Gas phase reactions of iodide and bromide anions with ozone: evidence for stepwise and reversible reactions

Despite the impacts - both positive and negative - of atmospheric ozone for life on Earth, there remain significant gaps in our knowledge of the products, mechanisms and rates of some of its most fundamental gas phase reactions. This incomplete understanding is largely due to the experimental challenges involved in the study of gas-phase reactions of ozone and, in particular, the identification of short-lived reaction intermediates. Here we report direct observation of the stepwise reaction of the halide anions iodide (I − ) and bromide (Br − ) with ozone to produce XO 3 − (where X = I and Br, respectively). These results substantially revise the rate constant for the I − + O 3 reaction to 1.1 (± 0.5) × 10 −12 cm 3 molecule −1 s −1 (0.13% efficiency) and the Br − + O 3 reaction to 6.2 (± 0.4) × 10 −15 cm 3 molecule −1 s −1 (0.001% efficiency). Exploiting five-orders of temporal dynamic range on a linear ion trap mass spectrometer enabled explicit measurement of the rate constants for the highly efficient intermediate, XO − + O 3 and XO 2 − + O 3 , reactions thus confirming a stepwise addition of three oxygen atoms ( i.e. , X − + 3O 3 → XO 3 − + 3O 2 ) with the first addition representing the rate determining step. Evidence is also presented for (i) slow reverse reactions of XO − and XO 2 − , but not XO 3 − , with molecular oxygen and (ii) the photodissociation of IO − , IO 2 − and IO 3 − to release I − . Collectively, these results suggest relatively short lifetimes for Br − and I − in the tropospere with direct gas-phase oxidation by ozone playing a role in both the formation of atmospheric halogen oxides and, conversely, in the ozone depletion associated with springtime polar bromine explosion events. Ion-trap mass spectrometry maps the discrete steps in the reaction of I − and Br − with gaseous ozone.