[Evolution of PM_2.5 Chemical Composition and Sources in Nanjing During the Implementation of the APPCAP]

To acquire a deeper understanding about the impact of the Air Pollution Prevention and Control Action Plan (APPCAP) on ambient fine particulate matter (PM_2.5), measurements of PM_2.5 chemical components were conducted in the urban area of Nanjing during the implementation of the APPCAP. Evolution characteristics of several properties of the PM_2.5 were studied, including the concentrations, proportions, and enrichment factors of major PM_2.5 components and the acidity of the PM_2.5. In addition, the source apportion of the PM_2.5 was analyzed using the positive matrix factorization (PMF) model. The results showed that during the implementation of the APPCAP, the proportion of sulfate (SO₄^2-) in PM_2.5 decreased, whereas the proportions of nitrate (NO^-₃) and ammonium (NH⁺₄) increased significantly together, from 25.58% in 2014 to 37.15% in 2017. The concentrations of carbon components (OC and EC) decreased in the early stage of the APPCAP; however, the decreasing trend slowed down in the later stage. In particular, the proportion of organic carbon (OC) increased, especially for the secondary organic carbon (SOC). The acidity of the PM_2.5 decreased significantly, with the molar equivalent ratio of cations and anions increasing from 0.83 in 2014 to 1.13 in 2017. The contributions of coal combustion and the metallurgical and petrochemical industry to the PM_2.5 generally showed a downward trend, whereas the contribution of vehicle emissions increased rapidly, from 8.78% in 2014 to 13.92% in 2017. The contribution of general fugitive dust fluctuated greatly, but that of soil fugitive dust showed an upward trend. This study suggested that the direct PM_2.5 emissions, such as those from coal combustion and industries, were effectively controlled during the implementation of the APPCAP; however, the contribution of secondary aerosols became increasingly prominent, which indicates that future policies of PM_2.5 pollution control should focus on emission reduction of the precursors such as NO, NH₃, and VOCs. In addition, the effects of increasing alkalization of the PM_2.5 and enhanced oxidizability of the atmosphere on the PM_2.5 pollution also require further attention.