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Chemical composition of atmospheric precipitation in the cloud, the cloud under a series of chemical reactions result of the interaction of precipitation by region, season, precipitation, and the duration of their pollution and other factors, its chemical composition there is a big difference. Even if the same location with a rainfall precipitation process in different time periods, its chemical composition is significantly different. At present, China Chemical Study on Precipitation mostly accumulated precipitation for the entire field of analysis of samples collected after the results, while the sub-sampling periods less detailed study. Cumulative sample analysis the conclusion is often associated with the actual situation of large deviations, sometimes come to the opposite conclusion, especially for rainwater acidity judgment, Beijing is a traditional non-acid rain, but with the rapid economic development, acidification more and more serious. To accurately grasp the Beijing area rainwater chemical composition of precipitation over the course of the duration of rainfall variation, in 2010 in Beijing 16 precipitation process was sub-period sampling, analysis of pH, conductivity, 10 kinds of water-soluble inorganic ions (Na , NH 4 , K , Mg 2 , Ca 2 < / sup>, and F - , Cl - , NO 2 - , NO 3 - , SO 4 2 - ) and 27 kinds of chemical elements (N, C, Ca, Mg, Al, Fe, K, Na, Zn, Mn, Pb, Ba, Sb, Cu, As, Co, Se, V, Ni, Cr, Cd, Mo, Tl, Be, U, Th, Ag) with the change in precipitation, obtained the following conclusions: 1, precipitation pH and conductivity (EC) values in precipitation are generally higher initial value, and then decreased rapidly, the late heavy precipitation pH value can reach 3.0 shows the strong acidity. In the late precipitation pH and conductivity values change over time precipitation and precipitation intensity showed a certain relationship. 2, Beijing precipitation 10 kinds of water-soluble inorganic ions Na , NH 4 , K , Mg 2 , Ca 2 and F - , Cl - , NO 2 - < / sup>, NO 3 - , SO 4 2 - -weighted average concentrations were 5.9,263.9, 2.8,17.4,96.6 and 6.5,14.2,3.9,90.4,181.0 μeq / L, where NH 4 , SO 4 2 - , Ca 2 and NO 3 - of major ions, 92.6% of the total ion concentration. Precipitation samples in 27 kinds of element concentrations in the order of: N gt; C gt; Ca gt; Mg gt; Al gt; Fe gt; K gt; Na gt; Zn gt; Mn gt; Pb gt; Ba gt; Sb gt ; Cu gt; As gt; Co gt; Se gt; V gt; Ni gt; Cr gt; Cd gt; Mo gt; Tl gt; Be gt; U gt; Th gt; Ag, except Al, Fe and Cr, the Most elements enrichment occurred. 3, various ionic components over time variation of precipitation and pH, electrical conductivity (EC) value relatively similar, the proportion of major ion composition changes over time precipitation analysis, post-precipitation SO 4 < sup> 2 - and NH 4 ratio increases, the compounds may be mainly rain to (NH 4 ) 2 SO 4 main form. Precipitation element concentrations in precipitation over the extension of time, showing a higher concentration values beginning, gradually declining trend (Pb elements except), but dropped to the lowest concentration enrichment element time earlier upon the occurrence of an element enrichment (mainly heavy metals). 4, for each sub-precipitation air mass analysis found that Southwest air mass ion concentrations in precipitation are basically the highest, where NH 4 , NO 3 - and SO 4 2 - can reach other air mass ion concentration in precipitation of 2-3 times. The greater the intensity of precipitation and precipitation, erosion under the cloud of pollutants in the ionic composition of precipitation greater impact. Most elements have a certain concentration and precipitation negatively correlated with precipitation pH value of the relationship is relatively complex, with the acidic enhanced, Pb, Fe, Cu, and U elements increased solubility concentration. Northeast air mass concentration of heavy metals in precipitation are basically the highest, Fe, Cr, Mn, Ni, Cu, Zn, Pb, As and Cd mean concentrations were: 77.2,0.5,20.4,0.3,3.7,127.7,52.3, 4.1 and 1.16μg / L.
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