西安市大气降水污染和沉降特征及其来源解析

基于东亚酸沉降监测网(Acid Deposition Monitoring Network in East Asia,EANET)的湿沉降观测数据,分析了2000～2017年西安市大气降水化学特征、沉降特征以及潜在来源.结果表明,2000～2017年西安市降水pH和电导率的变化分别呈上升和下降趋势;大气降水中离子平均浓度大小依次为SO_4~(2-) Ca~(2+) NH_4~+ NO_3~- Na+ Cl- Mg~(2+) K+,其中SO_4~(2-)浓度占总离子的比值由2000年的38. 6%降低到2017年的27. 9%.近年来的SO_4~(2-)和NO_3~-的比值表明,西安市大气污染物排放类型有由燃煤型向混合型转变的趋势.近十几年来SO_4~(2-)的沉降量整体呈下降趋势,与Mann-Kendall检验所得结果基本一致.西安市大气降水中氮和硫的沉降量分别为16. 89 kg·(hm~2·a)~(-1)和33. 52 kg·(hm~2·a)~(-1),并且NH_4~+-N为西安市大气活性氮的主要沉降成分.机动车排放既是SO_4~(2-)的主要贡献者,又是NO_3~-的主要贡献者,对SO_4~(2-)和NO_3~-的贡献占比分别为15. 43%和72. 99%.对NH_4~+贡献最大的是农业源,占比达到75. 47%.

Pollution and Deposition Characteristics of Precipitation and Its Source Apportionment in Xi'an City

Chemical and deposition characteristics as well as potential sources of precipitation in Xi''an from 2000 to 2017 were analyzed based on the acid deposition monitoring network in East Asia (EANET). The pH of precipitation showed increasing trends, whereas the conductivity of precipitation showed decreasing trends between 2000 and 2017 in Xi''an. The decreasing order of ion concentration in precipitation was as follows:SO₄^2- > Ca²⁺ > NH₄⁺ > NO₃^- > Na⁺ > Cl^- > Mg²⁺ > K⁺. The percentage of SO₄^2- in the total ion concentration decreased from 38.6% in 2000 to 27.9% in 2017. The ratio of SO₄^2- to NO₃^- in recent years indicated that the type of pollutant in Xi''an changed from the coal-burning type to the mixed type. The deposition of SO₄^2- in precipitation had a declining trend, which was consistent with the results of the Mann-Kendall test. Nitrogen and sulphur depositions in precipitation were 16.89 kg·(hm²·a)^-1 and 33.52 kg·(hm²·a)^-1, respectively, and ammonium nitrogen (NH₄⁺-N) was the main deposition component of atmospheric active nitrogen in the precipitation of Xi''an. Vehicle emissions were the major contributors to SO₄^2- (15.43%) and NO₃^- (72.99%). NH₄⁺ originated mainly from agriculture sources, and the percentage reached up to 75.47%.