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. To better understand the chemical controls of sub- and super-saturatedaerosol water uptake, we designed and conducted a series of chamberexperiments to investigate the evolution of secondary organicaerosol (SOA) particle physicochemicalproperties 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 hexperiments, the cloud condensation nuclei (CCN) activity atsuper-saturation of water (0.1 %–0.5 %), hygroscopic growthfactor at 90 % relative humidity (RH), and non-refractory PM1 chemical composition wererecorded concurrently. Attempts to use the hygroscopicity parameter κ to reconcile water uptake ability below and above water saturation fromvarious VOC precursor systems were made, aiming to predict the CCN activityfrom the sub-saturated hygroscopicity. The thermodynamic model AIOMFAC (aerosol inorganic-organic mixtures functional groups activity coefficients) wasused to simulate κ values of model compound mixtures to compare withthe observation and to isolate the controlling factors of water uptake atdifferent RHs. The sub- and super-saturated water uptake (in terms of both κHTDMA and κCCN) were mainly controlled by the SOA massfraction, which depended on the SOA production rate of the precursors, andthe SOA composition played a second-order role. For the reconciliation ofκHTDMA and κCCN, the κHTDMA/κCCN ratio increased with the SOA mass fraction and this was observedin all investigated single and mixed VOC systems, independent of initial VOCconcentrations and sources. For all VOC systems, the mean κHTDMA of aerosol particles was ∼25 % lower than theκCCN at the beginning of the experiments with inorganic seeds.With the increase of condensed SOA on inorganic seed particles throughoutthe experiments, the discrepancy of κHTDMA and κCCN became weaker (down to ∼0 %) and finally themean 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 cannotfully explain the κ discrepancy at high SOA mass fraction (0.8). Agood agreement in κCCN between model and observation wasachieved by doubling the molecular weight of the model compounds or byreducing the dry particle size in the CCN counter. This indicates that theevaporation of semi-volatile organics in the CCN counter together withnon-ideality could have led to the observed κ discrepancy. As aresult, the predicted CCN number concentrations from the κHTDMAand particle number size distribution were ∼10 % lowerthan CCN counter measurement on average at the beginning, and further eventurned to an overestimation of ∼20 % on average when theSOA mass fraction was ∼0.8. This chemicalcomposition-dependent performances of the κ-Köhler approach on CCNprediction can introduce a variable uncertainty in predicting cloud dropletnumbers from the sub-saturated water uptake, the influence of which onmodels still needs to be investigated.

On the evolution of sub- and super-saturated water uptake of secondary organic aerosol in chamber experiments from mixed precursors