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Montmagny, Quebec, is strategically positioned on the southern shore of the St. Lawrence River, approximately 70 kilometers northeast of Quebec City. The city occupies a relatively flat coastal plain, part of the larger St. Lawrence Lowlands, with an elevation averaging around 20 meters above sea level. This low-lying terrain, combined with its proximity to the expansive St. Lawrence, significantly influences local air quality. The river acts as a major conduit for air masses, potentially transporting pollutants from both upstream and downstream sources, including industrial areas around Quebec City and Trois-Rivières. Surrounding Montmagny is a mix of agricultural land – primarily dairy farming and crop cultivation – and forested areas. This blend creates a complex interplay of emissions; agricultural activities contribute ammonia and particulate matter, while forests can release biogenic volatile organic compounds (BVOCs) that react with other pollutants to form ozone. The urban-rural gradient is relatively sharp, with the city centre transitioning quickly to farmland. The prevailing winds, generally from the west-southwest, carry pollutants across this gradient, impacting air quality in the city. The coastal location also means potential for sea breeze circulations, which can trap pollutants near the shore, especially during stable atmospheric conditions. Understanding these geographical factors is crucial for predicting and mitigating air quality issues in Montmagny.

Montmagny experiences a distinct four-season climate, profoundly impacting its air quality. Winters (December-February) often see stable atmospheric conditions and temperature inversions, trapping pollutants from residential heating sources – wood burning being common – near the ground. While overall emissions may be lower due to reduced agricultural activity, the lack of atmospheric mixing leads to localized pollution build-up. Spring (March-May) brings increased agricultural activity, releasing ammonia and particulate matter from fertilizer application and livestock. Melting snow also contributes to runoff carrying pollutants into waterways and potentially volatilizing them into the air. Summers (June-August) are generally characterized by better air quality, aided by prevailing winds and increased atmospheric mixing. However, high temperatures and sunlight promote the formation of ground-level ozone, particularly downwind of agricultural areas. Autumn (September-November) sees a return to stable conditions, coupled with the burning of agricultural residue and increased residential heating. Sensitive individuals – children, the elderly, and those with respiratory conditions – should limit strenuous outdoor activity during prolonged periods of stagnation, particularly in winter and autumn. Monitoring wind patterns and avoiding peak heating hours can further minimize exposure. Fog, common in the fall and winter, can exacerbate particulate matter concentrations.