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Hygroscopicity of Organic Aerosols and Their Contributions to CCN Concentrations Over a Midlatitude Forest in Japan

The formation of biogenic secondary organic aerosols (BSOAs) in forest environments is potentially important to cloud formation via changes of the cloud condensation nuclei (CCN) activity of aerosols. In this study, the CCN activation of submicrometer aerosols and their chemical compositions and size distributions were measured at a midlatitude forest site in Japan during the summer of 2014 to assess the hygroscopicity of the organic aerosols and their contributions to the local CCN concentrations. The mean number concentrations of the condensation nuclei and CCN at supersaturation (SS) conditions of 0.11–0.80% were 1,238 and 166–740 cm−3, respectively. Organic aerosols and sulfate dominated the submicrometer aerosol mass concentrations. The particle hygroscopicity increased with increases in particle diameters. The hygroscopicity parameter for the organics, κorg, was positively correlated with the atomic O to C ratio. The product of κorg and the volume fraction of OA was 0.12, accounting for 38% of the water uptake by aerosol particles. The hygroscopicity parameter of the locally formed fresh BSOA was estimated to be 0.09. The contribution of OA to the CCN number concentration, which was assessed by subtracting the CCN concentration of the hypothetical inorganic aerosols from that of the ambient aerosols, was 50–182 cm−3 for the SS range of 0.11–0.80%. The increase of the CCN number concentrations per 1‐μg/m3 increase of the BSOA was 23–299 cm−3 at 0.11–0.80% SS. The contribution of the BSOA to the CCN number concentration can be enhanced by new particle formation.