AAAR 36th Annual Conference October 16 - October 20, 2017 Raleigh Convention Center Raleigh, North Carolina, USA
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Determination of the Activity Coefficients of Water in Aqueous Solutions at Sub-zero Temperatures using a Novel Chilled optical Aerosol Tweezers
Hassan Beydoun, Kyle Gorkowski, Jim Walker, Jonathan P. Reid, RYAN SULLIVAN, Carnegie Mellon University
Abstract Number: 494 Working Group: Instrumentation and Methods
Abstract The activity coefficient of water in aqueous and organic solutions is a crucial property in the determination of an aerosol particle’s size and its ability to act as a cloud condensation nucleus. While many accurate techniques exist to measure water activity coefficients in aerosol particles, none have yet to do so below 0 °C. The inability to measure this key aerosol property in the temperature range most relevant to the tropospheric aerosol remains a significant deficiency in existing instrumental techniques.
We present a novel method for determining the activity coefficient of water in aqueous and organic solutions at sub-zero temperatures using a chilled optical aerosol tweezers (COAT). The new temperature controlled system retains the powerful capabilities of traditional aerosol optical tweezers: retrieval of a cavity enhanced Raman spectrum that can be used to accurately determine the size and refractive index of a trapped droplet. It has already been shown that for aqueous droplets these two properties can lead to an accurate determination of the activity coefficient of water if the relative humidity (RH) around the droplet is known. In the newly developed system, single aqueous KCl, NaCl, and NaCl-adipic acid droplets were cooled down from room temperature to -12 °C to investigate the temperature dependence of the activity coefficient of water in all three mixtures in the RH range of 70%-90%. The COAT technique is therefore the first to examine the temperature dependent aerosol aqueous thermodynamic properties on the same individual droplet in the “sub-zero” temperature range.