Cloud condensation nuclei activity at Jeju Island, Korea in spring 2005

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 D-50. D-50 represents the activation dry diameter of atmospheric particles. The average D-50 at SS=0.097% and 0.97% was 136+/-17 nm and 31+/-3 nm, respectively. The temporal variation of D-50 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 (D-crit), which is the threshold dry diameter for CCN activation, was calculated from the observed aerosol chemical compositions by Kohler theory for comparison with D-50. The D-50 at SS=0.097% was correlated (r(2)=0.48) with calculated D-crit, although D-crit was larger than D-50 by 20-29% on average. The systematic difference between D-50 and D-crit 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.