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研究报告

Below-cloud/in-cloud scavenging and chemical components in precipitation of Beijing in 2014

基金项目：国家自然科学基金（No.41225019，41575123，41305113）；中国科学院战略性先导科技专项（B类）（No.XDB05030200）；国家科技支撑项目（No.2014BAC22B04）

摘要：为了解现阶段北京地区降水的化学特征和云下/云内的清除作用，于2014年采用分段采样的方式针对25场降水进行收集，分析了降水中各离子组分浓度.结果发现，2014年北京地区降水中NH₄⁺、SO₄^2-、NO₃^-和Ca²⁺浓度最高，雨量加权平均离子浓度分别为240、162、91和65 μeq·L^-1，与早期研究相比均有所下降.SO₄^2-/NO₃^-比值为1.8，NO₃^-和NH₄⁺的比重显著增加.综合利用相关性分析、主因子分析及后向轨迹等方法，探讨了北京地区降水中各离子组分的来源.结果表明，东南气团中主要离子NO₃^-、SO₄^2-及NH₄⁺的浓度较高，主要受人为活动排放的影响.总体上各离子的云下冲刷作用略大于云内清除，其中，Na⁺、Ca²⁺及NO₃^-云下冲刷的贡献大于SO₄^2-和NH₄⁺.与其他城市相比，北京地区云内清除的贡献略大，表明长距离输送的影响不可忽视.

Abstract：In order to understand characteristic of chemical compositions and washout/rainout effects of precipitation in Beijing, 25 precipitations were collected through the sequential sampling. The averaged concentrations of the four largest amounts of the total ions NH₄⁺, SO₄^2-, NO₃^- and Ca²⁺ were 240, 162, 91 and 65 μeq·L^-1, respectively. The ratio of SO₄^2-/NO₃^- was 1.8, NO₃^- and NH₄⁺ were gradually increasing in proportion recently. Through correlation analysis, principle component analysis and back trajectory analysis, the precipitation composition was mainly affected by anthropogenic ions in Beijing, such as NO₃^-, SO₄^2- and NH₄⁺, which mainly come from the southeast. Overall, analytical results of washout/rainout contribution show various ions had different contribution with each other, but the washout contributed more than 50%. Na⁺, Ca²⁺ and NO₃^- were scavenged more by washout process than SO₄^2- and NH₄⁺. Compared with other cities, the rainout process contributed more in Beijing and the effect of long-range transport can't be ignored.