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