Unexpected Increases of Severe Haze Pollution During the Post COVID-19 Period: Effects of Emissions, Meteorology, and Secondary Production

Unexpectedly frequent severe haze episodes were observed in Beijing during February-March in 2021 after two phases of clean air action plan (2013-2020), yet the causes remained unclear. Here, we conducted real-time fine particle (PM2.5) composition measurements during January-March in 2021 using a time-of-flight aerosol chemical speciation monitor and an aethalometer and compared with those during the coronavirus disease (COVID-19) period in 2020. Our results showed ubiquitously elevated concentrations of chloride, black carbon (BC), and primary organic aerosol (POA) in 2021, suggesting increased primary emissions during the post-COVID-19 period. By using the machine learning-based random forest (RF) algorithm, we found largely different responses of aerosol changes to meteorology in different months. After decoupling the effects of meteorology, the PM2.5 changes from 2020 to 2021 were reduced from -35.6% to -29.0% in January, -24.1% to -4.5% in February, and +92.6% to +34.2% in March, respectively. Our results demonstrate the dominant roles of stagnant meteorology and secondary production in the formation of severe haze episodes in March 2021. In particular, we found that the compositions of observed and deweathered PM2.5 were fairly similar between 2020 and 2021, and the ratios of secondary OA to secondary inorganic aerosols were close. Our study indicates that decoupling the influence of meteorological conditions is of great importance for better evaluation of mitigating strategies of air pollution due to the large impact of meteorology on the changes in PM2.5 species particularly in a short period.

Automated summary

Frequent severe haze episodes occurred in Beijing from February to March 2021, and the causes were unclear. The study found increased primary emissions and elevated concentrations of chloride, black carbon, and primary organic aerosol during this period. By analyzing the effects of meteorology, the changes in PM2.5 from 2020 to 2021 were reduced and showed the dominant roles of stagnant meteorology and secondary production in severe haze formation in March 2021.