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Nanjing, the capital of Jiangsu province, occupies a strategic position along the Yangtze River in eastern China, approximately 300 kilometers west of Shanghai. Situated at the crossroads of the Yangtze River Delta and the North China Plain, the city's geography profoundly influences its air quality patterns. Nanjing lies within a basin-like terrain surrounded by low mountains to the east, south, and northwest, including the Zhongshan and Zijinshan ranges, creating a natural amphitheater that traps pollutants. The city's elevation averages around 20 meters above sea level, with the Yangtze River flowing through its northern districts, while the Qinhuai River meanders through the urban core. This riverine location historically made Nanjing a vital transportation hub, but today it contributes to pollution through intensive shipping emissions and industrial activities concentrated along the waterfront. The Nanjing Chemical Zone, located northeast of the city center along the Yangtze, represents a major industrial cluster that releases petrochemical emissions into the regional airshed. The urban-rural gradient shows dense urbanization in central districts gradually transitioning to agricultural zones in surrounding counties, though construction dust from ongoing development blurs these boundaries. Nanjing's position within China's eastern industrial belt, combined with its topographic enclosure and heavy river traffic, creates a perfect storm for pollutant accumulation, particularly during stable atmospheric conditions when the surrounding mountains prevent dispersion.

Nanjing's air quality follows a distinct seasonal rhythm shaped by its humid subtropical climate and East Asian monsoon patterns. During winter (November-February), pollution peaks dramatically as cold, stable air masses from Siberia create frequent temperature inversions that trap pollutants near the surface. These months experience the highest PM2.5 concentrations, with December and January being particularly hazardous due to increased coal-fired heating emissions and reduced atmospheric mixing. Sensitive groups should minimize outdoor activities during this period and use air purifiers indoors. Spring (March-May) brings gradual improvement as warming temperatures increase vertical mixing and occasional rainfall helps wash pollutants from the air, though March can still see high pollution days from lingering winter conditions. Summer (June-August) offers the cleanest air despite heatwaves, as the East Asian summer monsoon brings southeasterly winds that disperse pollutants, and frequent precipitation scavenges particulate matter. However, high ozone levels can occur on hot, sunny days. Autumn (September-October) sees a transitional period with generally moderate air quality until late October when pollution begins building toward winter peaks. The best months for outdoor activities are typically June through September, while November through February require heightened precautions, especially for children, elderly residents, and those with respiratory conditions who should monitor air quality forecasts daily.