资源环境科技发展态势分析平台

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 (kappa(org)) 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 k org for particles with dry diameters of 100 and 300 nm were on average 0.091 and 0.096, respectively, and the difference in kappa(org) 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 kappa(org). 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 kappa(org), the use of time- and size-averaged kappa(org) leads to under- and overestimation 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 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 52%-84%.