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.