(270c) Decomposition of the Primary Ozonide: Towards a Better Understanding of Ozonolysis Kinetics

Ozonolysis is an important oxidation pathway for gaseous and condensed-phase alkenes in the troposphere. Since ozonolysis products often have lower vapor pressures than the reactant alkenes, these reactions are a significant source of secondary organic aerosol. In addition, ozonolysis reactions are a source of radicals that play a key role in tropospheric chemistry. Also, ozone uptake ages particles on a timescale that is relevant to the typical lifetime of an air mass.

The mechanism of ozonolysis involves many branch points, but in all cases it proceeds through an initial intermediate known as the primary ozonide (POZ). The decomposition of the POZ can proceed in several different ways leading to different subsequent chemistry and product distributions. However, the POZ decomposes too rapidly to be studied under normal atmospheric conditions.

We observe the decomposition of primary ozonides formed from different alkenes on a cold (>90 K) surface using Temperature Programmed Reaction Spectroscopy (TPRS) as the surface is warmed at multiple heating rates. Data from these experiments allows us to estimate the barrier heights for POZ decomposition and, in cases with multiple primary ozonides, the branching ratios of different reaction pathways. In addition, we observe formation of carbonyl products, secondary ozonides, and their subsequent desorption.

Two alkenes with only one unique primary ozonide, (2,3)-Dimethyl-2-butene and Cyclohexene, were previously studied. In addition, a few other linear alkenes with multiple POZ decomposition pathways were examined. Comparison of the POZ decomposition behavior will allow us to make predictions of the mechanistic kinetics for a suite of precursors ranging from fully symmetric straight chain alkenes to alkenes with R or Z isomers to cycloalkenes. These experiments will be extended to include alkenes that are significant secondary organic aerosol precursors in the atmosphere.