Abstract
Abstract. The rapid response to the COVID-19 pandemic led to unprecedented decreases in economic activities, thereby reducing the pollutant emissions. A random forest (RF) model was applied to determine the respective contributions of meteorology and anthropogenic emissions to the changes in air quality. The result suggested that the strict lockdown measures significantly decreased primary components such as Cr (−67 %) and Fe (−61 %) in PM2.5 (p<0.01), whereas the higher relative humidity (RH) and NH3 level and the lower air temperature (T) remarkably enhanced the production of secondary aerosol, including SO42- (29 %), NO3- (29 %), and NH4+ (21 %) (p<0.05). The positive matrix factorization (PMF) result suggested that the contribution ratios of secondary formation (SF), industrial process (IP), biomass burning (BB), coal combustion (CC), and road dust (RD) changed from 36 %, 27 %, 21 %, 12 %, and 4 % before the COVID-19 outbreak to 44 %, 20 %, 20 %, 9 %, and 7 %, respectively. The rapid increase in the contribution ratio derived from SF to PM2.5 implied that the intermittent haze events during the COVID-19 period were characterized by secondary aerosol pollution, which was mainly contributed by the unfavorable meteorological conditions and high NH3 level.
Highlights
In December 2019, a cluster of pneumonia cases with unknown etiology were first reported in Wuhan and quickly spread around the world (F. Wu et al, 2020)
We developed a random forest model to remove the impacts of meteorological conditions on air pollutants
The lockdown measures led to the shutdown of many industries, in turn resulting in significant decreases in primary components in PM2.5
Summary
In December 2019, a cluster of pneumonia cases with unknown etiology were first reported in Wuhan and quickly spread around the world (F. Wu et al, 2020). As a heavily industrialized region, the North China Plain (NCP) possesses many energy-intensive industries including coal-fired power plants, non-ferrous smelting industries, textiles, building materials, chemical engineering, and papermaking industries (Ren et al, 2011) Due to these intensive industrial emissions, the NCP suffered from poor air quality and frequent aerosol pollution in the past decades (Zhang et al, 2018; Luo et al, 2017). These strict lockdown measures during the COVID-19 period inevitably led to dramatic decreases in industrial emissions, and a study about the response of chemical compositions to emission reduction in the heavy-pollution city might be more sensible. The source apportionment was performed based on the meteorology-normalized datasets to compare the source difference for these pollutants before and after the COVID-19 lockdown
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