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

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 aerosol physicochemical properties during SOA formation from the photochemistry of single or mixed biogenic (α-pinene, isoprene) and anthropogenic (o-cresol) volatile organic compounds (VOCs) in the presence of ammonium sulphate seeds. During the six-hour experiments, the cloud condensation nuclei (CCN) activity at super-saturation of water (0.1 ~ 0.5 %), hygroscopic growth factor at 90 % RH, and non-refractory PM1 chemical composition were recorded concurrently. The hygroscopicity parameter κ was used to represent water uptake ability below and above water saturation, and the κ-Kӧhler approach was implemented to predict the CCN activity from the sub-saturated hygroscopicity.

The sub- and super-saturated water uptake (in terms of 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 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. This is possibly attributable to the non-ideality of solutes at different RH or the different co-condensation of condensable organic vapours within the two instruments. 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 κ-Kӧhler approach on CCN prediction can introduce a variable uncertainty in predicting cloud droplet numbers from the sub-saturated water uptake.