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São Francisco do Sul, nestled in the northern coastline of Santa Catarina, Brazil, presents a unique geographic setting influencing its air quality. The city occupies a relatively flat coastal plain, transitioning rapidly to the Serra do Mar mountains inland. This terrain creates a funnel effect, potentially trapping pollutants during periods of calm winds. Situated at -26.2433 latitude and -48.6381 longitude, the city benefits from Atlantic Ocean breezes, which generally aid in dispersing pollutants, but can also carry maritime aerosols. The surrounding landscape is a mosaic of Atlantic Forest remnants, agricultural lands – primarily sugarcane and banana plantations – and urban development. The port, a significant economic driver, introduces industrial activity and associated emissions. The urban-rural gradient is pronounced; the city centre experiences higher concentrations of traffic and industrial sources, while peripheral areas retain more natural air filtration. Proximity to the ocean moderates temperatures, but also contributes to humidity, potentially fostering the formation of secondary pollutants. The Itajaí-Açu River estuary, bordering the city, adds complexity to air circulation patterns and can influence local pollutant dispersion. Understanding these geographical factors is crucial for effective air quality management in São Francisco do Sul.

São Francisco do Sul experiences a humid subtropical climate, with distinct wet and dry seasons impacting air quality. The drier winter months (June-August) often see increased particulate matter concentrations due to reduced rainfall for atmospheric cleansing and potential agricultural burning. Temperature inversions, common during calm winter nights, exacerbate pollution by trapping emissions near ground level. Spring (September-November) brings increasing rainfall and a gradual improvement in air quality, though pollen counts can rise, affecting allergy sufferers. The wet summer (December-February) generally offers the best air quality, with frequent rainfall effectively washing away pollutants. However, increased humidity can promote the formation of ozone, particularly on sunny days. Autumn (March-May) is a transitional period, with fluctuating weather patterns and moderate pollution levels. Sensitive groups – children, the elderly, and those with respiratory conditions – should be particularly cautious during the drier winter months and periods of stagnant air. Monitoring wind direction is key; winds from inland agricultural areas may carry smoke and dust. Avoiding strenuous outdoor activity during peak pollution times, typically early mornings and evenings, is advisable.