  Coarse particulate matter (PM) is a serious air pollution problem in East
Asia. Analysis of air quality network observations in the North China Plain and
the Seoul Metropolitan Area shows that it is mainly anthropogenic and has
decreased by 21% over 2015-2019. This anthropogenic coarse PM is generally not
included in air quality models but scavenges nitric acid to suppress the
formation of fine particulate (PM2.5) nitrate, a major contributor to PM2.5
pollution. Including it in the GEOS-Chem model decreases simulated PM2.5
nitrate to improve agreement with observations. Decreasing anthropogenic coarse
PM over 2015-2019 directly increases PM2.5 nitrate in summer, offsetting the
effect of other emission controls, while in winter it increases the sensitivity
of PM2.5 nitrate to ammonia and sulfur dioxide emissions. Our work implies the
need for stronger ammonia and nitrogen oxides emission controls to improve
PM2.5 air quality as coarse PM continues to decrease.
