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

Abstract

We measured the number concentrations of cloud condensation nuclei (CCN) and the size distributions of CCN/CN (CN: condensation nuclei) ratios at supersaturations (<i>SS</i>s) 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 PM_2.5. The CCN/CN ratios increased with increasing particle diameter, and the diameter at CCN/CN=0.5 was defined as <i>D</i>₅₀. <i>D</i>₅₀ represents the activation dry diameter of atmospheric particles. The average <i>D</i>₅₀ at <i>SS</i>=0.097% and 0.97% was 136&plusmn;17 nm and 31&plusmn;3 nm, respectively. The temporal variation of <i>D</i>₅₀ at <i>SS</i>=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 (<i>D</i>_crit), which is the threshold dry diameter for CCN activation, was calculated from the observed aerosol chemical compositions by Köhler theory for comparison with <i>D</i>₅₀. The <i>D</i>₅₀ at <i>SS</i>=0.097% was correlated (<i>r</i>²=0.48) with calculated <i>D</i>_crit, although <i>D</i>_crit was larger than <i>D</i>₅₀ by 20&ndash;29% on average. The systematic difference between <i>D</i>₅₀ and <i>D</i>_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&plusmn;14% uncertainty in the CCN number concentration estimated from the observed particle number size distribution.