Drought, heatwave, and fires: The impact on air quality during São Paulo's record-breaking fire season in 2024

Extreme compound climate events, such as simultaneous heatwaves and droughts, have become more frequent and intense, increasing the risk of forest fires and air quality degradation. Although previous studies have linked compound events to fires in some biomes, a gap remains in understanding their cascading impacts, from climate extremes to fires and, subsequently, to air quality, particularly in densely populated subtropical regions such as São Paulo, Brazil. This study analyzed the evolution of active fires and their relationship with compound drought and heatwave events, as well as their effects on air quality (PM₁₀ and O₃), during the exceptional 2024 fire season. The year 2024 experienced one of the state's most severe droughts (SPI < −2.0) and the highest temperatures since 1961. This combination favored an unprecedented fire season in which São Paulo recorded 8712 active fires, a 118% increase relative to the historical mean and the highest value since 2003. Nearly 70% of these fires occurred in August (3612) and September (2522), months that registered four compound drought and heatwave events. During these periods, PM₁₀ and O₃ concentrations frequently exceeded WHO guidelines, and a strong positive correlation was observed between the maximum temperature and pollutant levels ( r = 0.79 for PM₁₀, r = 0.87 for O₃). These results underscore the importance of incorporating compound event analysis into environmental monitoring and developing targeted mitigation and adaptation policies. The 2024 season serves as a critical warning of this new climate reality.