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Sumidouro, nestled within the mountainous landscape of Rio de Janeiro state, Brazil, occupies a geographically significant position within the Paraíba do Sul River basin. Located at approximately -22.0500 latitude and -42.6750 longitude, the municipality sits at an elevation that contributes to its unique microclimate and influences air circulation patterns. The surrounding terrain is characterized by rolling hills and steep slopes, typical of the Serra do Mar mountain range, which significantly impacts local wind patterns and can trap pollutants under certain meteorological conditions. The Paraíba do Sul River, flowing nearby, provides a vital water source but also presents a potential pathway for pollutants to disperse, though its influence on air quality is generally secondary to topography. Sumidouro’s urban character is primarily rural, with agriculture – particularly sugarcane cultivation and cattle ranching – dominating the surrounding landscape. This agricultural activity introduces potential sources of particulate matter and volatile organic compounds (VOCs) into the air. The urban–rural gradient is relatively sharp, with the town itself being a small, localized settlement transitioning quickly into extensive agricultural lands and forested areas. While not directly adjacent to major industrial belts, Sumidouro’s proximity to larger urban centers in the Rio de Janeiro metropolitan area means it can be affected by regional pollution transport, particularly during periods of stagnant air. The valley setting, combined with the surrounding mountains, can create localized inversions, exacerbating air quality challenges.

Sumidouro’s tropical climate dictates a distinct wet and dry season pattern, profoundly influencing its air quality dynamics. The dry season, typically spanning from April to September, often sees a build-up of pollutants due to reduced rainfall and decreased atmospheric cleansing. Lower humidity during this period can also increase the concentration of particulate matter as dust and soil particles remain suspended in the air. Agricultural burning, a common practice for land clearing and sugarcane preparation, is most prevalent during the dry season, significantly contributing to elevated levels of smoke and particulate pollution. Conversely, the wet season (October to March) brings increased rainfall, which naturally washes pollutants from the atmosphere, leading to improved air quality. However, periods of intense rainfall can also mobilize soil erosion, temporarily increasing particulate matter. Fog, more common during the cooler months of June and July, can trap pollutants near the ground, creating localized pockets of poor air quality. Temperature inversions, where a layer of warm air sits above cooler air, are also more likely during the dry season, preventing vertical mixing and further concentrating pollutants. Sensitive groups, such as children, the elderly, and individuals with respiratory conditions, should exercise caution during the dry season, particularly on days with agricultural burning or fog. Limiting outdoor exertion and avoiding areas with visible smoke are advisable. During the wet season, while generally cleaner, periods of heavy rain and subsequent soil disturbance warrant similar precautions.