The Variation in Chemical Composition and Source Apportionment of PM[sub.2.5] before, during, and after COVID-19 Restrictions in Zhengzhou, China

Despite significant improvements in air quality during and after COVID-19 restrictions, haze continued to occur in Zhengzhou afterwards. This paper compares ionic compositions and sources of PM[sub.2.5] before (2019), during (2020), and after (2021) the restrictions to explore the reasons for the haze. The average concentration of PM[sub.2.5] decreased by 28.5% in 2020 and 27.9% in 2021, respectively, from 102.49 μg m[sup.−3] in 2019. The concentration of secondary inorganic aerosols (SIAs) was 51.87 μg m[sup.−3] in 2019, which decreased by 3.1% in 2020 and 12.8% in 2021. In contrast, the contributions of SIAs to PM[sub.2.5] increased from 50.61% (2019) to 68.6% (2020) and 61.2% (2021). SIAs contributed significantly to PM[sub.2.5] levels in 2020–2021. Despite a 22~62% decline in NO[sub.x] levels in 2020–2021, the increased O[sub.3] caused a similar NO[sub.3] [sup.−] concentration (20.69~23.00 μg m[sup.−3] ) in 2020–2021 to that (22.93 μg m[sup.−3] ) in 2019, hindering PM[sub.2.5] reduction in Zhengzhou. Six PM[sub.2.5] sources, including secondary inorganic aerosols, industrial emissions, coal combustion, biomass burning, soil dust, and traffic emissions, were identified by the positive matrix factorization model in 2019–2021. Compared to 2019, the reduction in PM[sub.2.5] from the secondary aerosol source in 2020 and 2021 was small, and the contribution of secondary aerosol to PM[sub.2.5] increased by 13.32% in 2020 and 12.94% in 2021. In comparison, the primary emissions, including biomass burning, traffic, and dust, were reduced by 29.71% in 2020 and 27.7% in 2021. The results indicated that the secondary production did not significantly contribute to the PM[sub.2.5] decrease during and after the COVID-19 restrictions. Therefore, it is essential to understand the formation of secondary aerosols under high O[sub.3] and low precursor gases to mitigate air pollution in the future.