Classification of large-scale circulation patterns and their spatio-temporal variability during High-PM10 events over the Korean Peninsula

The typical spatio-temporal evolutions of large-scale weather patterns before and after high-concentration events of particulate matter with an aerodynamic diameter <= 10 mu m (PM10) (High-PM10) over the Korean Peninsula were investigated. Using the K-means clustering method, five significant large-scale weather patterns associated with High-PM10 events were identified. Among the 91 High-PM10 events that occurred during winter/spring of 2007-2018, approximately 41% (38 cases) belonged to Cluster 1 and 5, characterized by upper-tropospheric downstream amplification of pressure anomaly originating from the Barents/Kara Sea of the Arctic Ocean. We refer to this pattern as the wave-train type. Concurrently with this pattern, southwesterly wind anomalies from southeastern China and significant descending air occurred over the Korean Peninsula, causing PM10 accumulation. Despite its dominance of occurrence, the wave-train type exhibited a relatively shorter mean duration of 1-2 days owing to its rapid development and traveling characteristics. Cluster 2 corresponded to approximately 29% (27 cases) of the total cases, and was characterized by a stagnant anticyclone over the northeastern Eurasia region. In the lower atmosphere, southerly winds dominated the Korean Peninsula. Vertical diffusion of PM10 was restricted by a vertically stable condition in the lower-to-mid-troposphere and a notably weak wind speed. Cluster 2 had the longest duration of 3 days and the highest mean PM10 concentration compared to the other clusters. Cluster 3 corresponded to approximately 13% (12 cases) of the total cases and exhibited a stagnant anticyclone centered at the Kamchatka region. In this case, weakened westerly is observed resulting in the characteristics of a circulation pattern that easily accumulates pollutants. Cluster 4 accounted for approximately 15% (14 cases) of all cases, and is a pattern in which air of PM10 inflows following the back of the cyclone located in the northern part of the Korean Peninsula. We emphasize that most of High-PM10 events occurring over the Korean Peninsula are linked to the preceding upper-tropospheric large-scale circulation patterns. They control the low-level transports and accumulation of pollutants.