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Primeira Cruz, nestled in the state of Maranhão, Brazil, occupies a low-lying position at coordinates -2.5100, -43.4378. The city’s geography is fundamentally shaped by its proximity to the Pindaré River, a major waterway influencing local humidity and potentially contributing to localised pollutant dispersion. The surrounding landscape is characterised by the transition zone between the Amazon rainforest and the Cerrado savanna, a mosaic of dense vegetation and open grasslands. Elevation is minimal, typical of the Maranhão coastal plain, meaning limited natural ventilation and a propensity for temperature inversions. While not a major industrial hub, Primeira Cruz supports a regional agricultural economy, primarily focused on subsistence farming and small-scale plantations, which can contribute to particulate matter from land preparation and seasonal burning. The urban-rural gradient is relatively sharp, with the city centre quickly giving way to agricultural lands. This proximity means pollutants from agricultural practices can readily impact the city’s air quality. The city’s small population of approximately 13,614 concentrates emissions within a limited area, exacerbating local air quality challenges. Understanding this interplay between the riverine environment, agricultural activity, and flat terrain is crucial for assessing and mitigating air pollution in Primeira Cruz.

Primeira Cruz experiences a distinct wet and dry season, heavily influencing air quality. The dry season, typically from June to November, sees significantly reduced rainfall and increased dust and particulate matter in the air. Agricultural burning, common during land preparation for the next planting cycle, peaks during these months, contributing to smoky conditions and respiratory irritation. Lower humidity also means pollutants become more concentrated. December to May marks the wet season, bringing relief from dust and washing away some pollutants. However, increased humidity can also foster mold growth, impacting indoor air quality. Temperature inversions, where a layer of warm air traps cooler air near the ground, are more frequent during the transition periods (May/June and November/December), exacerbating pollution build-up. Sensitive groups – children, the elderly, and those with respiratory conditions – should limit strenuous outdoor activity during the peak of the dry season (August-October). Fog, common during the wet season mornings, can trap pollutants at ground level. Monitoring wind patterns is crucial; winds from agricultural areas during burning season will carry pollutants directly into the city. Overall, the wet season offers better air quality, while the dry season demands increased vigilance.