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

We investigated variations of PM 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 concentration across all sites decreased by 41.9 µg/m 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. SIAs concentrations underwent a decline at urban locations, while sulfate (SO), nitrate (NO), and ammonium (NH) decreased by 49.5%, 31.3% and 31.6%, respectively. However, only SO decreased at rural sites, NO increased by 21% and NH decreased slightly. Those changes contributed to an overall SIAs increase in 2019. Higher molar ratios of NO to SO and NH to SO were observed at urban sites than rural sites, being highest in the heavily polluted days. Mean molar ratios of NH/NH were higher in 2019 than 2015 at both urban and rural sites, implying increasing NH remained as free NH. Our observations indicated a slower transition from sulfate-driven to nitrate-driven aerosol pollution and less efficient control of NO than SO 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 levels and an increasing fraction of NO to PM under NH-rich conditions. Our findings imply that synchronous reduction in NO and NH emissions especially rural areas would be effective to mitigate NO-driven aerosol pollution.