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Nghi Sơn is a coastal city in Thanh Hóa Province, northern Vietnam, situated along the Gulf of Tonkin at coordinates 19.4170°N, 105.7500°E. This strategic location places it within the Red River Delta region, approximately 200 kilometers south of Hanoi, where it serves as a key industrial and port hub. The city's terrain is characterized by low-lying coastal plains, with elevations generally below 50 meters, transitioning gently inland to rolling hills and agricultural zones. Proximity to the South China Sea significantly influences its microclimate, with sea breezes often moderating temperatures but also potentially trapping pollutants when atmospheric conditions stagnate. Nghi Sơn is home to the massive Nghi Sơn Refinery and Petrochemical Complex, one of Vietnam's largest industrial projects, which dominates the local economy and contributes substantially to air pollution through emissions of particulate matter, sulfur dioxide, and volatile organic compounds. The urban-rural gradient is stark, with dense urban areas around the port and refinery contrasting with surrounding rice paddies and fishing villages. This mix creates a complex air quality dynamic, where industrial emissions interact with agricultural burning residues and urban traffic. The coastal setting can both disperse pollutants via onshore winds and concentrate them during temperature inversions, particularly in the sheltered valleys inland. The city's geography, as a nexus of industry, agriculture, and maritime activity, makes it a focal point for air quality challenges in Southeast Asia, with its airshed influenced by regional transport from the industrial belts of northern Vietnam.

In Nghi Sơn, air quality follows a distinct seasonal pattern shaped by the East Asian monsoon. During the dry season from November to April, northeasterly winds prevail, bringing cooler, drier air from mainland Asia. This period often sees the worst pollution, particularly from December to February, when temperature inversions trap industrial and urban emissions close to the ground, exacerbated by low wind speeds and frequent fog. Sensitive groups, such as children, the elderly, and those with respiratory conditions, should limit outdoor activities on calm, hazy days in these months. The wet season from May to October brings southwesterly monsoon winds from the ocean, which increase rainfall and ventilation, scattering pollutants and leading to clearer skies. June to August typically offers the best air quality, favoring outdoor exercise and tourism. However, occasional tropical storms or typhoons can stir up dust and debris, causing short-term spikes. Transitional months like October and April may see variable conditions as monsoon shifts occur. Health guidance includes monitoring local air quality reports, using masks during peak pollution periods, and planning outdoor activities for early mornings when winds are fresher. The refinery's operations are continuous, but meteorological factors like humidity and wind direction modulate their impact, making seasonal awareness key for residents and visitors managing exposure risks.