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São Pedro da Água Branca, nestled in the state of Maranhão, Brazil, occupies a geographically significant position within the Cerrado biome, a vast tropical savanna ecoregion. Located at approximately -5.0850° latitude and -48.4289° longitude, the city’s terrain is characterized by gently undulating plains, typical of the Maranhão region, with elevations rarely exceeding 150 meters above sea level. This low elevation contributes to a relatively stable atmospheric layer, which can, under certain conditions, exacerbate air quality issues. The surrounding landscape is dominated by the Cerrado vegetation – a mosaic of grasslands, woodlands, and gallery forests – interspersed with patches of agriculture, primarily soybean and maize cultivation. This agricultural activity, while vital to the regional economy, introduces potential sources of particulate matter and volatile organic compounds (VOCs) into the atmosphere, particularly during planting and harvesting seasons. The city’s proximity to the Pindaré River, though providing a crucial water source, also influences local humidity and temperature, impacting atmospheric stability and pollutant dispersion. São Pedro da Água Branca exhibits a clear urban–rural gradient, with the city center showing denser population and infrastructure, gradually transitioning to more sparsely populated agricultural areas and the expansive Cerrado landscape. The lack of significant industrial belts directly adjacent to the city mitigates some pollution risks, but regional transportation corridors carrying goods to and from larger urban centers can contribute to transient air pollution events. The prevailing winds, generally easterly, play a crucial role in dispersing pollutants, though their effectiveness is often limited by the relatively flat terrain.

The air quality in São Pedro da Água Branca is intrinsically linked to the region’s distinct wet and dry seasons, a characteristic of tropical climates. The dry season, typically spanning from June to September, often sees a build-up of pollutants due to reduced rainfall and increased temperatures. The lack of precipitation means fewer opportunities for atmospheric cleansing through wet deposition, allowing particulate matter and gaseous pollutants to accumulate. Furthermore, the higher temperatures promote photochemical reactions, leading to the formation of ground-level ozone, a respiratory irritant. During this period, stagnant air masses are more common, hindering pollutant dispersion. Conversely, the wet season, from October to May, brings frequent rainfall, which effectively washes pollutants from the atmosphere, leading to improved air quality. However, intense rainfall events can temporarily increase particulate matter due to soil erosion and dust resuspension from agricultural fields. Fog, though infrequent, can occur during the cooler months (July-August), trapping pollutants near the ground and creating localized air quality concerns. The months of August and September are generally considered periods to exercise caution for individuals with respiratory sensitivities, as the combination of dry conditions, high temperatures, and potential stagnant air can lead to elevated pollutant concentrations. Sensitive groups, including children, the elderly, and those with pre-existing respiratory conditions, should limit prolonged outdoor activities during these months and monitor local air quality reports. Agricultural burning, a common practice in the region, can significantly impact air quality during the transition periods between wet and dry seasons, releasing substantial amounts of smoke and particulate matter.