ATMOSPHERIC PHOTOOXIDATION OF ISOPRENE .2. THE OZONE-ISOPRENE REACTION

A series of experiments have been performed to study the ozone-isoprene reaction in a smog chamber by adding externally produced O3 to the hydrocarbon in the dark. A chemical tracer, methyl cyclohexane, was added to probe the OH formation in the system. O(3P) formation was also examined using the known distribution of products that are unique to the O(3P)-isoprene reaction (part I). The results provide clear evidence that both OH and O(3P) are produced by the O3-isoprene reaction directly in large quantities; about 0.68 +/- 0.15 and 0.45 +/- 0.20 per O3-isoprene reaction, respectively. These additional radicals severely complicate the analysis of the O3 reaction, hence, computer kinetic modeling was necessary to ascertain the products of the O3 reaction itself, corrected for OH and O(3P) reactions. The product distribution, which differs dramatically from that published previously, is: 67 +/- 9% methacrolein, 26 +/- 6% methyl vinyl ketone, and 7 +/- 3% propene, accounting for 100 +/- 10% of the reacted isoprene. Applicability of these results to the gas-phase O3 reaction with other unsaturated hydrocarbons is briefly discussed.