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São José do Rio Preto, nestled in the northwestern region of Rio de Janeiro state, Brazil, presents a unique geographic context influencing its air quality. The city sits at an elevation of approximately 220 meters (722 feet) above sea level, within a gently undulating landscape transitioning from the coastal plains to the more rugged interior. This terrain, while not mountainous, can contribute to localized temperature inversions, trapping pollutants near the ground. The city is not directly on the coast, but lies within the broader hydrographic basin of the Paraíba do Sul River, with numerous smaller rivers and streams crisscrossing the area. Surrounding the urban core is a mosaic of agricultural land, primarily sugarcane and pasture, alongside pockets of Atlantic Forest remnants. A significant factor is the proximity to larger industrial centers further south, particularly the Baixada Fluminense, which can contribute to long-range transport of pollutants. The urban-rural gradient is relatively sharp, with rapid development encroaching upon previously agricultural areas. This expansion, coupled with increasing vehicle traffic and localized industrial activity within the city itself, creates a complex interplay of emission sources. The prevailing winds, generally from the southeast, can carry pollutants from these sources, impacting air quality within São José do Rio Preto.

São José do Rio Preto experiences a tropical climate with distinct wet and dry seasons, profoundly impacting air quality. The dry season, typically from April to September, sees significantly reduced rainfall and increased dust and particulate matter in the air. Agricultural burning, common during land preparation for the next planting season, exacerbates this issue, leading to periods of haze and respiratory irritation. August and September are often the months with the poorest air quality. Conversely, the wet season (October to March) brings frequent rainfall, effectively washing pollutants from the atmosphere and improving air quality. However, increased humidity can also foster the formation of mold and allergens, impacting sensitive individuals. November and December generally offer the most favourable conditions for outdoor activity. Temperature inversions, more frequent during the cooler months of June and July, can trap pollutants near the ground, even during the dry season. Strong southerly winds can occasionally bring cleaner air from the south, but these events are infrequent. Individuals with asthma or respiratory conditions should exercise caution during the dry season and monitor local conditions closely. Avoiding strenuous activity during peak pollution periods is advisable.