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Castrop-Rauxel, nestled within the Ruhr metropolitan region of North Rhine-Westphalia, Germany, occupies a geographically significant position shaped by its industrial past and evolving urban landscape. Located at approximately 51.55° N, 7.32° E, the city sits on the edge of the Westphalian Lowland, a gently undulating terrain characterized by loamy soils and a history of coal mining. Its elevation ranges from roughly 110 to 180 meters above sea level, contributing to localized microclimates and influencing air circulation patterns. The Ruhr River, though not directly bordering Castrop-Rauxel, flows nearby, impacting regional humidity and influencing weather systems. Historically, the city’s location within the Ruhr industrial belt meant it was heavily influenced by coal processing, steel production, and related industries, leaving a legacy of brownfield sites and a complex urban fabric. Today, Castrop-Rauxel is undergoing a transition, with efforts to redevelop former industrial areas into green spaces and residential zones. The urban–rural gradient is noticeable; the city blends seamlessly into the surrounding agricultural lands and forested areas, particularly to the south and west. This proximity to agricultural zones introduces potential sources of ammonia and particulate matter from farming activities. The surrounding landscape is a mix of cultivated fields, scattered woodlands, and remnants of the industrial era, all contributing to the city’s unique environmental profile and influencing how pollutants disperse within the local atmosphere. The relatively flat terrain can exacerbate the trapping of pollutants, especially during periods of stable atmospheric conditions.

Castrop-Rauxel’s air quality experiences a distinct seasonal cycle, largely dictated by meteorological conditions and residual industrial influences. Winter months (December-February) often present the greatest challenges. Cold temperatures combined with frequent temperature inversions – where a layer of warm air traps cooler air near the ground – lead to pollutant accumulation. Reduced wind speeds during this period further limit dispersion, resulting in periods of poorer air quality. Heating systems, reliant on fossil fuels, contribute significantly to particulate matter and nitrogen dioxide levels. Spring (March-May) brings a gradual improvement as temperatures rise and wind speeds increase, aiding in pollutant dispersal. However, agricultural activities in the surrounding areas can introduce ammonia emissions, impacting local air quality. Summer (June-August) generally sees the best air quality, with consistent winds and higher temperatures promoting efficient pollutant removal. Occasional heatwaves can, however, exacerbate ozone formation. Autumn (September-November) presents a transitional period. Cooler temperatures and decreasing daylight hours can lead to stagnant air conditions, while agricultural burning practices, though regulated, can contribute to particulate matter. Fog, common during autumn and winter, traps pollutants close to the ground, intensifying their impact. Sensitive groups, including children, the elderly, and individuals with respiratory conditions, should be particularly cautious during winter and periods of stagnant air, limiting prolonged outdoor exposure and avoiding strenuous activity near busy roads. Maintaining indoor air quality through ventilation and air purification can also be beneficial.