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Pôrto União, nestled in the westernmost reaches of Santa Catarina state, Brazil, occupies a unique geographic position within the Alto Vale do Rio do Peixe basin. The city’s coordinates (-26.2378, -51.0778) place it amidst rolling hills and a landscape transitioning between the Brazilian Highlands and the coastal plains. Elevation varies, contributing to localized microclimates and influencing air dispersion patterns. The Rio do Peixe, a significant waterway, flows nearby, providing a source of moisture but also potentially trapping pollutants during temperature inversions. Pôrto União’s urban character is defined by a blend of residential areas and commercial activity, with a noticeable rural-urban gradient as one moves away from the city center towards agricultural lands. The surrounding region is heavily agricultural, dominated by soybean, corn, and wheat cultivation, alongside livestock farming. This agricultural activity is a key factor influencing air quality, through dust from field preparation, pesticide drift, and emissions from agricultural machinery. While not a major industrial hub, the presence of smaller processing facilities related to agriculture contributes to localized emissions. The terrain, characterized by gentle slopes, doesn’t offer significant natural barriers to pollutant dispersal, making the city susceptible to regional air quality influences.

Pôrto União experiences a humid subtropical climate, with distinct wet and dry seasons rather than traditional four seasons. Air quality generally improves during the wet season (October to March) due to increased rainfall which effectively washes particulate matter from the atmosphere. However, high humidity can also exacerbate the effects of certain pollutants. The dry season (April to September) consistently presents poorer air quality. This is largely due to increased dust from agricultural activities – planting and harvesting – and a greater prevalence of wildfires in surrounding areas. August and September are typically the months with the most noticeable air quality decline. Temperature inversions, common during cooler, calmer nights in the dry season, trap pollutants near ground level, worsening conditions. Winds generally flow from the south and southwest, potentially carrying pollutants from agricultural zones. Sensitive groups, including children, the elderly, and individuals with respiratory conditions, should limit strenuous outdoor activity during dry periods, particularly when winds are light. Fog, frequent in the mornings, can also concentrate pollutants. Monitoring local weather patterns and agricultural practices is crucial for anticipating air quality fluctuations.