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Viseu, nestled in the state of Pará, Brazil, at coordinates -1.1969, -46.1400, presents a unique geographic context influencing its air quality. The city is situated within the Amazon rainforest biome, a vast expanse of tropical rainforest, and is characterized by a relatively flat terrain typical of the Amazon basin. Its location isn’t directly on the Amazon River itself, but is interwoven with a network of smaller tributaries and waterways that contribute to the region’s high humidity. Viseu’s urban character is that of a regional hub, serving surrounding smaller communities and agricultural areas. The surrounding landscape is predominantly dense rainforest, transitioning to more open areas with agricultural activity – primarily small-scale farming and cattle ranching – as one moves outwards from the city center. This agricultural gradient, coupled with potential slash-and-burn practices, is a key factor in seasonal air quality variations. The urban-rural gradient is relatively sharp, with a rapid transition from built-up areas to dense vegetation. The lack of significant industrial belts within Viseu itself means that the primary air quality concerns stem from biomass burning, dust from unpaved roads, and localized vehicle emissions. Elevation is low, contributing to limited atmospheric dispersion of pollutants.

Viseu experiences a pronounced wet and dry season, dictating the rhythm of its air quality. The dry season, typically from July to November, is when air quality deteriorates most significantly. Reduced rainfall and increased temperatures create ideal conditions for wildfires, both naturally occurring and those initiated for land clearing. Smoke from these fires, containing particulate matter (PM2.5 and PM10) and carbon monoxide, becomes the dominant pollutant. Wind patterns during this period can transport smoke over long distances, impacting Viseu even if the fires are located outside the immediate vicinity. The wet season, from December to June, brings relief. Frequent rainfall effectively suppresses wildfires and washes particulate matter from the atmosphere, leading to improved air quality. However, the increased humidity can also contribute to the formation of mold and allergens, potentially affecting respiratory health. Temperature inversions, while not as common as in temperate climates, can occur during calmer periods, trapping pollutants near the ground. Sensitive groups – children, the elderly, and those with pre-existing respiratory conditions – should be particularly cautious during the dry season, limiting outdoor activity and using air filtration when possible. Months of July, August, September and October are generally less favourable for prolonged outdoor exertion.