Effect of HITRAN Database Improvement on Retrievals of Atmospheric Carbon Dioxide from Reflected Sunlight Spectra in the 1.61-µm Spectral Window

Transcription

1 ADVANCES IN ATMOSPHERIC SCIENCES, VOL. 29, NO. 2, 2012, Effect of HITRAN Database Improvement on Retrievals of Atmospheric Carbon Dioxide from Reflected Sunlight Spectra in the 1.61-µm Spectral Window DAI Tie 1 ( ), SHI Guangyu 1 ( ), and ZHANG Xingying 2 ( ) 1 State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing National Satellite Meteorological Center, Beijing (Received 15 February 2011; revised 23 June 2011) ABSTRACT A large number of experimental and theoretical investigations of carbon dioxide (CO 2) spectra have been conducted since the most recent update of the High-Resolution Transmission Molecular Absorption (HITRAN) database. To maintain optimal parameters, the HITRAN 2004 CO 2 line list has been completely replaced by HITRAN 2008 data in the near-infrared region from 4300 cm 1 to 7000 cm 1. To examine the effect of this change on the retrieval of CO 2 vertical column data from reflected sunlight spectra in the 1.61-µm spectral window, synthetic measurements for a given atmospheric state and instrument setup were generated and compared using radiative transfer model with the line-transition parameters from the HITRAN 2004 and 2008 databases. Simulated retrievals were then performed based on the optimal estimation retrieval theory. The results show that large systematic errors in atmospheric CO 2 column retrievals were induced by the differences in the HITRAN laboratory line parameters in the 1.61-µm region. The retrieved CO 2 columns were underestimated by >10 ppm using the HITRAN 2004 data, and improvements resulting from the use of the improved HITRAN database were more pronounced at a higher spectral resolution. Key words: HITRAN database, retrieval of CO 2, reflected sunlight spectra Citation: Dai, T., G. Y. Shi, and X. Y. Zhang, 2012: Effect of HITRAN database improvement on retrievals of atmospheric carbon dioxide from reflected sunlight spectra in the 1.61-µm spectral window. Adv. Atmos. Sci., 29(2), , doi: /s Introduction Emission of carbon dioxide (CO 2 ) has increased remarkably over the past century due to mass consumption of fossil fuels associated with expanding industrial activities. The increase in atmospheric CO 2, a powerful greenhouse gas, is expected to lead to significant future climate change with adverse consequences, such as a rising sea level and an increase in extreme weather conditions (IPCC, 2001, 2007). Reliable prediction of future atmospheric CO 2 concentration and the associated global climate change requires an adequate understanding of CO 2 sources and sinks. Unfortunately, such understanding still has significant gaps and large areas of uncertainty (Stephens et al., 2007), because our current observation system is based on a sparse and inhomogeneous geographical distribution of 180 CO 2 measurement sites. Satellite-based high-resolution spectroscopic monitoring of the evolution of CO 2 in the terrestrial atmosphere is thus of great importance to scientists and policy makers. To obtain global space-based measurements of atmospheric CO 2 with the spatial resolution and accuracy needed to characterize surface sources and sinks, NASA selected the Orbiting Carbon Observatory (OCO) as the fifth mission in the Earth System Science Pathfinder (ESSP) Program (Crisp et al., 2004; Crisp and Johnson, 2005). The OCO was unfortunately lost in February 2009 due to launch failure. Nevertheless, its successors, along with the Green- Corresponding author: DAI Tie, daitie@mail.iap.ac.cn China National Committee for International Association of Meteorology and Atmospheric Sciences (IAMAS), Institute of Atmospheric Physics (IAP) and Science Press and Springer-Verlag Berlin Heidelberg 2012

2 228 HITRAN DATABASE IMPROVEMENT EFFECT ON ATMOSPHERIC CO 2 RETRIEVAL VOL. 29 house Gases Observatory Satellite (GOSAT), are using the CO 2 vibration-rotation transition bands in the 1.61-µm region (Yokota et al., 2009). The High-Resolution Transmission Molecular Absorption (HITRAN) database is the recognized international standard and is used for applications such as atmospheric radiation transmission simulations and atmospheric remote sensing. The effects of improvements to the HITRAN database (Rothman et al., 2005) in radiative transfer or transmittance calculations were noted respectively by Feng et al. (2007) and Feng and Zhao (2009). The HITRAN 2008 database includes data that have been added, modified, or enhanced since 2004 (Rothman et al., 2009). To investigate the effect of these line parameter data modifications on retrievals of atmospheric CO 2 data from 1.61-µm reflected sunlight, synthetic radiance measurement spectra using the line parameters from the HITRAN 2004 and 2008 databases were generated and compared. A detailed assessment of the effect of HI- TRAN database improvement on retrievals of atmospheric CO 2 column was then conducted based on the optimal estimation retrieval theory. 2. Calculation outline 2.1 Radiative transfer calculations The SCIATRAN (Rozanov et al., 2002, 2005) radiative transfer model was used to calculate the synthetic measurements in this study. A newly implemented spherical mode and an improved plane-parallel mode were used to give SCIATRAN the ability to solve almost any scientific task associated with scattered solar radiation measurements in the Earth s atmosphere in the ultraviolet, visible, and near-infrared (UV-Vis- NIR) spectral regions. The recent version of SCI- ATRAN was used for the following calculations. The discrete ordinate method (Stamnes et al., 1988; Siewert, 2000) was switched on to solve the radiative transfer equation in this study. Spectra were computed at a resolution on the order of the smallest line half-width (10 3 nm) by the accurate line-by-line model and then convolved with a Gaussian slit function. United States standard atmosphere data containing zonally and seasonally averaged vertical distributions of absorption gases, as well as pressure and temperature, was used for reference atmospheric profiles. The model atmosphere was assumed to be plane parallel and divided into 125 total layers. Vertical thickness of the layers was 0.5 km up to an altitude of 25 km and increased to 1 km above this height. Each layer was considered as a homogeneous path in the calculations. Seven major molecular absorption species (H 2 O, CO 2, O 3, N 2 O, CO, CH 4, and O 2 ) were included in the calculations. The line wings were truncated at 25 cm 1 from the line center in all cases. The solar zenith angle was given a value of 30, and the satellite was chosen for nadir viewing. The Earth s surface was treated as Lambertian, with reflectance included in the calculations. The spectral reflectance was assumed to be a fixed value of 0.3, representing global average surface conditions. The parameters used in these calculations are listed in Table Inverse method The retrieval of atmospheric CO 2 data from measurements was based on optimal estimation theory as described by Rodgers (1976, 2000). The relationship between the measurement vector and state vector can be described as y = F (x) + ε, (1) where the vector y is the measurements with error ε, x is the state vector describing all the unknowns to be retrieved from the measurements (in our case the atmospheric CO 2 ), and F (x) is a vector-valued function of the state that encapsulated our understanding of the physics of the measurements. In the framework of the optimal estimation method, the solution was found using the iterative Gauss-Newton scheme as described by Rodgers (2000): x i+1 = x 0 + (Ki T Sy 1 K i + Sa 1 ) 1 Ki T Sy 1 [y y i + K i (x i x 0 )]. (2) Here, K = F / x is called the weighting function matrix, x 0 is the a priori state vector, S a is the a priori covariance matrix, and S y is the noise covariance Table 1. List of baseline input and output parameters used in the simulated calculations. Input Output 1976 U.S. standard atmosphere profile Band coverage, nm CO 2 level, ppm constant column Slit function, Gaussian Solar zenith angle, 30 Spectral resolution, 0.02 nm, 0.05nm and 0.1 nm Lambertian surface with reflectance, R S=0.3 Spectral line data, HITRAN 2004 and HITRAN 2008 Satellite viewing angle, nadir

3 NO. 2 DAI ET AL. 229 Fig. 1. Top panel, the calculated monochromatic sun-normalized radiance; bottom panel, the sun-normalized radiance relative change corresponding to the 1-ppm CO 2 column mixing ratio increase for three spectral resolutions. matrix. An iterative approach was employed to account for the nonlinearity of the inverse problem. This means that at each iterative step the forward model was initialized by the retrieval results from the previous iterative step. Typically, after two to five steps, this iterative retrieval process has converged. In this study, the CO 2 vertical column was retrieved based on scaling (or shifting) the priori profile. 3. Results 3.1 Sensitivity of measurements to changing CO 2 concentrations Most of the relatively new CO 2 measurement techniques retrieve the CO 2 column by fitting the logarithm of the ratio of measured nadir radiance and solar irradiance spectrum (i.e., observed sun-normalized radiance), so the sensitivity of the sun-normalized radiance to changing CO 2 concentrations is explored in this section. The reference spectra were computed assuming a CO 2 concentration of ppm in a clear atmosphere, which was the estimated global mean atmospheric mixing ratio of CO 2 for To consider the effect of spectral resolution changes on measurements and measurement sensitivity, the same calculations were performed with three spectral resolutions: 0.02 nm, 0.05 nm, and 0.1 nm at full width at half-maximum (FWHM) of the Gaussian function. Figure 1 shows synthetic sun-normalized radiance over the spectral range from nm to nm and the relative changes corresponding to an increase of 1 ppm in the CO 2 atmospheric mixing ratio using the line-transition parameters from the HITRAN 2008 database. As shown in the bottom panel of Fig. 1, increasing the atmospheric CO 2 mixing ratio by 1 ppm ( 0.26%) uniformly in the whole column resulted in a negative change in the synthetic measurements because transmittance decreased with the increased amount of CO 2 in the atmospheric layers. Notably, measurements at the higher spectral resolution show greater sensitivity to atmospheric CO 2 change. Because the absorption lines in the 1.61-µm band were relatively weak, the transmittance of the atmospheric column rose to near unity between the absorption lines (O Brien and Rayner, 2002). Consequently, measurement at low spectral resolution was dominated by the contributions from the near transparent regions between the lines. These contributions respond only weakly to changes in CO 2 concentration. In contrast, measurements with high spectral resolution responded strongly to changes in CO 2 if the measurement frequency lay on an absorption line. As shown in the bottom panel of Fig. 1, when the spectral resolution was 0.02 nm, the maximal relative change of the sunnormalized measurements was as much as 0.34%, because the atmospheric CO 2 mixing ratio increased by 1 ppm uniformly in the whole column. In addition, we note that this relative change was proportional to

4 230 HITRAN DATABASE IMPROVEMENT EFFECT ON ATMOSPHERIC CO 2 RETRIEVAL VOL. 29 the CO 2 absorption, peaking near the strongest absorption line. For 0.05-nm and 0.1-nm resolution, the maximal relative changes were 0.16% and 0.08%, respectively. 3.2 Difference between radiative transfer calculations using the two HITRAN databases The uncertainty of the spectroscopic parameters is one of the most important aspects that affect the accuracy of the forward model. The HITRAN database is the recognized international standard for spectroscopic parameters; it is used as the input to most of the line-by-line radiative transfer model. Many recent developments have pushed the requirements of HITRAN in terms of accuracy and degree of completeness. HITRAN 2008 is the latest edition of the HI- TRAN database. The line parameters of the HITRAN 2008 database have been added, modified, or enhanced compared with the previous edition HITRAN 2004, becoming more accurate and complete. How do the improvements of the line parameters in the HITRAN database affect the retrieval of CO 2 concentrations using the 1.61-µm reflected sunlight spectrum? To answer this question, the line-by-line calculation results of the HITRAN 2008 edition were compared with those of the HITRAN Figure 2 shows the relative difference between sun-normalized radiance calculated using the HITRAN 2004 and 2008 databases at the three spectral resolutions, and in this study the relative difference was calculated with reference to the sun-normalized radiance for the HITRAN As shown in Fig. 2, the relative differences were negative for most of the cases, indicating that sunnormalized radiance were underestimated using the HITRAN 2004 database. Measurements at the higher spectral resolution showed larger differences, a result that is similar to the sensitivity to atmospheric CO 2 change. When the spectral resolution was 0.02 nm, the maximum relative difference was 8.72%. When the spectral resolution was degraded to 0.1 nm, the maximum relative difference was 2.12%. The relatively large discrepancy occurred between 1599 nm and 1604 nm. Comparison of Figs. 1 and 2 shows that the differences of the HITRAN line parameters in the 1.61-µm region induced large systematic errors in retrieval of atmospheric CO 2 data. The major line parameters of HITRAN 2004 and 2008 used in our calculations are listed in Table 2. Important improvements are obvious in the CO 2 line parameters. The HITRAN 2008 database has approximately six times more CO 2 absorption lines than the HITRAN 2004 database. However, the sum of CO 2 line intensities in HITRAN 2008 was less than that in HITRAN 2004, indicating that most of the added CO 2 lines are weak absorption lines. The intensities of the strong absorption lines in the HITRAN 2008 database were less than those in the HITRAN 2004 database. The top panel in Fig. 3 gives the calculated transmittance spectra for the major absorption species O 2, Fig. 2. Relative difference in sun-normalized radiance between HITRAN 2004 edition and 2008 edition for the three spectral resolutions.

5 NO. 2 DAI ET AL. 231 Table 2. Summary of HITRAN database improvement for the seven molecular species. Number of lines Sum of line intensities [cm 1 (molecule cm 2 ) 1 ] Molecule HITRAN2004 HITRAN2008 HITRAN2004 HITRAN2008 H 2O CO O N 2O CO CH O Fig. 3. A comparison of transmittance spectra for O 2, H 2O, CO 2, N 2O, CO, and CH 4. Top panel, the calculated transmittances using the HITRAN 2008 database; bottom panel, relative difference between the HITRAN 2008 and HITRAN 2004 databases. H 2 O, CO 2, N 2 O, CO, and CH 4, using the line parameters from the HITRAN 2008 database (spectral resolution = 0.02 nm). Obviously, the CO 2 absorption band over the spectral range from 1598 nm to 1616 nm was less contaminated by other absorption lines; the transmittances of the atmospheric column for the overlapping absorption gases O 2, H 2 O, N 2 O, CO, and CH 4 were near unity. The bottom panel in Fig. 3 shows the relative differences between the calculated transmittances using the HITRAN 2004 database and the HITRAN 2008 database. The most prominent differences between the calculations using the two databases occur in the CO 2 transmittance spectra. The relative differences were negative for most of the cases, indicating that transmittance was underestimated using the HITRAN 2004 database. This further explains the underestimation of measurements using the HITRAN 2004 database. Figure 4 shows the correlation of the Fig. 4. Correlation plot of the residual caused by CO 2 only and the total residual caused by all absorption gases between the HITRAN 2004 and HITRAN 2008 databases.

6 232 HITRAN DATABASE IMPROVEMENT EFFECT ON ATMOSPHERIC CO 2 RETRIEVAL VOL. 29 residual spectrum of total transmittance for all absorption gases and the residual spectrum for CO 2 only. There is a very good correlation between the total residual and the residual caused by the CO 2 only. This illustrates that the differences over this band were mainly due to the update of the line parameters of CO Retrieval results For all retrievals, synthetic measurements for a given atmospheric state were generated using the radiative transfer model SCIATRAN with the HITRAN 2008 database as the input. Simulated retrievals were then performed based on the optimal estimation theory, so that in each case the retrieved CO 2 column was compared to its true value. In this study, continuum signal-to-noise ratio (SNR) of 400 was assumed. This performance can be achieved with photo-noiselimited detectors and current spectrometer technologies. A principal feature of optimal retrieval theory is the use of a priori constraints representing the range of expected values for the parameters of the atmospheric state vector to be retrieved. For all retrievals, the a priori CO 2 profile was taken to be well mixed, with a value of 350 ppm in the whole column. The resulting retrieval profile was obtained after scaling the initial profile. The inverse procedure was to fit the measured sunnormalized radiance spectrum. The convergence of the iterative process was controlled using the rootmean-square (RMS) of the fit residual and the relative change of the retrieved parameters. In this study, the iterative process were stopped when the RMS of the fit residuum (relative difference between measurement and model after the fit) was <0.25% or the relative change of the retrieved CO 2 concentration was <1%. A series of simulated measurements were generated using the global mean CO 2 concentrations from 1980 to 2010 (NOAA, 2011). A retrieval experiment was then conducted for a CO 2 -only retrieval using the simulated measurement. The profiles of all other gases as well as temperature profile were assumed known. We compared the retrieved results using the HITRAN 2008 database with the true values, as shown in the Fig. 5. The results indicate that the CO 2 column concentration was inferred precisely for the three spectral resolutions. To consider the influence of the uncertainty in spectroscopic information on the retrievals of CO 2 concentration, the same retrieval experiments were performed using the HITRAN 2004 database. These simulations reveal that using the HITRAN 2004 database results in an underestimate of the column CO 2 mixing ratio, as shown in the Fig. 6. As shown in Fig. 7, when the spectral resolution was 0.02 nm, the maximum error was 15.8 ppm and the mean error was 14.8 ppm; when the spectral resolution was 0.05 nm, the maximum error was 14.2 ppm and the mean error was 13.3 ppm; for spectral resolution of Fig. 5. Comparison of the retrieved column CO 2 concentration and the true value for the three spectral resolutions using the HITRAN 2008 database.

7 NO. 2 DAI ET AL. 233 Fig. 6. Comparison of the retrieved column CO 2 concentration and the true value for the three spectral resolutions using the HITRAN 2004 database. Fig. 7. The absolute errors in the retrieved CO 2 column as a function of the actual CO 2 concentration.

8 234 HITRAN DATABASE IMPROVEMENT EFFECT ON ATMOSPHERIC CO 2 RETRIEVAL VOL nm, the maximum error was 13.4 ppm and the mean error was 12.5 ppm. These results indicate that higher spectral resolution corresponds to an increase in the influence of HITRAN database improvements. To consider the effect of temperature and pressure profiles on the molecular absorption coefficient in terms of line strength, lines shape, and even line position (Shi and Zhang, 2007), two more pressure and temperature profiles representative of the tropical atmosphere and subarctic winter atmosphere were used to simulate retrievals. The results and the absolute errors in the retrieved CO 2 column are also shown in Figs. 6 and 7, respectively. We can see that the retrieved results and the retrieved errors were less dependent on the pressure and temperature profiles. Notably, the retrievals errors increased when the actual CO 2 column concentration became progressively higher for all the spectral resolutions, as show in Fig. 7. To consider the effect of the overlapping absorption of CH 4 on the retrievals of CO 2 column concentration, a set of synthetic measurements were created by applying shifts of ±10% to the CH 4 profiles. The subsequent retrievals revealed that the retrieved results and the retrieved errors were almost unaffected by the errors of the CH 4 profiles. This further illustrates that the CO 2 absorption band over spectral range from 1598 nm to 1616 nm was less contaminated by other absorption gases. 4. Conclusions Assessing our current knowledge of spectroscopic parameters is crucial for achieving accurate retrievals of atmospheric constituents. The line parameters of major absorption gases must be determined with certain accuracies to minimize systematic errors. The HITRAN 2008 CO 2 line list has completely replaced HITRAN 2004 data in the NIR region from 4300 cm 1 to 7000 cm 1. To consider the effect of these data changes on the retrieval of CO 2 vertical column from reflected sunlight spectra in the 1.61-µm region, this study compared the synthetic measurements generated using the radiative transfer model SCIATRAN with the line-transition parameters from the HITRAN 2004 and HITRAN 2008 databases as the input. Simulated retrievals were then performed based on the optimal estimation retrieval theory. The main findings are summarized below. First, the relative differences between sunnormalized radiance calculated using the HITRAN 2004 and 2008 databases over the spectral range from 1598 nm to 1616 nm were negative for most of the cases, indicating that sun-normalized radiance is underestimated using the HITRAN 2004 database. The relatively large discrepancy occurred from 1599 nm to 1604 nm. Measurements at the higher spectral resolution showed larger differences. Second, the differences over this band were mainly due to the update of the line parameters of CO 2. The HITRAN 2008 database has approximately six times more CO 2 absorption lines than the HITRAN 2004 database. The effect of the improvements in the line parameters on the retrievals of CO 2 column should not be ignored, particularly for high spectral resolution. Third, the retrieved results indicate that column CO 2 can be underestimated by >10 ppm when using the HITRAN 2004 database. An increase in the spectral resolution resulted in a corresponding increase in the influence of the HITRAN database improvements. The retrievals errors increased when the actual CO 2 column concentration became progressively higher. The underestimation for the CO 2 column using the HITRAN 2004 database was less dependent on the pressure and temperature profiles and was almost unaffected by the errors of the CH 4 profiles. Thus to improve the accuracy of atmospheric CO 2 retrievals with high spectral resolution in the 1.61-µm region, it is important to consider the changes of the line parameters of the HITRAN database. Acknowledgements. This work was supported by the National Natural Science Foundation of China (Grant No ), Ministry of Science and Technology of China (Grant No. 2010DFA22770), the key projects from the 11th Five-Year Plan of national scientific and technological (Grant No. 2008BAC34B04-2), and the National Basic Research Program of China (also called 973 Program, Grant Nos. 2005CB422200x and 2006CB403702). REFERENCES Crisp, D., R. M. Atlas, F. M. Bréon, L. R. Brown, and J. P. Burrows, 2004: The Orbiting Carbon Observatory (OCO) mission. Adv. Space Res., 34, Crisp, D., and C. Johnson, 2005: The orbiting carbon observatory mission. Acta Astronautica, 56, Feng, X., and F. S. Zhao, 2009: Effect of changes of the HITRAN database on transmittance calculations in the near-infrared region. Journal of Quantitative Spectroscopy & Radiative Transfer, 110, Feng, X., F. S. Zhao, and W. H. Gao, 2007: Effect of improvement of HITRAN database on radiative transfer calculation. Journal of Quantitative Spectroscopy & Radiative Transfer, 108, IPCC, 2001: Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report (TAR). Houghton et al., Eds., Cambridge University Press, Cambridge, UK, New York, USA, 881pp. IPCC, 2007: Climate Change 2007: The Physical Sci-

9 NO. 2 DAI ET AL. 235 ence Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Solomon et al., Eds., Cambridge University Press, Cambridge, UK, New York, USA, 996pp. NOAA, 2011: Trends in atmospheric carbon dioxide. [Available online from esrl. noaa. gov/ gmd/ccgg/trends/] O Brien, D. M., and P. J. Rayner, 2002: Global observations of the carbon budget, 2, CO 2 column from differential absorption of reflected sunlight in the 1.61 µm band of CO 2. J. Geophys. Res., 107(D18), 4354, doi: /2001JD Rodgers, C., 1976: Retrieval of atmospheric temperature and composition from remote measurements of thermal radiation. Rev. Geophys., 14, Rodgers, C., 2000: Inverse Methods for Atmospheric Sounding: Theory and Practice. World Scientific, 240pp. Rothman, L. S., D. Jacquemart, A. Barbe, D. Chris Benner, M. Birk, and L. R. Brown. 2005: The HI- TRAN2004 molecular spectroscopic database. Journal of Quantitative Spectroscopy & Radiative Transfer, 96, Rothman, L. S., I. E. Gordon, A. Barbe, D. Chris Benner, P. F. Bernath, M. Birk, and L. R. Boudon, 2009: The HITRAN2008 molecular spectroscopic database. Journal of Quantitative Spectroscopy & Radiative Transfer, 110, Rozanov, V. V., M. Buchwitz, K. U. Eichmann, R. de Beek, and J. P. Burrows, 2002: SCIATRAN A new radiative transfer model for geophysical applications in the nm spectral region. Adv. Space Res., 29(11), Rozanov, A., V. Rozanov, and M. Buchwitz, 2005: SCIA- TRAN 2.0 A new radiative transfer model for geophysical applications in the nm spectral region. Adv. Space Res.. 36, Shi, G. Y., and H. Zhang, 2007: The relationship between absorption coefficient and temperature and their effect on the atmospheric cooling rate. Journal of Quantitative Spectroscopy & Radiative Transfer, 105, Siewert, C. E., 2000: A concise and accurate solution to Chandrasekhar s basic problem in radiative transfer. Journal of Quantitative Spectroscopy & Radiative Transfer, 64, Stamnes, K., S.-C. Tsay, W. Wiscombe, and K. Jayaweera, 1988: Numerically stable algorithm for discrete-ordinate-method radiative transfer in multiple scattering and emitting layered media. Appl. Opt., 27(12), Stephens, B. B., K. R. Gurney, P. P. Tans, C. Sweeney, W. Peters, L. Bruhwiler, and P. Ciais, 2007: Weak northern and strong tropical land carbon uptake from vertical profiles of atmospheric CO 2. Science, 316, Yokota, T., Y. Yoshida, N. Eguchi, Y. Ota, T. Tanaka, H. Watanabe, and S. Maksyutov, 2009: Global concentrations of CO 2 and CH 4 retrieved from GOSAT: First preliminary results. Science Online Letter on the Atmosphere, 5, , doi: /sola