Gas-Phase Reactions of Chlorine Oxide Anions with Ozone: Atmospheric Implications

Understanding the mechanism for catalytic destruction of stratospheric ozone by chlorine radicals led to international restrictions on the emission of harmful chemicals. Measurements of chlorate (ClO 3 – ) and perchlorate (ClO 4 – ) anions in the atmosphere suggest that chloride and chlorine oxide anions could also play a role in ozone destruction, but the pathways for these gas-phase reactions are poorly understood. Here, reactions of hypochlorite (ClO – ) and chlorite (ClO 2 – ) with ozone were directly observed using linear ion-trap mass spectrometry and rate constants determined to be 6.6 (±0.7) × 10 –11 cm 3 molecule –1 s –1 (6.7% collision efficient) and 3.5 (±0.4) × 10 –10 cm 3 molecule –1 s –1 (38% efficient), respectively. Compared to the analogous bromine and iodine oxides, which undergo efficient stepwise oxidation terminating in XO 3 – (X = Br or I), the reaction products of ClO 2 – with ozone include both (i) oxidation to ClO 3 – and neutral O 2 (30%) and (ii) charge transfer to yield the ozonide anion O 3 • – and neutral ClO 2 • (70%). Branching ratio measurements for ClO – show a more complex reaction manifold with production of Cl – (50%), ClO 2 – (9%), and O 3 • – (41%), with the change in redox reactivity rationalized by coupled-cluster CCSD(T) calculations. These experiments highlight that, in addition to consuming ozone, these anions are precursors to the formation of the neutral radicals ClO 2 • and ClO •, which themselves are critical intermediates in atomic chlorine pathways that are implicated in ozone depletion in the stratosphere.