Balance of Unimolecular\nand Bimolecular Pathways Control\nthe Temperature-Dependent Kinetics of Ozonolysis in Aerosols

To better understand the key kinetic\nmechanisms controlling\nheterogeneous\noxidation in organic aerosols, submicron particles composed of an\nalkene and a saturated carboxylic acid are exposed to ozone in a variable-temperature\nflow tube reactor. Effective uptake coefficients (γ<sub>eff</sub>) are obtained from the multiphase reaction kinetics, which are quantified\nby Vacuum Ultraviolet Photoionization Aerosol Mass Spectrometry. For\naerosols composed of only of alkenes, γ<sub>eff</sub> doubles\n(from 6 × 10<sup>–4</sup> to 1.2 × 10<sup>–3</sup>) when the temperature is decreased from 293 to 263 K. Alternatively,\nfor an alkene particle doped with a carboxylic acid, an efficient\nscavenger of stabilized Criegee Intermediates (sCI), γ<sub>eff</sub> is observed to be weakly temperature dependent. A kinetic model,\nbenchmarked to literature data, explains these results as arising\nfrom the temperature dependent competition between unimolecular pathways\nof sCI that promote radical chain cycling and those bimolecular pathways\nthat form stable chain termination products (i.e., α-acyloxyalkyl\nhydroperoxides). The implication of these results for the kinetics\nof aerosol aging at low temperatures is discussed.