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Dortmund is situated in the heart of Germany's densely populated Ruhr region, a post-industrial urban landscape characterized by its transformation from heavy industry to a diversified service and technology economy. The city lies within the Westphalian Basin, a geological depression surrounded by low mountain ranges to the south (Sauerland) and east, creating a topographical bowl that influences local meteorology. At an elevation of approximately 86 meters above sea level, Dortmund's terrain is relatively flat with gentle undulations, intersected by the Emscher River and the Dortmund-Ems Canal, which historically supported industrial transport but now contribute to urban green corridors. Its position within the Rhine-Ruhr metropolitan area—Germany's largest urban agglomeration—means it experiences regional air pollution transport from neighboring cities like Essen, Bochum, and Duisburg, all part of the historic industrial belt. The urban-rural gradient is pronounced, with dense inner-city districts giving way to suburban zones and nearby agricultural lands in the Münsterland region to the north. This geographical setting, combined with frequent temperature inversions in the basin, can trap pollutants from vehicular traffic, residual industrial activities, and domestic heating, particularly during calm weather conditions. The city's extensive green spaces, including the Westfalenpark and Rombergpark, provide local air quality benefits but cannot fully mitigate regional pollution influences.

Dortmund's air quality follows a distinct seasonal pattern shaped by its temperate maritime climate with continental influences. In winter (December to February), pollution typically peaks due to increased emissions from heating systems, combined with frequent temperature inversions that trap pollutants in the Ruhr Basin's topography. Cold, stagnant air and low wind speeds exacerbate particulate matter (PM2.5 and PM10) concentrations, making this season the least favorable for outdoor activities, especially for sensitive groups like asthmatics or the elderly. Spring (March to May) brings gradual improvement as temperatures rise and precipitation increases, helping to disperse pollutants; however, early spring can still see episodes of fine dust from agricultural practices in surrounding regions. Summer (June to August) generally offers the best air quality, with higher wind speeds, convective mixing, and reduced heating emissions, though occasional heatwaves can elevate ground-level ozone levels, particularly on sunny, windless days. Autumn (September to November) sees a return to deteriorating conditions as heating resumes and weather becomes more stable, with fog and low wind speeds contributing to pollutant accumulation. Sensitive individuals should monitor local air quality forecasts year-round, limit strenuous outdoor exercise during winter pollution episodes, and take advantage of summer's cleaner air for outdoor pursuits.