Characterizing the ratio of nitrate to sulfate in ambient fine particles of urban Beijing during 2018-2019
Characterizing the ratio of nitrate to sulfate in ambient fine particles of urban Beijing during 2018-2019
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2018-2019年北京城区环境细颗粒物硝酸盐与硫酸盐比例特征
DOI：
10.1016/j.atmosenv.2020.117662
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发表时间：
2020
影响因子：
5
通讯作者：
Li Zhanqing
中科院分区：
文献类型：
--
作者：
Li Shangze;Zhang Fang;Jin Xiaoai;Sun Yele;Wu Hao;Xie Conghui;Chen Lu;Liu Jieyao;Wu Tong;Jiang Sihui;Cribb Maureen;Li Zhanqing
自2013年以来，中国开展了多项改善空气质量的立法行动，排放控制措施显著降低了严重雾霾的发生频率。随着空气质量的改善，细颗粒物的成分可能会沿着发生变化，从而可能在未来产生明显的环境和气候效应。在这项研究中，使用四极杆气溶胶化学形态监测仪（Q-ACSM）在北京市区观测的非难处理化学成分的最近长期数据集（2018-2019），研究了PM2.5（直径小于2.5 μm的颗粒物）中硝酸盐（NO3−）与硫酸盐（SO 42 −）比例的变化。结果表明，NO3-与SO 42-的比值随季节变化，冬季最大（1.6 ± 1.2），夏季最小（0.7 ± 1.0）。与早期研究的结果相比，自1999年以来NO3−与SO 42 −的比例持续增加，2018-2019年期间该比例有所下降。这部分与硝酸盐形成减弱有关，可能是由于自2016年以来中国氮氧化物排放量减少。我们的研究结果表明，严格的减排控制措施只会改善冬季的SO 42 −污染，而不会改善夏季的SO 42 −污染。研究期间SO 42 −和NO3−的浓度占PM2.5的37-53%，在PM2.5的水平中起着重要作用。此外，我们发现，在温暖的季节，NO3−/SO 42 −的比例通常随着相对湿度（RH）的增加而增加，这是由于增强NO3−水解形成，但最大值仅为~1.0，这是由夏季高水平的硫酸盐拉低的，而在寒冷的季节，当多种因素（区域输送、边界层、边界层、硫酸盐来源等）会影响硝酸盐含量最后，我们描述了导致北京城市大气中硝酸盐快速积累的两个典型过程：区域输送和PBL变化，发现在寒冷季节驱动重霾，以及NO3-的水解形成和分配，这往往会影响硝酸盐的日变化模式。
A variety of legislative actions for air quality improvement have been conducted in China since 2013, and the emission control measures have achieved remarkable reduction in severe haze frequency. The composition of the fine particles may change along with the improved air quality, and thus may induce distinct environmental and climate effects in future. In this study, a very recent long-term dataset (2018–2019) of non-refractory chemical composition measured by a quadrupole aerosol chemical speciation monitor (Q-ACSM) observed in urban Beijing is applied to investigate the changes in ratio of nitrate (NO3−) to sulfate (SO42−) in PM2.5(particulate matter with diameters of less than 2.5 μm). We show that the ratio of NO3−to SO42−varies seasonally, with a maximum in winter (1.6 ± 1.2) and a minimum in summer (0.7 ± 1.0). Compared with results from earlier studies showing a continuous increase in the ratio of NO3−to SO42−since 1999, a decline in the ratio is found during the period of 2018–2019. This is partially associated with an attenuated nitrate formation likely due to reduced nitrogen oxides emissions since 2016 in China. Our results suggest that the strict reduction control measures in place serve only to improve SO42−pollution in winter but not in summer when high SO42−levels are still observed. SO42−and NO3−concentrations during study periods together comprise 37–53% of PM2.5, presenting significant role in dominating the levels of PM2.5. In addition, we show that the ratio of NO3−/SO42−in warm seasons generally increases with increasing relative humidity (RH) due to enhanced NO3−hydrolysis formation, but with a maximum value of only ~1.0 that is pulled down by the high levels of sulfate in summer, while the dependence of the ratios on RH is more pronounced in cold seasons when multiple factors (regional transportation, planetary boundary layer, PBL and sources of sulfate, etc.) can impact nitrate levels. We finally characterize two typical processes that lead to the rapid accumulation of nitrate in the atmosphere over urban Beijing: the regional transportation and PBL variations, which is found driving heavy haze in cold seasons, and the hydrolysis formation and partitioning of NO3−that tends to impact the diurnal patterns of nitrate in warm seasons.