Compound fire-meteorology extremes drove megacity air pollution during the 2024 South American fire season.

Extreme air pollution increasingly threatens major cities, yet the relative roles of urban emissions and biomass-burning smoke transported across multiple spatial scales remain difficult to quantify. Here, we investigate three multi-day pollution extremes that affected São Paulo State during the 2024 South American fire season, when daily fine particulate matter (PM2.5) reached 80-100 μg m-3 in the Metropolitan Area of São Paulo. We combined surface observations, aerosol optical thickness measurements, backward trajectories, and controlled global atmospheric simulations differing only in fire-emission representation to quantify the biomass-burning contribution. Simulations without fire emissions failed to reproduce the observed PM2.5 and carbon monoxide (CO) peaks, whereas fire-attributable PM2.5 increments reached 17-21 μg m-3 at the state scale and the ensemble-mean cumulative metropolitan fire-attributable burden reached ∼547 μg m-3 day across E1-E3 combined. The three events revealed contrasting source-receptor configurations: regional and longer-range transport combined with weak receptor ventilation during E1 (15-24 August), greater fire activity during E2 (29 August-5 September) produced a comparatively weaker receptor response under less persistent transport pathways, and E3 (7-15 September) occurred when intense fire activity across multiple spatial scales coincided with favourable transport configurations, anomalous heat and dryness, and weak ventilation. Fire-emission configurations producing larger aerosol enhancements generally showed better agreement with observations, although all inventories underestimated the most extreme episode. These results show that megacity pollution can arise from the combined effects of biomass-burning emissions, atmospheric transport, and adverse meteorological conditions, while highlighting substantial sensitivity to fire-emission representation in operational atmospheric-composition systems.