Discontinuities in hygroscopic growth below and above water saturation for laboratory surrogates of oligomers in organic atmospheric aerosols
Authors: Hodas Natasha , Zuend Andreas , Schilling Katherine , Berkemeier Thomas , Shiraiwa Manabu , Flagan Richard C. , Seinfeld John H. 
Published: April 2016

Abstract. Organic oligomers and other organic high molecular mass compounds are potential sources of viscous atmospheric aerosol components, which may inhibit the mass transport of water and introduce kinetic limitations to water uptake. This, in turn, impacts aerosol optical properties and the efficiency with which these particles serve as cloud condensation nuclei (CCN). To investigate the influence of particle viscosity on water-uptake behavior, measurements of hygroscopic growth under subsaturated relative humidity (RH) conditions and of the CCN activity of laboratory surrogates for oligomers in atmospheric aerosols were conducted with the Differential Aerosol Sizing and Hygroscopicity Spectrometer Probe and a CCN counter, respectively. In order to explore variability in water-uptake behavior across aerosol systems with differing viscosities, experiments were conducted for particles comprised of polyethylene glycol (PEG) with average molecular masses of 200, 1,000, and 10,000 g/mol, as well as mixtures of these PEG oligomers with ammonium sulfate (AS). Experimental results were compared with 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 to observed water uptake was further explored through estimations of water diffusivity in the PEG oligomers. Under subsaturated 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 latter finding is attributed to an increase in the efficiency of PEG as a surfactant with increasing molecular mass. This effect is most pronounced for PEG-AS mixtures, and a modest enhancement in CCN activity was observed for the PEG10,000-AS mixture as compared to pure AS, as evidenced by a 8 % reduction in critical activation diameter at a supersaturation of 0.6 %. The supplementation of experimental observations with the AIOMFAC-based model results and estimations of water diffusivity in PEG suggests that apparent discontinuities in hygroscopicity above and below water saturation can be attributed, at least in part, to a combination of RH-dependent differences in the sensitivity of water uptake behavior to non-ideal interactions and to surface tension effects. There was no evidence that kinetic limitations to water uptake due to the presence of viscous aerosol components influenced hygroscopic growth. This work provides insight into the factors likely to be contributing to discontinuities in aerosol water-uptake behavior below and above water saturation (e.g., low hygroscopic growth at subsaturated RHs, but enhanced CCN activity for a given aerosol population) that have been observed previously in the ambient atmosphere.