AAAR 35th Annual Conference October 17 - October 21, 2016 Oregon Convention Center Portland, Oregon, USA
Abstract View
Discontinuities in Hygroscopic Growth Below and Above Water Saturation for Laboratory Surrogates of Oligomers in Organic Atmospheric Aerosols
NATASHA HODAS, Andreas Zuend, Katherine Schilling, Thomas Berkemeier, Manabu Shiraiwa, Richard Flagan, John Seinfeld, California Institute of Technology
Abstract Number: 255 Working Group: Aerosol Physics
Abstract Organic oligomers are potential sources of viscous atmospheric aerosol components, which may inhibit the mass transport of water and introduce kinetic limitations to hygroscopic growth, particularly at low relative humidity (RH). As water content increases, however, particle viscosity is expected to decrease due to the plasticizing effect of water. To investigate the influence of particle viscosity on water-uptake behavior across the range of RH conditions occurring in the atmosphere, measurements of hygroscopic growth under subsaturated RH conditions and of the CCN activity of laboratory surrogates for oligomers in atmospheric aerosols were conducted. Experiments were conducted for particles comprised of polyethylene glycol (PEG) with average molecular masses ranging from 200 to 10,000 g/mol, as well as mixtures of PEG with ammonium sulfate (AS). Results were compared with calculations of hygroscopic growth at thermodynamic equilibrium, and the influence of kinetic limitations on observed water uptake was further explored through estimations of water diffusivity in the PEG oligomers. At RH < 100%, we observed little variability in hygroscopic growth across PEG systems; however, increases in CCN activity with increasing PEG molecular mass were observed. This was 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 an 8% reduction in critical activation diameter at a supersaturation of 0.6%. There was no evidence that kinetic limitations to water uptake due to the presence of viscous aerosol components inhibited water uptake at low RH. Discontinuities in hygroscopicity above and below water saturation are attributed to RH-dependent differences in the sensitivity of water uptake to non-ideal interactions between aerosol components and to surface tension effects. This work provides insight into the factors potentially contributing to discontinuities in aerosol water-uptake behavior below and above water saturation observed in the ambient atmosphere.