Kinetics of atmospheric oxidation of nitrous acid by oxygen in aqueous medium

The facts that the high concentrations of nitrous acid have been reported in dew, fog, rain and cloud water and that its oxidation by dissolved oxygen is very fast in freezing conditions have led us to study the kinetics of aqueous phase oxidation of nitrous acid by dissolved oxygen in the pH range 1.0-4.5 at 30 degrees C. The reaction was followed by measuring [02] and under pseudo-first-order conditions the results were in agreement with the rate law: -d[O-2](t)/dt = k(0)[NIII](0)(2) [H+](2) [O-2](t)/(K-a + [H+])(2), where k(0) is third-order composite rate coefficient and K-a is the dissociation constant of HNO2. The values of k(0) and K-a were determined to be 1 x 10(2) L-2 mol(-2) s(-1) and 3.84 x 10(-4), respectively. Consistent with the kinetics results two alternative mechanisms have been considered. The first of these mechanisms assumes an intermediate complex formation, [HNO2 center dot O-2], by the reaction of HNO2 and O-2 in a rapid pre-equilibrium, followed by the reaction of this intermediate with another molecule of HNO2. The second mechanism, originally proposed by Damschen and Martin [1983. Aqueous aerosol oxidation of nitrous acid by 2, O-3 and H2O2. Atmospheric Environment 17, 2005-2011], assumes the formation of a dimer, [HNO2](2), in a rapid pre-equilibrium followed by the reaction of the dimer with O-2 to form HNO3. The application of the mechanisms to fast oxidation of nitrite by dissolved oxygen under freezing conditions is discussed. (C) 2007 Elsevier Ltd. All rights reserved.