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Teresina, the capital of Piauí state in northeastern Brazil, occupies a distinctive geographic position at the confluence of the Parnaíba and Poti rivers, earning it the nickname 'Green City' despite its semi-arid regional context. Situated at approximately 72 meters above sea level on the Parnaíba River basin's western edge, the city lies within the transition zone between the Cerrado savanna to the south and the Caatinga dry forest to the north, creating a mosaic of vegetation that influences local air dynamics. Teresina's urban character is defined by its role as a regional administrative and commercial hub, with sprawling development patterns that blend dense central districts with peripheral informal settlements, creating an urban-rural gradient where biomass burning from land clearance occasionally impacts air quality. The city's location in Brazil's interior, distant from major industrial belts or coastal influences, means pollution sources are predominantly local: vehicle emissions from its growing fleet, construction dust, and occasional agricultural burning in surrounding rural areas. The flat to gently rolling terrain, combined with the moderating influence of the rivers, typically allows for reasonable atmospheric dispersion, though the semi-arid climate can concentrate pollutants during dry periods. Teresina's position as the only northeastern state capital not on the coast makes it particularly vulnerable to temperature extremes that can trap pollutants, while its riverine setting provides some humidity that occasionally interacts with emissions to form haze.

Teresina experiences a tropical savanna climate with distinct wet and dry seasons that dramatically shape its air quality narrative throughout the year. During the dry season from June to November, pollution typically peaks, especially in September and October when rainfall is minimal and temperatures soar above 35°C. These months see increased particulate matter from construction activities, road dust resuspension, and occasional biomass burning in surrounding rural areas, compounded by reduced atmospheric dispersion due to stable high-pressure systems. Sensitive groups like asthmatics and the elderly should limit prolonged outdoor exertion during afternoon heat. The wet season from December to May brings relief, with January to April experiencing the heaviest rains that effectively scrub pollutants from the air, resulting in the cleanest conditions. However, the transition months of May and November can be unpredictable, with occasional temperature inversions trapping pollutants near the surface overnight. Wind patterns generally follow seasonal shifts, with stronger easterly winds during the dry season providing some ventilation, while calmer conditions prevail in wetter months. For optimal air quality, visitors should target February through April when rains have cleared the atmosphere but before the intense heat returns. Year-round, morning hours before 10 AM typically offer better air quality before daytime heating and traffic increase pollution levels.