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Déressia is situated within the Tandjilé region of southern Chad, nestled in a landscape characterized by the transition between the semi-arid Sahel and the more humid Sudanian savanna. The city occupies a remarkably flat terrain, typical of the Chari-Logone basin, where the elevation remains low and the horizon expansive. This lack of significant topographical relief means there are no mountain ranges or high plateaus to act as windbreaks, leaving the urban center exposed to the full force of regional atmospheric currents. The surrounding landscape is dominated by extensive agricultural zones, primarily focused on cotton and cereal production, which create a distinct urban-rural gradient. As one moves from the denser residential core to the periphery, the influence of agricultural land use becomes paramount, introducing seasonal organic aerosols into the local atmosphere. Proximity to seasonal wetlands and floodplains introduces a level of ambient humidity that varies wildly throughout the year, influencing the suspension of particulate matter. The urban character of Déressia is defined by low-rise constructions and unpaved roads, which contribute significantly to the local dust load during dry spells. Consequently, the city's air quality is a complex interplay between the sweeping mineral dusts of the north and the localized emissions from biomass burning and soil erosion. This geographic vulnerability makes the city a focal point for studying the intersection of Sahelian climate patterns and small-scale urban development in the heart of Central Africa.

The air quality in Déressia is governed by a stark binary between the dry and wet seasons. From November through April, the city falls under the influence of the Harmattan, a northeasterly trade wind that transports vast quantities of mineral dust from the Sahara Desert. During these months, pollution peaks as particulate matter saturates the air, often reducing visibility and triggering respiratory distress. Temperature inversions are common during cool winter mornings, trapping dust and smoke from domestic cooking fires close to the ground. Conversely, the wet season, spanning from May to October, provides a natural cleansing mechanism. Heavy monsoon rains effectively scrub the atmosphere, washing away suspended particles and drastically lowering the air pollution levels. This period is the most favorable for outdoor activities and respiratory recovery. However, the transition months can be volatile, with sudden wind shifts creating localized dust storms. For sensitive groups, including children and the elderly, the dry season necessitates strict precautions, such as wearing protective masks during peak Harmattan surges and minimizing exertion during the early morning hours when pollutants are most concentrated. Health guidance emphasizes staying hydrated and utilizing indoor spaces with filtered ventilation during the dust-heavy months. By understanding these meteorological rhythms, residents can better navigate the seasonal fluctuations, favoring the humid summer months for outdoor labor while exercising extreme caution during the arid, dust-laden winter window that defines the Sahelian atmospheric experience.