Five-year record of atmospheric precipitation chemistry in urban Beijing, China
1College of Earth Science, Key Laboratory of Computational Geodynamics, Graduate University of Chinese Academy of Sciences, Beijing 100049, China
2School of Geography, Earth and Environmental Science, University of Birmingham, Edgbaston Birmingham B15 2TT, UK
3School of Environment, Tsinghua University, Beijing100084, China
4Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan
5Chongqing Environmental Protection Bureau, Chongqing 401147, China
6Tianjin Key Laboratory of Water Resources and Environment, Tianjin Normal University, Tianjin 300387, China
Abstract. Precipitation samples on an event basis were collected from March 2001 through August 2005 at an urban location in Beijing. Ionic species in the samples were measured with ion chromatography to understand the long-term changes in the precipitation chemistry and their causes. Most precipitation samples had an intermediate pH (6.1–7.3) and 16% were acidic. As the major ions, SO42− plus NO3−, and NH4+ plus Ca2+ comprised more than 80% of anionic and cationic mass, respectively. Different from their more or less reductions of gaseous precursors, the counterintuitive features of much less than expected decrease in SO42− levels and unexpected increase in NO3− concentrations in the precipitations are likely due to the combination of enhanced conversion of gaseous precursors to acid compounds, and increased regional transport. The average ratio of neutralizing to acidifying potential (i.e. NP/AP) was as high as 1.2 but it exhibited a significant decline pattern, indicative of a long-term increasing trend in the acidifying potential of the wet depositions. This is mainly ascribed to reduced input of two major alkaline agents – NH4+ and Ca2+ – over increased input of a minor alkaline agent – Mg2+ – according to the magnitudes of their neutralization factors. The equivalent mass ratio of NO3− to nss-SO42− presented an evident increasing trend with a still low mean value of 0.37 ± 0.11. This indicates that the relative contribution of NO3− to the wet deposition acidity was strengthened in recent years while the precipitation acidity in Beijing was still overwhelmingly from sulfur but not nitrogen.