Heterogeneous Interaction of N2O5 with HCl Doped H2SO4 under Stratospheric Conditions: ClNO2 and Cl2 Yields

The reaction of dinitrogen pentoxide, N2O5, with hydrogen chloride, HCl, in sulfuric acid solutions was studied at temperatures and compositions relevant to the upper troposphere/lower stratosphere. Experiments were performed using a rotating wetted wall flow tube reactor coupled to a chemical ionization mass spectrometer for the gas-phase detection of reactants (N2O5 and HCl) and products (nitryl chloride, ClNO2, and Cl2) using I− as the reagent ion. Uptake coefficients, γ, were measured under stratospheric conditions: 205 < T < 225 K; 50 and 60 wt % H2SO4 solutions; 5.8 × 10−5 < [HCl]liq < 0.1 M. Uptake coefficients of N2O5 on pure H2SO4/H2O (50 and 60 wt % H2SO4) and HCldoped H2SO4 were found to be independent of temperature and sulfuric acid composition (weight percent of H2SO4 and HCl concentration) consistent with previous studies. ClNO2 was observed to be a major gas-phase product with its yield strongly dependent on the liquid-phase HCl concentration (5.8 × 10−5 to 0.1 M HCl) and with a maximum yield of nearly unity at 0.005 M HCl in both 50 and 60 wt % sulfuric acid solutions. The Cl2 yield was <1% under all conditions studied. ClNO2 production was attributed to the heterogeneous reaction of NO2+(aq), or H2NO3+(aq) (formed in the dissociative ionization of N2O5), with Cl−. The variation of the ClNO2 yield with HCl concentration was attributed to the competition between the reaction of NO2+(aq), or H2NO3+(aq) with Cl− and H2O. Using our measured yields as a function of HCl concentrations in 50 and 60 wt % H2SO4 solutions at different temperatures, we calculated the variation of the ClNO2 yield under stratospheric conditions. The atmospheric implications of these findings were examined using a 2D atmospheric model. The contribution of this chemistry to ozone depletion was found to be a minor process under nonvolcanic background aerosol levels.