require different measurement techniques and strat- egies. These approaches can be grouped in three categories: spaceborne remote measurements, ground-based remote measurements, and point sampling in situ measurements.

A global view of the CO distribution in the troposphere was obtained during four U.S. space shuttle flights (November 1981, October 1984, April 1994, and October 1994) by the Measurement of Air Pollution From Satellite (MAPS) experiment [Connors et al., 1998; Reichle et al., 1986, 1990, 1999]. This type of remote measurement provides a "snapshot" of the global CO distribution (within latitudinal band 57øN to 57øS) during a mission period (about 10 days) with an accuracy of ---10% [Pougatchev et al., 1998; Reichle et al., 1999; Novelli et al., 1998a]. A few CO profiles in the stratosphere and upper troposphere have been obtained during the Atmospheric Trace

26,195

26,196 POUGATCHEV ET AL.: PEM-TROPICS CO MEASUREMENTS

MOlecule Spectroscopy (ATMOS) instrument during four shuttle flights between in May 1985 [Gunson et al., 1990] and November 1994 [Rinsland eta!., 1998a] also from a U.S. space shuttle. The short duration of the shuttle flights, the geometry of spaceborne occultation observations, and the gaps between individual measurements (because of clouds, calibration periods, etc.) resulted in relatively poor spatial sampling, giving low spatial and temporal resolution of these snapshots.

Ground-based infrared solar spectroscopy has been used to measure CO for many years [e.g., Dianov-Klokov and Yur- ganov, 1989; Notholt et al., 1997; Pougatchev and Rinsland, 1995; Wallace and Livingston, 1990; Zander et al., 1989; Rins- land et al., 1998b]. These data sets cover long time periods, giving information about long-term trends, seasonal variations, and irregular (e.g., day-to-day) changes. Ground-based spectroscopic measurements provide CO columns integrated along layers in the troposphere and lower stratosphere. Hence they can detect enhanced CO levels in plumes of pollution uplifted by deep convection into middle and upper troposphere and then rapidly transported horizontally to remote regions [Chat- field et al., 1996; Dickerson et al., 1987; Heitzenberg and Bigg, 1990; Fuelberg et al., 1999]. These plumes that are detected by remote sensing techniques [Connors et al., 1999; Rinsland et al., 1998b] and in situ aircraft measurements [Matsueda et al., 1998] are not observed by surface in situ measurements [No- velli et al., 1998b]. This capability of the spectroscopic technique is especially valuable for measurements in southern tropical and subtropical areas, where CO variations during austral spring are caused by the long-range transport of tropical biomass burning emissions [Fishman et al., 1991; Watson et al., 1990; Heitzenberg and Bigg, 1990].

Flask sampling CO measurements have been performed over the southern Pacific area for approximately a decade. Samples have been taken from ground stations [Novelli et al., 1998b], scientific and commercial aircraft [Fraser et al., 1986b; Pak et al., 1996; Matsueda et al., 1998], and ships [Novelli et al., 1998b]. Regular (once a month) measurements of CO vertical profile over Cape Grim, Tasmania (from the surface to 7.6 km), revealed a stable positive vertical gradient of CO volume mixing ratio (VMR). The CO seasonal cycle at all altitudes had its maximum in September-October, corresponding to the peak of biomass burning activity in the Southern Hemisphere [Fraser et al., 1986b; Pak et al., 1996; Novelli et al., 1998b]. The same measurements showed that the amplitude of the seasonal cycle increased with altitude. Measurements from commercial jets between Narita, Japan, and Sydney, Australia, at 9.5-13 km altitude also showed a CO seasonal cycle in the upper troposphere, with a maximum at 25øS-30øS [Matsueda et al., 1998]. Comparison of the surface measurements [Novelli et al., 1998b] with those in the upper troposphere [Matsueda et al., 1998] indicated that meridional variations are smaller in the marine boundary layer.

Summarizing the research mentioned above, we conclude that spatial distributions and temporal variations of CO have a stable, reproducible pattern from year to year (e.g., phase of the CO seasonal cycle, sign of the vertical gradient) with quantitative characteristics that can vary significantly depending on irregular phenomena such as the intensity and geographical distribution of biomass burning, the type and intensity of atmospheric circulation which affect long-range transport. For example, whereas CO content in the Southern Hemisphere always is maximum in September-October, its magnitude var-

ies significantly from place to place and from year to year [Novelli et al., 1998b; Rinsland et al., 1998b; Matsueda et al., 1998].

The purpose of this paper is to report CO measurements from the Pacific Exploratory Mission-Tropics (PEM-T) in the context of other available relevant CO data sets. This approach enables us to determine which features of the observed CO

distribution represent its climatology and which are deter- mined by irregular phenomena. As mentioned above, CO is a good indicator of processes which regulate many other atmospheric constituents. Thus our analysis of PEM-T CO data in a historical context can provide information about the climatological representativeness of the measurements of other atmospheric constituents performed during the PEM-T mission. We begin by comparing the PEM-T results with in situ surface and aircraft measurements, MAPS results for October flights in 1984 and 1994, and ground-based spectroscopic CO measurements at Lauder, New Zealand. We pay special attention to comparability of the different techniques and consistency of the calibrations. We also consider possible sources and transport mechanisms which can explain the observed phenomena.

2. Results and Discussion

Carbon monoxide measurements were conducted over the

southern Pacific Basin by two NASA aircrafts, i.e., a McDon- nell Douglas DC-8 and a Lockheed Orion P3-B. The PEM-T mission covered the period from the end of August to the beginning of October 1996, over the geographical area 155øE - 80øW, 20øN-72øS. The DC-8's area of operation was between 155øE and 110øW, while the P3-B flew east of 160øW. Because of the relatively low ceiling of the P3-B (--•6 km) and the location of available data for comparison (most were taken over the western part of the southern Pacific) this study only considers the DC-8 measurements. Most of the DC-8 flights were conducted between the equator and 30øS. A map of the flight tracks is presented in Figure 1. Bases of operation were located at Fiji, Tahiti, Easter Island, and Christchurch (New Zealand). The DC-8 nominal airspeed at cruise altitude was ---225 m s -•, with an altitude range 0.3-12 km and a flight duration of 12 hours. An overview of the PEM-T mission is

provided by Hoell et al. [1999]. CO measurements from the DC-8 platform were made with

a mid-IR diode laser instrument [Sachse et al., 1991], fre- quently referred to as Differential Absorption CO Measure- ment (DACOM). This instrument utilizes tunable diode lasers in the 4.7-/zm CO band to measure this gas using a differential absorption technique discussed by Sachse et al. [1987]. Fre- quent but short calibrations with well-documented, stable reference gases from National Oceanic and Atmospheric Admin- istration Climate Monitoring and Diagnostic Laboratory (NOAA CMDL) (Boulder, Colorado) are critical to achieving both high precision and accuracy. Calibration is accomplished by periodically (approximately every 12 min) flowing calibration gas through the instrument. By interpolating between these frequent calibrations, slow instrument response drifts are effectively suppressed, yielding high precision. Measurement precision (standard deviation) is the greater of 1 ppbv or 1% for 1-s time response data. Analytical precision of the CMDL measurements is 0.5-2% (depending on CO absolute value) [Novelli et al., 1998b]. Intercomparisons between CMDL and DACOM techniques [Novelli et al., 1994b] show an agreement to better than _+2%.

Plate 1 illustrates features of the atmospheric CO distribu-

POUGATCHEV ET AL.: PEM-TROPICS CO MEASUREMENTS 26,197

EQ

10S

20S

30S

40S

50S

New ZealandS.: ..

, -.. , : , .. , ., : :, .. : '.. : . : .. : Western :.. ß .': .'

...... : ....... •::": ........ i'' ':' 7' ........ :' 'S:ahi6• .............. •": .... !' ': ' ',; .......... : :.. . .: .: ...... • ...... : ... :...: , , '.. : ß , ....' , ".....• , . .".'.. , ß ß : : '. ß .': ..... '. .... ..' ...... , '.._.. ',Tah,t, : : Fiji'•..":' : .-' ! "• ..... ß':::; ............

ß

.,

, . ,

. . ,

,

,

,

,

,

,

, . ,

ß

130E 140E 150E 160E 170E 180 170W 160W 150W 140W 130W 120W

Figure 1. Map of the DC-8 flight tracks during PEM-T. The thick rectangular boxes indicate the assigned boarders of the eastern and western zones (see section 2).

tion measured during three selected flights, i.e., (1) flight 13, September 21, from New Zealand to Antarctic coast and back; (2) flight 14, September 23, from New Zealand to Fiji; and (3) flight 16, September 28, from Fiji to the equator and back. The color of each point represents 1-min-averaged CO VMR. Thin lines indicate portions of the flights where no CO data were taken (calibration, malfunction, etc.).

CO is distributed very nonuniformly both vertically and horizontally. Comparison between individual CO measurement that are not collocated in time and space is very difficult due to this variability and nonuniformity. This consideration and the relatively poor sampling (compared to the area and duration of the mission) determine the way the PEM-T data must be organized to permit comparison with other CO data sets. In this study we calculate statistical characteristics of CO spatial distributions integrated over the entire period of the PEM-T mission. We focus primarily on the area of the equator-30øS and 155øE-130øW, where most of the CO data were taken. Because the prevailing west to east transport is accompanied by dilution and chemical sink, there is no zonal uniformity in the above mentioned area. Therefore we consider two zones, i.e., eastern (155øE-160øW) and western (160øW-130øW). Boundaries of these zones are indicated on Figure 1.

Three PEM-T flights were conducted in the extratropical southern Pacific, defined by Manning et al. [1997] as the area south of 30øS. Regular CO aircraft measurements over Cape Grim, Tasmania [Pak et al., 1996], and ground-based solar spectroscopic measurements at Lauder, New Zealand [Rins- land et al., 1998b], provide high-quality data sets for interpret- ing the PEM-T data taken in this area. These results are presented in section 2.4.

2.1. CO Vertical Distribution

Plate 2 contains meridional cross sections of the CO distri-

bution in the eastern and western zones. Plate 2 indicates that

the tropospheric CO distribution between equator and -10øS is relatively clean and uniform. Conversely, major pollution is observed in the middle and upper troposphere of the eastern zone between 20øS and 25øS. Box-and-whiskers plots in Figure 2 give additional information about the CO vertical distribu-

tion in four geographical areas. Boxes indicate the 25-50 per- centiles, horizontal bars denote the 10-90% intervals, and open circles show individual measurements beyond the categories mentioned above. In the equatorial zone (Figures 2a and 2b) CO vertical distribution in both the eastern and western zones is practically uniform with a mean value of 55-58 ppbv. A different situation occurs in the area between 10øS and 30øS (Figures 2c and 2d). The CO vertical profile in this zone has a well-pronounced maximum between 5 and 7 km altitude. Although shape of the profiles is similar in both the eastern and western regions, absolute CO values in the western part average 15-20 ppbv greater at all altitudes. In the western zone, CO concentrations of 140-170 ppbv are observed at all altitudes. However, in the eastern zone the greatest CO values occur in the upper troposphere. Analysis of the meteorological situation during PEM-T period [Fuelberg et al., 1999] and distribution of other trace gas [Gregory et al., 1999; Nay et al., 1999] indicate that prevailing west to east transport and the existence of barriers provided by the Intertropical Conver- gence Zone (ITCZ) and South Pacific Convergence Zone (SPCZ) can explain the observed distributions. More discussion on this subject is presented in section 2.5.

2.2. CO in the Marine Boundary Layer

The geographical distribution of the CO within the marine boundary layer (-0.2-1 km) during PEM-T is presented in Plate 3. The data are grouped in 1 ø x 1 ø bins, and color represents averaged CO VMR within a bin. Although sampling is poor, the limited number of data points suggests that CO is distributed relatively uniformly in the boundary layer. Figure 3 presents these data compared with NOAA CMDL surface measurements. The CMDL shipboard samples were taken at intervals -5 ø latitude during regular cruises between New Zea- land and the west coast of the United States. The ships crossed the equator between 170øE and 150øE. The sampling frequency was low, only once or twice a month. More details about the NOAA CMDL measurement operations are given by Novelli et al. [1998b]. The 6-year time series and other CMDL data [Novelli et al., 1998b] show no distinctive meridional CO gradient between 10øS and 30øS. Positive gradient in equatorial


lO km

170W 70S

' 10km

' 8km

CO (ppbv)

105+

95

75

55

Plate 1. CO distribution as measured during PEM-T flights 13, 14, and 16. Each symbol represents a 1-min average; thin lines denote parts of a flight where no CO data were taken.

155E - 160W

lO

8

160W - 130W

12 ! .... • .... , .... • .... • ............. • ...... _

•

o

3os

o

25s 2os 15s lOS 5s EQ 3OS 25S 2OS 15S lOS 5S

Latitude Latitude

CO (ppbv)

EQ

<45 55 65 75 85 95 105 115 125 135 >145

Plate 2. CO distribution (PEM-T data) averaged over the eastern and western zones as functions of altitude and latitude.


EQ

10S ........ .q•.k,,• ...... •. , , ,

, , ,

15S ...................... • .......................................................... : ............... . .................................... •. • . '. .

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, , , , ,

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155E 160E 165E 170E 175E 180 175W 170W 165W 160W 155W 150W 145W 140W 135W 131 •W

CO Mixing Ratio (ppbv)

<45 55 65 75 85 95 105 115 125 135 >145

Plate 3. CO distribution (PEM-T data) in the marine boundary layer (0.2-1 km). The data are averaged within 1 ø x 1 ø bins.

EQ :11

i

25S ............................................................................. ,

,

30s : , ' , 3

, ,

, ß

,

!

,

,

,

,

,

155E 160E 165E 170E 175E 180 175W 170W 165W 160W 155W 150W 145W 140W 135W 130W


<45 55 65 75 85 95 105 115 125 135 >145

Plate 4. CO distribution in the middle and upper troposphere during PEM-T. Data are averaged within 1 ø x 1 ø bins and an altitude range from 4 to 12 km.

26,200 POUGATCHEV ET AL.' PEM-TROPICS CO MEASUREMENTS

12

11-

10-

9 -

• 8-

• 7 --

1:3 6-

I-- 5-

_.1 4-

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'155øE - '160øW Equator- 10øS__

a) '160øW - 130øW

b)

oo co

50 60 70 40 50 60 70 80 90 40 80 90

CO (ppbv) CO (ppbv)

30øS_ 10øS '155øE - '160øW '160øW - 130øW

c) d) 12 12

11 • o co o 11 10 10

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2 2

1

0 4o •o •o •oo •2o •4o •o •o 4o •o •o •oo •2o •4o •6o •o

•O (•v) •o (•v)

Figure •. CO vertical distribution (PEM-T d•t•) over four 6eoBr•phic•l •re•s: (•) •ø-1•ø8, 155øE-16•øW; (b) •ø-1•ø8, 16•ø-l•øW• (c) 1•ø-•ø8, 155øE-16•øW; •nd (d) 1•ø-•ø8, 16•ø-l•øW. Boxes indicate 25-5• percentile mnBe, horizontal b•rs denote the 1•-9•% inte•ls, •nd open circles show individual measurements beyond the c•teBories mentioned

area reflects a stable interhemispherical gradient [Novelli et al., 1998b, Figure 7]. Despite the uniformity of these averages, individual samples are scattered in the range of -50-100 ppbv. The vertical bars in Figure 3 indicate standard deviations of individual CMDL samples, while solid circles represent the PEM-T 1-min CO measurements. The lines connecting the CMDL averages are for purposes of clarity only. The CMDL 1996 data are lower than 6-year averages, and they are in a good agreement with PEM-T measurements.

2.3. CO in the Middle and Upper Troposphere

The geographical distribution of CO in the middle and upper troposphere (4-12 km altitude range) as measured during the PEM-T mission is presented in Plate 4. The data are organized similarly to those in Plate 3; however, sampling at these higher altitudes is significantly better than in the boundary layer (Plate 3). Interesting features are evident in the CO distribution. The well-sampled area between 0 ø and 10øS does

not indicate any trace of CO plumes. However, most measurements south of 10øS and west of 160øW indicate enhanced CO

values. The eastern zone (160øW-130øW) appears cleaner (CO-wise) than the western zone (155øE-160øW).

For comparison, we show the results from two October (1984 and 1994) MAPS missions. The MAPS data averaged within 1 ø x 1 ø bins are presented in Plate 5. It is appropriate to characterize the MAPS data before proceeding further.

The MAPS instrument is a nadir-viewing gas filter radiometer that operates in the region of the 4.67-/•m CO fundamen- tal band. The instantaneous field of view is -20 x 20 km 2, and a single measurement time is 1 s. A detailed discussion of the instrument, data reduction, and data validation is given by Reichle et al. [1999]. The MAPS instrument does not have a profiling capability; hence its signal depends on the CO concentration at all levels in the atmosphere. However, the signal is not equally sensitive to the presence of CO molecules at all altitudes. Instead, the MAPS instrument has its maximum sen-


sitivity to CO in the middle and upper troposphere, and it is practically insensitive to the CO variations in the boundary layer [Reichle et al., 1999]. Comparisons of the MAPS data and correlative aircraft in situ measurements show that the MAPS

readings agree with average CO concentrations in the middle and upper troposphere to + 10% [Reichle et al., 1999]. During the MAPS 1994 validation activity, aircraft in situ measurements were referenced to NOAA CMDL standards. The esti-

mated accuracy was found to be about ___3% [Reichle et al., 1999]. Thus we conclude that to 10% accuracy, the reported MAPS data can be interpreted as averaged CO VMR through- out the middle and upper troposphere.

Both plots in Plate 5 reveal CO distributions that are similar to those seen during PEM-T (Plate 4), i.e., relatively clean equatorial area, with the western zone containing more CO than the eastern zone. Absolute values during 1994 are notice- ably greater than those in the 1984 MAPS data and 1996 PEM-T data. For a quantitative comparison, the data from Plates 4 and 5 are presented in Figure 4 as zonal averages as a function of latitude. The thin curves represent zonally averaged MAPS data with 1 ø latitude steps. Because of the relatively poorer sampling, the PEM-T data are averaged over 5 ø latitudinal belts. The vertical bars indicate standard deviation

for the PEM-T data, while the thick connecting lines are for purposes of clarity only. Standard deviations of the MAPS data range from •<10 ppbv in equatorial area to •<20 ppbv at 25 ø- 30øS (not shown). The 1984 MAPS and PEM-T values are in reasonable agreement. However, the 1994 MAPS data are systematically 30-70% greater. We have some evidence indi- cating that these relations between MAPS and PEM-T data are not occasional.

Solar spectroscopic measurements of CO and other trace gases over the Atlantic Ocean were performed by J. Notholt

90-

80-

60-

50-

30 25 20 15 10 5 0

Latitude South

Figure 3. CO in the marine boundary layer as a function of latitude. Solid circles represent 1-min-averaged PEM-T data, open symbols indicate NOAA CMDL flask samples for the PEM-T period, squares represent average data for 1990-1996 time series, and circles represent measurements only during 1996. The vertical bars indicate standard deviations.

130

120 -

110

IO0

90

80 // '• '"

7ø-I

30 20 10

Latitude South

Figure 4. PEM-T (4-12 km) and MAPS data as a function of latitude. The thin solid and dashed lines represent the MAPS 1994 data for the western and eastern zones, respectively. The thin dashed and dashed-double dotted lines show the MAPS

1984 data for the western and eastern zones, respectively. Solid squares and circles represent PEM-T data for the eastern and western zones, respectively.

during a cruise in October 1996. The R/V Polarstem traveled from Europe to South America in a longitudinal corridor between 25øW and 35øW. Data were taken at a spectral resolution up to ---0.005 cm -• between a latitude range of 56øN and 30øS. CO content in three atmospheric layers 0-4 km, 4-12 km, and 12-100 km has been retrieved from high-resolution solar infrared spectra using a nonlinear least squares fitting technique, similar to that used by Rinsland et al. [1998b]. (An analysis of the data for all species will be published by J. Notholt (manuscript in preparation, 1999.) Pougatchev et al. [1998] have shown that the MAPS data and correlative solar spectroscopic measurements agree to +_ 10%. More discussion about the accuracy of spectroscopic measurements is presented in section 2.4. Daily average CO VMR in the free troposphere (between 4 and 12 km) are presented in Figure 5 (solid circles) along with the MAPS October 1984 (dashed line) and 1994 (dotted line) results. The MAPS data are zonally averaged between 25øW and 35øW with 1 ø step in latitude. Thus, similar to observations in the South Pacific, 1984 and 1996 measurements in the Atlantic agree very well, whereas MAPS 1994 numbers for the south tropical area (both Pacific and Atlantic) are on average ---50% greater.

The above mentioned relation between CO levels during 1984, 1996, and 1994 is consistent with the E1 Nifio-Southern Oscillation (ENSO) cycle. Analysis of sea surface temperature anomalies (SSTA) in the eastern equatorial Pacific (4øS-4øN, 150øW-90øW) indicates that the ENSO cycle had its weak cold phases in 1984 and 1996 and a weak warm phase in 1994 (Center for Ocean Atmospheric Prediction Studies, http:// www.coaps.fsu.edu/--•legler/jma_indexl.shtml, 1998). Biomass burning is the major factor which controls the CO content in the Southern Hemisphere during the austral spring [Novelli et al., 1998b; Garstang et al., 1996; Krishnamufti et al., 1996; Pick-


, ,

25S .................... ,

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, : ß . ß , , . ,

305 ' ' ' 155E 160E 165E 170E 175E 180 175W 170W 165W 160W 155W 150W 145W 140W 135W 130W


<45 55 65 75 85 95 105 115 125 135 >145

10S ...... i ......... : .................... . , , ,'" , .' ........ :' ......... ß . ' , , , . , , ' . ,

, ! , , , , •, ,

15S .......... ', .............. : ................. : ................. •, ......... ,•'-' . ' =•,. ', .... ,,

20S ................. : ......... m..,., ....... ; .... :-' .. _; ........ ; ............ ; ......... . ,

, .. . :. , ' .... . • , . , 25S ........................................ : ........................... :' ...............................

....... ; : : ß ', : ß , -:. . , , ; • '

, ß , . .d ; , , , 30S .............

155E 160E 165E 170E 175E 180 175W 17'0W 165W 160W 155W 150W 145W 140W 135W 130W


<45 55 65 75 85 95 105 115 125 135 >145

Plate 5. CO distribution as measure during two MAPS flights, October 1984 and October 1994.


ering et al., 1996; Talbot et al., 1996]. During any given year the intensity of the burning depends on current weather conditions and meteorological situation during previous years [Justice et al., 1996]. While weather conditions for warm (or cold) ENSO phases tend to have many common characteristics, there is considerable variability between individual cases. To firmly establish relations between biomass burning activity in the Southern Hemisphere and the ENSO phase, careful analysis of rainfall, wind, and burning patterns for several years would be required. However, that task is beyond the scope of the present study.

Hence we conclude that the CO distribution in 1984 and

1996 were similar and hypothesize that the observed differ- ences between 1984, 1996, and 1994 data sets reflect difference in the biomass burning intensity in the Southern Hemisphere tropics during the corresponding years, which is consistent with the ENSO cycle.

2.4. CO Over the Extratropical Southern Pacific

Three PEM-T flights were conducted over the extratropical southern Pacific, i.e., south of 30øS. Numerous CO measurements in the area have shown large seasonal and irregular variations [Fraser et al., 1986a, b; Khalil and Rasmussen, 1994; Pak et al., 1996; Novelli et al., 1998b; Rinsland et al., 1998b]. Primarily, these variations are caused by the long-range transport of tropical biomass burning products [e.g., Novelli et al., 1998b; Garstang et al., 1996]. To evaluate the representativeness of the PEM-T data, they should be compared to the existing CO climatology considering two CO databases for these purposes, i.e., aircraft in situ measurements over Cape Grim, Tasmania, and ground-based solar spectroscopic measurements at Lauder, New Zealand.

Regular aircraft measurements of the CO vertical profile are performed over the Southern Ocean (40ø33'S, 144ø18'E), 35 km west of Cape Grim, Tasmania [Pak et al., 1996]. Profiles are

200 '

180 -

160 -

140-

120-

100 -

80-

60-

-40 -30 -20 -10 0 10 20 30 40 50 60

Latitude

Figure 5. CO VMR in the middle and upper troposphere over the Atlantic. Solid circles connected by a solid line denote 1996 solar spectroscopic data, while dashed and dotted lines indicate 1984 and 1994 MAPS, respectively.

8 ß

7 -

6 -

5 -

2 -

[ ,,

I

-60 70 80 90

CO (ppbv)

I

I

I

I

I

I

I

i i

100 110 120

Figure 6. CO vertical profiles in extratropical area. Squares and solid line denote the average PEM-T profile, while dots and dashed line indicate the Cape Grim 1992-1995 average profile. Horizontal bars indicate standard deviations.

measured once a month in the altitude range 0.15-7.6 km. The available time series covers the period 1992-1995. Profiles were taken during flow from the southwest to minimize poten- tial pollution from Australia and Tasmania. Hence the data represent the "clean" regional background. Comparisons with the NOAA CMDL standard indicate agreement to better than 2 ppbv (-4%) [Novelli et al., 1998a]. During PEM-T flights 12, 13, and 14 (September 18, 21, and 23), several vertical profiles were taken at latitudes south of 30øS. We selected CO profiles taken between -40øS and 50øS (total of five profiles) for the present study. In Figure 6 the average PEM-T profile (solid line) is presented along with the average Cape Grim Septem- ber profile (dashed line). Horizontal bars represent the standard deviation of individual profiles from the corresponding average. Agreement between the two profiles is good; that is, they have similar CO concentrations in the boundary layer, strong positive vertical gradients, and apparent maxima at 5-6 km altitude. The observed difference at altitudes between 1

and 6 km is statistically insignificant. Thus we conclude that the CO vertical distribution averaged within this latitudinal belt during PEM-T is close to its climatological average.

Solar spectroscopic measurements of CO at Lauder, New Zealand (45øS, 169.7øS, 0.37 km altitude), were performed on a regular basis (when weather permits) over a 4-year period (1993-1997) [Rinsland et al., 1998b]. The entire data set con- sists of 390 days of observation. The relatively high frequency of spectroscopic measurements (compared to monthly aircraft sampling at Cape Grim) and longer time series make the Lauder database a good source for the CO climatology of the area.

The spectroscopic technique used by Rinsland et al. [1998b] enables one to retrieve the CO column in 0.37-12 km layer with an estimated accuracy of -5% [Rinsland et al., 1998b]. Maximum sensitivity of the retrieval to CO molecules occurs in the upper troposphere. Simulation of retrievals from spectra

26,204 POUGATCHEV ET AL.' PEM-TROPICS CO MEASUREMENTS

lOO

80-

70-

60-

50-

40-

30

20

.So.

ø i ) i i i [ i i i ;

I II III IV V Vl VII VIII IX X XI XII

Month

Figure 7. CO spectroscopic measurements at Lauder. Open circles indicate daily averages for the 1993-1997 time series with the exception of 1996 data, solid circles denote 1996 spectroscopic data, and solid diamonds represent PEM-T results.

calculated with a set of actual CO profiles indicates that the rms difference between "true" and retrieved CO columns is

-2%, with no systematic bias over the full factor of 2 range in the measurements [Rinsland et al., 1998b]. This indicates that the so called "smoothing error" caused by a finite vertical resolution [Rodgers, 1990; Pougatchev et al., 1995] contributes only to the random error.

Figure 7 displays results of the CO spectroscopic measurements at Lauder along with PEM-T data for the five selected profiles. The data are plotted as average CO VMR in the 0.37-12 km layer versus day of the year. PEM-T data are presented by solid diamonds, while solid and open circles represent spectroscopic daily averages for 1996 and the rest of the time series, respectively. Various data sets exhibit reasonable agreement, and the PEM-T mean of 80 ppbv is similar to the 1996 maximum. The Lauder data reveal strong scatter during the sharp maximum of the seasonal cycle. The CO contents typical of the minimum of the seasonal cycle have been observed, as well as the maximum values, which are similar to mean mixing ratios observed at midlatitude of the Northern Hemisphere. To understand this phenomenon, a simulation of the CO distribution over the Southern Hemisphere has been performed by R. B. Chatfield. The three-dimensional Global Regional Atmospheric Chemistry Event Simulator (GRAC- ES) was used for these calculations, the same model that was used to study CO during the Transport and Atmospheric Chemistry Near the Equator--Atlantic/Southern African Fire- Atmosphere Research Intiative (TRACE-A/SAFARI) period of September-October 1992 [Chatfield et al., 1996, 1998]. We have used a detailed, reconstructed meteorology of the period, employing the MM5 meteorological model. Results show that during this time of the year, Lauder can be either within an air mass containing biomass burning plumes or inside a relatively clean air mass originating from higher latitudes. Changes in air

mass occur irregularly on a timescale of several days, which explains the observed scatter.

2.5. CO Sources and Long-Range Transport

Major sources of CO in the atmosphere are fossil fuel combustion, biomass burning, and oxidation of methane and non- methane hydrocarbons through reaction with the hydroxyl radical [Logan et al., 1981; Novelli et al., 1998b]. Experiments with a simple two-box model [Novelli et al., 1998b] and a more sophisticated three-dimensional global transport model [Chat- field et al., 1996, 1998; Granier et al., 1996] show that biomass burning during the dry season is the major factor controlling global CO content in the Southern Hemisphere at the peak of the seasonal cycle. Main contributing areas are South America, Africa, and Australia. The relatively long atmospheric life time of CO (-50-60 days) [Granier et al., 1996; Hough, 1991] and high zonal wind speed (-20-30 m s -q) in the middle and upper troposphere [Fuelberg et al., 1999] diminish the role of CO chemical sources and sinks in shaping the structure of the CO distribution [McKeen et al., 1996]. Hence the observed CO distribution primarily reflects a long-range transport from the above mentioned sources. In this section we discuss how me-

teorological conditions explain some of the observed features of the CO distribution.

On the basis of a comprehensive meteorological analysis of the PEM-T period Fuelberg et al. [1999, p. 5619] concluded that "... the 1996 PEM-T period appears to be climatologically representative." Important stable meteorological features which determine trace gas distribution include the Intertropi- cal Convergence Zone (ITCZ) and South Pacific Convergence Zone (SPCZ) [Fuelberg et al., 1999; Gregory et al., 1999]. Both the ITCZ and SPCZ are effective barriers to the low-level

transport of the urban/industrialized air from Northern Hemi- sphere (ITCZ) and biomass burning plumes from the west and southwest (SPCZ) [Gregory et al., 1999]. The existence of a clean equatorial area north of 10øS (see Figures 2 and 3 and Plates 1-4) can be explained by this mechanism. The MAPS data, which confirm the PEM-T observations (see Plate 5 and Figure 4), enable us to conclude that the observed features can be considered climatologically representative.

As mentioned above, biomass burning in South America, Africa, and Australia is the main source of atmospheric CO in the Southern Hemisphere during austral spring ("dry season"). Two 1992 field measurements campaigns TRACE-A [Fishman et al., 1996] and SAFARI [Lindsay et al., 1996] characterized the impact of the outflow of biomass burning emission on the atmosphere over the Atlantic Ocean and western portion of the Indian Ocean [e.g., Talbot et al., 1996; Singh et al., 1996; Krish•amttrti et al., 1996; Gregory et al., 1996; Garstang et al., 1996]. In particular, biomass burning products can be rapidly transported into the middle and upper troposphere by deep convection [Pickering et al., 1996; Wang et al., 1996] and then transported horizontally over thousands of kilometers in a few days [Garstang et al., 1996]. For the present study it is important to note that ---90% of air transport from Africa in the middle and upper troposphere at latitude -20øS can flow into the Indian Ocean [Garstang et al., 1996]. Unfortunately, to our knowledge, the analogous study for Australia has not been performed.

To evaluate the relative impact of the above mentioned sources on the CO distribution, a backward trajectory clima-


tology for the PEM-T period has been performed by H. E. Fuelberg. A broad maximum of CO concentration has been observed at altitudes of ---5-7 km and latitudes of --•20øS-30øS

(see Plate 2 and Figures 2 and 5). The trajectory analysis shows that ---65-75% of trajectories arriving at 150øW in this altitude and latitude range had passed over Australia 3-5 days earlier. At the same time, only ---20-30% of the trajectories had passed over Africa 6-7 days earlier. The fraction of the trajectories that passed over South America and reached 150øW is significantly smaller.

Ratios of hydrocarbons with an atmospheric lifetime shorter than that of CO (e.g., ethyne C2H2) to CO can be used as an indicator of the chemical age of an air mass after it was pol- luted [Smyth et al., 1996; McKeen et al., 1996; Talbot et al., 1996; Gregory et al., 1999]. C2H2 (pptv)/CO (ppbv) ratios of approximately <0.5, 1, and 3 can be used to characterize air aged approximately >10 days, 5 to 7 days, and a few days, respectively, from an emission source [Gregory et al., 1999]. In a relatively clean equatorial area, the ratio typically is between 0.25 and 0.65 (aged marine air), which is consistent with the meteorological analysis. In the eastern zone (155øE-160øW) at latitudes south of 10øS this ratio is near --•1-2.3 (7 days and less), which also agrees with our back trajectory analysis. In the western zone (160øW-130øW) the ratio does not exceed 1, which is consistent with a mechanism of prevailing westerly zonal transport.

3. Conclusions

CO measurements over the southern Pacific during PEM-T have been analyzed in the context of other available CO data sets. Results demonstrated that CO measurements obtained

from different in situ and remote techniques can be used to construct a CO climatology over this large geographical area.

The multiplatform approach enables us to evaluate the climatological representativeness of the PEM-T campaign. It has been shown that the CO distribution measured during PEM-T mission is similar to its climatological average. This finding is consistent with the meteorological analysis. Relations between the PEM-T 1996 data and the 1984 and 1994 MAPS data are

consistent with the phase of ENSO cycle. The CO distribution over the western part of the southern

Pacific during austral spring is mainly controlled by emissions from biomass burning in Australia and Africa its and subsequent long-range transport. The prevailing westerly transport occurs in the middle and upper troposphere, whereas the marine boundary layer remains relatively clean and uniform. PEM-T measurements in the marine boundary layer are in good agreement with 1996 NOAA CMDL data.

The equatorial area generally does not experience upper level transport from the west. Thus it is shielded from the direct impact of biomass burning plumes from the north and southwest. The ITCZ and SPCZ also play important role in shielding the equatorial area. Consistency between the PEM-T data and spaceborne MAPS measurements indicates that the observed CO minimum is a stable feature.

Outside the equatorial area the CO vertical distribution has a broad maximum at the altitude range 5-8 km and latitudes between 20øS and 30øS. These observations agree reasonably with the MAPS data, in situ aircraft measurements over Cape Grim, Tasmania, and solar spectroscopic measurements at Lauder, New Zealand. This enhanced CO layer cannot be detected by surface level in situ measurements (e.g., by NOAA

CMDL network). Hence, to build a comprehensive picture of the CO distribution, remote sensing or aircraft survey should be performed in addition to surface level measurements.

Acknowledgments. Research at Christopher Newport University has been supported by NASA cooperative agreement NCC-1-204. R. Chatfield gratefully acknowledges the support of J. Kaye under NASA Research Program 579-24-13-10. We thank Scott Nolf for his help with the MAPS data analysis and preparation of the figures.

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R. B. Chatfield, NASA Ames Research Center, Earth Science Di- vision, Mail Stop 245-5, Moffet Field, CA 94035-1000. (chatfiel d @clio.arc.nasa.gov)

V. S. Connors, N. S. Pougatchev, C. P. Rinsland, and G. W. Sachse, NASA LaRC, MS 472, Hampton, VA 23681-0001. (vickie@stormy.


larc.nasa.gov; [email protected]; rinsland@mipsbox. larc.nasa.gov)

H. E. Fuelberg, Department of Meteorology, Florida State Univer- sity, Tallahassee, FL 32306.

N. B. Jones, Atmospheric Division, National Institute of Water and Atmospheric Research, Lauder, Omakau, Central Otago, New Zea- land. [email protected])

J. Notholt, Alfred Wegener Institute for Polar and Marine Re- search, Telegraphenberg, Potsdam, Germany. (jnotholt@awi- potsdam.de)

P. C. Novelli, NOAA/CMDL, R/E/CG1, 325 Broadway, Boulder, CO 80303. ([email protected])

H. G. Reichle Jr., Department of Marine, Earth, and Atmospheric Science, North Carolina State University, Box 8208, Raleigh, NC 27695. ([email protected])

(Received October 29, 1998; revised May 25, 1999; accepted June 16, 1999.)

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