Beyond Industrial Emissions: How Biomass Burning Drives Extreme Urban Air Pollution

Biomass burning has become an increasingly important driver of air quality degradation in Minas Gerais, southeastern Brazil, particularly during the dry season, when fire activity intensifies, and atmospheric dispersion is suppressed. This work addresses recent evidence on the role of regional biomass burning in shaping fine particulate matter (PM₂.₅) concentrations, with a particular focus on the extreme air pollution episodes observed during 2024. The analysis integrates long-term records of fire hotspots, ground-based air quality monitoring, and key meteorological variables to elucidate the coupled processes linking fire activity, atmospheric dynamics, and urban air quality.

Minas Gerais encompasses the third largest metropolitan region in Brazil and hosts major local atmospheric emission sources, including intensive mining activities, steelmaking, and a dense urban–industrial infrastructure. Despite the persistent contribution of these structural sources to baseline air pollution levels, fire occurrence in the state exhibits pronounced seasonality, with approximately two-thirds of annual hotspots concentrated between August and October. The year 2024 stands out as one of the most critical periods of the last decade, characterized by prolonged drought, anomalously high temperatures, and persistently low relative humidity. These conditions not only favored the ignition and spread of fires but also created a meteorological environment highly unfavorable to pollutant dispersion.

Time-series analyses indicate that peaks in PM₂.₅ concentrations closely coincided with periods of increased fire frequency and intensity, particularly in the Metropolitan Region of Belo Horizonte. Statistical analyses reveal a moderate-to-strong positive association between PM₂.₅ levels and fire hotspot counts, and a consistent negative association with relative humidity. Notably, even in a region with significant local emissions from mining, steel production, and vehicular traffic, biomass burning emerged in 2024 as the dominant driver of exceedances of the national PM₂.₅ air quality standards established by CONAMA Resolution No. 506/2024. These findings demonstrate that regional-scale transport and accumulation of biomass-burning emissions can outweigh the influence of traditional urban and industrial sources during extreme events.

Several monitoring stations recorded historically high PM₂.₅ concentrations in 2024, leading to recurrent violations of national air quality thresholds. The compounded effects of extreme meteorological conditions and biomass burning led to short-lived but severe pollution episodes that substantially deteriorated air quality across the metropolitan area. Beyond local emission sources, regional fire activity therefore represents a critical and recurrent contributor to urban particulate pollution, with direct implications for public health and regulatory compliance.

Overall, the results reinforce the need for integrated air quality management strategies that combine fire prevention and control, regional-scale monitoring, and meteorological forecasting. Such approaches are particularly relevant under a changing climate, in which the frequency and severity of droughts, heatwaves, and associated biomass-burning events are expected to increase, thereby amplifying their impacts on air quality even in heavily industrialized urban regions.