Changes in air composition driven by differences in biofuel consumption.

In São Paulo, the largest city in the Southern Hemisphere, the air pollution is mainly associated with vehicular traffic. The use of biofuels gives unique characteristics to the Brazilian vehicle fleet. In this study, PM2.5 samples were collected in the city at the beginning of the lockdown (from March to August 2020) and in the same period in 2022 (n = 93 samples), after the full economic reopening. The source apportionment by the FA-MLR approach showed that vehicular traffic, biomass burning, and secondary formation contributed to PM2.5 concentrations. Biomass burning made remarkable contributions in both years, and was the main source in 2020. Vehicular traffic was the primary source after the economic reopening. In 2020, concentrations of most pollutants (water-soluble ions, elements, PAHs, and their derivatives) decreased compared to previous studies, but the air pollution in 2022 reached levels close to the pre-pandemic. Additionally, Cu and Mo, elements linked to vehicular traffic, unexpectedly decreased in 2022 in relation to the lockdown and pre-pandemic period. These results were associated with the decrease in ethanol (and consequently, increase in gasoline) consumption in flexible-fuel vehicles, due to increased fuel prices. Biomass burning markers (Rb, retene, and anhydride monosaccharides) showed no change during and after the pandemic, indicating that this activity was not affected by social distancing measures. The resurgence of anthropogenic activities in 2022 resulted in increasing health risks due to exposure to carcinogenic species.