Aqueous-phase peroxynitric acid chemistry and its potential impact on the marine boundary layer chemistry.

摘要

Seasalt particles in the marine boundary layer have a chloride deficit relative to seawater [Eriksson, 1960]. It has been suggested that it could be due to in situ chloride oxidation to volatile and photolabile species like Cl2, HOCl or BrCl. These could induce chemistry in the gas-phase and have an impact comparable to OH radicals.;This work is a study of a potential chloride oxidiser, peroxynitric acid (HOONO2), in order to figure out whether it might have a significant impact on marine-boundary-layer chemistry. After developing a set of spectroiodometric techniques to quantify HOONO2 in dilute aqueous solutions, its thermal and base-catalysed decays were studied.;The thermal decay takes place by a radical mechanism involving its dissociation into HO2 and NO2, with a rate constant of 0.026 +/- 0.002 s-1 at 298 K. These radicals made the HOONO2 lifetime highly susceptible to impurities. At pH < 2, HOONO2 regeneration was observed and attributed to reactions between HNO2 and H2O2. The rate constant of the decay of the conjugate base NO4-- to NO2-- and O2 was measured to be 0.75 +/- 0.17 s -1 at 298 K and the equilibrium between NO4-- and the radicals NO2 and O2-- was also discussed. Several thermodynamic properties could be inferred, most notably the Henry's law constant of HOONO2 (12,600 M atm -1).;The oxidation rate constants of I--, Br-- and Cl-- by HOONO2 were found to be 890 +/- 90 M-1 s-1 (295 K), 0.4--1.9 M-1 s-1 (295 K) and (1.4 +/- 0.5) x 10-3 M-1 s-1, respectively, and independent of pH. Under certain conditions, the HO2 radicals in equilibrium with HOONO2 can also initiate the reduction of the halogen products back to halides. The mechanism proposed can account for most of the observations, but is still imperfect, particularly in regard to the bromide system.;The multiphase reactions of HOONO2 in a typical seasalt aerosol were studied theoretically. HOONO2 might play a significant role in highly polluted air masses, but would have trouble competing with the gas-phase production of Cl atoms from the reaction between OH and HCl. Furthermore, nighttime chemistry of NO3 and N2O5 seems much more capable of oxidising Cl-- at a substantial rate.