Long-Lived Oxygen Species Are Key To Ozone Reactions

Atmospheric ozone reacts with polycyclic aromatic hydrocarbons in aerosol particles through a multistep mechanism involving intermediate reactive oxygen species, proposes an international group led by Ulrich Pöschl of Germany’s Max Planck Institute for Chemistry (Nat. Chem., DOI: 10.1038/nchem.988). Researchers have puzzled over O₃’s behavior in such reactions, which are of central importance to air quality. Experimental data show surface O₃ lifetimes of 0.01 to 10 seconds, but computational studies predict lifetimes of nanoseconds. To reconcile the two, Pöschl and colleagues have devised a new kinetic model in which O₃ is first physically adsorbed at a particle surface. From the physically adsorbed state, O₃ can desorb back to the gas state, with a desorption lifetime of nanoseconds. Alternatively, O₃ can dissociate into O₂ and a chemically adsorbed reactive oxygen intermediate with a lifetime of about 100 seconds. The intermediate could be an oxygen atom bound to delocalized π electrons or an alkoxy radical species. The researchers also propose that similar reactive oxygen intermediates play a role in ozone reactions with pollen proteins and with other organic and inorganic material on airborne particles, as well as in the nucleation and growth of new atmospheric aerosols.