太原市五年降尘及降水中主要离子特征

随着我国工业化和城镇化的深入推进,人们对城市大气环境质量关注日益增加。连续五年对山西省太原市5个点位进行监测,考察降水中主要离子时间变异及大气干湿沉降规律。结果表明:太原市五个监测点年均干沉降量排序为太钢工业区坞城(东)桃园(市中)晋祠(景区)上兰(市郊),其中太钢年均沉降量是上兰的三倍。降尘量除了在不同点位差异较大外,相同点上不同季节也有较大波动,表现为各监测点春季和冬季降尘量为全年最高。而夏季由于降雨较多,干沉降最少。总体上,近五年各点干沉降量呈现逐年下降的趋势。湿沉降方面以桃园为市区代表分析,SO_4~(2-)、NO_3~-、Ca~(2+)和NH_4~+是太原市桃园降水中主要的无机离子,其加权平均浓度范围为3.1～19 mg/L,各离子浓度在干燥的冬春季较高,且近五年其浓度及沉降量快速增加。桃园降雨中SO_4~(2-)和NO_3~-仍然是最主要的致酸离子,SO_4~(2-)年均沉降量分别是NO_3~-、Ca~(2+)和NH_4~+的1.7、3.9、5.9倍。Ca~(2+)、NH_4~+和Mg~(2+)的中和因子分别是0.42、0.30、0.14,说明Ca~(2+)、NH_4~+对酸雨缓冲作用大于Mg~(2+)。2013～2017年太原市桃园降雨SO_4~(2-)/NO_3~-和NH_4~+/NO_3~-比值分别为1.7和0.3,表明酸雨类型由硫酸型向混合型发展,主要是近年来NO_3~-浓度增幅大于SO_4~(2-)和NH_4~+。总的来看,近些年太原市政府大力提倡节能减排带来明显效果,但工业厂矿区周边降尘污染依然严重,属于今后城市环境治理的重点。

Characteristics of 5-year Atmospheric Dry Deposition and Annual Variations of Main Ions in Wet Precipitation in Taiyuan City

With the industrialization and urbanization in China, people pay more and more attention to the quality of urban atmospheric environment. The temporal variation of atmospheric wet and dry deposition and the main ions in precipitation were quantified using dry and wet deposition samples and rain gauge at five monitoring stations of Taiyuan city, Shanxi Province, in five-year period (from Jan 2013 to Dec 2017). Results showed the annual average deposition rate decreased in the order of:Tai steel industry area, Wucheng (east), Taoyuan (downtown), Jinci (scenic spot), Shanglan (surburb), and the annual dry deposition rate of Tai steel industry area averaged three times that of Shanglan. Dry deposition rate varied in five monitoring stations, and also varied in four seasons, with highest in spring and winter, and lowest in summer with rich rainfall. Moreover, there was an evident declining trend on dry deposition rate over the study period. It was found that SO₄^2-, NO₃^-, Ca²⁺, and NH₄⁺ were the dominant inorganic ions in the precipitation in urban area (Taoyuan monitoring station), with weighted average concentration varying from 3.1 to 19 mg/L, and higher values were observed in spring and winter. The concentration and deposition rate of these ions in the studied 5-year period increased substantially. Annual precipitation rate of SO₄^2- was 1.7, 3.9, and 5.9 times of that NO₃^-, Ca²⁺, and NH₄⁺, respectively. Neutralization factors (NF) indicated that Ca²⁺ and NH₄⁺ were the predominant neutralizers in rainwater samples, and these two ions had greater buffer effect on acid rain than Mg²⁺. The mean ratios of SO₄^2- to NO₃^- and NH₄⁺ to NO₃^- in precipitation during 2013-2017 in Taoyuan site were 1.7 and 0.3, respectively, indicating that the type of acid rain changed gradually from sulfuric to sulfuric-nitrous mixed type, due to the larger increase in NO₃^- than that of SO₄^2- and NH₄⁺. Overall, the Taiyuan municipal government has vigorously promoted energy conservation and emission reduction measures in recent years, which has brought obvious cleaning effect on urban environment, but the dust pollution around the industrial plant and mining area is still serious, which belongs to the focus of urban environmental governance in the future.