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. Global coupled chemistry–climate models underestimate carbonmonoxide (CO) in the Northern Hemisphere, exhibiting a pervasive negativebias against measurements peaking in late winter and early spring. Whilethis bias has been commonly attributed to underestimation of directanthropogenic and biomass burning emissions, chemical production and lossvia OH reaction from emissions of anthropogenic and biogenic volatile organic compounds (VOCs) play animportant role. Here we investigate the reasons for this underestimationusing aircraft measurements taken in May and June 2016 from the Korea–United States Air Quality (KORUS-AQ) experiment in South Korea and the AirChemistry Research in Asia (ARIAs) in the North China Plain (NCP). Forreference, multispectral CO retrievals (V8J) from the Measurements ofPollution in the Troposphere (MOPITT) are jointly assimilated withmeteorological observations using an ensemble adjustment Kalman filter(EAKF) within the global Community Atmosphere Model with Chemistry(CAM-Chem) and the Data Assimilation Research Testbed (DART). With regard to KORUS-AQ data, CO is underestimated by 42 % in the control run and by 12 % with the MOPITT assimilation run. The inversion suggests anunderestimation of anthropogenic CO sources in many regions, by up to 80 % for northern China, with large increments over the Liaoning Provinceand the North China Plain (NCP). Yet, an often-overlooked aspect of theseinversions is that correcting the underestimation in anthropogenic COemissions also improves the comparison with observational O3 datasetsand observationally constrained box model simulations of OH and HO2.Running a CAM-Chem simulation with the updated emissions of anthropogenic CO reduces the bias by 29 % for CO, 18 % for ozone, 11 % for HO2, and 27 % for OH. Longer-lived anthropogenic VOCs whose model errors are correlated with CO are also improved, while short-lived VOCs, including formaldehyde, are difficult to constrain solely by assimilating satellite retrievals of CO. During an anticyclonic episode, better simulation of O3, with an average underestimation of 5.5 ppbv, and a reduction in the bias of surface formaldehyde and oxygenated VOCs can be achieved byseparately increasing by a factor of 2 the modeled biogenic emissions forthe plant functional types found in Korea. Results also suggest thatcontrolling VOC and CO emissions, in addition to widespread NOx controls,can improve ozone pollution over East Asia.

Correcting model biases of CO in East Asia: impact on oxidant distributions during KORUS-AQ