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Abstract
To better understand the key kinetic mechanisms controlling heterogeneous oxidation in organic aerosols, submicron particles composed of an alkene and a saturated carboxylic acid are exposed to ozone in a variable-temperature flow tube reactor. Effective uptake coefficients (γ_eff) are obtained from the multiphase reaction kinetics, which are quantified by Vacuum Ultraviolet Photoionization Aerosol Mass Spectrometry. For aerosols composed of only of alkenes, γ_eff doubles (from 6x10-4 to 1.2x10-3) when the temperature is decreased from 293 to 263 K. Alternatively, for an alkene particle doped with a carboxylic acid, an efficient scavenger of stabilized Criegee Intermediates (sCI), γ_eff is observed to be weakly temperature dependent. A kinetic model, benchmarked to literature data, explains these results as arising from the temperature dependent competition between unimolecular pathways of sCI that promote radical chain cycling and those bimolecular pathways that form stable chain termination products (i.e., ɑ-acyloxyalkyl hydroperoxides). The implication of these results for the kinetics of aerosol aging at low temperatures is discussed.
Supplementary materials
Title
Supporting information
Description
Schematics of the experimental setup and instrumentation (Figures S1–S2), table of reaction rate constants (Table S1) and discussion of parameters used in reaction-diffusion simulations, sensitivity tests of parameters (Figures S3–S4), kinetics of additional peaks identified in main text (Figures S5–S9), proposed ion fragmentation mechanisms (Schemes S1–S3), evaluations of Arrhenius parameters for kAAHP (Table S2, Figures S10–S11), and predicted kinetics of [sCI] (Figure S12).
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