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Trélazé, nestled in the Pays de la Loire region of France, occupies a geographically significant position just southwest of Nantes, the regional capital. Its coordinates (47.4460, -0.4650) place it within a gently undulating landscape, characterized by the alluvial plains formed by the Loire River and its tributaries. The terrain is predominantly flat, facilitating urban expansion and agricultural activity, but subtle rises provide panoramic views of the surrounding countryside. The city’s proximity to the Loire River, while offering recreational opportunities and a vital water source, also influences local weather patterns and potential for humidity-related air quality concerns. Trélazé sits within a transitional zone, exhibiting a clear urban–rural gradient. To the east lies the sprawling urban agglomeration of Nantes, with its associated industrial and transportation infrastructure. Westward, the landscape gradually shifts to agricultural fields, primarily dedicated to crops and livestock farming, contributing to potential biogenic emissions and, seasonally, agricultural dust. The surrounding landscape is a mosaic of cultivated fields, hedgerows, and scattered woodlands, typical of the western French countryside. Elevation is relatively low, rarely exceeding 50 meters above sea level, which can exacerbate temperature inversions during cooler months, trapping pollutants near the ground. The prevailing westerly winds, common to this region, generally disperse pollutants, but local topography can create sheltered pockets where concentrations build up. The city’s location within a fertile agricultural belt means that seasonal agricultural practices, such as ploughing and fertilizer application, can impact local air quality.

Trélazé’s air quality experiences a distinct seasonal rhythm dictated by meteorological conditions and regional activities. Spring (March-May) often sees a gradual improvement as temperatures rise and winds increase, dispersing accumulated winter pollutants. However, agricultural activities ramp up during this period, potentially releasing dust and ammonia from fertilizer use, impacting local air quality, particularly in areas closer to farmland. Summer (June-August) typically brings the best air quality, with consistent winds and higher temperatures promoting dispersion. Heatwaves, however, can lead to stagnant air and increased ozone formation, a secondary pollutant. Autumn (September-November) presents a more complex picture. While wind patterns remain generally favourable, cooler temperatures can trigger temperature inversions, especially during calmer periods, trapping pollutants released from Nantes and surrounding areas. The harvest season also contributes to agricultural emissions. Winter (December-February) often witnesses the poorest air quality. Cold, stable air masses frequently lead to temperature inversions, preventing vertical mixing and allowing pollutants to accumulate. Fog, common during this season, further exacerbates the problem by trapping particulate matter. Sensitive groups, including children, the elderly, and individuals with respiratory conditions, should limit prolonged outdoor activity during periods of stagnant air or fog, particularly in the mornings. During the harvest season, those with allergies or respiratory sensitivities should be mindful of potential dust exposure. Maintaining indoor air quality through ventilation and air purification can be beneficial during periods of elevated pollution.