AAAR 35th Annual Conference October 17 - October 21, 2016 Oregon Convention Center Portland, Oregon, USA
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Direct Measurements to Compare the Surface and Bulk Properties of Mixed Component Aerosol Droplets: Simultaneous Surface Tension and Viscosity Measurements
BRYAN R. BZDEK, Allen E. Haddrell, Young-Chul Song, David Topping, Jonathan P. Reid, University of Bristol
Abstract Number: 97 Working Group: Aerosol Chemistry
Abstract Aerosol surface tension and viscosity impact key atmospheric processes such as cloud droplet activation, semivolatile partitioning, and chemical aging. Direct measurements that compare the surface composition of an airborne particle to bulk properties are required. We will report studies that explore both the surface and bulk character of a range of organic aerosol surrogates for atmospheric aerosol. Both properties are simultaneously measured on the same droplet using holographic optical tweezers. Two optically trapped droplets with volumes ~1 pL (radii ~5-10 micrometres) are equilibrated to a desired relative humidity and are coalesced. The coalescence event is monitored using elastically backscattered light directed to an oscilloscope to extract highly time resolved changes in droplet shape (100 ns time resolution). These changes in shape follow a damped oscillator and permit simultaneous determination of both the droplet surface tension and viscosity to <2 mN/m and <1 mPa·s, respectively. Initial measurements of these properties on droplets with supersaturated solute states indicate that surface tension values can be inferred from models of subsaturated bulk measurements. The effect of surfactants on droplet surface tension and the response of droplet surface tension to rapid changes in relative humidity (designed to mimic rapid water vapour condensation during activation) for particles of varying viscosity will be discussed. Finally, surface tension measurements of droplets exposed to laboratory and ambient air suggest aerosol surface tension may trend towards values consistent with those of surfactant coated droplets in most environments, regardless of the droplet bulk composition and potentially impacting our understanding of aerosol activation.