云水和云下冲刷颗粒物与气态污染物对降水中硫酸盐和硝酸盐的贡献

为定量云水和云下冲刷分别对降水中SO₄2-、NO₃-的贡献，并进一步解析云下冲刷颗粒相和气相物质分别对降水样品中SO₄2-、NO₃-的贡献，于2016年4月-2017年2月采用APS-3A型降水自动采样仪对降水进行分段采集.采用离子色谱检测分段降水样品的ρ（SO₄2-）、ρ（NO₃-），分析其变化规律；在降水前、降水中及降水后同步采集并检测大气颗粒相ρ（SO₄2-）、ρ（NO₃-）和气相ρ（SO₂）、ρ（NO₂），分析颗粒相中ρ（SO₄2-）、ρ（NO₃-）和气相中ρ（SO₂）、ρ（NO₂）的变化与分布.结果表明:①ρ（SO₄2-）、ρ（NO₃-）在同一场降水的分段样品中呈逐渐降低至后期趋于平稳的趋势，说明降水对空气中污染物的冲刷使空气逐渐清洁，后期冲刷作用有限使得降水中离子质量浓度趋稳.②颗粒相中ρ（SO₄2-）、ρ（NO₃-）与气相中ρ（SO₂）、ρ（NO₂）在降水前较高，在降水中减小，并在降水后回升，说明降水对颗粒相SO₄2-、NO₃-和气相SO₂、NO₂均有清除作用，降水结束后无云下冲刷作用，污染物质量浓度逐步回升.③云水对降水中ρ（SO₄2-）、ρ（NO₃-）的贡献率分别为22%~56%（平均值为35%）、9%~49%（平均值为29%），云下冲刷颗粒相SO₄2-、NO₃-对降水中ρ（SO₄2-）、ρ（NO₃-）的贡献率分别为39%~69%（平均值为55%）、43%~73%（平均值为56%），云下冲刷气相SO₂、NO₂对降水中ρ（SO₄2-）、ρ（NO₃-）的贡献率分别为5%~17%（平均值为10%）、5%~19%（平均值为15%）.研究显示，降水中SO₄2-、NO₃-主要来源于云水和云下冲刷颗粒相SO₄2-、NO₃-，而来源于云下冲刷气相SO₂、NO₂较少. 

Contribution of Cloud Water and Below-Cloud Scavenging Particulate Matter and Gaseous Pollutants to Sulfate and Nitrate in Wet Precipitation

In order to quantify the contribution of cloud water and rain wash-out below the clouds to SO₄2- and NO₃- in wet precipitation and further to differentiate the contribution of scavenging SO₄2- and NO₃- in particulates and of scavenging SO₂ and NO₂ in the air below the clouds to SO₄2- and NO₃- in wet precipitation, the segmented wet precipitation were collected by APS-3A precipitation automatic samplers from April 2016 to February 2017. The ρ(SO₄2-) and ρ(NO₃-) in segmented wet precipitation were detected by ion chromatography and then the variations were analyzed. SO₄2- and NO₃- in particulates and SO₂ and NO₂ in the air before rainfall, during rainfall and after rainfall were synchronously monitored. Then we analyzed variation of ρ(SO₄2-) and ρ(NO₃-) in particulates and ρ(SO₂) and ρ(NO₂) in the air before rainfall, during rainfall and after rainfall. The results showed:(1) During the same wet precipitation process, the ρ(SO₄2-) and ρ(NO₃-) in segmented precipitation decreased gradually and tended to be steady in the final stage of wet precipitation, which illustrated that the wet precipitation scoured the atmospheric pollutants and kept the air clean gradually and that scour effect of the rain became limited, leading to stable of ρ(SO₄2-) and ρ(NO₃-) in later wet precipitation period. (2) The mass concentrations of SO₄2- and NO₃- in particles, and the mass concentrations of SO₂ and NO₂ in the air were the highest before rain; they decreased during rain, and increased after rain, illustrating that the wet precipitation could scavenge SO₄2- and NO₃- in particulates and SO₂ and NO₂ in the air; but the concentrations of those pollutants increased slightly without the effects of rain wash-out after rain. (3) The contributions to SO₄2- and NO₃- in wet precipitation ranged from 22% to 56%, with an average of 35%, and from 9% to 49%, with an average of 29%. The contributions to SO₄2- and NO₃- in wet precipitation of SO₄2- and NO₃- in particulates scavenged below the clouds ranged from 39% to 69%, with an average of 55%, and from 43% to 73%, with an average of 56%. The contributions to SO₄2- and NO₃- in wet precipitation of SO₂ and NO₂ in the air scavenged below the clouds ranged from 5% to 17%, with an average of 10%, and from 5% to 19%, with an average of 15%, respectively. These results indicated that SO₄2- and NO₃- in wet precipitation were mainly from cloud water and SO₄2- and NO₃- in particulates scavenged below the clouds, and minor from SO₂ and NO₂ in the air scavenged below the clouds.