Wet deposition of inorganic ions in 320 cities across China: spatio-temporal variation, source apportionment, and dominant factors
The acid deposition has been considered to be a severe environmental issue in China. The pH, electrical conductivity (EC), and concentrations of water soluble ions (NO3-, Cl−, Ca2+, K+, F−, NH4+, Mg2+, SO42-, and Na+) in the precipitation samples collected from 320 cities during 2011–2016 across China were measured. The mean concentrations of F−, NO3-, and SO42- were in the order of winter (6.10, 19.44, and 45.74 µeq L−1) > spring (3.45, 13.83, and 42.61 µeq L−1) > autumn (2.67, 9.73, and 28.85 µeq L−1) > summer (2.04, 7.66, and 19.26 µeq L−1). Secondary ions (SO42-, NO3-, and NH4+) and F− peaked in the Yangtze River Delta (YRD) and Sichuan basin (SB). Crustal ions (i.e. Ca2+, Mg2+), Na+, and Cl− showed the highest concentrations in the semi-arid regions and the coastal cities. The statistical methods confirmed that the mean anthropogenic contribution ratios to SO42-, F−, NO3-, and NH4+ at a national scale were 46.12 %, 71.02 %, 79.10 %, and 82.40 %, respectively. However, Mg2+ (70.51 %), K+ (77.44 %), and Ca2+ (82.17 %) mostly originated from the crustal source. Both Na+ (70.54 %) and Cl− (60.42 %) were closely linked to sea salt aerosols. On the basis of the stepwise regression (SR) analysis, it was proposed that most of the secondary ions and F− were closely related to gross industrial production (GIP), total energy consumption (TEC), vehicle ownership, and N fertilizer use, but the crustal ions (Ca2+ and K+) were mainly controlled by the dust events. The influence of dust days, air temperature, and wind speed on ions increased from southeast China (SEC) to central China, and then to northwest China (NWC), whereas the influence of socioeconomic factors on acid ions (SO42- and NO3-) displayed the higher value in east China.