北京采暖季PM_(2.5)水溶性无机离子污染特征及其影响因素

The Pollution Characteristics and Influencing Factors of PM_(2.5) Inorganic Water-soluble Ions in Beijing during Heating Season

为深入研究北京市采暖季PM_(2.5)中水溶性离子的污染特征及其影响因素,利用大流量采样器结合石英滤膜采集了2016年11月15日—2016年12月31日期间北京市典型污染天的PM_(2.5)样品(19个),采用离子色谱法测定了其中的水溶性无机离子成分,收集了同期北京市的日均气象数据和海淀区日均PM_(2.5)数据。应用热力学平衡模型ISORROPIA-Ⅱ分析了PM_(2.5)样品的酸度值,Traj Stat软件分析气流的72 h后向轨迹,并采用潜在源贡献因子分析法(PSCF)定位了PM_(2.5)潜在污染源的位置,浓度权重轨迹分析(WCWT)法定量解析了潜在污染源对北京PM_(2.5)质量浓度贡献的大小。结果表明:(1)PM_(2.5)的日均质量浓度变化范围为7.6~383μg·m^(-3),均值为114μg·m^(-3),污染天是清洁天的4.4倍;(2)10种水溶性离子的总质量浓度均值为44.61μg·m^(-3),SNA(NO_3^-、SO_4^(2-)、NH_4^+)占总水溶性离子的81.37%,污染天NO_3^-、SO_4^(2-)、NH_4^+质量浓度均值分别为20.35、16.16、8.68μg·m^(-3),分别是清洁天的4.7、3.5、3.6倍;(3)污染天PM_(2.5)酸性比清洁天强,污染天NH_4^+的存在形式主要是(NH_4)_2SO_4、NH_4HSO_4,清洁天NH_4^+的存在形式主要是(NH_4)_2SO_4、NH_4HSO_4、NH_4NO_3;(4)北京PM_(2.5)及其水溶性离子的污染除受本地污染源影响,还受河北省中部和南部以及内蒙古中部等区域传输的影响;(5)在北京采暖季低大气边界层以及三面环山的特殊条件下,风速和相对湿度是影响北京PM_(2.5)及其水溶性离子污染特征的2个主要气象因素,高湿度低风速的静稳天气条件可以造成以本地污染物为主的大气重污染,此外,一定范围内的低风速可以使周边地区高浓度的污染物传输至北京,加重大气污染。

In order to further study the pollution characteristics and influence factors of PM2.5 inorganic water-soluble ions during heating season in Beijing, fine particle （PM2.5）, collected by large flow sampler combining quartz filtration membrane fromNovember 15, 2016 to December 31, 2016 in Beijing, was analyzed for water-soluble ions. Besides, the daily meteorological data and the average daily concentrations of PM2.5 in Haidian District were collected at the same time. Finaly, the acidity of PM2.5 samples was calculated by thermodynamic equilibrium model ISORROPIA-II. The possible sources of PM2.5 in Beijing were analyzed with the method of potential source contribution function （PSCF）. The contributions of different sources of PM2.5 in Beijing were analyzed quantitatively with the method of concentration weighted field （CWT）. Results indicated that： （1） The range of daily average mass concentration of PM2.5 in Beijing was 7.6~383 μg·m-3, and the average value was 114 μg·m-3, especialy daily average mass concentration of PM2.5 in pollution days was 4.4 times that in clean days. The average mass concentration of total inorganic water-soluble ions was 44.61 μg·m-3, with SNA （NO3-, SO42-, NH4＋） accounting for 81.82% of the total inorganic water-soluble ions. The daily average mass concentration of NO3-, SO42-, NH4＋ in pollution days were 20.35, 16.16, 8.68 μg·m-3, which were respectively 4.7, 3.5, 3.6 times that in clean days. The acidity of PM2.5 in pollution days was stronger than that in clean days. The forms of NH4＋ were mainly （NH4）2SO4 and NH4HSO4 in pollution days, while that were （NH4）2SO4, NH4HSO4, and NH4NO3 in clean days. The pollution of PM2.5 and its water soluble ions in Beijing was mainly affected by local pollutions, but also affected by the regional transmission surrounding area such as central and southern area of Hebei Province, central inner area of Mongolia district and so on. Under the conditions of low atmospheric boundary layer and the special terrains with the three sides of the mountain, wind speed and relative humidity were two main meteorological factors influencing PM2.5 and its water soluble ions; high humidity and low wind speed could cause severe air pollution because of local pollutants. In addition, wind within a certain speed range could make the high concentration of pollutants in the surrounding area arrive in Beijing, which affected severe air pollution.