Ozone pollution mitigation in Guangxi (South China) driven by meteorology and anthropogenic emissions during the COVID-19 lockdown ^☆

Abstract

With the implementation of COVID-19 restrictions and consequent improvement in air quality due to the nationwide lockdown, ozone (O ₃) pollution was generally amplified in China. However, the O ₃ levels throughout the Guangxi region of South China showed a clear downward trend during the lockdown. To better understand this unusual phenomenon, we investigated the characteristics of conventional pollutants, the influence of meteorological and anthropogenic factors quantified by a multiple linear regression (MLR) model, and the impact of local sources and long-range transport based on a continuous emission monitoring system (CEMS) and the HYSPLIT model. Results show that in Guangxi, the conventional pollutants generally declined during the COVID-19 lockdown period (January 24 to February 9, 2020) compared with their concentrations during 2016–2019, while O ₃ gradually increased during the resumption (10 February to April 2020) and full operation periods (May and June 2020). Focusing on Beihai, a typical Guangxi region city, the correlations between the daily O ₃ concentrations and six meteorological parameters (wind speed, visibility, temperature, humidity, precipitation, and atmospheric pressure) and their corresponding regression coefficients indicate that meteorological conditions were generally conducive to O ₃ pollution mitigation during the lockdown. A 7.84 μg/m ³ drop in O ₃ concentration was driven by meteorology, with other decreases (4.11 μg/m ³) explained by reduced anthropogenic emissions of O ₃ precursors. Taken together, the lower NO ₂/SO ₂ ratios (1.25–2.33) and consistencies between real-time monitored primary emissions and ambient concentrations suggest that, with the closure of small-scale industries, residual industrial emissions have become dominant contributors to local primary pollutants. Backward trajectory cluster analyses show that the slump of O ₃ concentrations in Southern Guangxi could be partly attributed to clean air mass transfer (24–58%) from the South China Sea. Overall, the synergistic effects of the COVID-19 lockdown and meteorological factors intensified O ₃ reduction in the Guangxi region of South China.

Graphical abstract

Highlights

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  Overall air quality improvement was observed as a side-product of Guangxi lockdown.

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  O ₃ change during COVID-19 lockdown in South China exhibited a clear regional trend.

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  Meteorology-driven O ₃ decreased by 7.84 μg/m ³ under lockdown in Beihai city.

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  Synergistic effects of lockdown and meteorology intensified O ₃ mitigation in Guangxi.

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  O ₃ slump in Southern Guangxi was affected by clean onshore air masses (24–58%).