COMPARISON OF ATMOSPHERIC METHANE OBSERVATIONS FROM AIRS AND IASI

This paper presents a comparison of methane (CH4), one important greenhouse gas in terrestrial atmosphere, observed from two hyperspectral infrared sensors on satellites, one is the Atmospheric Infrared Sounder (AIRS) onboard NASA/AQUA and the other is the Infrared Atmospheric Sounding Interferometer (IASI) onboard METEOP-A and -B. Comparison using about 500 cases shows the mean DOFs from AIRS is slightly smaller than IASI, with the difference of -0.049 +/- 0.152. Overall the retrieved CH4 from AIRS is larger than IASI, and their difference is 10.2 +/- 23.8 ppb below 650 hpa and 27.4 +/- 26.9 ppb above 350 hPa. In the most sensitive layer of infrared sounder between 350 and 650 hPa, their difference is as small as 2.8 +/- 17.2 ppb. Compared to aircraft measurements, AIRS retrievals tend to be overestimated while IASI retrievals tend to be underestimated in most cases, suggesting the necessarity to further improve the algorithms and call for more dedicated aircraft measurements for validation.