南昌前湖区域夏季降水中低分子有机酸的分布与来源

Distribution and source of low-molecular-weight organic acids in summer precipitation in Qianhu,Nanchang

2020年5—9月,共采集南昌前湖区域20个降水事件的88个分段降水样品,测定降水中3种低分子有机酸(甲酸、乙酸、草酸)和4种无机阴离子(Cl^(-)、NO_(2)^(-)、NO_(3)^(-)、SO_(4)^(2-))浓度,分析讨论降水有机酸的分布、来源,定量解析云下冲刷、云水对降水有机酸的贡献.结果表明,降水中甲酸、乙酸、草酸占所测定阴离子总量的16%,降水有机酸与无机酸总量的月变化呈相反趋势;长降水事件的降水有机酸浓度在降水进程中呈现先逐渐降低,到降雨末期趋于平稳或稍稍反升的变化特征;降水进程中,云下冲刷对降水中3种有机酸的贡献率逐渐减小,而云水对其贡献率逐渐增大,降雨前期,云下冲刷为降水中有机酸根的主要来源,降雨后期,以云水贡献为主;前期降水中3种有机酸两两之间的相关性比末期降水中的弱,降水中草酸与SO_(4)^(2-)的相关性较甲、乙酸与SO_(4)^(2-)的相关性强,反映降水中草酸受二次污染影响大;基于甲酸/乙酸比值(F/A)分析,南昌地区5、9月降水有机酸受二次有机物影响较大,6—8月降水有机酸主要来源于植物直接排放;因子分析结果表明降水有机酸受酸性气溶胶、燃烧源、植物排放源的影响;PMF分析结果表明燃烧源、二次颗粒的冲刷和植被排放源对降水有机酸的影响.

88 segmented precipitation samples of 20 precipitation events in Qianhu area of Nanchang were collected during the period from May to September in 2020.Through analyzing the concentration of three low-molecular-weight organic acids(formic acid,acetic acid,oxalic acid)and four inorganic anions(Cl^(-),NO_(2)^(-),NO_(3)^(-),SO_(4)^(2-))and the distribution of the precipitated organic acids in the segmented samples,the contribution of cloud water and below-cloud scavenging to organic acids in precipitation was quantitatively revealed.The results indicated that formic,acetic and oxalic acids accounted for 16%of the total precipitation anions and the monthly variation of the organic and inorganic acids in precipitation showed an opposite trend.The concentration of organic acids in precipitation from long-term precipitation events first decreased and then stabilized or slightly increased at the end of the precipitation process.During the precipitation,the below-cloud scavenging showed a growing contribution to the three organic acids,contrasting to the case of cloud water(a decreased contribution).For organic acids in precipitation,below-cloud scavenging was the main source at the beginning of precipitation,while cloud water played an important role at the later stage of precipitation.Meanwhile,the correlations among the three organic acids at the beginning of precipitation were weaker than those derived from the end of precipitation.Compared to formic acid and acetic acid,oxalic acid showed a stronger dependence on SO_(4)^(2-)in precipitation,implying that the presence of oxalic acid in precipitation was significantly influenced by secondary pollution.Based on the formic acid to acetic acid ratio(F/A),it was found that the organic acids in precipitation were primarily caused by secondary organic matter in May and September and by direct plant emissions from June to August in Nanchang City.Finally,factor analysis results demonstrated that the organic acids in precipitation were derived from acid aerosol washed,combustion source and plant emission source.The influence of combustion source,secondary aerosol washed and plant emission source on the organic acids was further verified by PMF analysis results.