Evolution of Water-Soluble Ions in PM2.5 in the Northern Suburbs of Nanjing During Summer Before and After the Implementation of the Air Pollution Prevention and Control Action Plan

• • Total water-soluble ion concentrations in PM2.5 during summer decreased significantly from 2012 to 2017–2019, with SNA (SO4^2−, NO3^−, NH4^+) accounting for 69.98%–92.58% of total ion mass, confirming the effectiveness of APPCAP in reducing PM2.5 pollution. • • In 2013 and 2017, PM2.5 was alkaline with SNA present as NH4NO3 and (NH4)2SO4, whereas in 2019 PM2.5 became acidic with SNA as NH4NO3 and NH4HSO4, indicating a shift in atmospheric acidity and ammonia availability. • • NOR and SOR values revealed that SO4^2− and NO3^− were predominantly from secondary reactions, with SO2 undergoing secondary conversion more readily than NO2, and the degree of secondary conversion increased annually from 2013 to 2019. • • Source apportionment indicated a shift from long-range transport in 2013 to local and regional sources by 2017, emphasizing the growing importance of regional emission controls and the need for continued SO2 reduction strategies.