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

The properties of mixed particles depend on their shape and morphology. For example, 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 shield. 

We demonstrated the ability to generate size-selected particles with complex shapes and 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 particle mass-spectra, and at higher 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 liquid dioctyl phthalate (DOP), NaCl particles coated by secondary organic aerosol (SOA), SOA-coated DOP particles, DOP-coated SOA particles, SOA particles mixed with polycyclic aromatic hydrocarbons (PAHs), and SOA-coated soot particles revealing their complex morphologies.

We will present the results of a recent study, in which we characterized the morphology of particles formed by SOA condensation on pre-existing size-selected seed particles with different phase and volatility (DOP, PAHs, oleic acid, and soot) and quantified their evaporation kinetics and rates of heterogeneous oxidation as a function of SOA coating thickness. In addition, we examined the stability and characterized the morphology, evaporation kinetics, and reactivity of particles with reverse morphology, i.e. SOA core coated with the corresponding organic.

We find, for example, that the evaporation of SOA-coated DOP particles and DOP-coated SOA particles exhibit very different behavior: The DOP coating on SOA evaporates very fast, while SOA-coated DOP does not evaporate, showing that the highly viscous semi-solid SOA coating prevents DOP core from evaporation. Similarly, our data show that SOA coating significantly reduces the rate of the heterogeneous core oxidation by O₃.