Retrieval of tropospheric CO column from hyperspectral infrared sounders – application to four years of Aqua/AIRS and MetOp-A/IASI

Abstract. Four years of tropospheric integrated content of CO were retrieved from infrared hyperspectral observations of AIRS onboard Aqua and IASI onboard MetOp-A, for the period July 2007–June 2011. The retrieval method is based on a double differential approach that relies on the difference between brightness temperatures observed by the sounder and BT simulated by the Automatised Atmospheric Absorption Atlas (4A) radiative transfer model on colocated ECMWF reanalyses, for several couples of channels located in the 4.67 μm CO band. AIRS and IASI give access to similar integrated contents of CO with a maximum sensitivity near 450 hPa and a half-height width of the weighting function between 200 and 750 hPa depending on the thermal contrast (i.e., the difference between the surface temperature and the temperature of the first pressure level). However, differences in their spectral and radiometric characteristics yield differences in the retrieval characteristics with AIRS selected couples of channels being more sensitive to surface characteristics. Moreover, IASI covers the whole CO absorption band, with a 3 times better spectral resolution, giving access to channels presenting a 3 times higher signal to noise ratio. This results in a better precision and lower standard deviation of the IASI retrievals. Conservatively, comparisons with CARIBIC aircraft measurements yield an averaged relative difference of 3.4% for IASI and 4.9% for AIRS. On average, AIRS and IASI retrievals are in very good agreement, showing the same seasonality, seasonal amplitudes, interannual variability and spatial distribution. The analysis of the monthly evolution of CO particularly highlights the expected strong influence of biomass burning on the evolution of CO in several tropical regions. In particular, a sharp increase in CO in 2010 in the southern tropics, especially over South America and South Africa, is observed, and is shown to be related to El Niño and to the Atlantic Multidecadal Oscillation.