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 (ClO3–) and perchlorate (ClO4–) 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 (ClO2–) with ozone were directly observed using linear ion-trap mass spectrometry and rate constants determined to be 6.6 (±0.7) × 10–11 cm3 molecule–1 s–1 (6.7% collision efficient) and 3.5 (±0.4) × 10–10 cm3 molecule–1 s–1 (38% efficient), respectively. Compared to the analogous bromine and iodine oxides, which undergo efficient stepwise oxidation terminating in XO3– (X = Br or I), the reaction products of ClO2– with ozone include both (i) oxidation to ClO3– and neutral O2 (30%) and (ii) charge transfer to yield the ozonide anion O3• – and neutral ClO2• (70%). Branching ratio measurements for ClO– show a more complex reaction manifold with production of Cl– (50%), ClO2– (9%), and O3• – (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 ClO2• and ClO•, which themselves are critical intermediates in atomic chlorine pathways that are implicated in ozone depletion in the stratosphere.