Cluster analysis of particle number size distributions downwind of Seoul, Korea

We measured the particle number size distribution using a scanning mobility particle sizer (SMPS) during 13 months from February 2015 to June 2017. The size distribution was measured in the range from 5.4 to 358 nm with 89 channels at 10-min interval in 2015 and in the range from 10.27 to 1093.95 nm with 45 channels at 5-min interval in 2016 and thereafter. The measurement was made at the rooftop of a five-story building on the hill (37.34 °N, 127.27 °E, 167 m above sea level), about 35 km southeast of the center of Seoul, downwind of urban pollution in the prevailing westerlies. The site is located in a horseshoe-shaped forested valley with westward-facing slopes, over which biogenic emissions likely prevail. To the west side is a rural area with small-sized buildings, farmlands, and open spaces scattered on the sides of a 4-lane road and a river. Hourly size distributions prepared by averaging the size distributions at the same hour of day were classified using the k-means clustering method. The clusters were divided into characteristic groups representing typical sources, nucleation, and pollution phenomena. Occurrences of the clusters and interactions between them were examined.