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Mont-Saint-Aignan, nestled in the heart of Normandy, France, presents a unique geographic context influencing its air quality. Situated approximately 5km inland from the English Channel at coordinates 49.4630, 1.0883, the city benefits from a temperate maritime climate, but also experiences influences from continental air masses. The terrain is gently undulating, rising to a modest elevation of around 100-150m above sea level, providing some natural drainage but also potentially trapping pollutants during temperature inversions. The surrounding landscape is a mosaic of agricultural land – primarily pasture and arable farming – interspersed with woodland, particularly towards the south and east. Rouen, a significant industrial and port city, lies immediately to the south, representing a key source of regional pollution that can impact Mont-Saint-Aignan via prevailing westerly winds. The urban-rural gradient is relatively sharp; the city itself is densely populated, transitioning quickly to open countryside. This proximity to both agricultural activity (potential ammonia emissions) and industrial Rouen creates a complex interplay of pollution sources. The coastal influence moderates temperatures, but also introduces salt spray which can affect particulate matter composition. Understanding these factors is crucial for assessing and mitigating air quality challenges in Mont-Saint-Aignan.

Mont-Saint-Aignan’s air quality follows a distinct seasonal pattern dictated by meteorological conditions and human activity. Winter (December-February) often sees poorer air quality due to increased residential heating, particularly wood burning, combined with frequent temperature inversions trapping pollutants near the ground. Calm, damp conditions exacerbate this, leading to higher concentrations of particulate matter. Spring (March-May) brings improvement as temperatures rise and winds become more frequent, dispersing pollutants. However, agricultural activities, such as fertilizer application, can contribute to ammonia levels. Summer (June-August) generally offers the cleanest air, with strong prevailing winds and ample sunshine promoting dispersion. However, occasional heatwaves can trigger ozone formation. Autumn (September-November) sees a gradual decline in air quality as heating demand increases and meteorological conditions become more unstable. Fog, common during autumn mornings, can trap pollutants. Sensitive individuals – children, the elderly, and those with respiratory conditions – should be particularly cautious during winter and autumn. Months to favour for outdoor activity are June through August. Monitoring pollen levels is also important during spring and summer, as these can exacerbate respiratory issues.