北京市一次沙尘过程中降水化学组分的监测分析

摘要

An unusual precipitation occurred in a sand-dust storm process was sampled in Beijing area on April 5, 2010. Nine inorganic ions of the sample were analyzed using an ion chromatography approach. pH of the precipitation sample was valued 7.33, the conductivity 24.50 mS/m, the total concentration of 9 inorganic ions 3080μeq/L, the ratio of SO42/ NO3- (in equivalent concentration) 1.27, and Ca2+/NH4+ 1.25. To further understand the impact of the sandstorm on chemical compositions of the precipitation, 39 cases of precipitation were examined of the Aprils in 2005-2010 at three ground monitoring sites in Beijing. The chemical analysis of the 39 cases showed that the average pH was valued 6.84, the average conductivity 9.20 mS/m, SO42/NO3- 2.37, and Ca2+/NH4+ 1.87. Comparison between the 5-4-2010 case and the 39 April-cases indicated that a great increment was observed in concentration of NO3- in the sandstorm precipitation process. Compared to the annual averaged levels, however, the April-cases had lower SO427NO3- and higher Ca2+/NH4+, implying a relatively large increase in Ca2+ and NO3- concentration in precipitation processes during the sand-dust season in Beijing. The mass concentrations of the precursors of SO42- and NO3-, namely the gaseous pollutants SO2 and NO2 monitored at the same site on 5 April 2010 were further examined, and the result proved that the concentration of NO2 remained low and the concentration of SO2 obviously dropped down during the precipitation, indicating that atmospheric humidity could constrain the gas-particle conversion of SO2.%2010-4-5北京市出现了罕见的沙尘过程中降水,在市中心采集了雨水样品,用离子色谱方法分析了样品中9种离子组分浓度,结果表明:该次降水pH值较高(7.33)、电导率偏高(24.50mS/m)、可溶性离子浓度普遍高,9种离子浓度总和为3080μeq/L;降水中离子当量浓度之比SO42-/NO3-为1.27,Ca2+/NH4+为1.25.为了进一步研究沙尘对降水化学组成的影响,统计了2005年至2010年全市3个监测点每年4月(沙尘高发期)监测的39次降水,得到其平均pH值为6.84、电导率为9.20 mS/m;降水中当量浓度之比SO42-/NO3-和Ca2+/NH4+分别为2.37和1.87.比较2组数据可见,发生在沙尘过程中的这次降水NO3-离子浓度的增加较为突出.而与历年年平均水平相比,4月降水均凸显了Ca2+和NO3-大幅增加的特征.进一步分析该次过程中同一监测点酸根离子前体物SO2、NO2的浓度变化特征发现:在沙尘和降水持续期间NO2浓度较低,在降水持续的时段SO2浓度显著降低,表明大气湿度对于SO2气粒转化起到了制约作用.