Variability of depolarization of aerosol particles in the megacity of Beijing: implications for the interaction between anthropogenic pollutants and mineral dust particles
Variability of depolarization of aerosol particles in the megacity of Beijing: implications for the interaction between anthropogenic pollutants and mineral dust particles
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DOI：
10.5194/acp-18-18203-2018
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发表时间：
2018-12
影响因子：
6.3
通讯作者：
Yu Tian;Xiaole Pan;T. Nishizawa;H. Kobayashi;I. Uno;Xiquan Wang;A. Shimizu;Zifa Wang
中科院分区：
文献类型：
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作者：
Yu Tian;Xiaole Pan;T. Nishizawa;H. Kobayashi;I. Uno;Xiquan Wang;A. Shimizu;Zifa Wang
抽象的。由于大量人为排放和天然矿物粉尘气溶胶，东亚正遭受严重的空气污染问题。在输送过程中，气溶胶粒子经历复杂的混合过程，对区域空气质量、人类健康和气候产生重大影响。在这项研究中，我们进行了长期的观察，使用光学粒子计数器配备了偏振检测模块（POPC）在北京的城市站点。从POPC估算的PM2.5和PM10的质量浓度与地面测量结果相比很好。结果表明，由于不同气溶胶类型的优势，夏季观测到的细模式气溶胶粒子的退偏振比（δ，s偏振信号强度与120 μ m后向散射信号强度之比[s/（s+p）]）普遍比春季低得多。粗粒型矿物粉尘颗粒由于其非球形结构，其δ值通常较大（0.3±0.05），而细粒型矿物粉尘颗粒主要含有水溶性成分，在高相对湿度（RH）条件下，其δ值明显降低。由于春季观测点受到频繁沙尘事件的影响，1 µm颗粒物的δ值几乎是夏季的2倍（0.07±0.01）。基于粒度分辨的δ值，可以很好地区分人为污染物、矿物尘和污染矿物尘颗粒物及其对当地空气质量的贡献。北京冬春季约有26.7%的不达标天数（PM2.5日平均浓度大于75 µg m−3）的大气粗模颗粒物负荷较高。特别是在冬季的严重污染事件中，粗模粒子的δ值下降了13%，这意味着在污染形成中与灰尘相关的非均匀过程的可能性很高。在沙尘天气过程中，光学粒径（Dp）为5 µm的颗粒物δ值明显降低，PM2.5 scinPM 10比值和相对湿度增大，表明矿物尘的形态发生了变化。研究证实，高相对湿度有利于促进沙尘表面的水分吸收过程和可溶性化合物的覆盖，并提出由于城市地区沙尘和人为颗粒物的复杂混合，气溶胶遥感技术可能低估了沙尘颗粒物的影响，光学模式应充分考虑沙尘颗粒物和污染物之间的相互作用。
Abstract. East Asia is suffering from severe air pollution problems due to intensive anthropogenic emissions and natural mineral dust aerosols. During transport, the aerosol particles undergo complex mixing processes, resulting in great impacts on regional air quality, human health and climate. In this study, we conducted a long-term observation using an optical particle counter equipped with a polarization detection module (POPC) at an urban site in Beijing. Mass concentrations of both PM2.5 and PM10 estimated from POPC compared well with ground-based measurements. The results revealed that the observed depolarization ratio (δ, termed as the ratio of the intensity of the s-polarized signal to the intensity of the 120∘ backward scattering signal [s/(s+p)]) for aerosol particles in the fine mode was generally much lower in summer than that in spring as a result of predominance of different aerosol types. Mineral dust particles in the coarse mode normally had a large δ value (0.3±0.05) owing to their nonspherical shape; however, particles in the fine mode mostly had water-soluble compositions, which led to an apparent decrease of their δ values in particular high relative humidity (RH) conditions. Because the observation site was subject to the impact of frequent dust events in spring, the δ value of particle at 1 µm was almost twice as high as that (0.07±0.01) in summer. Based on size-resolved δ values, anthropogenic pollutants, mineral dust and polluted mineral dust particles and their contribution to local air quality could be well distinguished. About 26.7 % of substandard days (daily averaged PM2.5 concentration larger than 75 µg m−3) in Beijing featured high atmospheric loading of coarse-mode particles in winter and springtime. In particular, during severe pollution episodes in winter, the δ values of coarse-mode particles decreased by 13 %, which implies a high possibility of dust-related heterogeneous processes in pollution formation. During dust events, δ values of particles with optical size (Dp) of 5 µm evidently decreased, with an increase of the PM2.5 ∕ PM10 ratio as well as RH, indicating the morphological changes of mineral dust. This study confirmed that high RH tends to promote water absorption processes on the dust surface as well as the coating of soluble compounds, and suggested that remote sensing techniques for aerosols may underestimate the impact of dust particles due to the complex mixing of dust and anthropogenic particles in urban areas, and the interaction between dust particles and pollutants should be considered well by the optical model.