Chemical compositions of precipitation at three non-urban sites of Hebei Province, North China: Influence of terrestrial sources on ionic composition

Abstract

Studies on precipitation chemistry were carried out from April to October in 2008 at three non-urban sites in North China. A total of 122 rainwater samples were collected, and all the samples were analyzed for pH, EC and major ions (F−, Cl−, NO3−, SO42−, K+, Na+, Ca2+, Mg2+, and NH4+). The predominant ions in the precipitation were SO42−, NO3− and Ca2+ at all sampling sites. The low VWM pH value (5.29) reflected the increasing air pollution in this region. But there was still more than 60% of the total precipitation had a pH value higher than 6.0, indicating the considerable neutralizing effects of alkaline aerosols in northern China. The volume-weighted mean (VWM) concentrations of major ions and pH values showed obvious seasonal differences, the precipitation had higher pH values and VWM concentrations of major ions in dry season. This is mainly attributed to the different sources of air masses in different seasons and the dilution effect on suspended particles. According to the equivalent ratio of [nssCa2++NH4+]/[nssSO42−+NO3−] and the results of neutralization factors, the strong acid neutralization was mainly brought about by Ca2+ followed by NH4+ and Mg2+. Using Na as an indicator of marine origin, the proportions of sea salt and non-sea-salt were estimated from elemental ratios. Combining with the results of correlation analysis, the main acidic ions (SO42− and NO3−) and NH4+ were mainly derived from anthropogenic activities, and Ca2+ and Mg2+ mostly originated from terrestrial sources. In general, the results reflect that terrestrial source was an important source of major cations in rainwater and strongly influenced the precipitation chemistry, especially in dry season.