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. Single-footprint retrievals of carbon monoxide from theAtmospheric Infrared Sounder (AIRS) are evaluated using aircraft in situobservations. The aircraft data are from the HIAPER Pole-to-Pole Observations (HIPPO,2009–2011), the first three Atmospheric Tomography Mission (ATom,2016–2017) campaigns, and the National Oceanic and AtmosphericAdministration (NOAA) Global Monitoring Laboratory (GML) Global GreenhouseGas Reference Network aircraft program in years 2006–2017. The retrievalsare obtained using an optimal estimation approach within the MUlti-SpEctra,MUlti-SpEcies, MUlti-SEnsors (MUSES) algorithm. Retrieval biases andestimated errors are evaluated across a range of latitudes from thesubpolar to tropical regions over both ocean and land points. AIRS MUSES CO profiles were compared with HIPPO, ATom, and NOAA GML aircraftobservations with a coincidence of 9 h and 50 km to estimate retrievalbiases and standard deviations. Comparisons were done for different pressurelevels and column averages, latitudes, day, night, land, and oceanobservations. We found mean biases of +6.6±4.6 %, +0.6±3.2 %, and -6.1±3.0 % for three representative pressurelevels of 750, 510, and 287 hPa, as well as column average mean biases of1.4±3.6 %. The mean standard deviations for the threerepresentative pressure levels were 15 %, 11 %, and 12 %, and the columnaverage standard deviation was 9 %. Observation errors (theoreticalerrors) from the retrievals were found to be broadly consistent in magnitudewith those estimated empirically from ensembles of satellite aircraftcomparisons, but the low values for these observation errors requirefurther investigation. The GML aircraft program comparisons generally hadhigher standard deviations and biases than the HIPPO and ATom comparisons.Since the GML aircraft flights do not go as high as the HIPPO and ATomflights, results from these GML comparisons are more sensitive to the choiceof method for extrapolation of the aircraft profile above the uppermostmeasurement altitude. The AIRS retrieval performance shows littlesensitivity to surface type (land or ocean) or day or night but somesensitivity to latitude. Comparisons to the NOAA GML set spanning the years2006–2017 show that the AIRS retrievals are able to capture the distinctseasonal cycles but show a high bias of ∼20 % in the lowertroposphere during the summer when observed CO mixing ratios are at annualminimum values. The retrieval bias drift was examined over the same years2006–2017 and found to be small at &lt;0.5 %.

Validation and error estimation of AIRS MUSES CO profiles with HIPPO, ATom, and NOAA GML aircraft observations