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Cássia, nestled in the southwestern region of Minas Gerais, Brazil, occupies a unique geographic position within the expansive Cerrado biome. Situated at coordinates -20.5828, -46.9219, the city’s terrain is characterised by rolling hills and plateaus, typical of the Brazilian Highlands. The elevation, averaging around 850 meters, influences local weather patterns and contributes to temperature variations. Cássia isn’t directly adjacent to major bodies of water, though it’s within the watershed of the Grande River, a tributary of the Paraná River basin. The surrounding landscape is predominantly agricultural, dominated by soybean, corn, and sugarcane cultivation, alongside cattle ranching. This agricultural intensity is a key factor influencing air quality. While Cássia itself isn’t heavily industrialised, its regional position means it can be affected by transport emissions from larger cities and agricultural practices like burning for land clearing. The urban-rural gradient is relatively sharp; the city centre is compact, quickly transitioning to extensive farmland. This proximity to agricultural sources means that particulate matter from soil disturbance and biomass burning can significantly impact local air quality, particularly during the dry season. The prevailing winds, generally from the southeast, can carry pollutants from these sources directly over the city.

Air quality in Cássia follows a distinct seasonal pattern closely tied to the region’s wet and dry seasons. The dry season, typically from May to September, presents the most challenging conditions. Reduced rainfall leads to increased dust and particulate matter from agricultural activities, particularly land preparation and harvesting. Burning for pasture renewal and crop residue disposal is also more common during this period, releasing significant amounts of smoke and pollutants. Temperature inversions, frequently occurring during cooler nights in the dry season, trap pollutants near the ground, exacerbating air quality issues. October to April marks the wet season, bringing relief through rainfall that washes away pollutants and suppresses dust. However, even during the wet season, localised pollution can occur from vehicle emissions and smaller-scale burning. February and March, often the wettest months, generally offer the best air quality. Sensitive groups – children, the elderly, and individuals with respiratory conditions – should exercise caution during the dry season, limiting outdoor activity on days with visible haze or smoke. Monitoring local conditions and staying indoors when pollution levels are elevated is crucial. The prevailing southeasterly winds can sometimes bring smoke from distant agricultural fires, impacting air quality even during the wetter months.