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

Global coupled chemistry-climate models underestimate carbon monoxide (CO) in theNorthern Hemisphere, exhibiting a pervasive, negative bias against measurements peaking in late40 winter and early spring. While this bias has been commonly attributed to underestimation of directanthropogenic and biomass burning emissions, chemical production and loss via OH reaction fromemissions of anthropogenic and biogenic VOCs play an important role. Here we investigate thereasons for this underestimation using aircraft measurements taken in May and June 2016 fromthe Korea United States Air Quality (KORUS-AQ) experiment in South Korea and the Air45 chemistry Research In Asia (ARIAs) in the North China Plain (NCP). For reference, multispectralCO retrievals (V8J) from the Measurements of Pollution in the Troposphere (MOPITT) are jointlyassimilated with meteorological observations using an Ensemble Adjustment Kalman Filter(EAKF) within the global Community Atmosphere Model with Chemistry (CAM-chem) and the2Data Assimilation Research Testbed (DART). With regard to KORUS-AQ data, CO isunderestimated by 42 % in the Control-Run and by 12 % with the MOPITT 50 assimilation run. Theinversion suggests an underestimation of anthropogenic CO sources in many regions, by up to 80% for Northern China, with large increments over the Liaoning province and the North ChinaPlains (NCP). Yet, an often-overlooked aspect of these inversions is that correcting theunderestimation in anthropogenic CO emissions also improves the comparison with observational55 O3 datasets, and observationally constrained box model simulations of OH and HO2. Running aCAM-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 whosemodel errors are correlated with CO are also improved while short-lived VOCs, includingformaldehyde, are difficult to constrain solely by assimilating satellite retrievals of CO. During an60 anticyclonic episode, better simulation of O3, with an average underestimation of 5.5 ppbv and areduction in the bias of surface formaldehyde and oxygenated VOCs can be achieved by separatelyincreasing by a factor of two the modeled biogenic emissions for the plant functional types foundin Korea. Results also suggest that controlling VOC and CO emissions, in addition to wide spreadNOx controls, can improve ozone pollution over East Asia.