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Châteaubriant, nestled in the Pays de la Loire region of France, occupies a geographically significant position within the broader landscape of western France. Located approximately 60 kilometers southwest of Rennes, the city’s terrain is gently undulating, characteristic of the Armorican Massif’s southern fringes. The elevation averages around 130 meters above sea level, contributing to localized microclimates. The surrounding area is predominantly agricultural, with extensive fields of maize, wheat, and rapeseed dominating the rural panorama. This agricultural activity, while vital to the regional economy, can contribute to particulate matter emissions, particularly during harvest seasons. To the west, the Erve River meanders through the landscape, providing a natural drainage system and influencing local humidity levels. The urban fabric of Châteaubriant itself is relatively compact, with a historic core reflecting its medieval origins. The city’s location, while buffered by the surrounding agricultural lands, is also within reach of industrial zones further east, potentially exposing it to transboundary pollution. The urban–rural gradient is gradual, with small villages and hamlets dotting the countryside, creating a patchwork of agricultural and residential areas. The prevailing westerly winds, common to this region, play a crucial role in dispersing pollutants, but can also carry them from more industrialized areas. The relatively flat terrain, combined with the river valley, can sometimes lead to stagnant air conditions, particularly during periods of temperature inversion, impacting local air quality.

Châteaubriant’s air quality experiences a distinct seasonal cycle heavily influenced by meteorological patterns. Spring (March-May) generally sees improved air quality as temperatures rise and winds increase, dispersing any lingering winter pollutants. However, agricultural activities, such as fertilizer application, can contribute to ammonia emissions during this period. Summer (June-August) typically offers the best air quality, with consistent winds and warm temperatures promoting good ventilation. Occasional heatwaves can, however, lead to stagnant air and increased ozone formation, though this is less pronounced than in more southerly regions. Autumn (September-November) marks a shift, with increased agricultural activity and the onset of cooler temperatures. Harvest operations release particulate matter, and the decreasing wind speeds, coupled with the potential for temperature inversions, can trap pollutants near the ground. This is often the most challenging period for air quality. Winter (December-February) brings the highest risk of poor air quality. Cold, stable air masses frequently lead to temperature inversions, where a layer of warm air sits above cooler air near the ground, preventing vertical mixing and trapping pollutants. Fog, common during these months, further exacerbates the problem by reducing visibility and hindering dispersion. Sensitive groups, including children, the elderly, and individuals with respiratory conditions, should limit outdoor exertion during periods of stagnant air or fog, particularly in the mornings. Increased ventilation in homes is also advisable during these times. The agricultural calendar dictates much of the pollution profile, making awareness of planting and harvesting schedules crucial for managing exposure.