Abstract: Due to the high transmission rate of the virus causing COVID-19 (SARS-CoV-2), several measures were adopted to reduce its spread, among which mobility restrictions were highlighted. These measures, besides proving effective in reducing viral transmission, had a significant impact on decreasing pollutant levels in major urban centers, owing to the substantial reduction in vehicular traffic. In São Paulo, one of the world’s largest urban centers, significant reductions in primary pollutants such as carbon monoxide (CO) and nitrogen oxides (NOX) were observed during the restriction period. However, surprisingly, some areas experienced increases in fine particulate matter (PM2.5) concentrations, indicating a complex dynamic of atmospheric pollution sources. Given this scenario and considering the relevance of PM2.5 to human health, particularly amidst the mobility restrictions imposed by the pandemic, this study proposed a detailed assessment of PM2.5 sources using the Concentration Weighted Trajectory (CWT) model. The analysis period covered the phase of the strictest mobility restrictions, spanning from March 16 to June 30. Trajectories of air masses at 100 m and 1000 m above ground level (AGL) reaching São Paulo were meticulously considered. The HYSPLIT dispersion model, coupled with the CWT model, was utilized to gain a deeper understanding of the origins and transport patterns of pollutants. Air quality monitoring stations (20) distributed across the State of São Paulo served as the locations of arrival of the air masses. Analyses included not only air mass trajectories but also the Weighted Average Concentration (WAC) of particles, providing a comprehensive view of sources and impacts on air quality. CWT results indicated that regions with significant emissions from biomass burning had a notable impact on pollutant concentrations in São Paulo. These elevated concentrations were attributed primarily to airflows from the northwest and west directions, traversing biomes such as the Pantanal, as well as territories in Bolivia and Paraguay. While the impact of biomass burning is evident, the analysis of primary pollutant sources (WAC) revealed that local sources, including nearby biomass burning, played a more prominent role in concentrations than fires in remote areas. WAC analysis also highlighted that the influx of air masses from regions like Minas Gerais, Rio de Janeiro, and São Paulo is crucial for pollutant transport to the receptor site, adding to local emissions. Therefore, although the impact of wildfires on particle concentrations in São Paulo is notable during specific times of the year, it seems that they did not have a significant impact during the strictest mobility restrictions imposed by the COVID-19 pandemic. Consequently, PM2.5 concentrations during that time appear to be more influenced by local sources, particularly industrial and biomass burning, as well as regional sources, especially from Rio de Janeiro and Minas Gerais. These results underscore the complexity of the interaction between atmospheric emissions, meteorological patterns, and local responses to human interventions, providing valuable insights into understanding atmospheric pollutant behavior in urban contexts and during significant global events such as the COVID-19 pandemic.

Keywords: Weighted Average Concentration, Concentration Weighted Trajectory, Mobility Restrictions, Biomass Burning