{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,6,29]],"date-time":"2026-06-29T06:09:21Z","timestamp":1782713361333,"version":"3.54.5"},"reference-count":54,"publisher":"MDPI AG","issue":"11","license":[{"start":{"date-parts":[[2017,10,28]],"date-time":"2017-10-28T00:00:00Z","timestamp":1509148800000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Remote Sensing"],"abstract":"<jats:p>Satellite retrievals of the atmospheric dry-air column-average mole fraction of CO     2     (XCO     2    ) based on hyperspectral measurements in appropriate near (NIR) and short wave infrared (SWIR) O     2     and CO     2     absorption bands can help to answer important questions about the carbon cycle but the precision and accuracy requirements for XCO     2     data products are demanding. Multiple scattering of light at aerosols and clouds can be a significant error source for XCO     2     retrievals. Therefore, so called full physics retrieval algorithms were developed aiming to minimize scattering related errors by explicitly fitting scattering related properties such as cloud water\/ice content, aerosol optical thickness, cloud height, etc. However, the computational costs for multiple scattering radiative transfer (RT) calculations can be immense. Processing all data of the Orbiting Carbon Observatory-2 (OCO-2) can require up to thousands of CPU cores and the next generation of CO     2     monitoring satellites will produce at least an order of magnitude more data. For this reason, the Fast atmOspheric traCe gAs retrievaL FOCAL has been developed reducing the computational costs by orders of magnitude by approximating multiple scattering effects with an analytic solution of the RT problem of an isotropic scattering layer. Here we confront FOCAL for the first time with measured OCO-2 data and protocol the steps undertaken to transform the input data (most importantly, the OCO-2 radiances) into a validated XCO     2     data product. This includes preprocessing, adaptation of the noise model, zero level offset correction, post-filtering, bias correction, comparison with the CAMS (Copernicus Atmosphere Monitoring Service) greenhouse gas flux inversion model, comparison with NASA\u2019s operational OCO-2 XCO     2     product, and validation with ground based Total Carbon Column Observing Network (TCCON) data. The systematic temporal and regional differences between FOCAL and the CAMS model have a standard deviation of 1.0 ppm. The standard deviation of the single sounding mismatches amounts to 1.1 ppm which agrees reasonably well with FOCAL\u2019s average reported uncertainty of 1.2 ppm. The large scale XCO     2     patterns of FOCAL and NASA\u2019s operational OCO-2 product are similar and the most prominent difference is that FOCAL has about three times less soundings due to the inherently poor throughput (11%) of the MODIS (moderate-resolution imaging spectroradiometer) based cloud screening used by FOCAL\u2019s preprocessor. The standard deviation of the difference between both products is 1.1 ppm. The validation of one year (2015) of FOCAL XCO     2     data with co-located ground based TCCON observations results in a standard deviations of the site biases of 0.67 ppm (0.78 ppm without bias correction) and an average scatter relative to TCCON of 1.34 ppm (1.60 ppm without bias correction).<\/jats:p>","DOI":"10.3390\/rs9111102","type":"journal-article","created":{"date-parts":[[2017,10,30]],"date-time":"2017-10-30T12:16:23Z","timestamp":1509365783000},"page":"1102","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":34,"title":["A Fast Atmospheric Trace Gas Retrieval for Hyperspectral Instruments Approximating Multiple Scattering\u2014Part 2: Application to XCO2 Retrievals from OCO-2"],"prefix":"10.3390","volume":"9","author":[{"ORCID":"https:\/\/orcid.org\/0000-0001-9141-3895","authenticated-orcid":false,"given":"Maximilian","family":"Reuter","sequence":"first","affiliation":[{"name":"Institute of Environmental Physics, University of Bremen, P.O. Box 330440, 28334 Bremen, Germany"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-7616-1837","authenticated-orcid":false,"given":"Michael","family":"Buchwitz","sequence":"additional","affiliation":[{"name":"Institute of Environmental Physics, University of Bremen, P.O. Box 330440, 28334 Bremen, Germany"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Oliver","family":"Schneising","sequence":"additional","affiliation":[{"name":"Institute of Environmental Physics, University of Bremen, P.O. Box 330440, 28334 Bremen, Germany"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Stefan","family":"No\u00ebl","sequence":"additional","affiliation":[{"name":"Institute of Environmental Physics, University of Bremen, P.O. Box 330440, 28334 Bremen, Germany"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-8882-4108","authenticated-orcid":false,"given":"Heinrich","family":"Bovensmann","sequence":"additional","affiliation":[{"name":"Institute of Environmental Physics, University of Bremen, P.O. Box 330440, 28334 Bremen, Germany"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-6821-5580","authenticated-orcid":false,"given":"John","family":"Burrows","sequence":"additional","affiliation":[{"name":"Institute of Environmental Physics, University of Bremen, P.O. Box 330440, 28334 Bremen, Germany"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2017,10,28]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"665","DOI":"10.1175\/BAMS-D-15-00310.1","article-title":"How much CO2 is taken up by the European Terrestrial Biosphere?","volume":"98","author":"Reuter","year":"2017","journal-title":"Bull. Am. Meteorol. Soc."},{"key":"ref_2","doi-asserted-by":"crossref","unstructured":"Miller, C.E., Crisp, D., DeCola, P.L., Olsen, S.C., Randerson, J.T., Michalak, A.M., Alkhaled, A., Rayner, P., Jacob, D.J., and Suntharalingam, P. (2007). Precision requirements for space-based XCO2 data. J. Geophys. Res., 112.","DOI":"10.1029\/2006JD007659"},{"key":"ref_3","doi-asserted-by":"crossref","unstructured":"Chevallier, F., Br\u00e9on, F.M., and Rayner, P.J. (2007). Contribution of the Orbiting Carbon Observatory to the estimation of CO2 sources and sinks: Theoretical study in a variational data assimilation framework. J. Geophys. Res., 112.","DOI":"10.1029\/2006JD007375"},{"key":"ref_4","doi-asserted-by":"crossref","first-page":"781","DOI":"10.5194\/amt-3-781-2010","article-title":"A remote sensing technique for global monitoring of power plant CO2 emissions from space and related applications","volume":"3","author":"Bovensmann","year":"2010","journal-title":"Atmos. Meas. Tech."},{"key":"ref_5","doi-asserted-by":"crossref","first-page":"445","DOI":"10.1016\/0094-5765(94)00278-T","article-title":"SCIAMACHY\u2014Scanning imaging absorption spectrometer for atmospheric chartography","volume":"35","author":"Burrows","year":"1995","journal-title":"Acta Astronaut."},{"key":"ref_6","doi-asserted-by":"crossref","first-page":"127","DOI":"10.1175\/1520-0469(1999)056<0127:SMOAMM>2.0.CO;2","article-title":"SCIAMACHY\u2014Mission objectives and measurement modes","volume":"56","author":"Bovensmann","year":"1999","journal-title":"J. Atmos. Sci."},{"key":"ref_7","doi-asserted-by":"crossref","first-page":"6716","DOI":"10.1364\/AO.48.006716","article-title":"Thermal and near infrared sensor for carbon observation Fourier-transform spectrometer on the Greenhouse Gases Observing Satellite for greenhouse gases monitoring","volume":"48","author":"Kuze","year":"2009","journal-title":"Appl. Opt."},{"key":"ref_8","doi-asserted-by":"crossref","first-page":"700","DOI":"10.1016\/j.asr.2003.08.062","article-title":"The Orbiting Carbon Observatory (OCO) mission","volume":"34","author":"Crisp","year":"2004","journal-title":"Adv. Space Res."},{"key":"ref_9","doi-asserted-by":"crossref","first-page":"59","DOI":"10.5194\/amt-10-59-2017","article-title":"The on-orbit performance of the Orbiting Carbon Observatory-2 (OCO-2) instrument and its radiometrically calibrated products","volume":"10","author":"Crisp","year":"2017","journal-title":"Atmos. Meas. Tech."},{"key":"ref_10","doi-asserted-by":"crossref","first-page":"15231","DOI":"10.1029\/2000JD900191","article-title":"A near-infrared optimized DOAS method for the fast global retrieval of atmospheric CH4, CO, CO2, H2O, and N2O total column amounts from SCIAMACHY Envisat-1 nadir radiances","volume":"105","author":"Buchwitz","year":"2000","journal-title":"J. Geophys. Res."},{"key":"ref_11","doi-asserted-by":"crossref","first-page":"209","DOI":"10.5194\/amt-3-209-2010","article-title":"A method for improved SCIAMACHY CO2 retrieval in the presence of optically thin clouds","volume":"3","author":"Reuter","year":"2010","journal-title":"Atmos. Meas. Tech."},{"key":"ref_12","doi-asserted-by":"crossref","first-page":"2863","DOI":"10.5194\/acp-11-2863-2011","article-title":"Long-term analysis of carbon dioxide and methane column-averaged mole fractions retrieved from SCIAMACHY","volume":"11","author":"Schneising","year":"2011","journal-title":"Atmos. Chem. Phys."},{"key":"ref_13","doi-asserted-by":"crossref","first-page":"2375","DOI":"10.5194\/amt-5-2375-2012","article-title":"SCIAMACHY WFM-DOAS XCO2: Reduction of scattering related errors","volume":"5","author":"Heymann","year":"2012","journal-title":"Atmos. Meas. Tech."},{"key":"ref_14","doi-asserted-by":"crossref","unstructured":"Butz, A., Guerlet, S., Hasekamp, O., Schepers, D., Galli, A., Aben, I., Frankenberg, C., Hartmann, J.M., Tran, H., and Kuze, A. (2011). Toward accurate CO2 and CH4 observations from GOSAT. Geophys. Res. Lett., 38.","DOI":"10.1029\/2011GL047888"},{"key":"ref_15","doi-asserted-by":"crossref","unstructured":"Cogan, A.J., Boesch, H., Parker, R.J., Feng, L., Palmer, P.I., Blavier, J.F.L., Deutscher, N.M., Macatangay, R., Notholt, J., and Roehl, C. (2012). Atmospheric carbon dioxide retrieved from the Greenhouse gases Observing SATellite (GOSAT): Comparison with ground-based TCCON observations and GEOS-Chem model calculations. J. Geophys. Res. Atmos., 117.","DOI":"10.1029\/2012JD018087"},{"key":"ref_16","doi-asserted-by":"crossref","first-page":"99","DOI":"10.5194\/amt-5-99-2012","article-title":"The ACOS CO2 retrieval algorithm\u2014Part 1: Description and validation against synthetic observations","volume":"5","author":"Connor","year":"2012","journal-title":"Atmos. Meas. Tech."},{"key":"ref_17","doi-asserted-by":"crossref","first-page":"1533","DOI":"10.5194\/amt-6-1533-2013","article-title":"Improvement of the retrieval algorithm for GOSAT SWIR XCO2 and XCH4 and their validation using TCCON data","volume":"6","author":"Yoshida","year":"2013","journal-title":"Atmos. Meas. Tech."},{"key":"ref_18","doi-asserted-by":"crossref","unstructured":"Buchwitz, M., Reuter, M., Schneising, O., Hewson, W., Detmers, R.G., Boesch, H., Hasekamp, O.P., Aben, I., Bovensmann, H., and Burrows, J.P. (2017). Global satellite observations of column-averaged carbon dioxide and methane: The GHG-CCI XCO2 and XCH4 CRDP3 data set. Remote Sens. Environ.","DOI":"10.1016\/j.rse.2016.12.027"},{"key":"ref_19","doi-asserted-by":"crossref","first-page":"1771","DOI":"10.5194\/acp-13-1771-2013","article-title":"A joint effort to deliver satellite retrieved atmospheric CO2 concentrations for surface flux inversions: The ensemble median algorithm EMMA","volume":"13","author":"Reuter","year":"2013","journal-title":"Atmos. Chem. Phys."},{"key":"ref_20","doi-asserted-by":"crossref","first-page":"13739","DOI":"10.5194\/acp-14-13739-2014","article-title":"Satellite-inferred European carbon sink larger than expected","volume":"14","author":"Reuter","year":"2014","journal-title":"Atmos. Chem. Phys."},{"key":"ref_21","unstructured":"Boesch, H., Brown, L., Castano, R., Christi, M., Connor, B., Crisp, D., Eldering, A., Fisher, B., Frankenberg, C., and Gunson, M. (2015). Orbiting Carbon Observatory-2 (OCO-2) Level 2 Full Physics Retrieval Algorithm Theoretical Basis, Version 2.0 Rev 2."},{"key":"ref_22","doi-asserted-by":"crossref","first-page":"549","DOI":"10.5194\/amt-10-549-2017","article-title":"The Orbiting Carbon Observatory-2: First 18 months of science data products","volume":"10","author":"Eldering","year":"2017","journal-title":"Atmos. Meas. Tech."},{"key":"ref_23","doi-asserted-by":"crossref","unstructured":"Reuter, M., Buchwitz, M., Schneising, O., No\u00ebl, S., Rozanov, V., Bovensmann, H., and Burrows, J.P. (2017). A fast atmospheric trace gas retrieval for hyperspectral instruments approximating multiple scattering\u2014Part 1: Radiative transfer and a potential OCO-2 XCO2 retrieval setup. Remote Sens., submitted.","DOI":"10.3390\/rs9111159"},{"key":"ref_24","unstructured":"Eldering, A., Pollock, R., Lee, R., Rosenberg, R., Oyafuso, F., Crisp, D., Chapsky, L., and Granat, R. (2015). Orbiting Carbon Observatory-2 (OCO-2) - LEVEL 1B - Algorithm Theoretical Basis, Version 1.2 Rev 1."},{"key":"ref_25","unstructured":"Stammes, P. (2002). OMI Algorithm Theoretical Basis Document, Volume III, Clouds, Aerosols, and Surface UV Irradiance (ATBD-OMI-03), Royal Netherlands Meteorological Institute (KNMI)."},{"key":"ref_26","unstructured":"Ackerman, S., Frey, R., Strabala, K., Liu, Y., Gumley, L., Baum, B., and Menzel, P. (2010). Discriminating Clear-Sky from Cloud with MODIS\u2014Algorithm Theoretical Basis Document (MOD35), Cooperative Institute for Meteorological Satellite Studies, University of Wisconsin\u2014Madison. Version 6.1."},{"key":"ref_27","doi-asserted-by":"crossref","first-page":"2961","DOI":"10.5194\/amt-8-2961-2015","article-title":"Consistent satellite XCO2 retrievals from SCIAMACHY and GOSAT using the BESD algorithm","volume":"8","author":"Heymann","year":"2015","journal-title":"Atmos. Meas. Tech."},{"key":"ref_28","doi-asserted-by":"crossref","unstructured":"Hu, C., Lee, Z., and Franz, B. (2012). Chlorophyll-a algorithms for oligotrophic oceans: A novel approach based on three-band reflectance difference. J. Geophys. Res. Oceans, 117.","DOI":"10.1029\/2011JC007395"},{"key":"ref_29","doi-asserted-by":"crossref","unstructured":"Chevallier, F., Ciais, P., Conway, T.J., Aalto, T., Anderson, B.E., Bousquet, P., Brunke, E.G., Ciattaglia, L., Esaki, Y., and Fr\u00f6hlich, M. (2010). CO2 surface fluxes at grid point scale estimated from a global 21 year reanalysis of atmospheric measurements. J. Geophys. Res., 115.","DOI":"10.1029\/2010JD013887"},{"key":"ref_30","unstructured":"Chevallier, F. (2017, October 29). Available online: https:\/\/atmosphere.copernicus.eu\/sites\/default\/files\/FileRepository\/Resources\/Validation-reports\/Fluxes\/CAMS73_2015SC2_D73.1.4.2-1979-2016-v1_201707_final.pdf."},{"key":"ref_31","doi-asserted-by":"crossref","unstructured":"Rodgers, C.D. (2000). Inverse Methods for Atmospheric Sounding: Theory and Practice, World Scientific Publishing.","DOI":"10.1142\/9789812813718"},{"key":"ref_32","first-page":"2087","article-title":"The Total Carbon Column Observing Network (TCCON)","volume":"369","author":"Wunch","year":"2011","journal-title":"Philos. Trans. R. Soc. Lond. Ser. A Math. Phys. Eng. Sci."},{"key":"ref_33","doi-asserted-by":"crossref","unstructured":"Reuter, M., Bovensmann, H., Buchwitz, M., Burrows, J.P., Connor, B.J., Deutscher, N.M., Griffith, D.W.T., Heymann, J., Keppel-Aleks, G., and Messerschmidt, J. (2011). Retrieval of atmospheric CO2 with enhanced accuracy and precision from SCIAMACHY: Validation with FTS measurements and comparison with model results. J. Geophys. Res., 116.","DOI":"10.1029\/2010JD015047"},{"key":"ref_34","unstructured":"Kivi, R., Heikkinen, P., and Kyro, E. (2014). TCCON data from Sodankyla, Finland, Release GGG2014R0, TCCON Data Archive."},{"key":"ref_35","unstructured":"Deutscher, N., Notholt, J., Messerschmidt, J., Weinzierl, C., Warneke, T., Petri, C., Grupe, P., and Katrynski, K. (2014). TCCON data from Bialystok, Poland, Release GGG2014R1, TCCON Data Archive."},{"key":"ref_36","unstructured":"Notholt, J., Petri, C., Warneke, T., Deutscher, N., Buschmann, M., Weinzierl, C., Macatangay, R., and Grupe, P. (2014). TCCON data from Bremen, Germany, Release GGG2014R0, TCCON Data Archive."},{"key":"ref_37","unstructured":"Hase, F., Blumenstock, T., Dohe, S., Gro\u00df, J., and Kiel, M. (2014). TCCON data from Karlsruhe, Germany, Release GGG2014R1, TCCON Data Archive."},{"key":"ref_38","unstructured":"Te, Y., Jeseck, P., and Janssen, C. (2014). TCCON data from Paris, France, Release GGG2014R0, TCCON Data Archive."},{"key":"ref_39","unstructured":"Warneke, T., Messerschmidt, J., Notholt, J., Weinzierl, C., Deutscher, N., Petri, C., Grupe, P., Vuillemin, C., Truong, F., and Schmidt, M. (2014). TCCON data from Orleans, France, Release GGG2014R0, TCCON Data Archive."},{"key":"ref_40","unstructured":"Sussmann, R., and Rettinger, M. (2014). TCCON data from Garmisch, Germany, Release GGG2014R0, TCCON Data Archive."},{"key":"ref_41","unstructured":"Wennberg, P.O., Roehl, C., Wunch, D., Toon, G.C., Blavier, J.F., Washenfelder, R., Keppel-Aleks, G., Allen, N., and Ayers, J. (2014). TCCON data from Park Falls, Wisconsin, USA, Release GGG2014R0, TCCON Data Archive."},{"key":"ref_42","unstructured":"Wennberg, P.O., Wunch, D., Roehl, C., Blavier, J.F., Toon, G.C., Allen, N., Dowell, P., Teske, K., Martin, C., and Martin, J. (2014). TCCON data from Lamont, Oklahoma, USA, Release GGG2014R1, TCCON Data Archive."},{"key":"ref_43","unstructured":"Goo, T.Y., Oh, Y.S., and Velazco, V.A. (2014). TCCON data from Anmeyondo, South Korea, Release GGG2014R0, TCCON Data Archive."},{"key":"ref_44","unstructured":"Morino, I., Matsuzaki, T., and Shishime, A. (2014). TCCON data from Tsukuba, Ibaraki, Japan, 125HR, Release GGG2014R1, TCCON Data Archive."},{"key":"ref_45","unstructured":"Iraci, L., Podolske, J., Hillyard, P., Roehl, C., Wennberg, P.O., Blavier, J.F., Landeros, J., Allen, N., Wunch, D., and Zavaleta, J. (2014). TCCON data from Armstrong Flight Research Center, Edwards, CA, USA, Release GGG2014R1, TCCON Data Archive."},{"key":"ref_46","unstructured":"Wennberg, P.O., Wunch, D., Yavin, Y., Toon, G.C., Blavier, J.F., Allen, N., and Keppel-Aleks, G. (2014). TCCON data from Jet Propulsion Laboratory, Pasadena, California, USA, Release GGG2014R0, TCCON Data Archive."},{"key":"ref_47","unstructured":"Shiomi, K., Kawakami, S., Ohyama, H., Arai, K., Okumura, H., Taura, C., Fukamachi, T., and Sakashita, M. (2014). TCCON data from Saga, Japan, Release GGG2014R0, TCCON Data Archive."},{"key":"ref_48","unstructured":"Feist, D.G., Arnold, S.G., John, N., and Geibel, M.C. (2014). TCCON data from Ascension Island, Saint Helena, Ascension and Tristan da Cunha, Release GGG2014R0, TCCON Data Archive."},{"key":"ref_49","unstructured":"Griffith, D.W.T., Deutscher, N., Velazco, V.A., Wennberg, P.O., Yavin, Y., Aleks, G.K., Washenfelder, R., Toon, G.C., Blavier, J.F., and Murphy, C. (2014). TCCON data from Darwin, Australia, Release GGG2014R0, TCCON Data Archive."},{"key":"ref_50","unstructured":"De Maziere, M., Sha, M.K., Desmet, F., Hermans, C., Scolas, F., Kumps, N., Metzger, J.M., Duflot, V., and Cammas, J.P. (2014). TCCON data from Reunion Island (La Reunion), France, Release GGG2014R0, TCCON Data Archive."},{"key":"ref_51","unstructured":"Griffith, D.W.T., Velazco, V.A., Deutscher, N., Murphy, C., Jones, N., Wilson, S., Macatangay, R., Kettlewell, G., Buchholz, R.R., and Riggenbach, M. (2014). TCCON data from Wollongong, Australia, Release GGG2014R0, TCCON Data Archive."},{"key":"ref_52","unstructured":"Sherlock, V., Connor, B., Robinson, J., Shiona, H., Smale, D., and Pollard, D. (2014). TCCON data from Lauder, New Zealand, 125HR, Release GGG2014R0, TCCON Data Archive."},{"key":"ref_53","doi-asserted-by":"crossref","first-page":"973","DOI":"10.5194\/amt-9-973-2016","article-title":"Orbiting Carbon Observatory-2 (OCO-2) cloud screening algorithms: Validation against collocated MODIS and CALIOP data","volume":"9","author":"Taylor","year":"2016","journal-title":"Atmos. Meas. Tech."},{"key":"ref_54","doi-asserted-by":"crossref","first-page":"3477","DOI":"10.5194\/amt-6-3477-2013","article-title":"Carbon Monitoring Satellite (CarbonSat): Assessment of atmospheric CO2 and CH4 retrieval errors by error parameterization","volume":"6","author":"Buchwitz","year":"2013","journal-title":"Atmos. Meas. Tech."}],"container-title":["Remote Sensing"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/2072-4292\/9\/11\/1102\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2025,10,11]],"date-time":"2025-10-11T18:48:44Z","timestamp":1760208524000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/2072-4292\/9\/11\/1102"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2017,10,28]]},"references-count":54,"journal-issue":{"issue":"11","published-online":{"date-parts":[[2017,11]]}},"alternative-id":["rs9111102"],"URL":"https:\/\/doi.org\/10.3390\/rs9111102","relation":{},"ISSN":["2072-4292"],"issn-type":[{"value":"2072-4292","type":"electronic"}],"subject":[],"published":{"date-parts":[[2017,10,28]]}}}