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Discussion papers
https://doi.org/10.5194/acp-2018-891
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2018-891
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 16 Oct 2018

Research article | 16 Oct 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

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

Yange Deng1, Hikari Yai1, Hiroaki Fujinari1, Kaori Kawana2,a, Tomoki Nakayama3,b, and Michihiro Mochida1,3 Yange Deng et al.
  • 1Graduate School of Environmental Studies, Nagoya University, Nagoya, Japan
  • 2Institute of Low Temperature Science, Hokkaido University, Hokkaido, Japan
  • 3Institute for Space-Earth Environmental Research, Nagoya University, Nagoya, Japan
  • anow at: School of Materials and Chemical Technology, Tokyo Institute of Technology, Tokyo, Japan
  • bnow at: Graduate School of Fisheries and Environmental Sciences, Nagasaki University, Nagasaki, Japan

Abstract. Formation of biogenic secondary organic aerosol (BSOA) and its subsequent evolution can modify the hygroscopicity of the organic aerosol component (OA) in the forest atmosphere, and affect the concentrations of cloud condensation nuclei (CCN) there. In this study, size-resolved aerosol hygroscopic growth at 85% relative humidity and size-resolved aerosol composition were measured using a hygroscopic tandem differential mobility analyzer and an aerosol mass spectrometer, respectively, at a forest site in Wakayama, Japan, in August and September 2015. The hygroscopicity parameter of OA (κorg) presented daily minima in the afternoon hours, and it also showed increase with the increase of particle dry diameter. The magnitudes of the diurnal variations of κorg for particles with dry diameters of 100 and 300nm were on average 0.091 and 0.096, respectively, and the difference of κorg between particles with dry diameters of 100 and 300nm was on average 0.056. The relative contributions of the estimated fresh BSOA and regional OA to total OA could explain 40% of the observed diurnal variations and size-dependence of κorg. The hygroscopicity parameter of fresh BSOA was estimated to range from 0.089 to 0.12 for particles with dry diameters from 100 to 300nm. Compared with the use of time- and size-resolved κorg, the use of time- and size-averaged kappa_org leads to under- and over-estimation of the fractional contribution of OA to CCN number concentrations in the range from −4.9 to 26%. This indicates that the diurnal variations and size-dependence of kappa_org strongly affect the overall contribution of OA to CCN concentrations. The fractional contribution of fresh BSOA to CCN number concentrations could reach 0.28 during the period of intensive BSOA formation. The aging of the fresh BSOA, if it occurs, increases the estimated contribution of BSOA to CCN number concentrations by 50–84%.

Yange Deng et al.
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Short summary
Organic aerosol components (OA) take account for 20–90 % of submicrometer aerosol mass and have a broad distribution of hygroscopicity. For the first time, the diurnal variation and size-dependence of the hygroscopicity of OA were characterized for a forest in East Asia. The observed variation and dependence were assessed from the viewpoint of biogenic secondary organic aerosol formation, and they strongly affected the contribution of OA to the CCN number concentration.
Organic aerosol components (OA) take account for 20–90 % of submicrometer aerosol mass and have...
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