Heterogeneous Kinetics of the Uptake of HOBr on Solid Alkali Metal Halides at Ambient Temperature ² M. Mochida, ‡,§ H. Akimoto, ‡ H. van den Bergh, § and M. J. Rossi* ,§ Research Center for AdVanced Science and Technology, UniVersity of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153, Japan, and Laboratoire de Pollution Atmosphe ´ rique et Sol (LPAS), Swiss Federal Institute of Technology (EPFL), CH-1015 Lausanne, Switzerland ReceiVed: January 22, 1998; In Final Form: March 25, 1998 The heterogeneous reactions of HOBr with solid crystalline NaCl [HOBr(g) + NaCl(s) f BrCl(g) + NaOH- (s)] and KBr [HOBr(g) + KBr(s) f Br 2 (g) + KOH(s)] substrates at ambient temperature have been investigated using a Teflon coated Knudsen cell reactor. Powder, grain, and spray-deposited salt substrates were used for the measurement of the HOBr reactivity. The observed uptake probability depends on the total external surface area of the salt substrates. For NaCl substrates, Br 2 and BrCl are observed as products; for KBr substrates, Br 2 is observed as the sole product. In both cases, a dependence of the initial uptake probability γ 0 on HOBr flow rate has been observed. The initial uptake is large at low flow rate and 10 times smaller at high flow rate. Values of γ 0 e (6.5 ( 2.5) × 10 -3 for NaCl and γ 0 e 0.18 ( 0.04 for KBr are obtained under our experimental conditions of limiting low flow rates akin to atmospheric conditions. The production of Br 2 is observed even for HOBr interacting on solid NaNO 3 , a non-halogen containing substrate. The yield measurements imply that a HOBr self-reaction occurs on salt surfaces according to 2HOBr f Br 2 + H 2 O + 1 / 2 O 2 . The decrease in Br 2 yield with increasing HOBr flow rate from 100 to 50% indicates that a competition between the heterogeneous reaction of HOBr with NaCl or KBr and the self-reaction of HOBr takes place on the solid salt surface under laboratory experimental conditions. The decrease of γ 0 with time indicates that approximately 5-10% of the Br atoms on a KBr surface interact with HOBr. Introduction Bromine containing compounds are thought to play an important role in the ozone destruction in various regions of the atmosphere. Barrie et al. 1 first reported a relationship between springtime ozone depletion in the lower Arctic troposphere which shows a strong negative correlation between the concentration of ozone and bromine compounds. This phenomenon has been reconfirmed in subsequent field measurements, 2-6 and heterogeneous reactions on sea salt particles have been invoked as potential sources of photoactive bromine compounds which cause ozone destruction. From kinetic studies of heterogeneous reactions on salt particles, a few key reactions have been proposed in order to rationalize the Br sources and the cycling of bromine species. 7-21 Recent studies of atmospheric bromine chemistry suggest that HOBr is one of the important bromine compounds. 22-25 The rapid photolysis of HOBr in daytime releases bromine atoms and hydroxyl radicals, which causes ozone destruction according to the reactions in eqs 1-5. In addition to the ozone destruction mechanism written in eqs 1-5, another mechanism involving the heterogeneous reaction in or on liquid aerosols is also proposed: Because of the recent attention to HOBr, several measure- ments of HOBr properties have been reported: for example, ideal gas thermodynamic properties, 26 UV/visible absorption spectra, 27 near-threshold photodissociation dynamics, 28 and an ab initio study of the electronic absorption spectrum. 29 Regarding the heterogeneous reaction of HOBr, Abbatt and Oppliger et al. have reported the HOBr reaction with ice 30,31 and supercooled sulfuric acid solution. 32 Kirchner et al. reported the HOBr reaction on an ice surface doped with sea salt. 33 However, experimental studies of the HOBr heterogeneous reaction on solid salt surfaces have not been reported. Salt particles exist in the marine boundary layer, in the stratosphere after volcanic eruption, 34 and in plumes from burning oil wells. 35-37 Although salt particles most often occur as solution droplets in the marine boundary layer, some of these particles may be transported to a dry atmosphere where they are found as solid crystalline salt particles. 38 In this study, the reactivity of HOBr with solid alkali metal halides is reported which affords insights into the importance of heterogeneous reactions of HOBr with salt. In addition, emphasis is placed on mechanistic aspects of the heterogeneous reaction in order to be able to extrapolate the kinetics to environmental conditions with confidence. Experimental Section The experiments have been performed using a Knudsen cell reactor shown in Figure 1, which consists of a Knudsen reactor coupled to a differentially pumped quadrupole mass spectrom- eter (Balzers model QMG 421). A detailed description of this * To whom correspondence should be addressed. ² Experimental work carried out at EPFL under the auspices of the Alliance for Global Sustainability (AGS). ‡ University of Tokyo. § Swiss Federal Institute of Technology (EPFL). Br + O 3 f BrO + O 2 (1) HO 2 + BrO f HOBr + O 2 (2) HOBr + hν f OH + Br (3) OH + O 3 f HO 2 + O 2 (4) net: 2O 3 f 3O 2 (5) BrONO 2 + H 2 O 9 8 het. HOBr + HNO 3 (6) 4819 J. Phys. Chem. A 1998, 102, 4819-4828 S1089-5639(98)00849-4 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/02/1998