Carbon Monoxide in Optically Thick Wildfire Smoke: Evaluating TROPOMI Using CU Airborne SOF Column Observations

TROPOspheric Monitoring Instrument (TROPOMI) measurements of carbon monoxide (CO) vertical column enhancements in optically thick biomass burning plumes were evaluated using measurements from the University of Colorado Airborne Solar Occultation Flux (CU AirSOF) instrument during the 2018 Biomass Burning Fluxes of Trace Gases and Aerosols (BB-FLUX) field campaign in the northwestern United States. The different temporal and spatial scales and measurement geometries sampled from the aircraft and satellite are actively accounted for by (1) focusing on coincident measurements, (2) comparing spatial integrals of CO enhancements across plume transects, (3) using the FLEXible PARTicle (FLEXPART) dispersion model to correct for atmospheric transport, and (4) accounting for Averaging Kernels (AVK). TROPOMI is found to be systematically higher relative to the aircraft by +36% for the operational product (+27% preoperational product) without geospatial and temporal corrections. Consecutive transects by CU AirSOF revealed significant variations between integrated CO enhancements (on average 28% over 30 min) on the satellite sub-pixel scale. When the additional corrections are applied (FLEXPART, and to a lesser degree also AVK), the average bias is reduced to +10% for the operational product (+7.2% preoperational), which is insignificant within 15% uncertainty (variability among case studies, 95% confidence level). Radiative transfer simulations in synthetic plumes indicate that multiple scattering can enhance satellite CO signals by 5???10% at high aerosol loads, which warrants further attention. Smoke strongly reduces trace gas measurements at ultraviolet and visible wavelengths (by up to a factor of 6), highlighting the importance of multispectral aerosol properties in thick smoke.