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Saint-Ouen-l’Aumône, situated in the northwestern suburbs of Paris within the Île-de-France region, occupies a relatively flat landscape characteristic of the Parisian Basin. The town’s elevation averages around 80-100 meters above sea level, contributing to limited natural dispersion of pollutants. It’s positioned approximately 20 kilometers northwest of central Paris, placing it within a densely populated and highly urbanized corridor. The surrounding landscape is a mosaic of residential areas, light industrial zones, and increasingly, peri-urban agricultural land. The Epte river flows nearby, but its influence on local air circulation is moderate. The proximity to major transportation arteries – the A15 motorway and several national roads – is a significant factor impacting air quality. This location experiences a strong urban-rural gradient; air quality generally degrades as one moves closer to the city centre and major roadways, while conditions improve slightly towards the more rural peripheries. The concentration of industrial activity in nearby areas, particularly to the east and west, contributes to background pollution levels. The prevailing winds, typically from the southwest, can transport pollutants from Paris and surrounding industrial areas directly over Saint-Ouen-l’Aumône, exacerbating local air quality concerns. The town’s urban form, with a mix of detached housing and apartment blocks, further influences pollutant dispersion patterns.

Air quality in Saint-Ouen-l’Aumône exhibits a distinct seasonal pattern. Winter, from December to February, often sees the poorest air quality due to increased heating demands, leading to higher particulate matter (PM) concentrations from wood burning and fossil fuel combustion. Temperature inversions, common during calm winter nights, trap pollutants near the ground, worsening conditions. Spring (March-May) brings a gradual improvement as temperatures rise and heating needs decrease, but pollen counts can become a concern for allergy sufferers. Summer (June-August) generally offers the best air quality, aided by prevailing winds and higher atmospheric mixing. However, heatwaves can trigger ground-level ozone formation, a secondary pollutant, particularly during periods of stagnant air. Autumn (September-November) sees a return to deteriorating conditions as heating systems are reactivated and agricultural activities, such as harvesting, contribute to dust and ammonia emissions. Sensitive groups – children, the elderly, and those with respiratory conditions – should limit strenuous outdoor activity during winter and autumn months. Monitoring wind direction is crucial; periods with winds from the east or southeast are likely to bring pollutants from industrial areas. Foggy conditions, frequent in autumn and winter, can also trap pollutants, reducing visibility and increasing respiratory irritation.