Heterogeneous kinetics of the uptake of HOBr on solid alkali metal halides at ambient temperature

Bromine contg. compds. are thought to play an important role in the O3 destruction in various regions of the atm. Recent studies of atm. Br chem. suggest that HOBr is one of the important Br compds. The rapid photolysis of HOBr in daytime releases Br atoms and OH radicals, which causes ozone destruction. 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, which in the atm. act as substrate particles for the reactions. The heterogeneous reactions of HOBr with solid cryst. NaCl [HOBr(g) + NaCl(s) -> BrCl(g) + NaOH(s)] and KBr [HOBr(g) + KBr(s) -> Br2(g) + KOH(s)] substrates at ambient temp. 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 obsd. uptake probability depends on the total external surface area of the salt substrates. For NaCl substrates, Br2 and BrCl are obsd. as products; for KBr substrates, Br2 is obsd. as the sole product. In both cases, a dependence of the initial uptake probability g0 on HOBr flow rate has been obsd. The initial uptake is large at low flow rate and 10 times smaller at high flow rate. Values of g0 ? (6.5 +- 2.5) * 10-3 for NaCl and g0 ? 0.18 +- 0.04 for KBr are obtained under our exptl. conditions of limiting low flow rates akin to atm. conditions. The prodn. of Br2 is obsd. even for HOBr interacting on solid NaNO3, a non-halogen contg. substrate. The yield measurements imply that a HOBr self-reaction occurs on salt surfaces according to 2HOBr -> Br2 + H2O + 1/2O2. The decrease in Br2 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 lab. exptl. conditions. The decrease of g0 with time indicates that approx. 5-10% of the Br atoms on a KBr surface interact with HOBr