Abstract
Abstract. Global observations of column-averaged dry air mole fractions of carbon dioxide (CO2), denoted by XCO2 , retrieved from SCIAMACHY on-board ENVISAT can provide important and missing global information on the distribution and magnitude of regional CO2 surface fluxes. This application has challenging precision and accuracy requirements. In a previous publication (Heymann et al., 2012), it has been shown by analysing seven years of SCIAMACHY WFM-DOAS XCO2 (WFMDv2.1) that unaccounted thin cirrus clouds can result in significant errors. In order to enhance the quality of the SCIAMACHY XCO2 data product, we have developed a new version of the retrieval algorithm (WFMDv2.2), which is described in this manuscript. It is based on an improved cloud filtering and correction method using the 1.4 μm strong water vapour absorption and 0.76 μm O2-A bands. The new algorithm has been used to generate a SCIAMACHY XCO2 data set covering the years 2003–2009. The new XCO2 data set has been validated using ground-based observations from the Total Carbon Column Observing Network (TCCON). The validation shows a significant improvement of the new product (v2.2) in comparison to the previous product (v2.1). For example, the standard deviation of the difference to TCCON at Darwin, Australia, has been reduced from 4 ppm to 2 ppm. The monthly regional-scale scatter of the data (defined as the mean intra-monthly standard deviation of all quality filtered XCO2 retrievals within a radius of 350 km around various locations) has also been reduced, typically by a factor of about 1.5. Overall, the validation of the new WFMDv2.2 XCO2 data product can be summarised by a single measurement precision of 3.8 ppm, an estimated regional-scale (radius of 500 km) precision of monthly averages of 1.6 ppm and an estimated regional-scale relative accuracy of 0.8 ppm. In addition to the comparison with the limited number of TCCON sites, we also present a comparison with NOAA's global CO2 modelling and assimilation system CarbonTracker. This comparison also shows significant improvements especially over the Southern Hemisphere.
Highlights
Carbon dioxide (CO2) is the most important anthropogenic greenhouse gas contributing to global warming
In order to enhance the quality of the SCIAMACHY XCO2 data product, we have developed a new version of the retrieval algorithm (WFMDv2.2), which is described in this manuscript
We use the independent measurements from the groundbased Fourier transform spectrometers (FTS) of TCCON (Total Carbon Column Observing Network) for the validation of the SCIAMACHY WFMDv2.2 XCO2 data product
Summary
Carbon dioxide (CO2) is the most important anthropogenic greenhouse gas contributing to global warming. The goal of this study is to determine to what extent the WFM-DOAS XCO2 accuracy can be further improved by reducing scattering related errors For this reason, we have developed an improved cloud filtering and correction method for SCIAMACHY WFM-DOAS XCO2 retrievals that is based on a threshold technique for radiances from the saturated water vapour absorption band at 1.4 μm, which is described in this manuscript. 4 the new SCIAMACHY WFM-DOAS XCO2 version 2.2 retrieval algorithm is presented followed by a description of the used data sets for the comparison with the satellite data in Sect.
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