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Nestled in the mountainous interior of Fujian Province, Yong'an occupies a strategic position along the upper reaches of the Min River, approximately 250 kilometers west of the coastal metropolis of Fuzhou. The city's urban core sits at an elevation of around 200 meters, surrounded by the undulating terrain of the Wuyi Mountains to the north and the Daiyun Mountains to the south, creating a natural basin that profoundly influences its air quality. This topography often traps pollutants, particularly during stable atmospheric conditions, as the surrounding peaks inhibit dispersion. Yong'an lies within the Nanping–Sanming industrial corridor, a region historically focused on forestry, mining, and chemical production, which contributes to its 'general urban' pollution profile. The Min River flows through the city, providing some ventilation but also supporting agricultural zones in the valley floors. The urban–rural gradient is distinct, with dense development along the riverbanks giving way to terraced fields and forested slopes. Proximity to major transportation routes, including highways and railways connecting inland Fujian to coastal ports, adds mobile emission sources. This geographic setting—a river valley city in an industrial belt, encircled by mountains—creates a microclimate where pollutants can accumulate, especially when regional winds are weak, making terrain a key determinant of air quality alongside local emissions.

Yong'an's air quality follows a distinct seasonal rhythm shaped by the East Asian monsoon and local topography. In winter (December–February), cold, stable conditions often lead to temperature inversions in the valley, trapping pollutants from heating and industrial sources, resulting in the year's worst air quality; sensitive groups should limit outdoor activities during these months. Spring (March–May) brings increased rainfall and stronger winds from the southeast, which help disperse pollutants, making it a favourable time for outdoor pursuits, though occasional fog can temporarily degrade visibility. Summer (June–August) sees the peak of the monsoon, with heavy rains cleansing the air, but high temperatures and humidity can combine with ozone formation from vehicle emissions on sunny days, posing risks for those with respiratory conditions. Autumn (September–November) is generally the best season, with mild weather, reduced rainfall, and consistent breezes that maintain good air quality, ideal for hiking in the surrounding mountains. Throughout the year, wind patterns are crucial: southeasterly winds from the coast improve air quality, while calm days allow buildup. Residents should monitor local forecasts and plan activities for mornings or after rains when pollution levels are typically lower.