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

Abstract

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.

Diurnal variation and size-dependence of the hygroscopicity of organic aerosol at a forest site in Wakayama, Japan: their relationship to CCN concentrations

“…Previous observational studies at the site indicated that BSOA formation was intensive and that aging occurred after formation (Han et al, 2014;Deng et al, 2018). It was also observed that org was time and size dependent and that the contribution of OA and BSOA to CCN number concentrations could be substantial (Deng et al, 2018). This work is an extension of previous studies on the hygroscopicity 5 and CCN activity of aerosols, and the contributions of OA and BSOA to the CCN concentration, in the same forest (Kawana et al, 2017;Deng et al, 2018).…”

“…Furthermore, the influence of these variations on the fractional contribution of OA and BSOA to the CCN concentration was assessed. Previous observational studies at the site indicated that BSOA formation was intensive and that aging occurred after formation (Han et al, 2014;Deng et al, 2018). It was also observed that org was time and size dependent and that the contribution of OA and BSOA to CCN number concentrations could be substantial (Deng et al, 2018).…”

“…ground-based observations in the City of Nagoya in Japan under SUBS presented relatively low org in small particles (0.12-0.15; 60 and 100 nm) and high org in large particles (0.17-0.22; 200 and 359 nm) (Kawana et al, 2016). For aerosols in forest areas, whereas Deng et al (2018) reported a difference of 0.03 in mean org between sub-100 nm particles (org was 0.19) and ~150 nm particles (org was 0.22), Thalman et al (2017) did not identify any size-dependence of org for 94-171 nm particles.…”

“…Bougiatioti et al (2016) reported diurnal variation of org in the range 0.09-0.18 (day to night) under SUPS for particles influenced by biomass burning in the eastern Mediterranean. Deng et al (2018) reported diurnal variation ranges of org of 0.09 to 0.30 (day to night) and 0.16 to 0.24 (day to night) on days with and without evident new particle formation (NPF), respectively, under SUPS (0.23 % water vapor supersaturation (SS) condition) in a forest in Wakayama, Japan. Different from the above studies, at a rural site in the 20 southeastern United States, a small diurnal variation of org (~0.13 to ~0.17, night to day) under SUPS was observed (Cerully et al, 2015).…”

“…The temporal variation and size-dependence of org of ambient aerosol is reported to relate with variations in the chemical composition of OA, which can result from the mixing of aerosols of different origins, formation of SOA, and aging processes (Cerully et al, 2015;Bougiatioti et al, 2016;Shingler et al, 2016;Thalman et al, 2017;Deng et al, 2018). The size-dependent chemical composition of model SOA has been explained by the size-dependent contributions of different organic vapors to particle growth (e.g., Winkler et al, 2012;Ehn et al, 2014;Zhao et al, 2015;Zhao et al, 2016).…”