Balance of Unimolecular and Bimolecular Pathways Control the Temperature-Dependent Kinetics of Ozonolysis in Aerosols.

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 6 × 10^-4 to 1.2 × 10^-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., α-acyloxyalkyl hydroperoxides). The implication of these results for the kinetics of aerosol aging at low temperatures is discussed.