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São Sebastião do Caí, nestled in the heart of Rio Grande do Sul, Brazil, occupies a geographically significant position within the Campanha Plateau. Located approximately 85 kilometers west of the state capital, Porto Alegre, the city’s coordinates (-29.5869, -51.3758) place it within a gently undulating terrain characterized by rolling hills and fertile plains. The surrounding landscape is predominantly agricultural, with extensive fields of soybeans, wheat, and corn dominating the rural areas. This agricultural activity, while vital to the regional economy, contributes to potential sources of particulate matter, particularly during harvest seasons. The city sits at an elevation of roughly 520 meters above sea level, influencing local weather patterns and potentially affecting air stagnation. While not directly adjacent to a major river, the nearby Caí River, from which the city derives its name, moderates the local climate to some extent. The urban–rural gradient transitions relatively smoothly, with smaller settlements and agricultural land interspersed throughout the municipality. São Sebastião do Caí’s location within a broader agricultural belt means it’s susceptible to regional pollution events, such as smoke from agricultural burning, although the city itself is not a major industrial hub. The plateau’s topography can trap pollutants, especially during periods of calm winds, leading to localized air quality concerns. The prevailing winds generally flow from the east, bringing air masses from the Atlantic coast, which can influence the city’s air quality depending on coastal conditions.

São Sebastião do Caí experiences a humid subtropical climate, resulting in distinct seasonal variations that significantly impact air quality. The warmer months, typically December to March, are characterized by higher temperatures and increased humidity, often leading to stagnant air conditions. This, coupled with agricultural activities like harvesting and occasional controlled burns, can elevate particulate matter concentrations. The dry season, generally from April to September, brings cooler temperatures and lower humidity, but also a greater risk of temperature inversions, particularly during the mornings. These inversions trap pollutants near the ground, exacerbating air quality issues. The wet season, from October to November, brings frequent rainfall, which naturally helps to cleanse the atmosphere and reduce pollutant levels. However, heavy rainfall can also lead to increased soil erosion and dust emissions, temporarily impacting air quality. During the spring (September-November), pollen counts can rise, affecting individuals with allergies. The summer months (December-February) are generally best avoided for strenuous outdoor activity, especially for children, the elderly, and those with respiratory conditions. Autumn (March-May) often presents the most stable air quality, while winter (June-August) requires vigilance due to potential temperature inversions. Maintaining awareness of regional agricultural practices and weather forecasts is crucial for minimizing exposure to potential air pollution events throughout the year. Sensitive groups should consider limiting outdoor activities during periods of stagnant air or high pollen counts.