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Birigui, nestled in the northwest of São Paulo state, Brazil, occupies a gently undulating terrain within the Paraná Basin. Its coordinates (-21.2889, -50.3400) place it approximately 538 meters above sea level, a moderate elevation contributing to generally good atmospheric dispersion, though susceptible to localized temperature inversions. The city’s landscape is characterized by a transition zone between the cerrado savanna and Atlantic Forest biomes, historically influencing land use and agricultural practices. Surrounding Birigui are extensive agricultural lands, primarily sugarcane and soybean cultivation, alongside cattle ranching. This agricultural dominance is a key factor in regional air quality, with seasonal burning for land preparation releasing significant particulate matter. The urban-rural gradient is relatively sharp, with the city centre densely populated and surrounded by a ring of agricultural and increasingly, light industrial activity. The lack of substantial natural barriers like mountains or large bodies of water means wind patterns play a crucial role in pollutant transport. The Rio Turvo, a tributary of the Tietê River, flows near Birigui, but its influence on local air quality is limited compared to the prevailing winds and agricultural emissions. The city’s position within a major agricultural belt and its moderate elevation create a unique set of challenges for maintaining clean air.

Birigui experiences a distinct wet and dry season, rather than traditional four seasons. Air quality in Birigui is most compromised during the dry season (roughly May to September). This period coincides with increased agricultural burning for sugarcane harvesting and land preparation, leading to elevated levels of particulate matter (PM2.5 and PM10). Lower humidity and stable atmospheric conditions during these months exacerbate pollution, trapping pollutants near the ground. August and September are typically the worst months for air quality. Conversely, the wet season (October to April) brings relief. Rainfall effectively washes pollutants from the atmosphere, and higher humidity promotes dispersion. November to February generally offer the best air quality, though localized pollution from vehicle emissions and small-scale industrial activity persists. Temperature inversions, common during cooler nights throughout the year, can trap pollutants at ground level, even during the wet season. Sensitive groups – children, the elderly, and individuals with respiratory conditions – should limit strenuous outdoor activity during the dry season, particularly on days with visible haze. Monitoring wind direction is also crucial, as pollutants can be transported from surrounding agricultural areas.