High time-resolved PM2.5 composition and sources at an urban site in Yangtze River Delta, China after the implementation of the APPCAP.

In this study, hourly concentrations of PM2.5 water-soluble inorganic ions, bulk organic carbon (OC), and elemental carbon (EC) were monitored from 1/1/2017 to 12/31/2017 and validated using filter-based offline analysis at an urban site in Nanjing, China. Compared with 2013 or before, the annual average of PM2.5 concentration (36.5 ± 32.9 μg m-3) in 2017 decreased by more than 40%, NO3- (12.8 ± 11.4 μg m-3) became the most abundant water-soluble ion instead of SO42- (9.29 ± 6.07 μg m-3), and the relative contribution of OC (5.92 ± 3.40 μg m-3) and EC (2.95 ± 1.53 μg m-3) to bulk PM2.5 (24.9 ± 9.31%) increased substantially, indicating the effectiveness of the control policy for reducing gaseous precursor emissions. Based on the diurnal variations of water-soluble ions and gaseous pollutants, NH4+, SO42-, and NO3- were secondarily formed and NH4NO3 dominated the composition of ammonium salts in PM2.5. The diurnal changes of OC, EC, and OC/EC ratios reflected prominent influences from local traffic patterns. Positive matrix factorization was performed using hourly data of PM2.5 components (PMF1-h), of which the results were justified by comparing to those using 23-h averaged data (PMF23-h). Given that the secondary ion formation was still the dominant source (68.2%) of PM2.5, and the average PM2.5 concentration in urban Nanjing remained higher than Tier II limit (35 μg m-3) of the Chinese National Ambient Air Quality Standard, controlling emissions of PM2.5 precursor gases should be continued after the completion of Air Pollution Prevention and Control Action Plan in 2017.