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Ceará-Mirim is strategically positioned within the metropolitan region of Natal in Rio Grande do Norte, characterized by its location on the coastal plains known as Tabuleiros Costeiros. The urban character of the city is a blend of a colonial administrative center and an expanding residential periphery, deeply intertwined with the surrounding agricultural landscape. The terrain is predominantly flat with gentle undulations, dominated by the fertile basin of the Ceará-Mirim River, which serves as a vital hydrological artery for the region. This low-lying topography facilitates the movement of the moist Atlantic Trade Winds, which generally assist in the dispersion of pollutants. However, the city’s position creates a distinct urban-rural gradient where the air quality is heavily influenced by the vast sugarcane plantations that encircle the built-up area. These agricultural zones are the primary drivers of local atmospheric conditions, as the practice of biomass burning during harvest cycles introduces significant loads of particulate matter into the lower troposphere. While the proximity to the coast provides a buffering effect through maritime air masses, the inland shift toward the semi-arid interior begins to manifest in the local microclimate. This geographic transition means that while the city avoids the extreme stagnation of deep inland basins, it remains susceptible to localized pollution plumes from agricultural activities and the transit corridors connecting it to the state capital, creating a complex interplay between natural ventilation and anthropogenic emission sources.

Ceará-Mirim experiences a tropical climate defined by a stark contrast between the rainy season, typically spanning from March to August, and the dry season, occurring from September to February. During the rainy period, air quality reaches its annual peak purity. Frequent precipitation events trigger wet deposition, effectively scrubbing the atmosphere of suspended particulates and aerosols, while increased soil moisture suppresses wind-blown dust. Conversely, the dry season presents significant air quality challenges. This period coincides with the peak of the sugarcane harvest, where traditional burning practices release dense plumes of smoke and fine particulate matter. Meteorological factors, including lower humidity and shifting wind vectors, can trap these pollutants near the surface, although the consistent Atlantic breezes usually prevent prolonged stagnation. Temperature inversions are rare but can occur during clear, cool nights in the winter months, momentarily concentrating ground-level emissions. For outdoor activities, the months of May and June are most favorable due to the cleansing effect of the rains. In contrast, October and November are the most critical months for sensitive groups, including children and the elderly, who may experience exacerbated respiratory distress. Health guidance for these periods emphasizes reducing strenuous outdoor exertion during the late afternoon when smoke concentrations often peak. Monitoring local wind directions is essential, as shifts can either sweep pollutants toward the sea or push them deep into the urban residential core, altering daily exposure levels.