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PM2.5与臭氧复合污染共4次典型污染过程PM2.5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PM2.5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NOx的进一步减排是控制北京大气细颗粒的有效手段.
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 PM2.5 during four typical pollution episodes, namely haze pollution in winter, dust pollution in spring, high ozone pollution in summer, and PM2.5 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 PM2.5 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 PM2.5 was the lowest (22%). For PM2.5 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 NOx would be an effective means to control the fine particles in Beijing.
关键词
ISORROPIA-II模型 /
北京 /
典型污染过程 /
二次硝酸盐生成机制 /
气溶胶pH值
Key words
aerosol pH /
Beijing /
ISORROPIA-II model /
secondary nitrate formation mechanism /
typical pollution episodes
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基金
生态环境新型治理体系构建与示范应用项目(2021YFC1809004)
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