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Discontinuities in hygroscopic growth below and above water saturation for laboratory surrogates of oligomers in organic atmospheric aerosols

Hodas, Natasha and Zuend, Andreas and Schilling, Katherine and Berkemeier, Thomas and Shiraiwa, Manabu and Flagan, Richard C. and Seinfeld, John H. (2016) Discontinuities in hygroscopic growth below and above water saturation for laboratory surrogates of oligomers in organic atmospheric aerosols. Atmospheric Chemistry and Physics, 16 (19). pp. 12767-12792. ISSN 1680-7324.

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Discontinuities in apparent hygroscopicity below and above water saturation have been observed for organic and mixed organic–inorganic aerosol particles in both laboratory studies and in the ambient atmosphere. However, uncertainty remains regarding the factors that contribute to observations of low hygroscopic growth below water saturation but enhanced cloud condensation nuclei (CCN) activity for a given aerosol population. Utilizing laboratory surrogates for oligomers in atmospheric aerosols, we explore the extent to which such discontinuities are influenced by organic component molecular mass and viscosity, non-ideal thermodynamic interactions between aerosol components, and the combination of these factors. Measurements of hygroscopic growth under subsaturated conditions and the CCN activity of aerosols comprised of polyethylene glycol (PEG) with average molecular masses ranging from 200 to 10 000 g mol⁻¹ and mixtures of PEG with ammonium sulfate (AS) were conducted. Experimental results are compared to calculations of hygroscopic growth at thermodynamic equilibrium conducted with the Aerosol Inorganic Organic Mixtures Functional groups Activity Coefficients (AIOMFAC) model, and the potential influence of kinetic limitations on observed water uptake was further explored through estimations of water diffusivity in the PEG oligomers. Particle-phase behavior, including the prevalence of liquid–liquid phase separation (LLPS), was also modeled with AIOMFAC. Under subsaturated relative humidity (RH) conditions, we observed little variability in hygroscopic growth across PEG systems with different molecular masses; however, an increase in CCN activity with increasing PEG molecular mass was observed. This effect is most pronounced for PEG–AS mixtures, and, in fact, an enhancement in CCN activity was observed for the PEG10000–AS mixture as compared to pure AS, as evidenced by a 15 % reduction in critical activation diameter at a supersaturation of 0.8 %. We also observed a marked increase in apparent hygroscopicity for mixtures of higher molecular mass PEG and AS under supersaturated conditions as compared to subsaturated hygroscopic growth. AIOMFAC-based predictions and estimations of water diffusivity in PEG suggest that such discontinuities in apparent hygroscopicity above and below water saturation can be attributed, at least in part, to differences in the sensitivity of water uptake behavior to surface tension effects. There is no evidence that kinetic limitations to water uptake due to the presence of viscous aerosol components influenced hygroscopic growth. For the systems that display an enhancement in apparent hygroscopicity above water saturation, LLPS is predicted to persist to high RH. This indicates a miscibility gap and is likely to influence bulk-to-surface partitioning of PEG at high RH, impacting droplet surface tension and CCN activity. This work provides insight into the factors likely to be contributing to discontinuities in aerosol water-uptake behavior below and above water saturation that have been observed previously in the ambient atmosphere.

Item Type:Article
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URLURL TypeDescription
Zuend, Andreas0000-0003-3101-8521
Shiraiwa, Manabu0000-0003-2532-5373
Flagan, Richard C.0000-0001-5690-770X
Seinfeld, John H.0000-0003-1344-4068
Additional Information:© Author(s) 2016. This work is distributed under the Creative Commons Attribution 3.0 License. Received: 18 Mar 2016 – Published in Atmos. Chem. Phys. Discuss.: 28 Apr 2016. Revised: 08 Aug 2016 – Accepted: 20 Sep 2016 – Published: 13 Oct 2016. This work was supported by the Office for Naval Research under award no. N00014-14-1-0097. Natasha Hodas was supported by a National Science Foundation Atmospheric and Geospace Sciences Postdoctoral Research Fellowship, award no. 14433246. Andreas Zuend acknowledges support by the Natural Sciences and Engineering Research Council of Canada (NSERC, grant RGPIN/04315-2014). The authors gratefully acknowledge helpful discussions with Armin Sorooshian and Taylor Shingler regarding the DASH-SP. All data are available from the corresponding author by request. Please also see the Appendices and for additional details. Edited by: K. Lehtinen. Reviewed by: two anonymous referees.
Funding AgencyGrant Number
Office of Naval Research (ONR)N00014-14-1-0097
NSF Postdoctoral FellowshipAGS-14433246
Natural Sciences and Engineering Research Council of Canada (NSERC)RGPIN/04315-2014
Issue or Number:19
Record Number:CaltechAUTHORS:20161107-145058887
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Official Citation:Hodas, N., Zuend, A., Schilling, K., Berkemeier, T., Shiraiwa, M., Flagan, R. C., and Seinfeld, J. H.: Discontinuities in hygroscopic growth below and above water saturation for laboratory surrogates of oligomers in organic atmospheric aerosols, Atmos. Chem. Phys., 16, 12767-12792, doi:10.5194/acp-16-12767-2016, 2016.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:71780
Deposited By: George Porter
Deposited On:08 Nov 2016 00:04
Last Modified:29 May 2020 17:00

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