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Saint-Louis, nestled in the Grand Est region of France, occupies a unique and historically significant position at the confluence of the Ill and the Rhine rivers. Its coordinates (47.5900, 7.5700) place it within the Upper Rhine Plain, a gently sloping alluvial landscape formed over millennia by fluvial deposition. The city’s terrain is remarkably flat, averaging around 250 meters above sea level, which, while facilitating urban development, can also contribute to air quality challenges. Saint-Louis is geographically intertwined with its neighbour, Basel, Switzerland, forming a substantial cross-border urban area. This proximity means that air pollution events are rarely confined to one jurisdiction. The surrounding landscape is a mix of agricultural fields – primarily vineyards and pastures – and pockets of deciduous forest, typical of the Alsace region. The Rhine River acts as a crucial transport corridor, and historically, the Ill has been vital for local industry. While industrial activity has shifted somewhat, remnants of manufacturing, particularly related to the watchmaking industry (Saint-Louis is renowned for crystal production), remain. The urban–rural gradient transitions relatively quickly, with agricultural land dominating the immediate outskirts. The flat topography and riverine setting, combined with the presence of industry and cross-border influences, create a complex interplay of factors that influence Saint-Louis’s air quality, making it susceptible to pollutants transported from both local and regional sources. The lack of significant elevation also limits natural dispersion of pollutants.

Saint-Louis’s air quality exhibits a distinct seasonal pattern dictated by meteorological conditions. Winter months (December-February) often see the poorest air quality, primarily due to temperature inversions. Cold, stable air becomes trapped near the ground, preventing the vertical mixing of pollutants. This phenomenon, coupled with reduced sunlight hours and increased domestic heating (often using wood or other biomass fuels), leads to a build-up of particulate matter and nitrogen oxides. Fog, common during these months, further exacerbates the problem by trapping pollutants close to the ground. Spring (March-May) brings a gradual improvement as temperatures rise, and wind speeds increase, facilitating pollutant dispersion. However, agricultural activities, such as fertilizer application, can contribute to ammonia emissions. Summer (June-August) generally offers the best air quality, with consistent sunshine, higher temperatures, and frequent winds. However, occasional heatwaves can lead to ozone formation, a secondary pollutant. Autumn (September-November) sees a transition period, with decreasing sunlight and increasing humidity, which can lead to stagnant air conditions and a return of some winter-like pollution episodes. Sensitive groups, including children, the elderly, and individuals with respiratory conditions, should be particularly cautious during winter and early spring, limiting outdoor activity during periods of stagnant air or fog. During the warmer months, monitoring ozone levels is advisable, especially during heatwaves. Maintaining well-ventilated indoor spaces and avoiding exposure to high-traffic areas can further mitigate risks throughout the year.