Atmospheric photooxidation of isoprene part II: The ozone-isoprene reaction

Abstract

A series of experiments have been performed to study the ozone-isoprene reaction in a smog chamber by adding externally produced O_3 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 O_3-isoprene reaction directly in large quantities; about 0.68 ± 0.15 and 0.45 ± 0.20 per O_3-isoprene reaction, respectively. These additional radicals severely complicate the analysis of the O_3 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.