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

Abstract

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.