10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Diffusion Limitations and Shielding Effects in the Ozonolysis of Polycyclic Aromatic Hydrocarbons Embedded in Secondary Organic Aerosols
BRIAN HWANG, Shouming Zhou, Pascale Lakey, Jonathan Abbatt, Manabu Shiraiwa, University of California, Irvine
Abstract Number: 547 Working Group: Aerosol Modeling
Abstract Benzo[a]pyrene (BaP), a key polycyclic aromatic hydrocarbon (PAH), is known as a carcinogen and mutagen. BaP is often associated with soot particles emitted from biomass burning and incomplete combustion. In the atmosphere, BaP can react with oxidants such as ozone forming more toxic compounds including quinones and epoxides. Thus, it is important to quantify the kinetics of the heterogenous reaction between BaP and ozone, which is investigated in this study. The decay kinetics of BaP by ozone were measured by applying direct analysis in real time mass spectrometry (DART-MS) [1]. BaP was embedded in bis(2-ethylhexyl) sebacate or alpha-pinene secondary organic aerosol material. To investigate the impact of phase state on reaction kinetics, experiments were conducted at different relative humidities of < 5%, 50%, and 85%.
We applied the kinetic multi-layer model of aerosol surface and bulk chemistry, (KM-SUB) [2] to model the experiment data. KM-SUB explicitly treats mass transport and chemical reaction at the surface and in the bulk. To obtain the distribution of kinetic parameter values to fit the experiment data with modeling, we applied the Monte-Carlo genetic algorithm method [3]. In this method, kinetic parameters are randomly chosen to fit the experimental data and these values are optimized by genetic algorithm. By considering the decomposition of ozone and subsequent formation of reactive oxygen intermediates [4], the concentration dependence of BaP decay was reproduced using a single set of kinetic parameters. The obstruction theory was applied to account for inhibition of bulk diffusion of the BaP and ozone due to accumulation of reaction products at the surface and in the near-surface bulk [5]. With this approach, we were able to capture the initial fast kinetics followed by slow decay of BaP. We also analyzed how mass transport, rate of surface or bulk reaction, and diffusion of BaP and ozone may limit the overall reaction kinetics, finding that the reactions were controlled by bulk diffusion of BaP from the bulk to the surface at longer reaction times.
References [1] S. Zhou, M.W. Forbes and J. P. D. Abbatt, Anal. Chem., 87, 4733 (2015). [2] M. Shiraiwa, C. Pfrang and U. Pöschl, Atmos. Chem. Phys., 10, 3673 (2010). [3] T. Berkemeir et. al., Atmos. Chem. Phys., 17, 8021 (2017). [4] M. Shiraiwa et al., Nature Chem., 3, 291 (2011). [5] C. Pfrang, M. Shiraiwa, and U. Pöschl, Atmos. Chem. Phys., 11, 7343 (2011).