On the evolution of sub- and super-saturated water uptake of secondary organic aerosol in chamber experiments from mixed precursors
Authors: Wang Yu , Voliotis Aristeidis , Hu Dawei , Shao Yunqi , Du Mao , Chen Ying , Kleinheins Judith , Marcolli Claudia , Alfarra M. Rami , McFiggans Gordon 
Published: March 2022

Abstract. To better understand the chemical controls of sub- and super-saturated
aerosol water uptake, we designed and conducted a series of chamber
experiments to investigate the evolution of secondary organic
aerosol (SOA) particle physicochemical
properties during photo-oxidation of single and mixed biogenic (α-pinene, isoprene) and anthropogenic (o-cresol) volatile organic compounds
(VOCs) in the presence of ammonium sulfate seeds. During the 6 h
experiments, the cloud condensation nuclei (CCN) activity at
super-saturation of water (0.1 %–0.5 %), hygroscopic growth
factor at 90 % relative humidity (RH), and non-refractory PM1 chemical composition were
recorded concurrently. Attempts to use the hygroscopicity parameter κ to reconcile water uptake ability below and above water saturation from
various VOC precursor systems were made, aiming to predict the CCN activity
from the sub-saturated hygroscopicity. The thermodynamic model AIOMFAC (aerosol inorganic-organic mixtures functional groups activity coefficients) was
used to simulate κ values of model compound mixtures to compare with
the observation and to isolate the controlling factors of water uptake at
different RHs. The sub- and super-saturated water uptake (in terms of both κHTDMA and κCCN) were mainly controlled by the SOA mass
fraction, which depended on the SOA production rate of the precursors, and
the SOA composition played a second-order role. For the reconciliation of
κHTDMA and κCCN, the κHTDMA/κCCN ratio increased with the SOA mass fraction and this was observed
in all investigated single and mixed VOC systems, independent of initial VOC
concentrations and sources. For all VOC systems, the mean κHTDMA of aerosol particles was ∼25 % lower than the
κCCN at the beginning of the experiments with inorganic seeds.
With the increase of condensed SOA on inorganic seed particles throughout
the experiments, the discrepancy of κHTDMA and κCCN became weaker (down to ∼0 %) and finally the
mean κHTDMA was ∼60 % higher than κCCN on average when the SOA mass fraction approached ∼0.8. As indicated by AIOMFAC model simulations, non-ideality alone cannot
fully explain the κ discrepancy at high SOA mass fraction (0.8). A
good agreement in κCCN between model and observation was
achieved by doubling the molecular weight of the model compounds or by
reducing the dry particle size in the CCN counter. This indicates that the
evaporation of semi-volatile organics in the CCN counter together with
non-ideality could have led to the observed κ discrepancy. As a
result, the predicted CCN number concentrations from the κHTDMA
and particle number size distribution were ∼10 % lower
than CCN counter measurement on average at the beginning, and further even
turned to an overestimation of ∼20 % on average when the
SOA mass fraction was ∼0.8. This chemical
composition-dependent performances of the κ-Köhler approach on CCN
prediction can introduce a variable uncertainty in predicting cloud droplet
numbers from the sub-saturated water uptake, the influence of which on
models still needs to be investigated.