Long-term trend of chemical composition of atmospheric precipitation at a regional background station in Northern China.

Understanding the trend of chemical composition of precipitation is of great importance for air pollution control strategies in Northern China. A comprehensive study on the long-term chemical compositions of precipitation was carried out from 2003 to 2014 at the Shangdianzi (SDZ) regional background station in northern China. All samples were analyzed for pH, electrical conductivity and major ions (F-, Cl-, NO3-, SO42-, NH4+, Mg2+, Ca2+, K+ and Na+). The average pH during this period was 4.53±0.35, which is considerably lower than those reported in other background stations in China (Linan, Waliguan and Longfengshan). NH4+, SO42-, Ca2+ and NO3- were the dominant ions in precipitation, with concentrations (volume-weighted mean) of 212.99μeqL-1, 200.20μeqL-1, 116.88μeqL-1 and 98.56μeqL-1, respectively. The ion concentrations at SDZ were much higher than those of other background stations and megacities in China. A significantly increasing trend was observed for NO3- (7.26%year-1), and a decreasing trend was observed for SO42-/NO3-, suggesting that the precipitation of SDZ evolved from a sulfuric acid type to a mixed type dominated by both sulfuric and nitric acid. The source identification indicated that SO42-, NO3-, NH4+ and F- were dominated by secondary sources, Mg2+, Ca2+ and Na+ mostly originated from natural sources, and K+ and Cl- probably associated with anthropogenic sources. Long-range transport of air masses could influence the acidity, electrical conductivity and ion concentrations of precipitation at SDZ. The higher acidity and ion concentrations mainly occurred in the southerly and westerly trajectory pathways and partially in northwest pathways. Anthropogenic pollutants and crustal sources along these pathways were significant contributors to the chemical composition of precipitation in SDZ.