AAAR 35th Annual Conference October 17 - October 21, 2016 Oregon Convention Center Portland, Oregon, USA
Abstract View
Accelerated Free Radical Chemistry and Enhanced Volatilization in the Heterogeneous Oxidation of Semisolid Organic Aerosol by OH Radicals
AARON WIEGEL, Kevin Wilson, William Hinsberg, Frances Houle, Lawrence Berkeley National Laboratory
Abstract Number: 576 Working Group: Aerosol Chemistry
Abstract In regional climate and air quality models, organic aerosol is treated as a well-mixed liquid in thermodynamic equilibrium with the surrounding gas, but several recent studies have shown that organic aerosol often exists in soft semi-solid or glassy states as well. However, how these phase states influence the heterogeneous oxidation of these organic aerosols is not well understood. To investigate the effect of phase state on the free radical chemistry, a stochastic kinetics model is developed that provides spatial information about the particle as a function of time and self-diffusion coefficient. This model has been successfully validated against experiments investigating the liquid OH + squalane and semi-solid OH + triacontane organic aerosol systems, which are proxies for hydrocarbon organic aerosols.
This model is then used to investigate how chemical aging affects the physical and chemical properties of the aerosol as a function of diffusion coefficient. Typical kinetic decays observed for the initial starting organic material are found to be sensitive to the reactivity towards OH radicals, the self-diffusion coefficient, and the rate of volatilization from the aerosol. In more diffusion limited cases, chemical pathways that are normally too slow in well-mixed, liquid, or aqueous particles as well as volatilization from the particle are enhanced due to the confinement of the free radical intermediates near the surface of the semisolid particle. Intriguingly, incompletely mixed organic aerosols with diffusion coefficients in between fluid and rigid systems show the most volatilization from the organic aerosol during chemical aging. Under atmospheric conditions, these model results suggest that semisolid secondary organic aerosol formed under dry conditions may actually experience enhanced volatilization upon exposure to OH radicals.