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Bastos, a municipality nestled in the interior of São Paulo state, Brazil, occupies a gently undulating terrain within the Paraná River basin. Located at approximately 560 meters above sea level, the city benefits from a moderate elevation that influences local climate patterns. The surrounding landscape is predominantly agricultural, characterized by extensive sugarcane and citrus plantations – key economic drivers for the region. This agricultural dominance, while vital to the local economy, presents a significant factor in air quality, with seasonal burning of sugarcane fields contributing to particulate matter pollution. The city’s position isn’t directly adjacent to large bodies of water, though the Rio Grande river flows nearby, influencing humidity levels. Bastos isn’t situated within a major industrial belt, but the increasing presence of agricultural processing facilities and associated transport networks contribute to localised emissions. The urban-rural gradient is relatively sharp; the city centre is compact, quickly transitioning to expansive agricultural lands. This proximity to agricultural activity means that wind patterns carrying pollutants from field burning directly impact the city’s air quality, particularly during the dry season. The terrain’s gentle slopes don’t significantly impede dispersion, but localised temperature inversions can trap pollutants near ground level.

Bastos experiences a tropical climate with distinct wet and dry seasons, profoundly impacting air quality. The dry season, typically from May to September, is when air quality deteriorates most significantly. This is primarily due to the widespread practice of sugarcane field burning prior to harvest, releasing substantial amounts of particulate matter (PM2.5 and PM10) and other pollutants. Lower humidity during these months also reduces the natural scavenging of pollutants by rainfall. October to April marks the wet season, bringing relief from the dry season’s pollution. Frequent rainfall effectively washes pollutants from the atmosphere, improving air quality considerably. However, even during the wet season, increased agricultural activity and vehicle emissions contribute to background pollution levels. Temperature inversions, common during cooler nights throughout the year, can exacerbate pollution by trapping pollutants near ground level. Sensitive individuals – children, the elderly, and those with respiratory conditions – should limit strenuous outdoor activity during the dry season, particularly in the mornings when pollutant concentrations are often highest. November and December generally offer the most favourable conditions for outdoor activities, with cleaner air and moderate temperatures.