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
To assess the impact of the Air Pollution Prevention and Control Action Plan (APPCAP) on the chemical composition of PM2.5, this study analyzed the concentrations, existing forms, and sources of water-soluble ions in PM2.5 collected during summer (June–August) in the northern suburbs of Nanjing for the years 2012, 2013, 2017, and 2019. The results demonstrate a significant reduction in total water-soluble ion concentrations in 2017–2019 compared to 2012, indicating the effectiveness of APPCAP in mitigating PM2.5 pollution. Sulfate (SO4^2−), nitrate (NO3^−), and ammonium (NH4^+) (collectively SNA) were the dominant ionic species, contributing 69.98%–92.58% of the total ion mass, with SO4^2− being the most abundant. In the summers of 2013 and 2017, PM2.5 exhibited alkaline properties, and SNA primarily existed as NH4NO3 and (NH4)2SO4. Conversely, in 2019, PM2.5 became acidic, with SNA present as NH4NO3 and NH4HSO4. The nitrogen oxidation ratio (NOR) and sulfur oxidation ratio (SOR) indicated that NO3^− and SO4^2− predominantly originated from secondary reactions, with SO2 undergoing secondary conversion more readily than NO2, and the degree of secondary conversion increasing annually. Source apportionment revealed a shift from long-range transport in 2013 to local and regional sources by 2017. These findings underscore the success of APPCAP in reducing primary emissions and altering the chemical speciation of secondary inorganic aerosols, while highlighting the persistent dominance of sulfate and the need for continued SO2 emission controls.