(5ab) Constraining Ozonolysis Product Distributions with Temperature Programmed Reaction Spectroscopy and Computational Chemistry

Reactions between biogenic alkenes and ozone are a significant source of atmospheric particulate matter (PM), contribute to the aging of PM, and in the gas phase, produce hydroxyl radicals?another important atmospheric oxidant. All ozonolysis reactions proceed through an initial highly unstable 1,2,3-trioxolane, the primary ozonide (POZ). Depending on the structure of the parent alkene, decomposition of the POZ can proceed through two different reaction pathways, sometimes leading to drastically different subsequent chemistry. We strive to constrain product distributions by examining the POZ decomposition branching using laboratory experiments and computational chemistry models.

In order to isolate the highly unstable POZ, an apparatus was constructed to perform temperature programmed reaction spectroscopy (TPRS) on a liquid nitrogen cooled surface with Fourier transform infrared spectroscopy. We will show results from experiments with SOA precursors and their surrogates?cyclohexene, 1-methyl-cyclohexene as a surrogate for α-pinene, and methylene-cyclohexene as a surrogate for β-pinene. With this instrument, we can determine POZ decomposition rate constants and characterize decomposition products for these three alkenes. We will also show that Density Functional Theory (DFT) calculations are a reliable way to predict POZ decomposition branching and extend these calculations to include several other atmospherically relevant terpenes.