Size‐segregated measurements of cloud condensation nucleus activity and hygroscopic growth for aerosols at Cape Hedo, Japan, in spring 2008

Abstract

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 (dact) were close to that of (NH4)2SO4. 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 dact and median g (gmedian) was observed for Aitken‐mode particles, and backward air mass trajectories indicate lower dact 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 gmedian suggests that less hygroscopic carbonaceous components were more enriched in Aitken‐mode particles and therefore affect the CCN activity and hygroscopicity. The CCN activation diameters were predicted on the basis of gmedian using a core‐shell model. The modeled activation diameters reasonably agreed with measured dact, 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.