北京市四季典型污染过程PM<sub>2.5</sub>酸度与二次硝酸盐形成机制

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摘要基于北京市城区点位2022年观测数据,运用ISORROPIA-II模型分析冬季霾、春季沙尘、夏季高臭氧与秋季PM_2.5与臭氧复合污染共4次典型污染过程PM_2.5的酸度特征,获得气溶胶pH值与硝酸盐快速增长的演变规律.结果表明,北京市气溶胶pH值为中度酸性,四次污染过程pH值范围分别为3.59~5.07,3.70~7.76,2.44~6.15和2.80~4.69. 4次污染过程气溶胶pH值呈正态分布,pH值中位数分别为4.60、4.59、3.91和4.09.冬季霾污染过程气溶胶水含量最高,其气溶胶pH值最大.春季沙尘污染过程气溶胶pH值呈双峰分布,受到人为源与天然源共同影响.夏、秋两次污染过程PM_2.5酸性分别为最强和次强,可能与大气氧化性增强促进酸性气体被氧化有关.夏季气温高气溶胶pH值低,HNO₃倾向于向气相分配,硝酸盐占比最低(22%);秋季气溶胶pH值昼低夜高,有利于硝酸盐的夜间积累,硝酸盐占比与冬、春两次污染过程相当(27%~28%).北京大气气态NH₃充分富余,HNO₃与NH₃的中和(均相)反应为NO₃^-的主要生成机制,气溶胶中NH₄⁺也相对富余,气溶胶中(NH₄)₂SO₄、NH₄NO₃和NH₄Cl均可以充分耦合.研究显示,较高的气溶胶水含量和气溶胶pH值是污染期间硝酸盐快速增长的原因,针对气态前体物NH₃和NO_x的进一步减排是控制北京大气细颗粒的有效手段.

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	王友峰
	景宽
	沈秀娥
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	王陈婧
	富佳明
	张博韬
	张健
	曹阳
	张珂
	刘保献

关键词 ：北京, 典型污染过程, ISORROPIA-II模型, 气溶胶pH值, 二次硝酸盐生成机制

Abstract：Based on the online monitoring data from urban sites in Beijing during 2022, the ISORROPIA-II model was used to analyze the acidity of PM_2.5 during four typical pollution episodes, namely haze pollution in winter, dust pollution in spring, high ozone pollution in summer, and PM_2.5 and O₃ combined pollution in autumn. The evolution of aerosol pH with the rapid formation of nitrate was obtained. The results show that the aerosol pH in Beijing was moderately acidic, in ranges of 3.59~5.07, 3.70~7.76, 2.44~6.15, and 2.80~4.69 for the four episodes, respectively. The aerosol pH exhibited normal distribution for the four pollution episodes with median values of 4.60, 4.59, 3.91 and 4.09, respectively. During winter haze pollution, aerosol water content and the aerosol pH were the highest. During spring dust pollution, the aerosol pH presented bimodal distribution, affected by both anthropogenic and natural sources. The acidity of PM_2.5 in summer and autumn episodes was the strongest and the second strongest respectively, which might be related to the oxidation of acid gas enhanced by strong atmospheric oxidation conditions. During high ozone pollution in summer, with high temperature and low pH, HNO₃ tended to be distributed in the gas phase, and the nitrate proportion in PM_2.5 was the lowest (22%). For PM_2.5 and O₃ combined pollution in autumn, pH was low in the daytime and high in the nighttime, conducive to the nitrate accumulation at night after the gaseous HNO₃ formation in the daytime. The nitrate proportion was comparable to that during pollution episodes in winter and spring, reaching 27%~28%. Gaseous NH₃ was abundant, and the neutralization (homogenization) reaction between HNO₃and NH₃ was the main formation mechanism of NO₃^-. Abundant of NH₄⁺, (NH₄)₂SO₄, NH₄NO₃ and NH₄Cl in aerosol could also be fully coupled. This research shows that high aerosol water content and aerosol pH were responsible for the rapid growth of nitrate during pollution. Further reduction of the precursors NH₃ and NO_x would be an effective means to control the fine particles in Beijing.

Key words： Beijing typical pollution episodes ISORROPIA-II model aerosol pH secondary nitrate formation mechanism

收稿日期: 2024-01-11

PACS:

X51

基金资助:生态环境新型治理体系构建与示范应用项目(2021YFC1809004)

作者简介: 王友峰(1988-),女,山东临沂人,高级工程师,博士,主要研究方向为大气污染成因分析.发表论文8篇.wyfandy@163.com.

引用本文:

王友峰, 景宽, 沈秀娥, 王琴, 王陈婧, 富佳明, 张博韬, 张健, 曹阳, 张珂, 刘保献. 北京市四季典型污染过程PM_2.5酸度与二次硝酸盐形成机制[J]. 中国环境科学, 2024, 44(8): 4167-4178. WANG You-feng, JING Kuan, SHEN Xiu-e, WANG Qin, WANG Chen-jing, FU Jia-ming, ZHANG Bo-tao, ZHANG Jian, CAO Yang, ZHANG Ke, LIU Bao-xian. PM_2.5 acidity and secondary nitrate formation during typical pollution episodes of four seasons in Beijing. CHINA ENVIRONMENTAL SCIENCECE, 2024, 44(8): 4167-4178.

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http://www.zghjkx.com.cn/CN/ 或 http://www.zghjkx.com.cn/CN/Y2024/V44/I8/4167

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