Haze caused by NOx oxidation under restricted residential and industrial activities in a mega city in the south of North China Plain. (October 2022)

• Record Type: Journal Article
• Authors: Ma, Qingxia; Wang, Weisi; Wu, Yunfei; Wang, Fang; Jin, Liyuan; Song, Xiaoyan; Han, Yan; Zhang, Renjian; Zhang, Daizhou
• Is Part Of: Chemosphere. Volume 305(2022)
• Publication Date: 2022-10
• Subjects: Haze -- NOx oxidation -- Urban pollution -- Reduced emission -- O3 chemistry; Pollution -- Periodicals; Pollution -- Physiological effect -- Periodicals; Environmental sciences -- Periodicals; Atmospheric chemistry -- Periodicals; 551.511
• DOI: 10.1016/j.chemosphere.2022.135489
• Languages: English
• ISSNs: 0045-6535

Abstract

Abstract: The formation of secondary aerosol species, including nitrate and sulfate, induces severe haze in the North China Plain. However, despite substantial reductions in anthropogenic pollutants due to severe restriction of residential and industrial activities in 2020 to stop the spread of COVID-19, haze still formed in Zhengzhou. We compared ionic compositions of PM2.5 during the period of the restriction with that immediately before the restriction and in the comparison period in 2019 to investigate the processes that caused the haze. The average concentration of PM2.5 was 83.9 μg m −3 in the restriction period, 241.8 μg m −3 before the restriction, and 94.0 μg m −3 in 2019. Nitrate was the largest contributor to the PM2.5 in all periods, with an average mass fraction of 24%–30%. The average molar concentration of total nitrogen compounds (NO x + nitrate) was 0.89 μmol m −3 in the restriction period, which was much lower than that in the non-restriction periods (1.85–2.74 μmol m −3 ) . In contrast, the concentration of sulfur compounds (SO2 + sulfate) was 0.34–0.39 μmol m −3 in all periods. The conversion rate of NO x to nitrate (NOR) was 0.35 in the restriction period, significantly higher than that before the restriction (0.26) and in 2019 (0.25). NOR was higher with relative humidity in 40–80% in the restriction period than in the other two periods, whereas the conversion rate of SO2 to sulfate did not, indicating nitrate formation was more efficient during the restriction. When O3 occupied more than half of the oxidants (O x = O3 + NO2 ), NOR increased rapidly with the ratio of O3 to O x and was much higher in the daytime than nighttime. Therefore, haze in the restriction period was caused by increased NO x -to-nitrate conversion driven by photochemical reactions. Graphical abstract: Image 1 Highlights: Haze formed during the COVID-19 restriction period despite reductions in air pollutants. Increased NO x oxidation caused by enhanced photochemical reactions drove haze occurrence. High O3 concentration and relative humidity increased nitrate formation under low NO x .