[1] Size-segregated measurements of cloud condensation nucleus (CCN) activity and hygroscopic growth were performed simultaneously for sulfate-rich aerosols at Cape Hedo, Okinawa, Japan, in spring 2008. The CCN fractions as functions of particle size at water vapor supersaturations of 0.44%, 0.25%, and 0.10% had nearly stepwise increases, and the diameters for 50% activation of its maximum (d act ) were close to that of (NH 4 ) 2 SO 4 . The size-resolved hygroscopic growth factor g measured using a hygroscopicity tandem differential mobility analyzer at 85% relative humidity (RH) mainly showed unimodal and highly hygroscopic characteristics. The observed characteristics as well as aerosol mass spectrometer data suggest the dominance of internally mixed ammoniated sulfate-rich particles. A clear negative correlation between d act and median g (g median ) was observed for Aitken-mode particles, and backward air mass trajectories indicate lower d act and higher g of the aerosols from China and the Pacific and the opposite tendency for those from Korea and Japan. The size dependence of g median suggests that less hygroscopic carbonaceous components were more enriched in Aitkenmode particles and therefore affect the CCN activity and hygroscopicity. The CCN activation diameters were predicted on the basis of g median using a core-shell model. The modeled activation diameters reasonably agreed with measured d act , suggesting that the surface tension lowering effect due to organics and the enhancement of bulk hygroscopicity at high RH due to sparingly soluble or polymeric compounds were small. The results suggest that CCN activity of sulfate-rich aerosol particles is predicted well in regional and global aerosol models without incorporating these effects.

Abstract. Tethered balloon-borne aerosol measurements were conducted at Syowa Station, Antarctica during the 46th Japanese Antarctic expedition (2005)(2006). The CN concentration reached a maximum in the summer, although the number concentrations of fine particles (D p >0.3 µm) and coarse particles (D p >2.0 µm) increased during the winterspring. The CN concentration was 30-2200 cm −3 near the surface (surface -500 m) and 7-7250 cm −3 in the lower free troposphere (>1500 m). During the austral summer, higher CN concentration was often observed in the lower free troposphere, where the number concentrations in fine and coarse modes were remarkably lower. The frequent appearance of higher CN concentrations in the free troposphere relative to continuous aerosol measurements at the ground strongly suggests that new particle formation is more likely to occur in the lower free troposphere in Antarctic regions. Seasonal variations of size distribution of fine-coarse particles show that the contribution of the coarse mode was greater in the winterspring than in summer because of the dominance of sea-salt particles in the winter-spring. The number concentrations of fine and coarse particles were high in air masses from the ocean and mid-latitudes. Particularly, aerosol enhancement was observed not only in the boundary layer, but also in the lower free troposphere during and immediately after Antarctic haze events occurring in May, July and September.

[1] Hygroscopic growth of aerosol particles at 85% relative humidity and the number fraction of cloud condensation nuclei (CCN; 0.42%, 0.23%, and 0.10% supersaturation) as a function of dry diameter (24.1-359 nm) were measured simultaneously on board R/V Hakuho-Maru over the western North Pacific during August-September 2008. Highly hygroscopic and unimodal growth distributions were observed, except for aerosols, which showed lower hygroscopic growth over the northern Pacific. The measured particle hygroscopicity, CCN activation diameters, and chemical composition data suggest the dominance of internally mixed sulfate aerosols. Backward air mass trajectory analysis exhibits an intrusion of free tropospheric aerosol, which was likely influenced by Kasatochi's volcanic plume and which was linked to the low-hygroscopicity event. Frequent observation of the Hoppel minimum suggests that in-cloud processing over the Pacific enhanced and/or maintained the high hygroscopicity of accumulation mode particles. The CCN activation diameters predicted from median hygroscopic growth factors (g median ) agreed well with those determined from the CCN efficiency spectra, without assuming surface tension reduction caused by organics or enhancement of bulk hygroscopicity at high RH caused by sparingly soluble or polymeric compounds. The CCN spectra predicted from g median and measured CCN activation diameters suggest that the high CCN activities of particles over the North Pacific are sustained by high hygroscopicity, while sporadic changes of aerosol origins produce the diversity of the aerosol properties.Citation: Mochida, M., C. Nishita-Hara, H. Furutani, Y. Miyazaki, J. Jung, K. Kawamura, and M. Uematsu (2011), Hygroscopicity and cloud condensation nucleus activity of marine aerosol particles over the western North Pacific, J. Geophys.

Abstract. The size distribution and volatility of ultrafine aerosol particles were measured using scanning mobility particle sizer and thermodenuder at Syowa Station during the 46-47 Japanese Antarctic Research Expeditions (2005)(2006)(2007). The relative abundance of non-volatile particles in a 240 • C scan was approximately 20 % during the summer, whereas the abundance of non-volatile particles increased by >90 % during the winter-spring. Most ultrafine particles were volatilized at temperature of 150-210 • C. This volatility was consistent well to major aerosol constituents (NH + 4 , SO 2− 4 and CH 3 SO − 3 ) during the summer. In contrast, major constituents of ultrafine particles were sea-salts (Na + and Cl − ) in winter-spring. Therefore, the seasonal feature of volatility of ultrafine particles at Syowa was associated with seasonal variations of the major aerosol constituents. Although the relative abundance of non-volatile particles was usually higher during the winter-spring, the abundance dropped occasionally to <30 %. The lower abundance of non-volatile ultrafine particles during winter-spring corresponded to the lower number concentration of ultrafine particles and transport from the free troposphere over Antarctica.