Using the COVID-19 lockdown to identify atmospheric processes and meteorology influences on regional PM2.5 pollution episodes in the Beijing-Tianjin-Hebei, China

A major tool for curtailing the spread of COVID-19 pandemic in China was a nationwide lockdown implemented by the Chinese authorities, which also led to significant reductions in anthropogenic emissions and fine particulate matter (PM2.5) concentrations. However, the lockdown measures did not prevent high PM2.5 pollution episodes (EPs). Three severe EPs were identified in the Beijing-Tianjin-Hebei (BTH) region during the lockdown. The integrated process rate (IPR) analysis tool in the Community Multiscale Air Quality (CMAQ) model was employed to identify and quantify the contributions of individual atmospheric processes to PM2.5 formation during the EPs in four representative cities (Beijing, Tianjin, Shijiazhuang, and Baoding) of the BTH. The influence of meteorological conditions on the accumulation and dispersion of PM2.5 during the EPs was also elucidated. The results of the IPR showed that emissions and aerosol processes were the dominant sources of net surface PM2.5 in Beijing and Tianjin, constituting a total of 86.2% and 92.9%, respectively, while emissions, horizontal transport, and aerosol processes dominated the net surface PM2.5 formation in Shijiazhuang and Baoding. In addition, the three pollution episodes in Beijing and Tianjin were primarily driven by local emissions, while the pollution events in Shijiazhuang and Baoding were attributed to the combined local emissions and regional transport. Also, the EPs were driven by low planetary boundary layer heights, low vertical export of PM2.5 from the boundary layer to the free troposphere, and substantial horizontal import, especially in Shijiazhuang and Baoding. Furthermore, the elevated PM2.5 concentrations during the EPs were greatly enhanced by unfavorable meteorological conditions, with the first two EPs being characterized with low ventilation coefficient (VC) values in the four representative cities. This study improves the understanding of buildup of PM2.5 during the pollution episodes, and the results provide insights for designing more effective emissions control strategies to mitigate future PM2.5 pollution episodes.