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Hyesan, nestled in the Ryanggang Province of North Korea, occupies a geographically challenging position bordering both China and Russia. Situated at coordinates 41.4000, 128.1833, the city is deeply embedded within a mountainous terrain, part of the Changbai Mountains range. This rugged landscape, with elevations rising sharply around the city, significantly influences local air quality. The Tumen River forms a natural border with China to the south, but its influence on Hyesan’s immediate air circulation is limited by the surrounding peaks. Hyesan’s urban character is defined by its relatively compact layout, constrained by the topography. The surrounding region is predominantly rural, characterized by forestry and limited agricultural activity, primarily subsistence farming on terraced slopes. Industrial activity within Hyesan itself is focused on mining and processing of local mineral resources, including coal and magnesite, contributing to localised pollution sources. The urban-rural gradient is steep; moving just a few kilometers from the city centre quickly leads to sparsely populated mountainous areas. This topography creates potential for temperature inversions, trapping pollutants within the valley during stable atmospheric conditions, exacerbating air quality issues. The city’s location within a basin further restricts air dispersal.

Hyesan experiences a harsh continental climate, profoundly impacting its air quality throughout the year. Winters (November to March) are exceptionally cold and dry, with frequent high-pressure systems leading to stagnant air and increased concentrations of particulate matter from coal burning for heating. Temperature inversions are common during these months, trapping pollutants near the ground. Spring (April-May) brings a gradual warming trend, but dust storms originating from the Gobi Desert can significantly degrade air quality. Summer (June-August) is relatively warm and wetter, with monsoon rains offering some respite by washing away pollutants, though humidity can also contribute to secondary pollutant formation. However, increased agricultural activity can release ammonia. Autumn (September-October) is generally considered the most pleasant season, with clear skies and moderate temperatures, offering the best air quality. However, the burning of agricultural waste after harvest can cause temporary spikes in particulate matter. Sensitive groups – children, the elderly, and those with respiratory conditions – should avoid strenuous outdoor activity during winter and spring. Monitoring wind direction is crucial; winds from China can carry pollutants into Hyesan, and vice versa. Fog is frequent, particularly in the colder months, further reducing visibility and trapping pollutants.