Substantial changes of gaseous pollutants and chemical compositions in fine particles in North China Plain during COVID-19 lockdown period: anthropogenic vs meteorological influences

Abstract. The rapid response to COVID-19 pandemic led to the unprecedented decreases of 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 of air quality. The result suggested the strict lockdown measures significantly decreased primary components such as Cr (−201 %) and Fe (−154 %) in PM_2.5, whereas the higher relative humidity (RH) and NH₃ level, and the lower air temperature (T) enhanced the production of secondary aerosol including SO₄²⁻ (47.2 %), NO₃⁻ (38.6 %), and NH₄⁺ (22.7 %). 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 35.2 %, 28.9 %, 19.4 %, 11.8 %, and 4.75 % before COVID-19 outbreak to 42.7 %, 20.5 %, 19.45 %, 9.80 %, and 7.56 %, respectively. The rapid increase of the contribution ratio derived from SF to PM_2.5 implied the intermittent haze events during COVID-19 period were characterized with secondary aerosol pollution, which was mainly contributed by the unfavorable meteorological conditions and high NH₃ level.