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São José de Piranhas, nestled in the semi-arid heartland of Paraíba, Brazil, presents a unique geographic context influencing its air quality. Located at -7.1208, -38.5019, the city sits on the right bank of the São Francisco River, a vital artery in the region. The terrain is characterised by gently rolling hills and *caatinga* vegetation, a thorny scrubland adapted to prolonged droughts. Elevation is relatively low, averaging around 160 meters, contributing to limited atmospheric dispersion. The urban area is compact, with a clear urban-rural gradient transitioning quickly into agricultural lands dedicated primarily to subsistence farming and, increasingly, irrigated fruit production. While not a major industrial hub, the surrounding region experiences activity related to agriculture – including pesticide application – and some small-scale processing of agricultural products. The São Francisco River, while a source of life, can also contribute to localised humidity and potential for stagnant air masses. The city’s position within the broader São Francisco Valley means it’s susceptible to dust transport from drier areas further inland, particularly during the dry season. This combination of factors – limited dispersion, agricultural activity, and dust – shapes the air quality challenges faced by the 19,067 residents.

São José de Piranhas experiences a pronounced wet and dry season, dictating the rhythm of air quality. The dry season, typically from August to December, is when air quality concerns peak. Reduced rainfall leads to increased dust resuspension from the *caatinga* and agricultural lands, exacerbated by stronger winds. Lower humidity also means particulate matter remains airborne for longer. Temperature inversions, common during cooler nights in the dry season, trap pollutants near the ground. January to July marks the wet season, bringing significant improvements in air quality. Rainfall effectively washes away particulate matter, and higher humidity promotes settling. However, even during the wet season, localised pollution from agricultural activities – such as pesticide spraying – can occur. February and March, during peak rainfall, generally offer the most favourable conditions for outdoor activity. Sensitive groups, including children, the elderly, and those with respiratory conditions, should limit strenuous outdoor exertion during the dry season, particularly in September and October. Monitoring wind patterns is crucial; winds from the west and northwest often carry dust from drier regions. Fog, common during the transition periods, can trap pollutants and reduce visibility.