PM 2.5 and water-soluble inorganic ion concentrations decreased faster in urban than rural areas in China

We investigated variations of PM 2.5 and water-soluble inorganic ions chemical characteristics at nine urban and rural sites in China using ground-based observations. From 2015 to 2019, mean PM 2.5 concentration across all sites decreased by 41.9 pg/m 3 with a decline of 46% at urban sites and 28% at rural sites, where secondary inorganic aerosol (SIAs) contributed to 21% (urban sites) and 17% (rural sites) of the decreased PM 2.5 . SIAs concentrations underwent a decline at urban locations, while sulfate (SO 4 2-), nitrate (NO 3 -), and ammonium (NH 4 + ) decreased by 49.5%, 31.3% and 31.6%, respectively. However, only SO 4 2- decreased at rural sites, NO 3 - increased by 21% and NH 4 + decreased slightly. Those changes contributed to an overall SIAs increase in 2019. Higher molar ratios of NO 3 - to SO 4 2- and NH 4 + to SO 4 2were observed at urban sites than rural sites, being highest in the heavily polluted days. Mean molar ratios of NH 3 /NH X were higher in 2019 than 2015 at both urban and rural sites, implying increasing NH X remained as free NH 3 . Our observations indicated a slower transition from sulfate-driven to nitrate-driven aerosol pollution and less efficient control of NO X than SO 2 related aerosol formation in rural regions than urban regions. Moreover, the common factor at urban and rural sites appears to be a combination of lower SO 4 2- levels and an increasing fraction of NO3- to PM2.5 under NH4+-rich conditions. Our findings imply that synchronous reduction in NOx and NH3 emissions especially rural areas would be effective to mitigate NO3--driven aerosol pollution. (c) 2022 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.