Characterization and source analysis of water-soluble inorganic ionic species in PM2.5 during a wintertime particle pollution episode in Nanjing, China

Water-soluble inorganic ions (WSIIs) are major components of PM2.5, and play a prominent role in atmospheric acidification. In January 2019, a large-scale persistent particle pollution episode occurred in the Yangtze River Delta (YRD) region of China. High time resolution on-line monitoring of WSIIs in PM2.5 were conducted at the Atmospheric Environment Vertical Observation Station of Nanjing University by using In-situ Gas and Aerosol Compositions Monitor. From January 1 to 13, the mean concentration of WSIIs was 76.7 ± 48.2 μg·m−3. WSIIs accounted for 64.2% of PM2.5 concentration. Sulfate, nitrate and ammonium (SNA) were the major components of WSIIs, and accounted for 25.5%, 45.6% and 26.3%, respectively. The values of NO3−/SO42− increased from 1.37 to 2.86, indicating that the contribution of mobile sources to fine particle pollution was significantly enhanced than that of stationary sources. The main factors affecting the values of sulfur oxidation ratio (SOR) and nitrogen oxidation ratio (NOR) were gaseous precursor concentrations, relative humidity and air temperature. The values of SOR and NOR exceeded 0.1, implying the secondary transformation processes played critical roles during this pollution episode. To investigate the potential source areas of WSIIs in PM2.5, the Principal Component Analysis - Multiple Linear Regression (PCA-MLR) analysis and the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model were used. By using PCA-MLR, it was found that the main pollution sources in Nanjing were secondary transformation (58%), manmade dust (26%) and biomass combustion (16%). Based on the backward trajectory calculation, the potential source contribution function (PSCF) analysis and the concentration weighted trajectory (CWT) analysis, it was revealed that the air pollutants mainly came from the local sources in and around Nanjing. With the aggravation of pollution, the contribution of regional transport to PM2.5 concentrations decreased and the contribution of local emissions increased. Among the main components of WSIIs in Nanjing, NH4+ was mainly caused by agricultural activities, and generally came from Jiangsu, eastern Henan, southern Shandong, and northeastern Anhui. SO42− was mainly derived from coal burning in the nearby industrial areas or from the north through the long-distance transmission. NO3− mainly concentrated in the local areas, which was greatly affected by human activities. The weighted concentration weighted trajectory (WCWT) peak values of NH4+ and SO42− were about 10 μg·m−3, while that of NO3− was more than 20 μg·m−3, which further indicated that the local emission of NO3− was the main potential source. The findings can contribute to understand the behavior of WSIIs and their roles in the wintertime haze event, reveal potential source areas for the corresponding components, and enlighten beneficial suggestions for particle pollution prevention and control in high polluted areas.