Comment on egusphere-2022-1485

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Air quality network data in China and South Korea show very high year-round mass concentrations of coarse particulate matter (PM), as inferred by difference between PM₁₀ and PM_2.5. Coarse PM concentrations in 2015 averaged 52 &mu;g m^-3 in the North China Plain (NCP) and 23 &mu;g m^-3 in the Seoul Metropolitan Area (SMA), contributing nearly half of PM₁₀. Strong daily correlations between coarse PM and carbon monoxide imply a dominant source from anthropogenic fugitive dust. Coarse PM concentrations in the NCP and the SMA decreased by 21 % from 2015 to 2019 and further dropped abruptly in 2020 due to COVID-19 reductions in construction and vehicle traffic. Anthropogenic coarse PM is generally not included in air quality models but scavenges nitric acid to suppress the formation of fine particulate nitrate, a major contributor to PM_2.5 pollution. GEOS-Chem model simulation of surface and aircraft observations from the KORUS-AQ campaign over the SMA in May&ndash;June 2016 shows that consideration of anthropogenic coarse PM largely resolves the previous model overestimate of fine particulate nitrate. The effect is smaller in the NCP which has a larger excess of ammonia. Model sensitivity simulations show that decreasing anthropogenic coarse PM over 2015&ndash;2019 directly increases PM_2.5 nitrate in summer, offsetting half the effect of other emission controls, while in winter it increases the sensitivity of PM_2.5 nitrate to ammonia and sulfur dioxide emissions. Decreasing coarse PM helps to explain the flat wintertime PM_2.5 nitrate trends observed in the NCP and the SMA despite decreases in nitrogen oxides and ammonia emissions. The continuing decrease of coarse PM from abating fugitive dust pollution will require more stringent nitrogen oxides and ammonia emission controls to successfully decrease PM_2.5 nitrate.