A mechanistic study of the reaction of HO2 radical with ozone

Abstract

The mechanism of the reaction of hydroperoxyl radical with ozone has been studied as a function of temperature by measuring product branching ratios using isotopic labeling. In these studies H18O2 was reacted with unenriched ozone in order to measure the relative importance of hydrogen abstraction and oxygen abstraction by ozone. In either case hydroxyl radical is a product. However, with isotopic labeling the two channels can be distinguished by detecting the OH product with an isotopically specific method: H18O2 + O3 → 18OH + O2 + 18OO; H18O2 + O3 → OH + O2 + 18O2. We detect both 16OH and 18OH with high sensitivity and high specificity using laser-induced fluorescence in the 1-1 and 0-0 bands after pumping the P1(2) transition of the 1-0, A-X band. The branching fractions were measured as a function of temperature between 226 and 355 K. At the highest temperatures studied, hydrogen abstraction by ozone accounts for 88% of the reactive encounters. This fraction increases with decreasing temperature to ∼95% at 226 K. This temperature dependence is analyzed to find the difference in activation energy for the two channels. We find that the barrier to oxygen abstraction by ozone exceeds that for hydrogen abstraction by 1000 ± 400 cal/mol. We conclude that hydrogen atom abstraction by ozone is the dominant pathway for this reaction at atmospherically relevant temperatures. © 1994 American Chemical Society.