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Rui'an, situated in southeastern Zhejiang Province along China's eastern coast, occupies a distinctive position within the Wenzhou metropolitan area, approximately 50 kilometers south of Wenzhou city proper. The city lies in the Ou River basin, with its urban core positioned at the confluence of the Feiyun River and its tributaries, creating a complex delta landscape that significantly influences local air dynamics. Surrounded by the Yandang Mountains to the northwest and the Dongtou Archipelago to the east, Rui'an's terrain transitions from coastal plains to rolling hills, with elevations generally below 100 meters except for peripheral mountainous areas. This geography creates a semi-enclosed basin effect that can trap pollutants, particularly during stable atmospheric conditions. The city's location within the Yangtze River Delta economic zone—one of China's most industrialized regions—means it experiences both local emissions from its manufacturing sectors (notably automotive parts, machinery, and textiles) and regional transport of pollution from neighboring industrial centers. Proximity to the East China Sea provides some maritime ventilation, but the urban-rural gradient shows concentrated pollution in the densely populated urban core, where vehicle emissions combine with residential and commercial sources. Agricultural zones in surrounding lowlands contribute occasional biomass burning, while the coastal location brings humid air that can interact with pollutants to form secondary aerosols.

Rui'an's air quality follows a distinct seasonal rhythm shaped by the East Asian monsoon system and local meteorological patterns. During winter (December-February), cold, dry northerly winds from the Asian continent often bring pollutants from northern industrial regions, while temperature inversions in the basin trap local emissions, creating the year's worst air quality—particularly in January and February when heating demands increase. Sensitive groups should limit prolonged outdoor exposure during these months. Spring (March-May) sees gradual improvement as increasing temperatures and more frequent southeasterly winds from the ocean begin dispersing pollutants, though March can still experience haze from lingering winter conditions. Summer (June-August) offers the cleanest air, with the prevailing summer monsoon bringing strong, humid southerly winds that effectively ventilate the basin, and frequent rainfall scrubbing pollutants from the atmosphere—July and August are ideal for outdoor activities. Autumn (September-November) presents a transitional period where decreasing monsoon winds and occasional temperature inversions allow pollution to gradually accumulate, with October and November showing deteriorating conditions as winter approaches. Throughout the year, sea fog from the East China Sea can occasionally mix with pollutants to reduce visibility, particularly in spring and early summer. Residents with respiratory conditions should monitor daily forecasts closely during transitional seasons when wind patterns shift unpredictably.