Heterogeneous Kinetics of the Uptake of ClONO 2 on NaCl and KBr Franc ¸ ois Caloz, Frederick F. Fenter, and Michel J. Rossi* Laboratoire de Pollution Atmosphe ´ rique et Sol (LPAS), Swiss Federal Institute of Technology (EPFL), 1015 Lausanne, Switzerland ReceiVed: October 20, 1995; In Final Form: February 15, 1996 X The uptake kinetics of ClONO 2 on NaCl (reaction 1) and on KBr (reaction 2) have been studied in a low- pressure, Teflon-coated Knudsen reactor at room temperature. The initial uptake coefficient for both reactions has been measured as 0.23 ( 0.06 and 0.35 ( 0.06 for reactions 1 and 2, respectively, and is independent of reactant density in the range 10 10 -10 13 molecules cm -3 . The values of the uptake coefficients are independent of presentation of the salt substrates: identical results are obtained on powder, grains, single-crystal surfaces, and thin deposited salt layers. The only product of reaction 1 is Cl 2 . Reaction 2 initially produces Br 2 , followed by BrCl and Cl 2 . In our proposed mechanism, BrCl is the product of reaction 2, and secondary reactions between BrCl and the KBr substrate yield Br 2 at short reaction times and Cl 2 under prolonged exposure. Introduction The heterogeneous reactions of nitrogen oxides with salt are coming under increased scrutiny, with the goal being to understand the role they play in atmospheric chemistry. In particular, nitrogen oxides such as N 2 O 5 , HNO 3 , and ClONO 2 have been found to react with salt to produce gaseous chlorine- containing species. If these products are photolyzed in sunlight, then the chlorine atoms thus produced may affect the oxidative potential of the local atmosphere. 1,2 To this end, many efforts are currently underway, including field studies, 3,4 laboratory measurements, 2,5-10 and atmospheric modeling. 11-15 Here, we consider the reactions of gaseous chlorine nitrate with salt, which are characterized by a displacement of halide for nitrate in the condensed phase with a release of the corresponding molecular halogen: In the atmosphere, the halogen will photolyze to produce reactive chlorine and bromine atoms. It has been proposed that the chlorine atoms liberated indirectly by the reactions of nitrogen oxides with sea salt aerosol may provide a source of atomic chlorine in the marine troposphere. 16 This source may be competitive with the homogeneous gas-phase reaction between the OH radical and HCl: Also, the chlorine and bromine atoms liberated by reactions 1 and 2 may play a role in the catalytic loss of ozone after volcanic injection of salt in the stratosphere, as suggested by in situ measurements 17 and modeling. 14 Keyser and co-workers performed the first quantitative study on reaction 1 at two different temperatures using a flow reactor that significantly deviated from cylindrical symmetry. 5 They found Cl 2 to be the sole product, and they determined the uptake coefficient to be γ ) (4.6 ( 3.0) × 10 -3 after having applied a correction factor for internal diffusion. These results are in line with the more qualitative conclusions of earlier studies by Finlayson-Pitts and co-workers. 16,18 Here, we present our kinetic results for reactions 1 and 2, obtained using a low-pressure flow reactor (Knudsen cell). We include a study of the effects of the “internal” surface area of salt samples, presented as powder, grain, single crystals, or spray-deposited films, discussed in the frame of a model proposed by Keyser and co-workers 5,6,19-22 and in light of previous work. 2,9 By systematically varying the presentation of the salt substrate, the reactivity of ClONO 2 is studied as a function of surface morphology. Experimental Section The experiments are carried out in a low-pressure, Teflon- coated Knudsen reactor described elsewhere. 2 The apparatus consists of a gas-handling system, a Knudsen cell reactor, and a vacuum chamber that houses the mass spectrometer (see Figure 1). Gaseous samples are prepared in the vacuum line and injected into the reactor. From the pressure drop in a calibrated volume, the gas flow (molecules s -1 ) is directly determined using the ideal gas relation, and the mass-spectrometer signal, for all species of interest, is calibrated against the known flow rate. Flow as small as 5 × 10 14 molecules s -1 can be directly measured, whereas smaller flow rates are found by extrapolation of the calibration curve. The Knudsen cell is a two-chamber reactor in which the exposure of the reactive substrate is controlled with a plunger (cf. Figure 1). In the absence of reaction, the gas flowing into the cell subsequently leaves via a small orifice into the vacuum chamber. Because the cell is operated in the molecular flow regime, the loss through the aperture is a first-order process with respect to the gas-phase density. We experimentally determine the value of the rate of effusive loss, k esc , for each species and for each aperture size. The parameters for the Knudsen cell are summarized in Table 1. The molecules that effuse out of the Knudsen cell enter the upper volume of a differentially pumped vacuum chamber. The effusive beam is modulated by a tuning fork chopper located at the orifice separating the two chambers and is subsequently sampled by a quadrupole mass spectrometer (Balzers QMA 421). Phase-sensitive detection of the chopped signal using a lock-in amplifier (SRS 830) eliminates background sources. The amplified mass-spectrometer signal is proportional to the flow of molecules exiting the Knudsen cell. * Author to whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, April 1, 1996. ClONO 2 + NaCl f Cl 2 + NaNO 3 (1) ClONO 2 + KBr f BrCl + KNO 3 (2) HCl + OH f H 2 O + Cl (3) 7494 J. Phys. Chem. 1996, 100, 7494-7501 S0022-3654(95)03099-1 CCC: $12.00 © 1996 American Chemical Society