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Zhoushan is an archipelago city situated in the East China Sea, comprising over 1,390 islands at the confluence of the Yangtze River Delta and Hangzhou Bay. As China's only prefecture-level island city, its urban character blends maritime heritage with modern port development, creating a unique urban-rural gradient where dense fishing villages transition to industrial zones on larger islands like Zhoushan Island. The city's coordinates at 29.9856°N, 122.2074°E place it within the Zhejiang coastal industrial belt, approximately 150 kilometers southeast of Shanghai across Hangzhou Bay. Terrain varies from low-lying coastal plains to modest hills, with average elevations under 50 meters, though some islands feature peaks up to 500 meters. This archipelagic geography profoundly influences air quality through natural ventilation from sea breezes that typically disperse pollutants, yet proximity to major shipping lanes and the Yangtze River Delta's industrial heartland means Zhoushan receives transported pollution from mainland sources. The surrounding landscape includes extensive fishing grounds, aquaculture zones, and strategic port facilities, with the Zhoushan Port being among the world's busiest. While the islands themselves have limited heavy industry beyond shipbuilding and fisheries processing, their position downwind from Shanghai and Ningbo's industrial complexes means Zhoushan's air quality reflects regional pollution patterns more than purely local emissions.

Zhoushan's air quality follows distinct seasonal patterns shaped by the East Asian monsoon and maritime influences. Spring (March-May) brings variable conditions as winter pollution gradually dissipates with increasing southeasterly winds, though occasional temperature inversions can trap pollutants during calm periods, particularly in April when agricultural burning upwind may contribute haze. Summer (June-August) offers the cleanest air as prevailing southerly monsoon winds from the Pacific Ocean sweep across the islands, dispersing pollutants effectively; this season is ideal for outdoor activities with minimal pollution concerns. Autumn (September-November) sees a transition as winds shift northerly, sometimes carrying industrial emissions from the Yangtze River Delta, with October-November potentially experiencing increased particulate matter as heating season begins upwind. Winter (December-February) presents the most challenging conditions with frequent temperature inversions that trap cold air near the surface, compounded by northerly winds transporting pollution from mainland industrial zones; January typically sees peak pollution levels. Sensitive groups should monitor air quality forecasts year-round, with particular caution during winter inversions and autumn transition periods when wearing masks outdoors may be advisable. The maritime climate generally prevents extreme pollution episodes, but regional transport means Zhoushan rarely experiences pristine air even during favorable seasons.