Simultaneous Measurement of CCN Activity and Chemical Composition of Fine-Mode Aerosols at Noto Peninsula, Japan, in Autumn 2012

Size resolved CCN activity and bulk chemical composition of atmospheric aerosols in the submicrometer size range were measured at Noto Ground-based Research Observatory (NOTOGRO), facing the Sea of Japan at the tip of Noto Peninsula, in autumn 2012. In the atmospheric measurement, the CCN efficiency spectra, where the CCN number fraction is plotted against the diameter of aerosols, were obtained for four different supersaturation (SS) conditions. The hygroscopicity parameter κ, which depends on the chemical composition of aerosols, was estimated from analysis of the CCN spectra. The CCN activation diameters of ambient aerosols were clearly larger than those of pure ammonium sulfate under all SS conditions. The bulk chemical composition derived by an aerosol chemical speciation monitor (ACSM) also indicated the significant mass fraction of organics in the submicrometer size range. The relationship between the estimated κ values and the CCN activation diameters suggests that organics contribute the aerosol mass especially in the size range of less than 100 nm. The contribution of organics observed in this study was more apparent than those of other sites in East Asia. The mass concentrations and chemical compositions of the aerosols varied with trajectory pathway, which caused large fluctuations in κ values especially at higher SS conditions. The negative correlations between organic mass fraction and the diameters of cloud droplet diameters were observed under low SS conditions (0.13% and 0.25%), suggesting that less-hygroscopic organics, that can suppress the initial growth rates of cloud droplets, were included during the periods of large organic mass concentrations.