To characterize organic and elemental carbon (OC and EC), and water-soluble organic carbon (WSOC) contents, daily $PM_{2.5}$ measurements were performed in August 2006 (summer) and Jan $11{sim}Feb$ 12 2007 (winter) at an urban site of Gwangju. Daily size-segregated aerosol samples were also collected for WSOC analysis. No clear seasonal variations in EC and WSOC concentrations were observed, while seasonal differences in OC concentration, and OC/EC and WSOC/EC ratios were shown. The WSOC/OC ratio showed higher value in summer (0.56) than in winter (0.40), reflecting the greater enhancement of secondary WSOC formation at the site in summer. Secondary WSOC concentrations estimated using EC tracer method were in the range $0.0{sim}2.1;{mu}g/m^3$ (average $0.42;{mu}g/m^3$) and $0.0{sim}1.1;{mu}g/m^3;(0.24;{mu}g/m^3)$, respectively, accounting for $0{sim}51.6%$ (average 16.8%) and $0{sim}52.5%$ (average 13.1 %) of the measured WSOC concentrations in summer and winter. Sometimes higher WSOC/OC ratio in winter than that in summer could be attributed to two reasons. One is that the stable atmospheric condition often appears in winter, and the prolonged residence time would strengthen atmospheric oxidation of volatile organic compounds. The other is that decrease of ambient temperature in winter would enhance the condensation of volatile secondary WSOC on pre-existing aerosols. In summertime, atmospheric aerosols and WSOC concentrations showed bimodal size distributions, peaking at the size ranges $0.32{sim}0.56;{mu}m$ (condensation mode) and $3.2{sim}5.6;{mu}m$ (coarse mode), respectively. During the wintertime, atmospheric aerosols showed a bimodal character, while WSOC concentrations showed a unimodal pattern. Size distributions of atmospheric aerosols and WSOC with a peak in the size range $0.32{sim}0.56;{mu}m$ were observed for most of the measurement periods. On January 17, however, atmospheric aerosols and WOSC exhibited size distributions with modal peaks in the size range $1.0{sim}1.8;{mu}m$, suggesting that the aerosol particles collected on that day could be expected to be more aged, i.e, longer residence time, than the aerosols at other sampling periods.