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Yongji, situated in southwestern Shanxi Province at coordinates 34.8670°N, 110.4480°E, occupies a strategic position in China's Loess Plateau region. The city lies approximately 50 kilometers north of the Yellow River, which flows eastward through the fertile Fen River valley, creating an agricultural belt that surrounds the urban core. Yongji's elevation of roughly 350 meters above sea level places it within a basin-like terrain, flanked by the Zhongtiao Mountains to the south and the Lüliang Mountains to the north, forming a natural amphitheater that can trap airborne pollutants. This topographic enclosure, combined with its location in the industrial heartland of Shanxi—a province renowned for coal mining and heavy manufacturing—exacerbates air quality challenges. The urban–rural gradient is pronounced, with the city center transitioning rapidly to farmlands and then to mining zones in the hinterlands. Proximity to major transportation corridors, including highways linking to provincial capital Taiyuan and neighboring Henan Province, adds vehicular emissions to the mix. The absence of large water bodies nearby limits natural cleansing effects, while the loess soil contributes to particulate matter during dry spells. This geographic setting creates a perfect storm for pollution accumulation, particularly under stable atmospheric conditions.

In Yongji, air quality follows a distinct seasonal rhythm shaped by the temperate continental monsoon climate. Winter, from December to February, brings the worst pollution due to coal-fired heating, industrial emissions, and frequent temperature inversions that trap pollutants in the basin. Cold, stagnant air with low wind speeds exacerbates PM2.5 and sulfur dioxide levels, making outdoor activities hazardous, especially for children, the elderly, and those with respiratory conditions. Spring, from March to May, sees gradual improvement as temperatures rise and winds pick up, dispersing pollutants, though dust storms from the Loess Plateau can spike particulate matter in April. Summer, from June to August, offers the cleanest air, with monsoon rains from the southeast washing away pollutants and higher temperatures promoting vertical mixing; this is the best time for outdoor exercise. Autumn, from September to November, begins with relatively good air quality but deteriorates by late October as heating season resumes and calm conditions return. Sensitive groups should monitor local air quality indexes closely in winter and early spring, limit outdoor exposure during peak pollution days, and use air purifiers indoors. Planning visits for summer months maximizes health benefits while avoiding the hazardous winter smog.