Abstract
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 an increase with the increase in particle dry diameter. The magnitudes of the diurnal variations in κorg for particles with dry diameters of 100 and 300 nm were on average 0.091 and 0.096, respectively, and the difference in κorg between particles with dry diameters of 100 and 300 nm 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 300 nm. Compared with the use of time- and size-resolved κorg, the use of time- and size-averaged κorg leads to under- and over-estimation of the fractional contribution of OA to CCN number concentrations in the range from −5.0 % to 26 %. This indicates that the diurnal variations and size dependence of κorg 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 52 %–84 %.
Highlights
The hygroscopicity of organic aerosol (OA) components is governed by their chemical composition, and has two important roles in the atmosphere
The difference in κt between particles with ddry of 100 and 300 nm was 0.13, and that of κorg was 0.056. The ranges of both the diurnal variations and the size dependence of κt are similar to those reported from a previous study at the same site in 2010 (Kawana et al, 2017), which pointed out the importance of the variation in particle hygroscopicity with time and size to the cloud condensation nuclei (CCN) number concentration
The size-resolved hygroscopicity at 85 % relative humidity (RH), chemical composition, and number–size distributions of atmospheric aerosols were observed at a forest site in Wakayama, Japan, in August and September 2015
Summary
The hygroscopicity (ability to absorb water) of organic aerosol (OA) components is governed by their chemical composition, and has two important roles in the atmosphere. It is important to study the temporal variation and size dependence of κorg in more locations where OA dominates the aerosol chemical composition, to characterize the κorg values and to represent κorg appropriately in model predictions of CCN number concentrations. The temporal variation and size dependence of κorg of ambient aerosol is reported to relate to 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). Tions (e.g., Han et al, 2014; Thalman et al, 2017) Such processes could result in time- and size-dependent variation in the chemical composition of OA and time- and sizedependent κorg in the forest atmosphere.
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