AAAR 34th Annual Conference
October 12 - October 16, 2015
Hyatt Regency
Minneapolis, Minnesota, USA
Abstract View
A Stochastic Reaction Diffusion Kinetics Model of the Fragmentation Processes during Heterogeneous Oxidation of Organic Aerosol
AARON WIEGEL, Kevin Wilson, William Hinsberg, Frances Houle, Lawrence Berkeley National Laboratory
Abstract Number: 236 Working Group: Aerosol Chemistry
Abstract While progress has been made in understanding the heterogeneous oxidation of atmospheric organic aerosols, a more detailed understanding of the underlying chemical mechanisms is crucial for improving models of their chemical evolution in the atmosphere, particularly in regard to the effect of phase on evaporation rates. Previous experimental work in our lab has shown two general reaction pathways for organic aerosol upon oxidation: functionalization, which adds additional oxygen functional groups to the carbon skeleton, and fragmentation, which leads to C-C bond scission and lower molecular weight oxidized products. A kinetics model for the OH + Squalane model system can describe the underlying molecular processes behind these two pathways with the particle treated as a uniformly mixed volume. Using a lumped species approach to describe the free radical chemistry, the model can predict the measured aerosol mass, volume, density, carbon number distribution of fragmentation products, and the elemental composition of the aerosol over 10 oxidation lifetimes. The functionalization chemistry is found to be primarily controlled by the site preference of the initial hydrogen abstraction by OH radicals. The fragmentation chemistry is found to be driven by “activated” alkoxy radicals with a functional group on the adjacent carbon. As a result, as the O/C ratio of the aerosol increases, the decomposition of these radicals is the primary pathway leading to lower volatility products.
While useful, this model does not explore where in the particle functionalization and fragmentation are most likely to occur. Extension of the lumped species approach to a full coupled reaction-diffusion model provides full spatial information about the particle as a function of time. This model is explored to understand the liquid, well-mixed case of OH + Squalane system and then is extended to study the solid, diffusion-limited case of OH + Triacontane. Several hypothetical cases with intermediate diffusion coefficients are also tested to investigate how the chemistry and evaporation rates change as the diffusion within the particle gets progressively slower. This approach provides insight into the issues with climate and air quality models that over-predict the partitioning of volatile components into and out of the particle phase.