Constraining the mechanism of OH+NO2 using isotopically labeled reactants:Experimental evidence for HOONO formation

The reaction of OH with NO2 is central to atmospheric chemistry, and its dy namics can be constrained by studying the kinetics of isotopically labeled (OH)-O-18 with NO2. This labeling opens an isotopic scrambling pathway in t he reaction coordinate for nitric acid formation, providing experimental co nstraints on the high-pressure behavior of the reaction with data obtained at low pressures. This reaction, however, is complicated by the presence of a second product isomer, peroxynitrous acid (HOONO), which does not have a scrambling pathway. We present data for the reaction of (OH)-O-18 with NO2 at room temperature between 4 and 200 Torr. The reaction is rapid and inde pendent of pressure. We also locate the H-atom isomerization transition sta te and show that the isomerization rate constant is at least an order of ma gnitude faster than adduct dissociation. These results allow us to accurate ly constrain the formation rate constant of HONO2, which is a factor of 5 s lower than the observed OH removal rate constant at high pressure. We concl ude that the difference is due to HOONO formation. Our conclusion is consis tent with recent theoretical predictions of this branching, and also provid es the only self-consistent reconciliation of the high-pressure data with t he remainder of the experimental data set.