On aerosol hygroscopicity, cloud condensation nuclei (CCN) spectra and critical supersaturation measured at two remote islands of Korea between 2006 and 2009

Abstract. Aerosol size distribution, total concentration (i.e. condensation nuclei (CN) concentration, N_CN), cloud condensation nuclei (CCN) concentration (N_CCN), hygroscopicity at ~90% relative humidity (RH) were measured at a background monitoring site at Gosan, Jeju Island, south of the Korean Peninsula in August 2006, April to May 2007 and August to October 2008. Similar measurements took place in August 2009 at another background site (Baengnyeongdo Comprehensive Monitoring Observatory, BCMO) on the island of Baengnyeongdo, off the west coast of the Korean Peninsula. Both islands were found to be influenced by continental sources regardless of season and year. Average values for all of the measured N_CCN at 0.2, 0.6 and 1.0% supersaturations (S), N_CN, and geometric mean diameter (D_g) from both islands were in the range of 1043–3051 cm⁻³, 2076–4360 cm⁻³, 2713–4694 cm⁻³, 3890–5117 cm⁻³ and 81–98 nm, respectively. Although the differences in D_g and N_CN were small between Gosan and BCMO, N_CCN at various S was much higher at the latter, which is closer to China.

Most of the aerosols were internally mixed and no notable differences in hygroscopicity were found between the days of strong pollution influence and the non-pollution days for both islands. During the 2008 and 2009 campaigns, critical supersaturation for CCN nucleation (S_c) for selected particle sizes was measured. Particles of 100 nm diameters had mean S_c of 0.19 ± 0.02% during 2008 and those of 81 and 110 nm diameters had mean S_c of 0.26 ± 0.07% and 0.17 ± 0.04%, respectively, during 2009. The values of the hygroscopicity parameter (κ), estimated from measured S_c, were mostly higher than the κ values obtained from the measured hygroscopic growth at ~90% RH.

For the 2008 campaign, N_CCN at 0.2, 0.6 and 1.0% S were predicted based on measured dry particle size distributions and various ways of representing particle hygroscopicity. The best closure was obtained when temporally varying and size-resolved hygroscopicity information from the HTDMA was used, for which the average relative deviations from the measured values were 28 ± 20% for 0.2% S (mostly under-prediction), 25 ± 52% for 0.6% (balanced between over- and under-prediction) and 19 ± 15% for 1.0% S (balanced). Prescribing a constant hygroscopicity parameter suggested in the literature (κ = 0.3) for all sizes and times resulted in average relative deviations of 28–41% where over-prediction was dominant. When constant hygroscopicity was assumed, the relative deviation tended to increase with decreasing N_CCN, which was accompanied by an increase of the sub-100 nm fraction. These results suggest that hygroscopicity information for particles of diameters smaller than 100 nm is crucial for more accurate predictions of N_CCN. For confirmation when κ = 0.17, the average κ for sub-100 nm particles in this study, was applied for sub-100 nm and κ = 0.3 for all other sizes, the CCN closure became significantly better than that with κ = 0.3 for all sizes.