Three years of greenhouse gas column-averaged dry air mole fractions retrieved from satellite – Part 1: Carbon dioxide

Abstract. Carbon dioxide (CO₂) and methane (CH₄) are the two most important anthropogenic greenhouse gases. SCIAMACHY on ENVISAT is the first satellite instrument whose measurements are sensitive to concentration changes of the two gases at all altitude levels down to the Earth's surface where the source/sink signals are largest. We have processed three years (2003–2005) of SCIAMACHY near-infrared nadir measurements to simultaneously retrieve vertical columns of CO₂ (from the 1.58 μm absorption band), CH₄ (1.66 μm) and oxygen (O₂ A-band at 0.76 μm) using the scientific retrieval algorithm WFM-DOAS. We show that the latest version of WFM-DOAS, version 1.0, which is used for this study, has been significantly improved with respect to its accuracy compared to the previous versions while essentially maintaining its high processing speed (~1 min per orbit, corresponding to ~6000 single measurements, and per gas on a standard PC). The greenhouse gas columns are converted to dry air column-averaged mole fractions, denoted XCO₂ (in ppm) and XCH₄ (in ppb), by dividing the greenhouse gas columns by simultaneously retrieved dry air columns. For XCO₂ dry air columns are obtained from the retrieved O₂ columns. For XCH₄ dry air columns are obtained from the retrieved CO₂ columns because of better cancellation of light path related errors compared to using O₂ columns retrieved from the spectrally distant O₂ A-band. Here we focus on a discussion of the XCO₂ data set. The XCH₄ data set is discussed in a separate paper (Part 2). In order to assess the quality of the retrieved XCO₂ we present comparisons with Fourier Transform Spectroscopy (FTS) XCO₂ measurements at two northern hemispheric mid-latitude ground stations. To assess the quality globally, we present detailed comparisons with global XCO₂ fields obtained from NOAA's CO₂ assimilation system CarbonTracker. For the Northern Hemisphere we find good agreement with the reference data for the CO₂ seasonal cycle and the CO₂ annual increase. For the Southern Hemisphere, where significantly less data are available for averaging compared to the Northern Hemisphere, the CO₂ annual increase is also in good agreement with CarbonTracker but the amplitude and phase of the seasonal cycle show systematic differences (up to several ppm) arising partially from the O₂ normalization most likely caused by unconsidered scattering effects due to subvisual cirrus clouds. The retrieved XCO₂ regional pattern at monthly resolution over various regions show clear correlations with CarbonTracker but also significant differences. Typically the retrieved variability is about 4 ppm (1% of 380 ppm) higher but depending on time and location differences can reach or even exceed 8 ppm. Based on the error analysis and on the comparison with the reference data we conclude that the XCO₂ data set can be characterized by a single measurement retrieval precision (random error) of 1–2%, a systematic low bias of about 1.5%, and by a relative accuracy of about 1–2% for monthly averages at a spatial resolution of about 7°×7°. When averaging the SCIAMACHY XCO₂ over all three years we find elevated CO₂ over the highly populated region of western central Germany and parts of the Netherlands ("Rhine-Main area") reasonably well correlated with EDGAR anthropogenic CO₂ emissions. On average the regional enhancement is 2.7 ppm including an estimated contribution of 1–1.5 ppm due to aerosol related errors and sampling.