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Nantes, situated in western France's Pays de la Loire region, occupies a strategic position at the confluence of the Loire and Erdre rivers, approximately 50 kilometers inland from the Atlantic coast. This historic port city lies within the Armorican Massif's southern edge, characterized by gentle hills with elevations typically between 5 and 30 meters above sea level, creating a basin-like topography that can trap pollutants under certain meteorological conditions. The urban core, home to over 325,000 residents, transitions gradually into suburban areas and then to the fertile agricultural plains of the Loire Valley, known for vineyards, cereal crops, and livestock farming. Nantes' location along the Loire River—France's longest river—has historically facilitated industrial development, though today's economy is more service-oriented with some manufacturing in sectors like aerospace and food processing concentrated in peripheral zones. The city's proximity to the Atlantic Ocean moderates its climate but also means maritime winds can disperse pollutants effectively when blowing inland. However, the surrounding rural areas contribute agricultural emissions, particularly ammonia from livestock, which can combine with urban traffic emissions to form secondary particulate matter. The urban-rural gradient sees decreasing pollution levels moving eastward toward more sparsely populated areas, though the city's position in a river valley can occasionally create stagnation episodes when winds are calm.

Nantes experiences distinct seasonal air quality patterns influenced by its temperate oceanic climate. During winter (December-February), colder temperatures increase residential heating emissions, particularly from wood-burning in suburban areas, while frequent anticyclonic conditions can lead to temperature inversions that trap pollutants near the ground, making late January and February the peak pollution months. Sensitive groups like asthmatics should limit prolonged outdoor exertion during these cold, still periods. Spring (March-May) brings improving conditions as rising temperatures reduce heating needs and increased Atlantic winds disperse pollutants, though April can see occasional pollution spikes from agricultural ammonia emissions combining with urban nitrogen oxides. Summer (June-August) generally offers the cleanest air, with strong sea breezes and frequent rainfall cleansing the atmosphere, making it ideal for outdoor activities; however, heatwaves in July or August can elevate ground-level ozone, particularly in afternoon hours, requiring caution for those with respiratory conditions. Autumn (September-November) sees a gradual deterioration as temperatures drop and wind patterns become more variable, with October and November often experiencing foggy mornings that can temporarily concentrate pollutants until winds pick up. Year-round, traffic emissions remain a concern in the urban center, but Nantes' coastal proximity ensures most pollution episodes are short-lived compared to inland French cities.