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Chang'an, situated in northeastern Hunan Province at coordinates 29.4768°N, 113.4504°E, occupies a strategic position within China's East Asia region. The city lies in the Dongting Lake Plain, approximately 50 kilometers southwest of Yueyang City and 150 kilometers north of Changsha, Hunan's capital. This location places Chang'an within the Yangtze River Basin's middle reaches, with the Yangtze flowing about 40 kilometers to the north and Dongting Lake—China's second-largest freshwater lake—positioned roughly 30 kilometers to the west. The terrain is predominantly flat alluvial plain with an average elevation of 30-50 meters above sea level, characterized by fertile agricultural land interspersed with low hills to the southeast. This low-lying topography creates natural drainage basins but also facilitates pollutant accumulation under certain meteorological conditions. Chang'an's urban character reflects a typical Chinese county-level city transitioning from agricultural dominance to mixed urban development, with a population of 433,200 creating distinct urban-rural gradients. The city serves as a regional hub for light manufacturing, food processing, and agricultural distribution, situated within Hunan's broader industrial-agricultural belt. Proximity to Dongting Lake influences local microclimates through lake-effect moisture, while the surrounding agricultural zones contribute seasonal biomass burning emissions. The flat landscape allows relatively unimpeded wind flow from the northwest during winter, but during calm periods, pollutants from urban activities, transportation networks, and regional industries can become trapped, particularly when temperature inversions develop over the basin-like terrain.

Chang'an experiences distinct seasonal air quality patterns influenced by the East Asian monsoon system and local topography. During winter (December-February), cold, dry northerly winds from the Siberian High transport pollutants from northern industrial regions, while frequent temperature inversions trap local emissions from heating and industry, creating the year's worst air quality—particularly in January when stagnant conditions prevail. Sensitive groups should limit outdoor activities during these months and use air purifiers indoors. Spring (March-May) brings transitional weather with increasing rainfall that helps cleanse the atmosphere, though March can still experience poor air quality from lingering winter patterns and agricultural burning. By April-May, conditions generally improve as monsoon winds shift. Summer (June-August) features the East Asian summer monsoon with southerly winds from the South China Sea bringing cleaner maritime air, though high temperatures and sunlight can generate ground-level ozone, particularly in July-August. Morning outdoor activities are recommended before ozone peaks. Autumn (September-November) begins with relatively good air quality in September-October as monsoon retreats gradually, but November sees deteriorating conditions as winter patterns establish, with increased biomass burning from agricultural residue contributing to particulate pollution. The annual cycle shows pollution typically peaks in winter months, particularly January, while summer months offer the most favorable conditions for outdoor activities.