2022年北京市城区PM2.5水溶性离子含量及其变化特征

为探究北京市大气细颗粒物（PM_2.5）水溶性离子含量及其变化特征,有针对性地提出污染防治方案,对2022年全年PM_2.5水溶性离子、气态前体物（SO₂、NO₂）和气象因素（温度、RH）进行分析测定.结果表明,北京市城区PM_2.5中占比最高的水溶性离子为NO₃^-、NH₄⁺和SO₄^2-,占PM_2.5的52.7%,ρ（PM_2.5）（33.2 μg·m^-3）和ρ（SNA）（18.9 μg·m^-3）低于历史研究结果,但SNA占比（52.7%）、SOR（0.45）和NOR（0.15）高于历史研究结果,体现出北京市细颗粒物污染得到明显改善,但仍具有较强的二次污染特征.NO₃^-/SO₄^2-为2.2,高于历史及附近省市研究结果,反映出移动源的影响不断扩大.从季节变化上看,PM_2.5呈现秋高夏低的变化特征,秋、春、冬这3个季节NO₃^-的占比最高,夏季SO₄^2-占比最高,而NH₄⁺在各季节占比变化不大.NOR与SOR的季节变化规律几乎相反,反映出二者的转化形成因素存在差异.北京城区SNA的主要存在形式为NH₄NO₃和（NH₄）₂SO₄,其中冬季阴阳离子中和度最高,夏季阳离子NH₄⁺稍显不足,而春秋两季NH₄⁺处于过量状态,北京城区为富氨环境.从污染级别看,水溶性离子质量浓度均随污染加重有不同程度的增长,增长最快的是SNA,其在PM_2.5中占比出现先上升后稳定的变化特征.从空间分布特征来看,中心城区和东南西北部郊区的SNA质量浓度大小均为：NO₃^->SO₄^2->NH₄⁺,体现了以NO₃^-为主导的污染特征；SNA对PM_2.5的贡献率最高的区域发生在东部、中心城区和传输点,表明在中心城区和东部地区二次反应相对活跃,同时区域传输也是二次离子的重要来源.

Water-soluble Inorganic Ion Content of PM2.5 and Its Change Characteristics in Urban Area of Beijing in 2022

To explore the content and variation characteristics of water-soluble ions of atmospheric fine particles (PM_2.5) in a Beijing urban area and put forward the pollution prevention and control scheme, the water-soluble ions, gaseous precursors (SO₂, NO₂), and meteorological factors (temperature, RH) of PM_2.5 in 2022 were analyzed and determined. The results showed that the water-soluble ions with the highest proportion in PM_2.5 in the Beijing City urban area were NO₃^-, NH₄⁺, and SO₄^2-, accounting for 52.7% of PM_2.5. The mass concentrations of PM_2.5 and SNA were lower than the historical results, whereas the proportion of SNA, SOR, and NOR was higher than the historical results. This showed that the fine particulate matter pollution in Beijing has been significantly improved, but it still has strong secondary pollution characteristics. NO₃^-/SO₄^2-(2.2) was higher than those of historical and nearby provinces and cities, reflecting the expanding influence of mobile sources. In terms of seasonal variation, PM_2.5 showed the characteristic of high in autumn and low in summer. The proportion of NO₃^- was the highest in autumn, spring, and winter; the proportion of SO₄^2- was the highest in summer; and the proportion of NH₄⁺ changed little in each season. The seasonal variation rules of NOR and SOR were almost opposite, which reflected the difference in transformation factors between NOR and SOR. The main forms of SNA in the Beijing urban area were NH₄NO₃ and (NH₄)₂SO₄. The neutralization degree of cations and anions was the highest in winter, the cation NH₄⁺ was slightly insufficient in summer, and NH₄⁺ was in excess in spring and autumn. The Beijing urban area was an ammonia-rich environment. In terms of pollution level, RH, particulate matter moisture, and water-soluble ions mass concentration all increased with the increase in pollution level, and SNA increased fastest, with its proportion in PM_2.5 increasing first and then stabilizing, whereas the contribution rate of other water-soluble ions decreased gradually. In terms of spatial distribution, the mass concentration relationship of SNA at the central urban area and suburbs was NO₃^- > SO₄^2- > NH₄⁺, which reflected the pollution characteristics dominated by NO₃^-. The highest contribution rate of SNA to PM_2.5 occurred in the eastern region, the central urban area, and the transmission point, indicating that the secondary reaction was relatively active in the central urban area and the eastern region, and the regional transport was also an important source of secondary ions.