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Osório, nestled in the northern coastal region of Rio Grande do Sul, Brazil, presents a unique geographic setting influencing its air quality. Situated at coordinates -29.8869, -50.2700, the city occupies a gently undulating terrain transitioning from coastal plains to the foothills of the Serra Geral. This location, approximately 120km north of Porto Alegre, places Osório within a region characterized by a humid subtropical climate. The Atlantic Ocean exerts a significant moderating influence, contributing to relatively mild temperatures and consistent humidity. The city itself is built around the Tramandaí River estuary, a feature impacting local wind patterns and potential for stagnant air. Surrounding Osório are extensive agricultural lands, primarily dedicated to rice cultivation and livestock farming, alongside pockets of Atlantic Forest remnants. While not heavily industrialized, regional economic activity centers on agriculture, tourism (particularly beach tourism), and some small-scale manufacturing. The urban-rural gradient is relatively sharp, with agricultural areas immediately bordering the city limits. This proximity means agricultural practices, including fertilizer use and seasonal burning, can directly affect local air quality, particularly during drier periods. The coastal location also introduces the possibility of salt spray and marine aerosols influencing particulate matter concentrations.

Osório’s air quality follows a distinct seasonal pattern dictated by the humid subtropical climate. Winter (June-August) often sees the poorest air quality, due to temperature inversions trapping pollutants near the ground. Calm winds and increased residential heating using wood or coal exacerbate this, leading to higher concentrations of particulate matter and carbon monoxide. Conversely, spring (September-November) brings improving conditions as temperatures rise and winds become more frequent, dispersing pollutants. Summer (December-February) is generally the period of best air quality, with strong sea breezes flushing out contaminants. However, increased tourism and associated vehicle traffic can locally elevate nitrogen dioxide levels near beaches and tourist hotspots. Autumn (March-May) experiences a transition, with decreasing temperatures and increasing humidity. Rainfall becomes more frequent, helping to cleanse the air, but can also contribute to mold spore concentrations. Sensitive individuals, such as those with respiratory conditions, should be particularly cautious during winter and periods of stagnant air. Farmers often engage in controlled burns of rice stubble after harvest in April/May, which can cause temporary spikes in particulate matter. Monitoring wind direction during these times is crucial. Outdoor activity is generally most favourable during the summer months.