A major tool for curtailing the spread of COVID-19 pandemic in China was a nationwide lockdown, which led to significant reductions in anthropogenic emissions and fine particulate matter (PM2.5). 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 quantify the contributions of individual atmospheric processes to PM2.5 formation during the lockdown in the BTH region. The IPR results 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 in Shijiazhuang and Baoding. In addition, the EPs in Beijing and Tianjin were primarily driven by local emissions, while the EPs in Shijiazhuang and Baoding were attributed to combined local emissions and regional transport. The reductions in PM2.5 in Case 2 relative to Case 1 were attributed to the weaker PM2.5 formation from emissions and aerosol processes. However, the EPs were enhanced 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. This study improves the understanding of buildup of PM2.5 during the EPs, and the results provide insights for designing more effective emissions control strategies to mitigate future PM2.5 episodes.

In 2021, the World Health Organization (WHO) ranked Nigeria among the most polluted nations in the world, an indication of a deteriorating air quality, especially in the major urban areas of the country, which might pose adverse human health impacts. In this study, the Integrated Source Apportionment Method (ISAM) tool in the Community Multiscale Air Quality (CMAQ) model (CMAQ-ISAM) was employed to quantify the contributions of eight emissions sectors to fine particulate matter (PM2.5) and its major components in Lagos during a prolonged severe atmospheric pollution episode (APE) in January 2021. The influence of meteorological conditions on the formation and dispersion of PM2.5 during the APE was also elucidated. Spatially, elevated PM2.5 concentrations were found in the northwestern region of Lagos, an urban area with larger anthropogenic emissions. Residential and industry were the two major sources of PM2.5. Residential contributed the most to total PM2.5 (~40 μg/m3), followed by industry (~20 μg/m3). High concentrations of secondary inorganic aerosols (SIA) at the northwest and upper northern areas of Lagos were majorly attributed to residential and industry sectors. In addition, sulfate accounted for the largest fraction of PM2.5, with residential, industry, and energy being its major sources. Residential, industry, and on-road sectors dominated the contributions to nitrate, while residential and industry were the major contributors to ammonium. Furthermore, the elevated PM2.5 concentrations during the APE were greatly enhanced by unfavorable meteorological conditions. This study provides insights for designing effective emissions control strategies to mitigate future severe PM2.5 pollution episode in Lagos.