Two Reactivity Modes in the Heterogeneous Cyclohexene Ozonolysis under Tropospherically Relevant Ozone-Rich and Ozone-Limited Conditions

Important mechanistic differences regarding C═C double-bond oxidation processes under ozone-limited and ozone-rich reaction conditions for cyclohexene-functionalized fused silica substrates serving as model systems for studying heterogeneous C═C double bond oxidation chemistry in the troposphere are evaluated. By using broadband vibrational sum frequency generation (SFG), we track heterogeneous ozone reactions in real time. Ozone levels span three orders of magnitude and represent environments ranging from pristine remote continental regions to highly polluted urban centers, ranging from 30 ppb to 3 ppm (from 7 × 1011 molecules cm−3 to 7 × 1013 molecules cm−3). We determine reaction rates and reactive uptake coefficients (γ values). At these tropospherically relevant ozone levels, the heterogeneous reaction rates follow a Langmuir−Hinshelwood-type mechanism. The product formation rates, which we determine as a function of ozone concentrations, are found to be half of the olefin reaction rates. This ratio ...