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Saint Petersburg occupies a distinctive geographic position at the northeastern extremity of the Baltic Sea, where the Neva River flows into the Gulf of Finland. Built across 42 islands in the Neva delta, the city's terrain is predominantly flat and low-lying, with an average elevation of just 3 meters above sea level, making it vulnerable to flooding and atmospheric stagnation. This urban landscape is encircled by the Leningrad Oblast, which features mixed forests, agricultural lands, and scattered industrial zones to the south and east, creating a complex urban–rural gradient. The city's location on the Baltic coast subjects it to maritime influences, with the Gulf of Finland moderating temperatures but also contributing to high humidity and frequent fog. Proximity to major transport corridors, including ports and railways, intensifies local emissions from diesel vehicles and shipping. The surrounding landscape, lacking significant topographic relief, offers little natural ventilation, allowing pollutants from urban sources—such as vehicle exhaust, residential heating, and industrial activities in the city's southern districts—to accumulate under stable atmospheric conditions. This flat, water-bound setting exacerbates air quality issues, particularly during temperature inversions in colder months, when cold air traps pollutants near the surface.

In winter, from December to February, Saint Petersburg experiences its worst air quality due to temperature inversions that trap pollutants from heating systems and vehicle emissions, compounded by low wind speeds and frequent fog. Sensitive groups, such as those with respiratory conditions, should limit outdoor activities during these months, especially on calm, cold days. Spring, from March to May, sees gradual improvement as temperatures rise and winds increase, dispersing pollutants, though lingering cold snaps can still cause brief spikes. Summer, from June to August, offers the best air quality, with longer daylight hours, stronger Baltic breezes, and reduced heating needs, making it ideal for outdoor recreation; however, occasional heatwaves can elevate ground-level ozone. Autumn, from September to November, brings declining air quality as temperatures drop, heating resumes, and atmospheric stability returns, with October and November often marked by increased particulate matter. Throughout the year, weather hazards like fog and low wind exacerbate pollution, so monitoring local forecasts is advised for planning activities.