A temperature-dependent study of the ozonolysis of propene

The ozonolysis of propene has been investigated in a temperature controlled reaction chamber at 295, 260, and 230 K. Experiments were performed using a total zero air pressure of 760 Torr (STP) and propene/ozone reactant mixing ratios ranging from 2.3 to 23 ppmv. An analysis of FTIR spectra collected at the conclusion of each reaction revealed that methane was formed with a yield of 0.14 +/- 0.03 (precision) for all the temperatures investigated. In addition, the yield of HCHO decreased from 0.67 +/- 0.04 to 0.43 +/- 0.03 upon cooling from 295 to 230 K, whereas the yield of HCOOH increased from 0.11 +/- 0.02 to 0.53 +/- 0.04. Experiments were also performed using an excess of cyclohexane (to scavenge OH) and it was found that the formaldehyde yield was 0.79 +/- 0.05 and 0.61 +/- 0.04 at 295 and 260 K, respectively. Finally, to more fully understand the reaction energies involved in product formation, we have performed molecular orbital calculations of heats of formation of reactants, stable intermediates, and products. Three conclusions can be made of this work. First, the reaction CH2OO + Aldehyde --> Secondary Ozonide --> HCOOH + Aldehyde is not an important mechanism in formic acid production. Second, the decomposition of the primary ozonide products (e.g., C-2 radical species) appears to occur, in part, by a thermal mechanism (e.g., thermalized to chamber temperature). Third, ab initio results combined with experiment reveal no correlation between reaction exothermicity and products formed (e.g., kinetically dictated product formation occurs). The ab initio database is provided nevertheless as a starting point for transition state calculations to be performed in the future. Finally, since formaldehyde yield decreases by at most similar to 35% with decreasing temperature and formic acid is relatively unreactive in the atmosphere, our results suggest that temperature-dependent HCHO yield will constitute only a minor perturbation to HOx formation in the middle troposphere.