Satellite observation of atmospheric methane: intercomparison between AIRS and GOSAT TANSO-FTS retrievals

<jats:p>Abstract. Space-borne observations of atmospheric methane (CH4) have been made using the Atmospheric Infrared Sounder (AIRS) on the EOS/Aqua satellite since August 2002 and the Thermal and Near-infrared Sensor for Carbon Observation Fourier Transform Spectrometer (TANSO-FTS) on the Greenhouse Gases Observing Satellite (GOSAT) since April 2009. This study compared the GOSAT TANSO-FTS thermal infrared (TIR) version 1.0 CH4 product with the collocated AIRS version 6 CH4 product using data from 1 August 2010 to 30 June 2012, including the CH4 mixing ratios and the total column amounts. The results show that at 300–600 hPa, where both AIRS and GOSAT-TIR CH4 have peak sensitivities, they agree very well, but GOSAT-TIR retrievals tend to be higher than AIRS in layer 200–300 hPa. At 300 hPa the CH4 mixing ratio from GOSAT-TIR is, on average, 10.3 ± 31.8 ppbv higher than that from AIRS, and at 600 hPa GOSAT-TIR retrieved CH4 is -16.2 ± 25.7 ppbv lower than AIRS CH4. Comparison of the total column amount of CH4 shows that GOSAT-TIR agrees with AIRS to within 1 % in the mid-latitude regions of Southern Hemisphere and in tropics. In the mid to high latitudes in the Northern Hemisphere, GOSAT-TIR is ~ 1–2 % lower than AIRS, and in the high-latitude regions of Southern Hemisphere the difference of GOSAT from AIRS varies from −3 % in October to +2 % in July. The difference between AIRS and GOSAT TANSO-FTS retrievals is mainly due to the difference in retrieval algorithms and instruments itself, and the larger difference in the high latitude regions is associated with the low information content and small degree of freedoms of the retrieval. The degree of freedom of GOSAT-TIR retrievals is lower than that of AIRS also indicates that the constraint in GOSAT-TIR retrieval may be too strong. From the good correlation between AIRS and GOSAT-TIR retrievals and the seasonal variation they observed we are confident that the thermal infrared measurements from AIRS and GOSAT-TIR can provide valuable information to capture the spatial and temporal variation of CH4, especially in the mid-upper troposphere, in most time and regions.</jats:p>