Three years of greenhouse gas column-averaged dry air mole fractions retrieved from satellite – Part 2: Methane

Carbon dioxide (CO 2) and methane (CH 4) 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 5 have processed three years (2003–2005) of SCIAMACHY near-infrared nadir measurements to simultaneously retrieve vertical columns of CO 2 (from the 1.58 µm absorption band), CH 4 (1.66 µm) and oxygen (O 2 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 10 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 XCO 2 (in ppm) and XCH 4 (in ppb), by dividing the greenhouse gas columns by simultaneously retrieved dry air columns. For 15 XCO 2 dry air columns are obtained from the retrieved O 2 columns. For XCH 4 dry air columns are obtained from the retrieved CO 2 columns because of better cancellation of light path related errors compared to using O 2 columns retrieved from the spectrally distant O 2 A-band. Here we focus on a discussion of the XCH 4 data set. The XCO 2 data set is discussed in a separate paper (Part 1). For 2003 we present detailed 20 comparisons with the TM5 model which has been optimally matched to highly accurate but sparse methane surface observations. After accounting for a systematic low bias of ∼2% agreement with TM5 is typically within 1–2%. We investigated to what extent the SCIAMACHY XCH 4 is influenced by the variability of atmospheric CO 2 using global CO 2 fields from NOAA's CO 2 assimilation system CarbonTracker. We show 25 that the CO 2 corrected and uncorrected XCH 4 spatio-temporal pattern are very similar but that agreement with TM5 is better for the CarbonTracker CO 2 corrected XCH 4. In line with previous studies (e.g., Frankenberg et al., 2005) we find significantly higher 8274 Abstract Introduction Conclusions References Tables Figures ◭ ◮ ◭ ◮ Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion methane over the tropics …