Article pubs.acs.org/JPCA
Heterogeneous Interaction of N2O5 with HCl Doped H2SO4 under Stratospheric Conditions: ClNO2 and Cl2 Yields Ranajit K. Talukdar,*,†,‡ James B. Burkholder,† James M. Roberts,† Robert W. Portmann,† and A. R. Ravishankara† †
Earth System Research Laboratory, Chemical Sciences Division, National Oceanic and Atmospheric Administration, 325 Broadway, Boulder, Colorado 80305-3328, United States ‡ Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado 80309, United States ABSTRACT: 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 0.01), the radial concentration gradient of N2O5 becomes significant and km is replaced by a corrected first-order loss rate coefficient, kc.32,33 kc values were roughly twice the km values for γ of 0.1. The measured first-order loss rate coefficients, km, were converted into reaction probabilities, γ, using the Brown32 solution to the continuity equation that assumed laminar flow and accounted for diffusion. When the reaction probabilities γ > 0.05, these values need to be corrected for the non-Maxwellian velocity distribution using the eq IV.2,34,35
(I)
where HHCl * is the Henry’s law solubility constant in sulfuric acid solutions. The solubility constant, HHCl * , as a function of temperature, T, and activity of water in sulfuric acid, aH2O, is given by eq II.29 * (T ,a (6250/ T − 10.414)(a 3.49 HHCl H2O) = e H2O)
aH2O
The acid solution was analyzed for H2SO4 content before and after the experiments by titration with a standard NaOH solution. The acid content determined in this way was found to be close (99.995%) was passed through a molecular sieve trap held at 195 K before it entered the ozone generator. During the experiments, a high purity He flow was used to flush N2O5 vapor out of the trap held at 195 K. Synthesis of ClNO2. Nitryl chloride was synthesized from the reaction of HCl with excess N2O5. Dry and pure HCl gas was condensed over excess N2O5 at 195 K.40 The trap was slowly warmed to 273 K for 30 min and then cooled to 77 K. Subsequently, the mixture was warmed to 195 K again and pumped on briefly to remove Cl2. ClNO2 was drawn off the cold trap and examined using FTIR to ensure there were no detectable impurities present. As noted earlier, the impurity of
Figure 2. Upper panel: Uptake of N2O5 on 60 wt % H2SO4 at 220 K and [HCl]liq = 0.00085 M. Exposure of N2O5 to H2SO4 shows that the uptake is irreversible: There was no indication of saturation or regeneration of N2O5 when the injector was past the H2SO4 solution (position B in Figure 1). IP = 4 cm (position B in Figure 1), exposure: IP = 21 cm (position A in Figure 1). ΦClNO2 = 0.28 and ΦCl2
