| Summary: | Chuncheon, a medium-sized city in South Korea, frequently shows high PM<sub>2.5</sub> concentrations despite scarce anthropogenic emission sources. To identify factors increasing PM<sub>2.5</sub> concentrations, PM<sub>2.5</sub> and its major chemical components were concurrently measured at two different sites, namely, downtown and suburban areas. The average PM<sub>2.5</sub> concentrations at the two sites were similar, but the daily and monthly variations in PM<sub>2.5</sub> and its components were significantly larger at the suburban site. NH<sub>4</sub><sup>+</sup> was significantly higher at the suburban site than at the downtown site, whereas organic carbon (OC) showed the opposite trend. Several PM<sub>2.5</sub> samples showed an abrupt increase during winter at the suburban site, along with an increase in the amount of OC, NH<sub>4</sub><sup>+</sup>, and K<sup>+</sup>, and the correlations between water-soluble OC, K<sup>+</sup>, and NH<sub>4</sub><sup>+</sup> were considerably strong, implying that local biomass burning in the suburban site was an important source of high PM<sub>2.5</sub> episodes. Secondary <i>OC</i> (SOC) concentration was generally lower at the suburban site than at the downtown site, but its contribution to <i>OC</i> increased during winter with an increase in relative humidity, indicating the significance of heterogeneous SOC formation reactions at the suburban site. These results indicate that relevant local measures can be put into place to alleviate the occurrence of high PM<sub>2.5</sub> concentration episodes even in medium-sized residential cities where medium-and long-range transport is anticipated to be significant.
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