30 Jul 2021

30 Jul 2021

Review status: this preprint is currently under review for the journal ACP.

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

Yu Wang1, Aristeidis Voliotis1, Dawei Hu1, Yunqi Shao1, Mao Du1, Ying Chen2, M. Rami Alfarra1,3,4, and Gordon McFiggans1 Yu Wang et al.
  • 1Centre for Atmospheric Science, Department of Earth and Environmental Sciences, The University of Manchester, Manchester M13 9PL, UK
  • 2Exeter Climate Systems, University of Exeter, Exeter, EX4 4QE, UK
  • 3National Centre for Atmospheric Science, Department of Earth and Environmental Sciences, The University of Manchester, Manchester, M13 9PL, UK
  • 4Environment & Sustainability Center, Qatar Environment & Energy Research Institute, Doha, Qatar

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 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.

Yu Wang et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2021-577', Anonymous Referee #1, 23 Aug 2021
  • RC2: 'Comment on acp-2021-577', Anonymous Referee #2, 07 Sep 2021

Yu Wang et al.


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Short summary
Aerosol water uptake plays a key role in atmospheric physicochemical processes. We designed chamber experiments on aerosol water uptake of secondary organic aerosol (SOA) from mixed biogenic and anthropogenic precursors with inorganic seed. Our results highlight the chemical composition influences the reconciliation of the sub- and super-saturated water uptake, providing laboratory evidence for understanding the chemical controls of water uptake of the multi-component aerosol.