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Sainte-Marie, Quebec, nestled in the picturesque Lac-Saint-Jean region of Saguenay–Lac-Saint-Jean, occupies a unique geographic position significantly influencing its air quality. Located at 46.45°N, 71.03°W, the town sits on the southwestern shore of Lac Saint-Jean, a vast inland lake that moderates local temperatures and influences wind patterns. The surrounding landscape is characterized by the Canadian Shield, a bedrock of ancient geological formations resulting in gently rolling hills and rocky terrain. Elevation varies slightly within the town, generally ranging from 200 to 300 meters above sea level. This relatively low elevation, combined with the lake's presence, can contribute to localized temperature inversions, particularly during colder months, trapping pollutants near the ground. Sainte-Marie’s regional position is crucial; it lies within a predominantly rural area, with agricultural lands (primarily dairy farming and forestry) extending outwards. While there aren't major industrial belts immediately adjacent to Sainte-Marie, the broader Saguenay–Lac-Saint-Jean region does contain pulp and paper mills and other resource-based industries, whose emissions can occasionally be transported by prevailing winds. The urban–rural gradient is gradual, with smaller settlements and extensive forests defining the surrounding area. The lake itself acts as a natural barrier, influencing airflow and potentially dispersing pollutants, though calm conditions can conversely lead to stagnation. The town's relatively small size (population 13,134) means that local emissions from vehicles and residential heating are the primary contributors to air quality concerns.

Sainte-Marie’s air quality experiences a distinct seasonal cycle dictated by Quebec’s continental climate. Winter (December-February) presents the most challenging period. Cold temperatures and frequent temperature inversions, where a layer of warm air sits above colder air near the ground, trap pollutants, leading to potential stagnation. Residential heating, primarily wood stoves and oil furnaces, becomes a significant source of particulate matter. Calm, foggy conditions are common, further exacerbating the issue. Spring (March-May) brings a gradual improvement as temperatures rise and the inversion layer weakens. However, early spring can still experience lingering cold snaps and inversions. Agricultural activities, such as fertilizer application, can contribute to ammonia emissions. Summer (June-August) generally offers the best air quality, with warmer temperatures promoting atmospheric mixing and dispersing pollutants. Prevailing winds across Lac Saint-Jean help to ventilate the area. However, occasional wildfires in the boreal forests of Quebec can introduce smoke plumes, impacting air quality significantly. Autumn (September-November) sees a transition period. Cooler temperatures return, and the risk of temperature inversions increases again, though typically less severe than in winter. Leaf burning, a common practice in rural areas, can also contribute to localized particulate matter. Sensitive groups, including children, the elderly, and individuals with respiratory conditions, should limit outdoor activity during periods of stagnant air, particularly in winter and during wildfire events. During summer months, while generally good, monitoring for smoke from distant fires is advisable.