The Effect of Particle Phase and Morphology on Its Evaporation Kinetics and Rates of Heterogeneous Reactions

ALLA ZELENYUK, Jacqueline Wilson, Kaitlyn Suski, ManishKumar Shrivastava, Pacific Northwest National Laboratory

     Abstract Number: 520
     Working Group: Aerosol Physical Chemistry and Microphysics

Abstract
The properties and behavior of mixed aerosols particles depend on their phase and morphology. In particular, the rates of heterogeneous reactions strongly depend on whether the condensed-phase reacting substances are on the particle surface or inside the particle, behind a protective shell.

We demonstrated the ability to generate size-selected particles with tailored morphologies and to characterize their three-dimensional structures utilizing our “depth-profiling” approach, in which by controlling laser fluence, we control the depth to which the laser penetrates the particle. At low laser fluence, the surface compounds dominate the single particle mass spectra, and at high fluence, the mass spectra represent the composition of the entire particle. This approach was successfully applied to characterize morphology of NaCl seeds coated with solid pyrene and with liquid dioctyl phthalate (DOP), NaCl particles coated by secondary organic aerosol (SOA) formed by ozonolysis of α-pinene, SOA-coated DOP particles, and DOP-coated SOA particles, revealing their complex layered morphologies.

We will present the results of a recent study, in which we prepared particles containing SOA formed by oxidation of different precursors and condensed on pre-existing size-selected seed particles with different compositions, viscosities, and volatility (different SOA material, oleic acid, DOP, and polyaromatic hydrocarbons), characterized their morphology, and investigated the effects of SOA coating thickness, and viscosity on the rates of heterogeneous reactions, evaporation kinetics, and particle coalescence.

We find, for example, that the viscous semi-solid SOA coatings can impede evaporation of more volatile organic seed particles. Similarly, our data show that SOA coatings of oleic acid particles significantly reduces the rate of their heterogeneous oxidation by ozone.