Characterizing the errors in AIRS mid-tropospheric CO2 retrievals

Abstract:

Mid- to upper-tropospheric carbon dioxide (CO₂) has been retrieved since 2002 from thermal infrared channels of NASA’s Atmospheric Infrared Sounder (AIRS) instrument. Unlike retrievals using near-IR data from the Japanese Greenhouse gas Observing SATellite (GOSAT) or NASA’s Orbiting Carbon Observatory 2 (OCO2), the sensitivity of AIRS measurements does not peak near the surface; however, the AIRS tropospheric CO₂ data cover nearly the entire globe across a decadal time period – they should provide a good constraint on long-term surface CO₂ fluxes at broad spatial scales, in the absence of significant biases. In this study, we attempt to characterize the systematic and random errors in AIRS CO₂ retrievals by comparing to CO₂ fields generated by the PCTM transport model using CarbonTracker-optimized fluxes. We examine both the standard and support products of AIRS Version 5 Release Level 2 CO₂, together with other parameters (e.g., cloud top pressure/temperature) retrieved using coincident microwave measurements from the Advanced Microwave Sounding Unit (AMSU). We formulate a bias correction for AIRS CO₂ against these parameters, remove the bias, and use the bias-corrected data to solve for weekly flux corrections across 2009-2011 at a 3.0 x 3.75 deg resolution (lat/lon).