Direct Determination of Melting Temperatures for Individual, Sub-Micron α-Pinene Derived Secondary Organic Aerosol Particles
KATHERINE KOLOZSVARI, Natasha Garner, David Bell, Jens Top, Markus Ammann, Andrew Ault, University of Michigan
Abstract Number: 458
Working Group: Aerosol Chemistry
Abstract
The phase state of atmospheric aerosol particles – solid, semi-solid, or liquid – influences their ability to take up water and participate in heterogeneous chemical reactions. Changes in phase state have been predicted by glass transition temperature (Tg) and viscosity; however, direct measurements of these properties are challenging for sub-micron particles. Historically, bulk measurements have been used, but this does not account for particle-to-particle variation or the impacts of internal phase separation. Melting temperature (Tm) is the most significant predictor of Tg, and the two properties can be related through the Boyer-Beaman rule. Herein, we apply a recently developed method utilizing a nano-thermal analysis (nanoTA) module coupled to an atomic force microscope (AFM) to determine the Tm of individual secondary organic aerosol (SOA) particles generated from α-pinene ozonolysis. NanoTA works by using a specialized AFM probe which can be heated while in contact with a particle of interest. As the temperature increases, the probe deflection will first increase due to thermal expansion of the particle followed by a decrease at its Tm. We investigated the influence of inorganic material on the viscosity of this SOA by comparing the Tm of SOA formed with and without the presence of ammonium sulfate seed particles. Additionally, we determined the impact of higher volatility molecules on particle viscosity by comparing the Tm of particles before and after undergoing thermal denuding to evaporate any high volatility material.