北京市冬季重污染期间PM2.5及其组分演变特征

通过实时在线监测了2018年11月27日~2019年1月15日北京市城区PM_2.5、水溶性无机离子（Na⁺、NH₄⁺、K⁺、Mg²⁺、Ca²⁺、F^-、Cl^-、NO₂^-、NO₃^-、SO₄^2-、PO₄^3-）、碳质组分（有机碳OC、元素碳EC）的质量浓度以及气态污染物浓度和气象要素，收集整理了近20年北京市冬季PM_2.5、主要离子组分以及碳质组分浓度，分析研究了1999~2018年北京市冬季PM_2.5、离子、碳质组分的变化特征，重点探讨了监测期间清洁日与两个典型重污染事件PM_2.5及其组分的演变特征.结果表明：研究期间PM_2.5浓度为53.5μg/m³，达到近20年北京市冬季较低值，且大气主要污染源由煤烟型污染源转变为燃煤型与机动车尾气复合型污染源.监测期间，湿度高、微弱的西南风导致重污染产生，清洁日、污染事件I与污染事件II PM_2.5平均浓度分别为32.5，138.9，146.8μg/m³且不同时段PM_2.5日变化趋势存在差异.各离子浓度变化为：NO₃^- > NH₄⁺ > SO₄^2- > Cl^- > K⁺ > Ca²⁺ > Na⁺ > PO₄^3- > F^- > NO₂^-~Mg²⁺，总水溶性离子浓度为24.6μg/m³占PM_2.5总浓度的46.0%，其中SNA浓度占总离子浓度的83.7%，是离子中最主要的组分.碳质组分浓度达到近二十年北京市冬季最低值，变化为：一次有机碳POC > EC > 二次有机碳SOC，OC与EC相关系数达到0.99，一次燃烧源对污染过程有较大贡献.NH₄⁺在清洁日与污染II中富集，主要以（NH₄）₂SO₄、NH₄NO₃和NH₄Cl形式存在，在污染I中较少，仅以（NH₄）₂SO₄和NH₄NO₃存在.在污染I和II期间，SO₄^2-的形成昼夜均受相对湿度与NH₃影响；NO₃^-的形成白天受O₃与NH₃的影响，夜间受相对湿度和NH₃的影响.

Evolution of PM2.5 and its components during heavy pollution episodes in winter in Beijing

In this study, the mass concentrations of PM_2.5, water-soluble inorganic ions (Na⁺, NH₄⁺, K⁺, Mg²⁺, Ca²⁺, F^-, Cl^-, NO₂^-, NO₃^-, SO₄^2-, PO₄^3-), carbonaceous components (organic carbon OC, element carbon EC) and gaseous pollutants, and meteorological elements were monitored in real time in Beijing urban area from November 27, 2018 to January 15, 2019. At the same time, the concentrations of PM_2.5, its major water-soluble ion components and carbonaceous components in winters from 1999 to 2018 in Beijing, were collected to analyze their variation characteristics. The evolutions of PM_2.5 and its components in the clean days and two typical heavy pollution processes during the monitoring period were mainly discussed. The results showed that the average mass concentration of PM_2.5 during the study period was 53.5μg/m³, almost lower than those in winters in Beijing since 1999, and the main atmospheric pollution sources displayed a significant change from traditional soot-type pollution source to both coal-fired and motor vehicle exhaust-type pollution sources. High relative humidity, and weak southwest wind were favorable for the formation of heavy pollution. The average concentrations of PM_2.5 during the clean days, pollution episode I and II were 32.5, 138.9, and 146.8μg/m³, respectively. The diurnal profiles of PM_2.5 were different in the different periods. The ion concentrations presented the order of NO₃^- > NH₄⁺ > SO₄^2- > Cl^- > K⁺ > Ca²⁺ > Na⁺ > PO₄^3- > F^- > NO₂^-~Mg²⁺, and their mean total concentration was 24.6μg/m³, accounting for 46.0% of PM_2.5, of which SNA was the most important component, accounting for 83.7% of the total water-soluble ions. The concentrations of carbonaceous composition reached the lowest value in winters in Beijing since 1999, and followed the order of primary organic carbon POC > EC > secondary organic carbon SOC, and the correlation coefficient between OC and EC was 0.99, indicating that the primary combustion source had a greater contribution to atmospheric pollution. NH₄⁺ was enriched in PM_2.5 during the clean days and pollution II, and existed mainly in the form of (NH₄)₂SO₄, NH₄NO₃ and NH₄Cl, but lower in pollution I, and existed only as (NH₄)₂SO₄ and NH₄NO₃. During the periods of pollution I and II, the formation of SO₄^2- was mainly affected by relative humidity and NH₃during the daytime and nighttime, however, the formation of NO₃^- was mainly affected by O₃ and NH₃ during the daytime_e, and by relative humidity and NH₃ during the nighttime.