Direct Determination of Melting Temperatures for Individual, Sub-Micron Isoprene Epoxydiol-Derived Secondary Organic Aerosol Particles

KATHERINE KOLOZSVARI, Cara Waters, Alison Fankhauser, N. Cazimir Armstrong, Jin Yan, Madeline Cooke, Yao Xiao, Rebecca Parham, Zhenfa Zhang, Avram Gold, Jason Surratt, Andrew Ault, University of Michigan

     Abstract Number: 381
     Working Group: Instrumentation and Methods

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 (T_g) and viscosity; however, direct measurements of these properties is 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 particle size. Melting temperature (T_m) is the most significant predictor of T_g, 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 T_m of individual secondary organic aerosol (SOA) particles generated from the reactive uptake of isoprene-derived epoxydiols (IEPOX) onto acidic ammonium sulfate aerosol particles. 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 T_m. The direct measurements are compared with model predictions based on molecular composition from hydrophilic interaction liquid chromatography coupled to electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (HILIC/ESI-HR-QTOF-MS) analysis. We compared the T_m of the SOA particles formed from IEPOX uptake onto acidic ammonium sulfate particles created at 30, 65, and 80% relative humidity (RH), and found that increasing RH from 30 to 80% led to an overall decrease in average T_m, indicating less viscous particles at higher RH conditions. Our measurements with this technique will allow for more accurate representations of the phase state of aerosols in the atmosphere.