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Aix-les-Bains, nestled in the Auvergne-Rhône-Alpes region of France, occupies a uniquely advantageous yet geographically complex position. Situated on the eastern shore of Lac du Bourget, France’s largest natural lake, the city benefits from a stunning landscape dominated by the Chartreuse Mountains to the east and the Jura Mountains to the west. Its coordinates (45.6885, 5.9153) place it at an elevation of approximately 380 meters (1,250 feet) above sea level, contributing to a generally temperate climate. The lake itself plays a crucial role in the local microclimate, moderating temperatures and influencing wind patterns. The surrounding terrain is a blend of rolling hills and steeper slopes, impacting air circulation and potential for pollutant accumulation. While primarily a tourist and spa town, Aix-les-Bains lies within a broader agricultural zone, with vineyards and farmland extending outwards. The urban–rural gradient is relatively gradual, with smaller villages and agricultural land transitioning seamlessly into the city’s periphery. Historically, the area saw some industrial activity, particularly related to mining and timber, though this has largely diminished. The proximity to the industrial belt of Lyon, approximately 60km south, introduces a potential source of transboundary pollution, though the mountainous terrain often mitigates its direct impact. The lake’s basin acts as a natural filter, but also can trap pollutants under certain meteorological conditions, creating localized air quality challenges. The city’s layout, with its historic centre and expanding residential areas, further influences local airflow and pollutant dispersion.

Aix-les-Bains experiences a distinct seasonal cycle impacting air quality. Spring (March-May) often sees a gradual improvement as temperatures rise and winds increase, dispersing accumulated winter pollutants. However, agricultural activities, including fertilizer application, can contribute to ammonia emissions, particularly in the surrounding rural areas. Summer (June-August) generally offers the best air quality, with consistent sunshine, strong daytime convection currents, and frequent breezes from the lake, effectively ventilating the basin. The lake’s evaporative cooling also helps to moderate temperatures, reducing photochemical smog formation. Autumn (September-November) presents a more variable picture. As temperatures cool, the potential for temperature inversions increases, particularly during calm, clear nights. These inversions trap pollutants near the ground, leading to periods of reduced visibility and elevated concentrations. The burning of agricultural residues after harvest can also contribute to particulate matter. Winter (December-February) is typically the most challenging season. Cold, stable air masses, coupled with frequent fog and temperature inversions, lead to pollutant stagnation. Residential heating, primarily using wood or gas, becomes a significant source of particulate matter and nitrogen oxides. Sensitive groups, including children, the elderly, and individuals with respiratory conditions, should limit prolonged outdoor exposure during periods of fog or calm, cold weather. Increased ventilation in homes is recommended, and avoiding wood-burning stoves during periods of poor air quality is advisable. The lake’s influence is most pronounced in mitigating extreme events, but localized pollution episodes can still occur.