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

摘要

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℃. The reaction was followed by measuring [O_2] and under pseudo-first-order conditions the results were in agreement with the rate law: -d[O_2]_t/dt = k_0[N~Ⅲ]_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 HNO_2. The values of k_0 and K_a were determined to be 1 x 10~2L~2mol~(-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, [HNO_2.O_2], by the reaction of HNO_2 and O_2 in a rapid pre-equilibrium, followed by the reaction of this intermediate with another molecule of HNO_2. The second mechanism, originally proposed by Damschen and Martin [1983. Aqueous aerosol oxidation of nitrous acid by O_2, O_3 and H_2O_2. Atmospheric Environment 17, 2005-2011], assumes the formation of a dimer, [HNO_2]_2, in a rapid pre-equilibrium followed by the reaction of the dimer with O_2 to form HNO_3. The application of the mechanisms to fast oxidation of nitrite by dissolved oxygen under freezing conditions is discussed.