Seoul, a megacity and the economic center of South Korea, frequently grapples with some of the highest concentrations of fine particulate matter globally, often experienced as a persistent, visible haze. This poor air quality, characterized by elevated levels of PM2.5 and PM10, is a serious public health concern. The problem results from a combination of local pollution generated within the metropolitan area, external influences, and the region’s unique geography and weather patterns.
Domestic Emission Sources
The sheer density of population and economic activity within the Seoul Metropolitan Area (SMA) creates a continuous stream of localized pollution. Vehicular traffic is a primary domestic contributor, particularly emissions from diesel-fueled vehicles, including older trucks and buses. These vehicles release substantial amounts of nitrogen oxides and fine particulate matter directly into the urban environment.
Local industrial activity, especially in the surrounding Gyeonggi province, adds to the burden through manufacturing and power generation. South Korea’s reliance on coal-fired power plants, even those outside the city limits, releases sulfur dioxide and other precursors that form secondary aerosols. During colder months, residential heating systems also contribute as demand for warmth increases the burning of fossil fuels.
Construction and road dust from continuous urban development across the SMA also account for coarse particulate matter (PM10). Studies have estimated that domestic sources often contribute over 40% of the PM2.5 concentration in the metropolitan area.
The Transboundary Factor
A major portion of Seoul’s air quality challenge originates outside of South Korea’s borders, transported across the Yellow Sea from highly industrialized regions in East Asia. This transboundary pollution is predominantly carried by the prevailing westerly winds, particularly during the winter and spring months. South Korea acts as a receptor for the long-range transport of industrial and agricultural emissions from neighboring nations.
These winds carry primary pollutants, such as sulfur dioxide and nitrogen oxides, which are often byproducts of coal combustion. As these gases travel, they undergo chemical reactions, forming secondary aerosols that contribute significantly to the fine particulate matter (PM2.5) measured in Seoul. During high-pollution episodes, the contribution of transboundary sources can rise to between 60% and 70%.
In the spring, Asian dust, or Hwangsa, storms originating from the deserts of northern China and Mongolia, also contribute. While primarily natural, these massive dust clouds often pick up and transport anthropogenic pollutants, including soot and heavy metals, from industrialized areas. This influx of foreign particles is a powerful external driver of Seoul’s worst air quality days.
Meteorological and Geographical Constraints
Once pollutants enter the atmosphere above the city, Seoul’s unique meteorological and geographical features conspire to trap them close to the ground. The city is situated within a basin, partially encircled by mountains, which restricts the horizontal movement and dispersal of air masses. This basin geography makes it difficult for winds to effectively ventilate the city, allowing pollutants to accumulate.
Synoptic weather conditions, such as migratory high-pressure systems, frequently lead to periods of atmospheric stagnation. These stagnant conditions are characterized by weak winds and stable air, preventing the vertical mixing and horizontal transport necessary to clear the air. The stability is often intensified by temperature inversions, especially during the cold season.
A temperature inversion occurs when a layer of warmer air sits above cooler air near the ground, acting like a lid over the city. Since the cooler, denser air cannot rise, low-lying emissions from traffic and heating are trapped beneath this boundary. This mechanism dramatically increases the concentration of fine particulate matter, turning the Seoul basin into an effective receptacle for accumulated pollution.