Validation of 12 years (2008-2019) of IASI-CO with IAGOS aircraft observations.

IASI-A, B and C (Infrared Atmospheric Sounding Interferometer) are nadir looking thermal infrared sensors which are monitoring the atmospheric composition since 2008. Atmospheric Carbon monoxide (CO) is retrieved from IASI radiances with two algorithms: the SOftware for a Fast Retrieval of IASI Data (SOFRID) and the Fast Optimal Retrievals on Layers for IASI (FORLI). The airborne in-situ observations from the In-service Aircraft for a Global Observing System (IAGOS) European Research Infrastructure have been used to validate the IASI CO retrievals. The validation study of IASI CO data performed in 2011 whith IAGOS data was limited to two airports (Frankfurt and Windhoek) and 2 years because of the limited sampling at the other IAGOS sites. The extension of the IAGOS infrastructure during the last decade enables a validation with enough temporal sampling at 33 airports worldwide over the whole IASI-A period (2008–2020). The retrievals provide between 1.5 and 3 independent pieces of information about the CO vertical profile and we have selected to validate the surface-600 hPa and 600–200 hPa partial columns in addition to the total column. The ability of the retrievals to capture the CO variabilities is slightly different for the two retrieval algorithms. The correlation coefficients are generally larger for SOFRID, especially for the total and lower tropospheric columns, meaning a better representation of the phase of the variability, while the amplitude of the variations of FORLI are in better agreement with IAGOS in the mid-upper troposphere. On average SOFRID and FORLI retrievals are underestimating the IAGOS total columns of CO (TCC) by 8±16 % and 6±14 % respectively. This global TCC agreement between the algorithms is hiding significant vertical and geographical differences. In the lower troposphere (Surface-600 hPa) the bias is larger for FORLI (-11±27 %) than for SOFRID (-4±24 %). In the mid-upper troposphere the situation is reversed with a bias of -6±15 % for FORLI and of -11±13 % for SOFRID. The largest differences between the retrievals are detected south of Bangkok where SOFRID underestimation is systematically larger for the TCC and mid-upper tropospheric column. North of Philadelphia FORLI biases are significantly larger than SOFRID ones for the TCC and the lower tropospheric columns. Our validation results will provide a better characterisation of IASI-CO data to the users and help improve the retrievals for future versions. [ABSTRACT FROM AUTHOR]