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

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 sizeresolved aerosol composition were measured using a hygroscopic tandem differential mobility analyzer and an aerosol mass 15 spectrometer, respectively, at a forest site in Wakayama, Japan, in August and September 2015. The hygroscopicity parameter of OA (org) 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 org for particles with dry diameters of 100 and 300 nm were on average 0.091 and 0.096, respectively, and the difference of 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 20 observed diurnal variations and size-dependence of 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 org, the use of time-and size-averaged 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 org strongly affect the overall contribution of OA to CCN concentrations. The fractional contribution of fresh BSOA to CCN number 25 Atmos. Chem. Phys. Discuss., https://doi.