基于高分辨率MARGA分析桂林市PM2.5水溶性离子特征

利用高时间分辨率MARGA于2017年2月17日~3月24日在桂林市开展PM_2.5组分监测,结合同一点位环境和气象监测数据,分析桂林市大气PM_2.5水溶性无机离子组分特征及气溶胶酸性.结果表明：MARGA监测的PM_2.5中8种水溶性离子与PM_2.5变化趋势一致.8种水溶性离子总浓度均值29.27μg/m³,3种二次水溶性离子SO₄^2-、NO₃^-和NH₄⁺浓度均值26.91μg/m³,占水溶性离子总浓度的93.50%,是桂林市大气PM_2.5的主要组分.二次水溶性离子SO₄^2-、NH₄⁺和NO₃^-两两之间存在显著正相关性（相关系数均>0.80）,提示二次离子产生的机制及在大气中的演化、沉积具有一定的相似性.无论有无降雨,能见度（Vis）均随着水溶性离子,尤其是二次水溶性离子浓度的增加呈幂函数规律递减.24h累计降雨量≥ 10.0mm时,湿清除作用明显.晴天及降雨量不大的天气下,需注意管控机动车尾气、生物质燃烧和扬尘污染.SOR、NOR分别为0.35、0.12,SO₂同时通过均相和非均相氧化反应转化为SO₄^2-,NO_x主要是通过白天光化学反应转化为NO₃^-.大多数离子和气态前体物均存在明显的日变化规律,这与物质的来源、形成机制和气象条件不同有关.CE/AE摩尔浓度均值为1.5,桂林市PM_2.5总体偏碱性.PM_2.5中SO₄^2-、NO₃^-、Cl^-主要以（NH₄）₂SO₄、NH₄NO₃和NH₄Cl形式存在.PM_2.5中NH₄⁺可能与监测点位交通源排放有关,桂林市应加强交通污染物排放管控.

Analysis of characteristics of water-soluble ions in PM2.5 in Guilin based on the MARGA

The composition of PM_2.5 was serially on-line monitored by using Monitor for Aerosols and Gases in Ambient Air (MARGA ADI 2080) at atmosphere monitoring sites of Guilin in a time span of February 17 to March 24, 2017 continuously. Composition characteristics of water-soluble inorganic matter, secondary conversion mechanism and aerosol acidity were analyzed combining with environmental monitoring data and meteorological monitoring data. The results shown that variability of 8water-soluble ions concentration was in consistent with the trend of PM_2.5 varieties in Guilin. The average mass concentration of the total water-soluble ions was 29.27μg/m³, and the average mass concentration of three secondary water soluble ions SO₄^2-, NH₄⁺and NO₃^- was 26.91μg/m³, accounting for 93.50% of PM_2.5, indicating that secondary water-soluble ions were the major components of PM_2.5 in Guilin. There was a significant positive correlation between secondary water-soluble ions, namely SO₄^2-, NH₄⁺ and NO₃^- (all the correlation coefficients were over 0.80), indicating that the mechanism of evolution and deposition of secondary water-soluble ions in the atmosphere are similar to each other. Visibility declined in terms of power function with the increase of the water-soluble ions, especially secondary water-soluble ions despite if there was rain fell or not. When the rain fell significantly (accumulated rainfall of 24hours≥10mm), the effect of wet removal was obvious. In sunny days and days of light rainy, the focal point should be the control of vehicle exhaust, biomass burning and dust pollution. SOR and NOR were 0.35 and 0.12, respectively. SO₂ was converted to SO₄^2- by homogeneous and heterogeneous oxidation reactions at the same time. NO_x was mainly converted to NO₃^- by photochemical reactions during the day. A pronounced diurnal cycle was found for most ions and gaseous precursors which could be attributed to their respective sources, formation mechanisms and meteorological conditions. The mean value of CE/AE was 1.5, indicating that most aerosols in Guilin were relatively alkaline. In PM_2.5, SO₄^2-, NO₃^- and Cl^- mainly exist in the forms of (NH₄)₂SO₄, NH₄NO₃ and NH₄Cl, respectively. NH₄⁺ in PM_2.5 was related to local vehicle emissions. Regulatory actions in minimizing traffic emissions may represent the critical step in mitigating haze in Guilin.