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Sầm Sơn, a coastal city in Thanh Hóa province, Vietnam, occupies a strategically important position on the South China Sea, approximately 90 kilometers north of Vinh, the provincial capital. Its geography is dominated by a narrow coastal plain fringed by the expansive and often turbulent waters of the sea. The city itself is built around and along the sandy beaches of Sầm Sơn Beach, a major draw for tourism. Behind the beach, the terrain rises gently, transitioning into low-lying hills and agricultural land. This gradient creates a localized urban–rural interface, with rice paddies and smaller settlements interspersed within the city limits. The surrounding landscape is largely characterized by fertile deltas fed by rivers flowing from the Trường Sơn (Annamite Range) mountains to the west. These rivers, while vital for agriculture and freshwater resources, can also carry pollutants into the coastal zone. The city’s elevation is relatively low, averaging around 10 meters above sea level, which limits dispersion of pollutants and can exacerbate ground-level concentrations. While Sầm Sơn benefits from sea breezes, the proximity to agricultural zones, particularly those utilizing fertilizers and pesticides, and the potential for industrial activity further inland, contribute to localized air quality challenges. The coastal location also means the city is susceptible to sea spray and salt deposition, which can interact with other pollutants. The prevailing winds generally blow from the west, carrying potential contaminants from inland sources towards the coast.

Sầm Sơn’s tropical monsoon climate significantly influences its air quality throughout the year. The wet season, typically spanning from September to January, brings frequent rainfall and high humidity, generally leading to improved air quality as precipitation washes away particulate matter. However, periods of prolonged rainfall can also result in stagnant air conditions, particularly in low-lying areas, trapping pollutants released from agricultural activities and occasional industrial emissions. The dry season, from February to August, is characterized by lower rainfall and higher temperatures. This period often sees a build-up of pollutants, especially during the hottest months (June-August), due to increased photochemical reactions and reduced atmospheric cleansing. Temperature inversions, common during the cooler evenings of the dry season, can trap pollutants near the ground, leading to localized spikes in air quality concerns. The monsoon winds, while generally beneficial in dispersing pollutants, can occasionally bring in dust and smoke from regional sources. Fog, particularly prevalent during the transitional months of spring and autumn, can also reduce visibility and trap pollutants. Sensitive groups, such as children, the elderly, and individuals with respiratory conditions, should exercise caution during the dry season, particularly during periods of stagnant air or fog. Outdoor activities are generally more favorable during the wet season, although heavy rainfall can still pose challenges. The agricultural cycle, with burning of crop residue after harvest, can contribute to localized pollution events, particularly in the months following the main harvest season.