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Landerneau, nestled in the heart of Brittany (Bretagne) in northwestern France, occupies a strategic position near the mouth of the Élorn river, just inland from the Bay of Biscay. The city’s geography is characterised by a gently undulating landscape, typical of the Armorican Massif, with an average elevation of around 30 meters. This low-lying terrain, combined with its proximity to the Atlantic, influences local weather patterns and, consequently, air quality. Landerneau isn’t situated within a major industrial belt; however, the surrounding Finistère department features significant agricultural activity, particularly dairy farming, which contributes to ammonia emissions. The urban-rural gradient is relatively sharp, transitioning quickly from the town’s built environment to expansive fields and coastal heathland. The Élorn river acts as a partial corridor for air movement, but can also trap pollutants during stable atmospheric conditions. The coastal influence brings frequent rainfall and strong winds, generally aiding dispersion, but also periods of high humidity and fog which can exacerbate particulate matter concentrations. The city’s historic centre, with its narrow streets and traditional granite buildings, can create localised pockets of reduced air circulation, impacting micro-climate air quality.

Landerneau’s air quality follows a distinct seasonal pattern. Winter (December-February) often sees the poorest air quality, not due to high emissions, but due to stable, high-pressure systems bringing cold, damp air and temperature inversions. These inversions trap pollutants near the ground, particularly particulate matter from wood burning for heating. Spring (March-May) brings improved conditions as winds increase and rainfall washes away accumulated pollutants. However, agricultural activity intensifies, leading to localised ammonia spikes. Summer (June-August) generally offers the cleanest air, with prevailing westerly winds effectively dispersing pollutants over the Atlantic. However, heatwaves can occasionally lead to ozone formation. Autumn (September-November) is a transitional period, with increasing rainfall and wind, but also a return to wood burning as temperatures drop. October and November can experience periods of stagnant air. Sensitive individuals – children, the elderly, and those with respiratory conditions – should limit strenuous outdoor activity during winter inversions and periods of calm weather. Monitoring pollen levels is also crucial in spring and summer. Foggy conditions, common throughout the year, can increase particulate matter exposure.