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Barra de São Francisco, nestled in the northern part of Espírito Santo state, Brazil, occupies a unique geographic position within the Doce River Valley. The city’s coordinates (-18.7550, -40.8908) place it on relatively flat terrain, typical of the valley floor, with an elevation generally below 50 meters. This low-lying landscape significantly influences air quality, as it can trap pollutants during periods of atmospheric stability. The Doce River, a vital waterway, runs nearby, providing both ecological benefits and potential pathways for pollutant dispersion, particularly following the 2015 Mariana dam disaster which heavily impacted the river’s ecosystem and sediment load. Surrounding the urban core is a mosaic of agricultural land, primarily focused on coffee, cocoa, and pasture for cattle, alongside remnants of Atlantic Forest. The urban-rural gradient is relatively sharp, with agricultural activities contributing to seasonal biomass burning, a key source of particulate matter. While not a major industrial hub itself, Barra de São Francisco is situated within a region containing some mining and processing activities further upstream along the Doce River, potentially contributing to long-range transport of pollutants. The city’s location experiences a tropical climate, influencing pollutant behaviour through temperature and humidity.

Air quality in Barra de São Francisco follows a distinct seasonal pattern dictated by the region’s tropical climate. The drier months, typically from May to September, witness a peak in particulate matter pollution. This is largely due to increased agricultural burning for land preparation and crop residue removal, coupled with lower rainfall which reduces the natural scavenging of pollutants from the atmosphere. Temperature inversions, common during these cooler, drier nights, exacerbate the problem by trapping pollutants near ground level. Conversely, the wet season, from October to April, generally brings improved air quality. Increased rainfall effectively washes pollutants from the air, and higher humidity can suppress dust and smoke. However, the intense rainfall can also lead to localised flooding and the mobilization of sediment, potentially impacting water quality and indirectly affecting air quality through increased fungal spores. Sensitive individuals, such as those with respiratory conditions, should be particularly cautious during the dry season. November and December, transitioning into the wet season, often offer the most favourable conditions for outdoor activity. Fog, common in the mornings, can temporarily trap pollutants, but typically dissipates with solar heating.