2007年北京夏季降水分段采样酸度和化学成分分析

Variation of pH and Chemical Composition of Precipitation by Multi-step Sampling in Summer of Beijing 2007

为掌握降水酸度和污染性化学成分随降水持续时间的变化规律,对北京2007年夏季5次典型降水过程分时段采样,分别测定了pH值、电导率,并分析了其中水溶性大气污染成分SO4^2-、NO3^-、NH4^＋等随时间的变化特征.结果表明,pH加权平均值在降水初期为5.70±0.73,为非酸雨;平缓期为4.35±0.56,呈酸雨特征.pH、电导率和各离子浓度随降水时间的持续而迅速下降,10-45 min后进入平缓期,各污染离子随之下降率为50%～90%.来自北京西北方向云团的降水（8月1日）pH值较高、污染物浓度较低,SO4^2-、NO3^-和NH4^＋分别为65.4、23.9和117.3μeq/L;而来自偏南云团降水（8月6日）pH值较低、污染物浓度较高,SO4^2-、NO3^-和NH4＋浓度分别为310.8、95.7和249.8μeq/L.北京夏季二次粒子浓度升高,将使降水酸化加剧.

In order to understand variations of pH and chemical composition of precipitation in Beijing, 5 precipitation events in summer time of 2007 were sampled step by step sequentially on time, and the variations of pH, EC and the characterization of water-soluble ion such as NH4^＋ , SO4^2- and NO3^- were analyzed. The results showed that pH was 5.70 ± 0.73 in the beginning （before-35 min） and the precipitation was not acidic, but in the steady period （after-35 min） pH was 4.35 ±0.56 and the raining water presented acidity actually. Simultaneously, pH, EC and concentration of ions decreased rapidly with the raining continuously, 10-45 min later that became nearly constant and the concentration of water soluble ions decreased 50%-90% compare to beginning. The major pollutants in the precipitation on Aug. 1, which the raining air mass came from the northwest, was low and the concentrations of NH4 , SO4^2- and NOr were 65.4, 23.9 and 117.3 μeq/L, respectively; while the major pollutants in the precipitation on Aug. 6, which the air mass came from south, was high and the concentrations of NH4^＋, SO4^＋ and NO4^2- were 310.8, 95.7 and 249.8μeq/L. But acidic precipitation was much higher when the air mass came from south than from northwest. The increasing fine particles from photo-chemical reaction in summer time of Beijing will result that the precipitation of the rain will be more and more acidic.