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Rheinfelden, nestled in the canton of Aargau, Switzerland, occupies a unique geographic position at the border with Germany, straddling the Rhine River. The city’s terrain is characterised by gentle rolling hills transitioning into the Swiss Plateau, with an elevation around 320 meters. This location, while picturesque, significantly influences its air quality. The Rhine acts as both a potential pollutant pathway and a dispersing agent, depending on flow rates and industrial activity upstream. To the north and west lie predominantly agricultural lands in Germany, contributing to potential ammonia emissions, while the south and east feature a mix of residential areas and light industrial zones within Switzerland. Rheinfelden’s urban-rural gradient is relatively sharp; the city centre quickly gives way to farmland and forested areas. The surrounding landscape, including the Black Forest to the north, can trap pollutants during stable atmospheric conditions. The city’s proximity to Basel, a major industrial hub, also means it can experience transboundary pollution events. The valley configuration along the Rhine can exacerbate temperature inversions, particularly during winter months, leading to pollutant accumulation. Understanding these geographical factors is crucial for managing and mitigating air quality challenges in Rheinfelden.

Rheinfelden’s air quality follows a distinct seasonal pattern. Winter, from December to February, often sees the poorest air quality due to temperature inversions trapping pollutants from heating systems and limited atmospheric mixing. Calm, cold conditions exacerbate particulate matter concentrations. Spring (March-May) brings gradual improvement as temperatures rise and winds increase, dispersing pollutants, though agricultural activities can contribute to ammonia spikes. Summer (June-August) generally offers the cleanest air, with strong convective currents and ample sunshine promoting dispersion. However, occasional heatwaves can lead to increased ozone formation. Autumn (September-November) presents a transitional period. As heating demand increases and atmospheric stability returns, pollutant levels begin to rise again. Fog, common in the Rhine valley during autumn and winter, can further worsen air quality by trapping pollutants near the ground. Sensitive individuals – children, the elderly, and those with respiratory conditions – should limit strenuous outdoor activity during winter and periods of fog. Monitoring wind direction is key; pollutants from the Basel industrial area are more likely to impact Rheinfelden with easterly winds. Overall, spring and summer offer the most favourable conditions for outdoor activities.