QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETYQ. J. R. Meteorol. Soc. 133: (S3) 233–241 (2007)Published online in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/qj.130

NAST-I tropospheric CO retrieval validation duringINTEX-NA and EAQUATE†

Daniel K. Zhou,a* Allen M. Larar,a Xu Liu,a William L. Smith,b,c Jonathan P. Taylor,d

Stuart M. Newman,d Glen W. Sachsea and Stephen A. Mangoe

a NASA Langley Research Center, Hampton VA, USAb Hampton University, Hampton VA, USA

c University of Wisconsin-Madison, Madison WI, USAd Met Office, Exeter, UK

e NPOESS Integrated Program Office, Silver Spring MD, USA

ABSTRACT: Troposphere carbon monoxide (CO), as well as other trace species retrieved with advanced ultraspectralremote sensors of Earth observing satellites, is critical in air quality observation, modelling, and forecasting. The retrievalalgorithm and the accuracy of the parameters retrieved from passive satellite remote sounders must be validated. TheIntercontinental Chemical Transport Experiment - North America (INTEXNA) and the European Aqua ThermodynamicExperiment (EAQUATE) provide important validation of satellite observations with ongoing satellite measurementprogrammes such as Terra, Aura, and Aqua. One of the experimental objectives is to validate chemical species observed fromultraspectral sounders with aircraft in situ measurements, such as the NPOESS Airborne Sounder Testbed-Interferometer(NAST-I). Detailed intercomparisons between aircraft in situ measured and NAST-I retrieved CO profiles were performedto assess the retrieval capability of a passive infrared spectral remote sounder. Validation results illustrate that the COvertical structure can be obtained by the NAST-I. The thermal radiances are most sensitive to CO emissions from the freetroposphere. However, the profile retrieval accuracy depends on the CO uncertainty in the terrestrial boundary layer. It isshown here that the CO distribution in the terrestrial boundary layer over the sea cannot be obtained with reliable accuracywhere there is little contrast between the surface air and surface skin temperature. Copyright 2007 Royal MeteorologicalSociety

KEY WORDS ultraspectral sounder; NPOESS; CO; retrieval

Received 3 November 2006; Revised 26 April 2007; Accepted 14 May 2007

1. Introduction

Tropospheric chemical reactions involving carbon mon-oxide (CO) extend their influence to the global climatethrough the accumulation of greenhouse gases. Traceamounts of CO are found in the atmosphere, and Levy(1971) has recognized the significance in atmosphericchemistry of CO, which, due to its relatively long life-time, can be transported a great distance from its origi-nal source. Air particles downwind of combustion (e.g.biomass, fossil fuels) often show elevated CO and ozone(O3) resulting from photochemical production (Fishmanet al., 1990). Crutzen et al. (1979) have recognized theimportance of improving our knowledge of troposphericchemical gases relating to environmental health and themagnitude of global climate change. The critical role of

* Correspondence to: Daniel K. Zhou, Mail stop 401A, NASA LangleyResearch Center, Hampton, VA 23681, USA.E-mail: daniel.k.zhou@nasa.gov† The contributions of Jonathan P. Taylor and Stuart M. Newman ofthe Met Office, Exeter were prepared as part of their official duties asemployees of the UK Government. It is published with the permissionof the Controller of HER Majesty’s Stationery Office and the Queen’sPrinter for Scotland.

satellite and aircraft observations has been established toprovide the necessary global and regional data neededto understand the complex chemistry and transport pro-cesses involved in regional air pollution chemistry andits influence on the global environment.

The Measurement of Pollution in the Troposphere(MOPITT) instrument was launched on 18 December1999 aboard the Terra satellite (Drummond, 1992; Panet al., 1998; Deeter et al., 2003) for space-based mea-surement of CO and methane. The Atmospheric InfraRedSounder (AIRS) on the Aqua satellite, launched on 4 May2002, monitors atmospheric thermodynamic structure aswell as trace species like O3 and CO (e.g. Aumann et al.,2003). The Tropospheric Emission Spectrometer (TES)instrument, aboard the Aura satellite, was launched on15 July 2004 and is detecting tropospheric trace species(e.g. Worden et al., 2004). One of the objectives of thesemissions is to monitor global CO distribution (Beer et al.,2001). A great deal of effort has been given to vali-dating satellite observations and their retrievals as wellas model analyses (e.g. Emmons et al., 2004). Scientistshave been using aircraft in situ measurements as wellas chemical model predictions (e.g. Deeter et al., 2004;

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234 D. K. ZHOU ET AL.

Kulawik et al., 2006) to understand the retrieval accuracywith satellite measurements.

Since the inversion of the radiative transfer equationis an ill-posed problem, a priori information may beused to stabilize the retrieval process. The a priori infor-mation used and the vertical resolution of the retrievaldetermined by the averaging kernels may be taken intoaccount when validating the retrieval accuracy. Dedi-cated field campaigns, such as Transport And ChemicalEvolution over the Pacific (TRACE-P), the Interconti-nental Chemical Transport Experiment - North America(INTEX-NA) and the European Aqua ThermodynamicExperiment (EAQUATE) were conducted with intensiveaircraft in situ measurements as well as other measure-ments from different airborne sensors in order to accu-rately capture the features of atmospheric species with ahigh spatial and vertical resolution (e.g. Crawford et al.,2004). To fully understand the vertical resolution of theretrievals from satellite measurements, the informationcontent of the satellite infrared radiometric measurementsmust be known. It is difficult to evaluate the accuracyof satellite retrievals without knowledge of the chemicalabundance, and its vertical distribution, used as a prioriinformation.

To investigate tropospheric CO vertical profile retrievalaccuracy from a satellite ultraspectral sounder, theNational Polar-Orbiting Operational Environmental Satel-lite System (NPOESS) Airborne Sounder Testbed-Inter-ferometer (NAST-I) field campaign data can be usedto provide radiometric measurements including tropo-spheric trace species. NAST-I is an ultraspectral res-olution sounder (e.g. Cousins and Smith, 1997) simi-lar to current and future satellite sounders, such as theAIRS, the Infrared Atmospheric Sounding Interferome-ter (IASI), and the Cross-track Infrared Sounder (CrIS).NAST-I provides high spatial resolution spectral radiancemeasurements, and profile retrievals obtained from them.The NAST-I measurements, in conjunction with coinci-dent in situ measurements which have a very high spatialand vertical resolution, are vital for validating the infor-mation content and retrieval accuracy achievable withsatellite instruments (i.e. in this case, the CO verticalprofile retrieval from satellite radiometric observations).

NAST-I radiometric radiance samples collected duringfield campaigns, together with the in situ measurementsregarded as ground ‘truth’, are used to validate the accu-racy of a CO profile retrieval algorithm and to investigateretrieval sensitivity to CO variations. This study providesa means of understanding the accuracy achievable fromradiance emission measurements from satellite, as wellas airborne, ultraspectral resolution instruments. In thispaper, the focus is on CO retrieval validation in order todemonstrate the capability of capturing the troposphericCO vertical distribution from spectral radiance emissionobservations. The scope of this work is limited to validat-ing the NAST-I CO retrievals, but the results are believedto be useful in understanding the sensitivity of satelliteinfrared radiometric observations to the vertical and hor-izontal distribution of CO.

2. Experiment, algorithm, and validation

NAST-I instrumentation, measurements, calibration, andradiance validation are documented elsewhere (e.g.Cousins and Smith, 1997; Gazarik et al., 1998; Smithet al., 1999). NAST-I is a Fourier Transform Spectrom-eter of the Michelson Interferometer design. It possesseshigh spectral resolution (0.25 cm−1) and high spatial res-olution (0.13 km linear ground resolution per km of air-craft flight altitude at nadir). The NAST-I spatially scanscross-track to the aircraft motion at +/−48.2 degrees,thereby providing a 2.3 km ground track swath widthper km of aircraft flight altitude (e.g. a 46 km swathfrom a flight altitude of 20 km). The radiometric noise isnominally 0.3 K, spectrum to spectrum, depending uponthe spectral region and scene temperature. The spectrallyrandom noise, spectral point to spectral point, is gener-ally less than 0.3 K within a given radiance spectrum.It is the spectrally random component of the radiancemeasurement noise which limits the ability to decon-volute the radiance spectrum with the precision neededto retrieve small-scale vertical features of atmospherictemperature and the absorbing constituents (e.g. watervapour). NAST-I is designed to support the developmentand performance validation of high-spectral-resolution(i.e. vertical) temperature and moisture sounders beingflown on Earth-orbiting satellites. While a large amountof data has been collected since July 1998 under a vari-ety of meteorological conditions, NAST-I observationsthat are coincident with in situ CO measurements arevery limited. However, such coincident measurements didoccur during the INTEX-NA and EAQUATE campaigns.During these campaigns, NAST-I flew on the Proteusaircraft while the NASA DC-8 and the UK Facility forAirborne Atmospheric Measurements (FAAM) BAE 146aircraft provided in situ measurements during INTEX-NA and EAQUATE, respectively. Only one INTEX-NAflight (22 July 2004) off the US east coast and twoEAQUATE flights (14 and 18 September 2004) over theUK Celtic Sea provided data valuable for this study. COprofiles from these three flights were observed in situ,which enabled the validation of retrievals obtained fromNAST-I ultraspectral radiance measurements.

NAST-I profile retrieval algorithms have been devel-oped in which clouds are detected and their geometri-cal and microphysical properties are accounted for inthe profile retrieval process (Zhou et al., 2007a). TheNAST-I CO profile retrieval scheme is briefly describedhere. Under cloud-free conditions, the NAST-I COinversion scheme is combined with a three-step proce-dure: (1) EOF (empirical orthogonal function) regres-sion retrieval, (2) simultaneous matrix inversion, and(3) CO profile enhancement inversion. Since the retrievalproblem is ill-posed, additional information is neededto constrain the solution. The radiosonde temperatureand moisture profiles with regional and seasonal vari-ations are used as a training dataset. Statistical sam-ples of CO profiles are used with radiosonde tempera-ture and water vapour profiles to obtain coefficients for

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NAST-I TROPOSPHERIC CO RETRIEVAL VALIDATION 235

regression retrieval. Unfortunately, CO profiles are notsampled simultaneously with the radiosonde temperatureand moisture profiles used for the statistical regressionretrieval. As a result, CO abundance profiles must beassembled for each radiosonde profile using the relation-ship between thermodynamic parameters (i.e. temperatureand moisture) and CO abundance profiles based on mea-surements from aircraft made during several previousatmospheric chemistry field campaigns. The EOF regres-sion relations are based on the radiosonde and CO profiletraining dataset. Although a model analysis might providea more accurate ‘first guess’ for the retrieval process, theemphasis here is placed on how the information contentof the radiance measurements are able to improve theCO vertical profile specified as the first guess for theretrieval. It is shown that the CO profile retrieval accu-racy depends heavily on the accuracy of the first guess asexpected from the CO profile radiance weighting func-tions and the averaging kernels resulting from the inversesolution of the radiative transfer equation.

The last two steps of this retrieval approach employphysical inversion, which is a one-dimensional varia-tional (1D-Var) solution, also known as the regulariza-tion algorithm or the minimum information method (e.g.Twomey, 1963; Tikhonov, 1963; Rodgers, 1976; Hansen,1998). Detailed descriptions of thermodynamic parame-ters and CO profile retrievals under clear conditions arefound in Zhou et al. (2002, 2005). The averaging kernelsare the vertical resolution functions of the retrieval andthey represent how a true atmospheric state is transformedto the atmospheric state specified by the retrieval pro-cess. Detailed formulation and definition of the averagingkernel can be found elsewhere (e.g. Pan et al., 1998;Rodgers, 1990). The averaging kernels for NAST-I CO,presented by Zhou et al. (2005), were based on NAST-Iradiances simulated from the US standard atmospherewith a surface/atmosphere thermal contrast of 3.0 K.Selected significant layers plotted in Figure 1 indicatethat NAST-I radiances are most sensitive to the CO varia-tions within the 2 to 10 km altitude region. The averagingkernels, as well as the profile weighting functions (Zhouet al., 2005), assist us to understand how well the COvertical structure can be captured using radiance observedwith the NAST-I.

NAST-I CO retrieval error and the error contributionsources (e.g. the retrieval accuracies for temperature,moisture, surface properties, as well as the forwardradiative transfer model accuracy) have been addressedpreviously by Zhou et al. (2005). NAST-I CO retrievalstandard deviation of error (STDE) was estimated as afunction of the altitude. These are approximately 80,60, 30, and 20 parts per billion by volume (ppbv) at1.0, 1.5, 4.5, and 7.0 km, respectively. The estimatedSTDE may change depending on the nature of COdistributions. The retrieval error should be consideredas the intercomparisons between NAST-I retrievals andin situ observations are presented hereafter.

Figure 1. Selected significant averaging kernels calculated for retrievingthe US standard atmosphere with NAST-I instrument specification.Perturbed layers are indicated in the legend (after Zhou et al., 2005).

2.1. INTEX-NA case of 22 July 2004

INTEX-NA is an integrated atmospheric field experi-ment performed over North America. The objective ofthe experiment is to understand the transport and trans-formation of gases and aerosols on transcontinental andintercontinental scales and their impact on air quality andclimate. NAST-I, flown on the Proteus aircraft, partici-pated in the INTEX-NA campaign. The particular focusof this study is to quantify and characterize NAST-ICO profile retrieval sensitivity and accuracy. During oneparticular flight on 22 July 2004, a NASA DC-8 air-craft was ascending and descending to collect trace gasprofiles, including CO, with an instrument described bySachse et al. (1987), while the Proteus aircraft, carryingthe NAST-I, cruised over the DC-8.

As shown in Figure 2(a), flight tracks of Proteus andDC-8 aircraft were close and covered a large geophysicalarea over the water. NAST-I retrieved temperature, mois-ture, and CO profiles are plotted in Figures 2(b), (c) and(d), respectively. Based on the validation of tempera-ture and moisture profiles obtained during numerous fieldcampaigns, NAST-I retrieved thermodynamic parametershave been shown to be relatively accurate. The NAST-Itemperature and moisture profiles retrieved here com-pare favorably with nearby radiosondes (e.g. Smith et al.,1999; Smith et al., 2005; Zhou et al., 2007b). As bothaircraft covered a large geophysical region, a large COvariation from both NAST-I retrieved profiles (with view-ing angles less than 23°) and in situ observations areshown in Figure 2(e) indicating CO variation in thisarea. Detailed intercomparison is given in Figure 3. Itis noted that a relatively large CO variation (and/or gra-dient) was also observed by the MOPITT satellite instru-ment for the same geographical region. The MOPPITobservations show similar CO column enhancement off

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236 D. K. ZHOU ET AL.

Figure 2. Case of 22 July 2004: (a) Proteus and DC-8 aircraft flight tracks, (b) NAST-I retrieved temperature (K) vertical cross-section,(c) NAST-I retrieved relative humidity (%) vertical cross-section, (d) NAST-I retrieved CO (ppbv) vertical cross-section, and (e) NAST-I COprofiles (coloured curves) and in situ CO observations (black circles). In (b)–(d), regions where cloud prevents accurate tropospheric retrievals

are shown as white stripes.

the US eastern seaboard where the aircraft data wereobtained. It is believed that this variation was due to thetransport of CO produced by Alaskan wildfires (Pfisteret al., 2005). Tropospheric CO enhancement (4–5 km)

observed by both in situ measurements and NAST-Iretrievals (Figures 2(d) and (e)) may also be due tolong-range transport of Alaskan wildfire produced CO.The terrestrial boundary layer (TBL) CO enhancement

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NAST-I TROPOSPHERIC CO RETRIEVAL VALIDATION 237

Figure 3. (a) Comparison between NAST-I retrievals and in situ measurements at co-located points, with background of all in situ data (seelegend). (b) The difference between NAST-I retrievals and in situ measurements at the collocated points (see text).

observed by the in situ sensor may be due to urbanemissions.

In order to compare NAST-I retrievals with in situmeasurements, it is necessary to find relatively close ‘col-located’ spots (i.e. within a cube of 2 × 2 × 2 km) forintercomparison. The latitude, longitude, and altitude ofin situ data points were used to search for their ‘collo-cated’ NAST-I data. Despite a small time difference (i.e.within a few hours) and a sample resolution difference,there are 44 ‘collocated’ data points. Figure 3 is a plotof in situ observations and NAST-I retrievals; the verti-cal mean difference between NAST-I CO retrievals andin situ CO measurements is ∼5 ppbv and their STDE is∼22 ppbv. The mean difference illustrates the bias causedby differences in the vertical resolution and absoluteaccuracy between the retrievals and in situ observations.In contrast, the STDE illustrates the residual randomerror caused by these differences. Here, NAST-I aver-aging kernels and a priori information are not applied toin situ data to make the in situ data vertical resolution thesame (i.e. the retrieval equivalent). Therefore, the differ-ent vertical resolution associated with these two datasetsaffects the outcome of the comparison. Thus, these statis-tics reveal the difference between the retrievals using apassive ultraspectral remote sensor and in situ observa-tions. As a consequence, these results reveal how well theretrieval can capture tropospheric CO vertical features.Although NAST-I CO in the TBL is heavily dependent onthe a priori condition, the retrievals compare favourablyto the in situ measurements, as shown in Figure 3.

2.2. EAQUATE cases of 14 and 18 September 2004

During the second phase of the EAQUATE campaignover the United Kingdom, the Proteus aircraft, together

with the UK FAAM BAE 146 aircraft, made two ded-icated AIRS validation flights on 14 and 18 Septem-ber 2004. Numerous in situ sensors and remote-soundinginstruments were deployed aboard the BAE 146 aircraft(Taylor et al., 2008) including a fast-response vacuum-UV resonance fluorescence CO instrument (Gerbig et al.,1999). Dropsondes were also released from BAE 146aircraft during the experiments. Detailed radiometricand thermodynamic parameter retrieval validation stud-ies (Larar et al., 2006; Zhou et al., 2007b) indicate thathigh-quality datasets were collected. Both flights wereconducted in the same geophysical location and at almostthe same local time, thus providing an excellent case forverifying retrieval sensitivity of day-to-day atmosphericvariations of CO.

Figure 4 plots the flight tracks of Proteus and BAE146 aircraft and NAST-I retrievals for both 14 and18 September 2004. NAST-I temperature and moistureretrievals are validated with the dedicated dropsondesreleased from BAE 146 aircraft. Detailed validationindicating that the agreement is within approximately1 K for temperature STD and 15% for relative humiditySTD is found in Zhou et al. (2007b). The day-to-dayvariation of the atmosphere is clearly shown. A relativelylarge amount of CO in the free troposphere (FT) isshown in NAST-I retrievals for 14 September comparedwith those of 18 September 2004. Since the geophysicallocations covered by both aircraft were almost the same,and the in situ observed CO are not scattered as shownin the INTEX-NA case of 22 July 2004, the NAST-Icross-section averaged CO profile is used and plottedagainst the in situ profile. As shown in Figure 5, COprofiles indicating a day-to-day difference as validatedfrom the comparison between NAST-I retrievals and the

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238 D. K. ZHOU ET AL.

Figure 4. Data from 14 September 2004: (a) Proteus and BAE 146 aircraft flight tracks, (b) NAST-I temperature vertical cross-sections,(c) NAST-I relative humidity vertical cross-sections, and (d) NAST-I CO vertical cross-sections. (e)–(h) are as (a)–(d), but for 18 September

2004.

in situ observations of the 14 and 18 September 2004are plotted. A relatively small amount of CO abundancewithin the TBL indicates that strong urban emissionswere not nearby, and the TBL CO day-to-day variationis barely noticed. CO variations observed in the FT areattributed to its long-range transport from other regions.Unlike the case of 22 July 2004 shown in Figure 3, thefirst guess is relatively poor (especially in the TBL).There is nearly no difference in the first guess betweenthe two cases of 14 and 18 September 2004 (shownin Figure 5(a)). This situation provides an exceptionalopportunity to test the retrieval algorithm with realmeasurements to understand vertical retrieval sensitivityfrom NAST-I type remote sounders. Figure 5(b) plotsthe NAST-I CO physical retrieval profiles. The COprofile separation in the FT, as shown by the in situobservations between 14 and 18 September, illustratesthe importance of the physical inversion in the retrievalprocess. The ultraspectral thermal emissions, measuredby remote sounders such as NAST-I, are sensitive to

CO column content in the FT. However, the NAST-ICO profile within the TBL is hardly changed from thefirst guess to final physical retrieval, which is morepronounced than in situ observations. This is explainedby the NAST-I CO weighting functions (i.e. Jacobianmatrices) and averaging kernels shown in Figure 1.As a consequence, a vertical CO distribution must becompensated for a total CO column amount, which isexactly what happened as shown in Figure 5(b). NAST-ICO in the FT is less pronounced than that shown by thein situ measurements.

Another way to assess the retrieval accuracy is to com-pare the spectral radiance measurements with simulatedradiances using the true and retrieved atmospheric pro-files. Here, using the case of 18 September 2004, NAST-Ispectral radiances are simulated using a retrieved COprofile as well as in situ measurements. Other parame-ters, such as temperature, moisture, ozone, and surfaceproperties used in the radiance simulation, are from thesame retrieval. Figure 6(a) shows two spectra, one which

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NAST-I TROPOSPHERIC CO RETRIEVAL VALIDATION 239

Figure 5. CO profile intercomparisons: (a) NAST-I regression versusin situ, and (b) NAST-I final physical retrieval versus in situ. Data from14 (18) September are plotted in grey (black); in situ data are plotted

as dots, and NAST-I section means as solid curves.

is based on the retrieved CO profile and the other onthe in situ measured CO profile shown in Figure 5(b),and the difference is plotted in Figure 6(b). In order toinvestigate error contributions from TBL and FT COabundance, the radiance differences contributed by theCO differences in the FT and the TBL are also plotted inFigure 6(b). The differences are associated with the extraamount of retrieved CO in the TBL and the lesser amountof retrieved CO in the FT in comparison with the in situmeasured CO profile shown in Figure 5(b). With verylittle CO retrieval sensitivity in the TBL, the TBL errorintroduced by the first guess is not significantly reducedthrough physical inversion. This TBL error affects theFT CO retrieval accuracy; in other words, the FT erroris dependent on the TBL error. The extra or lack of COin the TBL compensates for the lack of or extra CO inthe FT, in order to achieve an accurate total CO column

density. A CO profile retrieved from spectral radiancemeasurements by an instrument with a given radiancemeasurement capability is influenced by the first-guessprofile as well as the accuracy of the other retrieved ther-modynamics parameters.

3. Conclusions and future work

Field campaigns with aircraft in situ observations provideprecise distributions of chemical species needed to vali-date retrieval algorithms and results from satellite/aircraftremote sounders. This CO evaluation study, having coin-cident radiance and in situ measurement datasets, enablesan understanding of the accuracy of our current COretrieval algorithm and validates the results based on the-oretical simulations (Zhou et al., 2005). In addition tomeasurement quality (e.g. instrument noise, spectral res-olution, cloud contamination, thermal property accuracy),the CO first-guess profile (or a priori condition) plays amajor role in the CO profile accuracy. The TBL CO accu-racy, mainly determined by the first guess (or a prioriinformation), affects the free tropospheric CO retrieval.However, CO variations in the FT can still be capturedwhile CO amount variations in the TBL are not retrievedvery well. Thus, it is a challenge to obtain accurate COprofiles (especially in the TBL) from remote sounderssuch as NAST-I. The situation may change when thedifference of air and surface skin temperatures is morepronounced over the land during the local daytime. Thelarger the contrast between surface air and surface skintemperatures, the greater the radiance sensitivity is to theTBL gas concentrations. At the same time, the land sur-face properties, mainly the surface emissivity, need to beaccounted for in order to obtain accurate CO retrievalsfrom a thermal IR ultraspectral sounder. While this workevaluates the NAST-I CO retrieval algorithm and itsretrieval products, additional profile validation analysesfor different geophysical location and seasonal conditionsare desired.

Acknowledgements

The authors greatly appreciate the contributions of theNASA Langley Research Center, the Space Scienceand Engineering Center of the University of Wiscon-sin – Madison, and the UK Met Office. The NAST-I program is supported by the NPOESS IPO, NASAHeadquarters, and NASA Langley Research Center. Theauthors acknowledge support from NASA HeadquartersEarth Science Division Associate Director for ResearchDr. Jack A. Kaye and IPO Algorithm Division Chief Dr.Karen St.Germain. The FAAM is jointly funded by theNatural Environment Research Council and the UK MetOffice. The personnel who contributed to INTEX-NA andEAQUATE field campaigns are too numerous to men-tion by name; nonetheless their personal contributionsare greatly appreciated.

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240 D. K. ZHOU ET AL.

Figure 6. (a) NAST-I spectra simulated with the retrieved and in situ measured CO profile of 18 September 2004. (b) Spectral difference betweenretrieved and in situ measured CO profile (dashed black), difference due to free tropospheric CO (light grey), and difference due to TBL CO

(dark grey).

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