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Deux-Montagnes, Quebec, occupies a unique position within the Laurentian Mountains north of Montreal, a landscape profoundly shaping its geography and, consequently, its air quality. Situated at approximately 45.53°N, 73.88°W, the city’s terrain is characterized by rolling hills and valleys carved by glacial activity, with elevations ranging from around 100 to 250 meters above sea level. The Rivière des Milles Îles, a complex network of waterways and islands, meanders through the municipality, influencing local microclimates and providing a natural buffer against some pollutants. The surrounding landscape is predominantly rural, a mosaic of agricultural fields, forested areas, and smaller settlements, creating a gradual urban–rural gradient. While not directly adjacent to major industrial belts, Deux-Montagnes benefits from its proximity to Montreal, which can occasionally contribute to transboundary pollution events. The Laurentian Mountains to the north act as a barrier, sometimes trapping pollutants, while prevailing westerly winds generally disperse them. The city’s relatively small population (17,915) contributes to lower local emissions compared to larger urban centers. The soil composition, largely derived from glacial till, can also influence the deposition of particulate matter. The presence of deciduous and coniferous forests surrounding the city plays a vital role in air purification, absorbing pollutants and releasing oxygen, though seasonal changes in vegetation affect this process. The overall geographic setting presents a complex interplay of factors impacting air quality, requiring careful monitoring and mitigation strategies.

Deux-Montagnes experiences a distinct four-season climate, significantly influencing its air quality profile throughout the year. Spring (March-May) often sees a gradual improvement as snow cover recedes, and vegetation begins to absorb pollutants. However, early spring can be characterized by lingering temperature inversions, trapping emissions from residential heating and occasional agricultural burning. Summer (June-August) generally offers the best air quality, with warm temperatures promoting atmospheric mixing and dispersing pollutants. Prevailing winds are typically more consistent, carrying away any localized emissions. Autumn (September-November) presents a mixed picture. While cooler temperatures can lead to some inversions, the vibrant foliage absorbs pollutants, and the reduced agricultural activity minimizes emissions. The most concerning period is often late autumn, when stagnant air and cooler temperatures can combine to trap pollutants. Winter (December-February) frequently brings the poorest air quality. Cold temperatures and frequent temperature inversions, particularly during periods of high pressure, trap emissions from residential heating (primarily natural gas and wood stoves) and vehicle traffic. Fog, common during winter mornings, further exacerbates the problem by reducing visibility and concentrating pollutants near ground level. Sensitive groups, including children, the elderly, and individuals with respiratory conditions, should limit outdoor activity during periods of stagnant air and fog, especially in the mornings. Increased ventilation in homes can also help mitigate indoor air pollution from heating sources. Public health advisories are crucial during winter months to inform residents about potential air quality concerns.