Ozonolysis of Uric Acid at the Air/Water Interface

Uric acid (UA) epoxide, peroxide, and ozonide species produced in aqueous UA microdroplets exposed to O3(g) are detected by online mass spectrometry within ∼1 ms. UA conversions are independent of its initial concentration below ∼0.1 mM and are unaffected by addition of excess H2O2 or t-butanol. UA reactivity increases ∼380 times from pH 4 to 7, which is at variance with the pH-independent rates reported for the UA + O3(aq) reaction in bulk water. At pH ∼7, UA and ascorbic acid (AH2) microdroplets react with O3(g) at similar rates, although UA is ∼40 times more reactive than AH2 toward O3(aq) in bulk water. Only the UA epoxide, plus traces of UA peroxide, are formed upon mixing UA(aq) and O3(aq) solutions. We infer that the gas−liquid ozonolysis of UA proceeds in an interfacial aqueous medium quite distinct from bulk water. Thus, UA, a component of the pulmonary epithelial lining fluid that scavenges atmospheric O3(g) into less deleterious species (similar to AH2), is rendered inactive below pH ∼5. The potential implications of these findings on synergistic health effects between tropospheric ozone and acidic particulates are briefly analyzed