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Change of chemical compositions of the regional free troposphere in North China during COVID-19: Observational evidence from the Mt. Tai observatory.

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Title: Change of chemical compositions of the regional free troposphere in North China during COVID-19: Observational evidence from the Mt. Tai observatory.
Authors: Sun, Yue1 (AUTHOR), Zhu, Yujiao1 (AUTHOR) zhuyujiao@sdu.edu.cn, Dong, Can1 (AUTHOR), Zhang, Yingnan1 (AUTHOR), Bi, Yujian2 (AUTHOR), Wu, Di2 (AUTHOR), Yin, Xiangkun2 (AUTHOR), Shen, Hengqing1 (AUTHOR), Chen, Tianshu1 (AUTHOR), Li, Hongyong1,3 (AUTHOR), Zhang, Yuqiang1 (AUTHOR), Li, Qinyi1 (AUTHOR), Wang, Wenxing1 (AUTHOR), Xue, Likun1 (AUTHOR) xuelikun@sdu.edu.cn
Source: Atmospheric Environment. Jan2026, Vol. 365, pN.PAG-N.PAG. 1p.
Subject Terms: *Air quality, *Troposphere, *Pollutants, *Atmospheric aerosols, *Particulate matter, SARS-CoV-2, COVID-19
Geographic Terms: China, North China Plain (China)
Abstract: To prevent the spread of the novel coronavirus 2019 disease (COVID-19), China adopted a series of rigorous containment measures, resulting in dramatic anthropogenic emission reductions, and thus exerted great influences on air quality. Ground-based measurements reported notable decrease in major air pollutants such as CO, NO x , SO 2 , PM 2.5 during the lockdown period, however, O 3 concentration increased markedly due to the alleviated NO-titration. However, the impact of these emission reductions on the regional air quality in the free troposphere remains largely unexplored. In this study, we investigated the influence of the emission reductions on air pollutant concentrations at the summit of Mt. Tai, which lies above the identifiable residual layer (RL) and planetary boundary layer (PBL) during wintertime and is therefore representative of the lower free troposphere over the central North China Plain (NCP). Our results showed that during the lockdown period, NO 2 , SO 2 , and CO concentrations decreased significantly by 37.1 %, 27.6 % and 23.4 %, respectively, compared to the pre-COVID period. O 3 and Ox concentrations remained largely unchanged, but PM 2.5 concentrations increased significantly by 21.9 %, diverging from most ground-level stations. Combined with the RACM-CAPRAM model simulations, the result suggests that the increased in PM 2.5 was mainly attributed to enhanced secondary aerosol formation, as indicated by enhanced formation of sulfate, nitrate, ammonium and organic aerosol component associated with aqueous-phase reactions. This enhancement was more pronounced during nighttime. These findings suggest that enhanced secondary aerosol formation during the lockdown period can offset primary emission reductions, contributing to the elevation of PM 2.5 in the free troposphere of the NCP. [Display omitted] • Primary air pollutants decreased from the ground surface to the free troposphere during COVID-19. • O 3 and O x levels at Mt. Tai remained stable, contrasting with sharp increases at ground-level sites. • PM 2.5 rose significantly at Mt. Tai, driven by enhanced secondary aerosol formation. [ABSTRACT FROM AUTHOR]
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Abstract:To prevent the spread of the novel coronavirus 2019 disease (COVID-19), China adopted a series of rigorous containment measures, resulting in dramatic anthropogenic emission reductions, and thus exerted great influences on air quality. Ground-based measurements reported notable decrease in major air pollutants such as CO, NO x , SO 2 , PM 2.5 during the lockdown period, however, O 3 concentration increased markedly due to the alleviated NO-titration. However, the impact of these emission reductions on the regional air quality in the free troposphere remains largely unexplored. In this study, we investigated the influence of the emission reductions on air pollutant concentrations at the summit of Mt. Tai, which lies above the identifiable residual layer (RL) and planetary boundary layer (PBL) during wintertime and is therefore representative of the lower free troposphere over the central North China Plain (NCP). Our results showed that during the lockdown period, NO 2 , SO 2 , and CO concentrations decreased significantly by 37.1 %, 27.6 % and 23.4 %, respectively, compared to the pre-COVID period. O 3 and Ox concentrations remained largely unchanged, but PM 2.5 concentrations increased significantly by 21.9 %, diverging from most ground-level stations. Combined with the RACM-CAPRAM model simulations, the result suggests that the increased in PM 2.5 was mainly attributed to enhanced secondary aerosol formation, as indicated by enhanced formation of sulfate, nitrate, ammonium and organic aerosol component associated with aqueous-phase reactions. This enhancement was more pronounced during nighttime. These findings suggest that enhanced secondary aerosol formation during the lockdown period can offset primary emission reductions, contributing to the elevation of PM 2.5 in the free troposphere of the NCP. [Display omitted] • Primary air pollutants decreased from the ground surface to the free troposphere during COVID-19. • O 3 and O x levels at Mt. Tai remained stable, contrasting with sharp increases at ground-level sites. • PM 2.5 rose significantly at Mt. Tai, driven by enhanced secondary aerosol formation. [ABSTRACT FROM AUTHOR]
ISSN:13522310
DOI:10.1016/j.atmosenv.2025.121671