Chemical Compositions of Precipitation at Three Non-urban Sites of Hebei Province, North China: Influence of Terrestrial Sources on Ionic Composition

Abstract:

Rapid economic development and increased energy demand have resulted in severe air pollution problems throughout China, and the increasing emissions of SO2 and NOx have led to significant deposition of acid rain in China. Despite high SO2 concentrations are observed in both southern and northern China, acid rain is mainly observed in the south area in history. It is believed that high content of alkaline matters in the soil in the North China is one of the key factors that contribute to the neutralization of acidic ions in the rainwater. However, an increasing trend of the precipitation acidity was observed at many sites in North, Central and East China after 2000, with more pronounced trends in the Beijing-Tianjin region and Bohai Sea surrounding area. In this study, precipitations were collected from April to October in 2008 at three non-urban sites (Chicheng, Huailai and Xuanhua) in Hebei Province, North China. The pH values of precipitation events ranged from 4.08 to 7.53 for the three sampling sites. About 61.8% of the total precipitation had a pH value higher than 6.0. The predominant ions in the precipitation were SO42-, NO3- and Ca2+, and their concentrations were significantly higher than other sites in South China. The high concentration of acidic ions reflects that atmospheric pollution has already been severe in this area. The volume-weighted mean (VWM) concentration of major ions and pH value showed obvious seasonal differences, precipitation in dry season had high pH values and VWM concentrations of major ions. According to the equivalent ratio of NP/AP and the results of neutralization factors, the strong acid neutralization is 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 and enrichment factors, the main acidic ions (SO42- and NO3-) and NH4+ were mainly derived from anthropogenic activities, and Ca2+ and Mg2+ mostly originated from terrestrial sources. The results reflect that terrestrial source is an important source of major cations in rainwater and strongly influence the precipitation chemistry in North China.