2014年北京地区云内云下的降水化学分析

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

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NH_4^+、SO_4^(2-)、NO_3^-和Ca^(2+)浓度最高,雨量加权平均离子浓度分别为240、162、91和65μeq·L^(-1),与早期研究相比均有所下降.SO_4^(2-)/NO_3^-比值为1.8,NO_3^-和NH_4^+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NO_3^-、SO_4^(2-)及NH_4^+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^+、Ca^(2+)及NO_3^-云下冲刷的贡献大于SO_4^(2-)和NH_4^+.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.

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＋, SO42-, NO3 and Ca2＋ were 240, 162, 91 and 65 μeq · L-1, respectively. The ratio of SO24-/N03 was 1.8, NO~ and NH~ 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-, SO42- 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 SO24- and NH4＋. Compared with other cities, the rainout process contributed more in Beijing and the effect of long-range transport can＇ t be ignored.