1Research Center for Advanced Science and Technology, the University of Tokyo, Tokyo, Japan
2Department of Atmospheric Science, Yonsei University, Seoul, Korea
3Atmospheric Environmental Division, National Institute for Environmental Studies, Tsukuba, Japan
4Geochemical Research Department, Meteorological Research Institute, Tsukuba, Japan
*now at: Institute of Low Temperature Science, Hokkaido University, Japan
Abstract. We measured the number concentrations of cloud condensation nuclei (CCN) and the size distributions of CCN/CN (CN: condensation nuclei) ratios at supersaturations (SSs) of 0.097, 0.27, 0.58, and 0.97% at Jeju Island, Korea during March-April 2005. We made simultaneous measurements of aerosol inorganic ions, water-soluble organic carbon (WSOC), organic carbon (OC), and elemental carbon (EC) in PM2.5. The CCN/CN ratios increased with increasing particle diameter, and the diameter at CCN/CN=0.5 was defined as D50. D50 represents the activation dry diameter of atmospheric particles. The average D50 at SS=0.097% and 0.97% was 136±17 nm and 31±3 nm, respectively. The temporal variation of D50 at SS=0.097% was correlated with the mass fraction of water-soluble components (inorganic ions + WSOC), indicating that the temporal variation of CCN activity was mainly controlled by changes in the water-soluble components fraction. The critical dry diameter (Dcrit), which is the threshold dry diameter for CCN activation, was calculated from the observed aerosol chemical compositions by Köhler theory for comparison with D50. The D50 at SS=0.097% was correlated (r2=0.48) with calculated Dcrit, although Dcrit was larger than D50 by 20–29% on average. The systematic difference between D50 and Dcrit could be caused by the size dependence of the aerosol chemical compositions or surface tension lowering caused by the mixing of water-soluble organic compounds. This difference corresponds to a 27±14% uncertainty in the CCN number concentration estimated from the observed particle number size distribution.