Hygroscopicity and cloud condensation nucleus activity of forest aerosol particles during summer in Wakayama, Japan

Abstract

Size-resolved distributions of the hygroscopic growth factor (g) and the ratios of cloud condensation nuclei (CCN) to condensation nuclei were observed at a forest site during summer in Japan. The g distributions at 85% relative humidity were unimodal. During 0900-2100 Japan Standard Time (JST) on new particle formation (NPF) event days, less hygroscopic particles (g ~ 1.1) were dominant in the Aitken-mode range and the CCN activation diameters of the aerosols were large. These results are explained by the substantial contribution from newly formed biogenic secondary organic aerosol (BSOA). Hygroscopicity parameter κ for newly formed Aitken particles, calculated from g and CCN activation diameters, were 0.12 and 0.16, respectively, which were estimated to be the κ of organics. The κ values of particles were higher during 2100-0900 JST on NPF event days, in which the aerosols were characterized by the dominance of large and more hygroscopic particles. The number fractions of CCN that were predicted from time- and size-resolved g at 0.23% and 0.41% supersaturations better matched the measured values compared to the cases with the time-averaged g and/or g for the bulk composition, which suggests that the differences in particle hygroscopicity with time and size are important to CCN activation. A cloud parcel model indicates that the contributions from less hygroscopic particles to the number concentrations of CCN and cloud droplets were potentially large during NPF event days, which suggests a marked contribution from locally formed BSOA particles alongside particles from background air.