北京市典型区域降水特性及其对细颗粒物影响

通过采集北京市典型区域2016年12月~2017年11月期间大气降水样品，实时监测降水前、期间和后大气非难熔亚微米颗粒物NR-PM₁及其组分浓度，研究了降水的理化特性、典型降水过程离子组分变化特征、以及对大气非难熔亚微米颗粒物NR-PM₁及其组分的影响，同时采用后向轨迹聚类分析法研究了气团长距离传输对降水组分的影响.结果表明，2017年北京市典型区域降水主要集中在夏季，约占总降水量的82.2%，降水主要呈中性或碱性，酸雨发生率很低.降水pH值表现为冬季 > 春季 > 夏季~秋季的季节变化特征.降水中总离子浓度、总阴、阳离子浓度均表现为春季 > 夏季 > 冬季 > 秋季，且呈污染日显著高于清洁日的变化特征.降水中主要水溶性离子年均浓度表现为NH₄⁺ > Ca²⁺ > NO₃^- > SO₄^2- > Na⁺ > Cl^- > Mg²⁺ > F^- > K⁺，其中NH₄⁺、Ca²⁺、NO₃^-和SO₄^2-是降水中最主要的离子组分，占总离子浓度的80%以上，各离子浓度均呈污染日高于清洁日的变化特征.降水期间不同时段，降水中各离子浓度大多表现为：降水初期最高，降水中期显著低于降水初期，降水后期均略有增加.降雨量和降雨速率较小的降水对污染日大气NR-PM₁及其组分的清除作用较强，而降雨量和降雨速率较大的降水对清洁日NR-PM₁及其组分的清除作用较小.值得关注的是在降水不同时段，始终存在2个重要的过程，即污染物的累积和二次污染物的形成过程，以及降水的云下冲刷和云内雨除过程.研究期间，降水主要受到东南和西南方向气团影响，分别约占总降水的60%和23%，且主要发生在夏季，这些气团对降水中离子组分都有不同程度的影响.

The characteristics of precipitation and its impact on fine particles at a representative region in Beijing

A campaign of sampling atmospheric precipitation and real-time measurements of atmospheric non-refractory submicron particulate matter NR-PM₁ and its component concentrations before, during and after precipitation at a representative region in Beijing from December 2016 to November 2017 was performed to investigate the physical and chemical characteristics of precipitation, changes of major ion components in rainwater during the course of typical precipitation events, and impacts of typical precipitation processes on atmospheric non-refractory submicron particulate matter NR-PM₁ and its components. Backward trajectory clustering analysis was deployed to study the effects of air mass through long-range transport on main ion components in rainwater. The results showed that the precipitation at the representative region of Beijing in 2017 mainly occurred in summer, accounting for 82.2% of the total rainfall amount. The precipitation was mainly neutral or alkaline, accounting for 86.7% of the total precipitation, and the frequency of acid rain was very low. The seasonal variation of pH values in the rainwaters was characterized by winter > spring > summer~autumn. The seasonal variations of the total ion concentrations, total anion and cation concentrations in the precipitations, which in polluted days were significantly higher than those in clean days, followed the order of spring > summer > winter > autumn. The annual average concentrations of water-soluble ions in the rainwaters were characterized by NH₄⁺ > Ca²⁺ > NO₃^- > SO₄^2- > Na⁺ > Cl^- > Mg²⁺ > F^- > K⁺, among which NH₄⁺, Ca²⁺, NO₃^- and SO₄^2- were the most important components in the rainwaters, which accounted for about more than 80% of the total ion concentration. The concentrations of all ions in pollution days were higher than those in clean days. During the course of the rainfall with three stages such as initial, middle and latter stage, the ion concentrations in the initial stage were mostly highest in all stages of the rainwater, and were much lower in the middle stage than those in the initial phase, however, were a little higher in the latter stage than those in the latter middle stage. The precipitation with smaller rainfall and rainfall rate had stronger scavenging effects on atmospheric NR-PM₁ and its components in pollution days, while precipitation with larger rainfall and rainfall rate showed weak scavenging effects in clean days. It is noted that there were always two important processes at each stage of rainfall, with accumulation of pollutants and formation of secondary pollutants, as well as the washout and rainout of precipitation. During this study, the precipitation in the typical area was mainly affected by air masses from the southeast of Beijing, accounting for 60% of the total precipitation, and the southwest, accounting for 23%, which mainly occurred in summer. These air masses had different effects on the ion components in precipitation.