Where there is smoke, there is fire: long- and mid-range biomass burning role on São Paulo's state air quality

Prolonged exposure to fine particulate matter (PM_2.5) is the leading environmental risk factor for human health. In Brazil, frequent wildfires during dry seasons, especially in the Amazon, Cerrado, and more recently the Pantanal, significantly increase PM_2.5 levels, often exceeding national and international air quality standards. This study analyzed ten years of PM_2.5 data from 25 monitoring stations across São Paulo State and used 1464 backward trajectories from the HYSPLIT model to assess the influence of open biomass burning and long-range pollutant transport. Results show that after declining trends, 2024 recorded a sharp increase in PM_2.5, with concentrations peaking up to seven times higher during fire season and some daily averages exceeding 100 μg/m³. The analysis revealed that air masses arriving in the state frequently crossed major fire hotspots, including neighboring states and countries. Consequently, the estimated avoidable deaths rose significantly in 2024, with interior municipalities showing the highest rates. These findings underscore the urgent need to address transboundary pollution sources linked to open biomass burning within Brazil's air quality management and climate adaptation strategies.