AAAR 32nd Annual Conference
September 30 - October 4, 2013
Oregon Convention Center
Portland, Oregon, USA
Abstract View
Phase Transitions and Phase Miscibility of Mixed Particles of Ammonium Sulfate, Toluene-Derived Secondary Organic Material, and Water
Mackenzie Smith, Yuan You, Mikinori Kuwata, Allan Bertram, SCOT MARTIN, Harvard University
Abstract Number: 340 Working Group: Aerosol Chemistry
Abstract The phase transitions and phase miscibility of particles composed of ammonium sulfate, toluene-derived secondary organic material (SOM), and water were studied in two types of experiments, specifically tandem differential mobility analysis and optical microscopy. The SOM was produced in the Harvard Environmental Chamber by photo-oxidation of toluene at chamber relative humidities of < 5% and 40%. The efflorescence and deliquescence relative humidities (ERH and DRH, respectively) of ammonium sulfate decreased as the SOM organic fraction epsilon in the particle increased, dropping from DRH = 80% and ERH = 31% for epsilon = 0.0 to DRH = 58% and ERH = 0% for epsilon = 0.8. The approximately flat response of the DRH and ERH curves for epsilon < 0.2 were consistent with independent behavior by aqueous ammonium sulfate and toluene-derived SOM and therefore suggestive of immiscibility. The optical images confirmed immiscibility between aqueous ammonium sulfate and toluene-derived SOM for epsilon = 0.12 for RH < 85% and complete miscibility for epsilon = 0.79 to RH < 0.5%. Prolonged exposure time of the aerosol from 0.5 s to 30 s in the relative humidity conditioning portion of the apparatus affected within error neither the ERH(epsilon) nor DRH(epsilon) curves, implying an absence of kinetic effects on the observations. For epsilon > 0.6 the DRH values for particles produced at < 5% RH were measurably lower by 0 to 4% RH than for particles produced at 40% RH. The observed miscibility gap for toluene-derived SOM can be compared to previous reports of full miscibility for isoprene-derived SOM and full immiscibility for α-pinene-derived SOM. The differences among the SOMs can be related to their physicochemical properties, for which the oxygen-to-carbon (O:C) ratio is an indicator.