HOBr in Sulfuric Acid Solutions:  Solubility and Reaction with HCl as a Function of Temperature and Concentration

Abstract

A detailed study of the interaction of HOBr and HCl in cold sulfuric acid solutions has been performed using a coated-wall flow tube coupled to an electron-impact mass spectrometer. The liquid-phase bimolecular rate constants, measured over a temperature range from 213 to 238 K and in solutions from 59.7 to 70.1 wt % composition, show a strong positive dependence on both acid composition and temperature. The solubility of HOBr has also been measured in these solutions by analyzing its time-dependent uptake. Henry's Law constants (H) determined from the measured values of HD1/2 and the liquid-phase diffusion coefficient (D) are independent of acid composition over the above range of solution compositions. The values of H demonstrate a clear Clausius−Clapeyron temperature dependence, with a heat of solution of −9 ± 1 kcal/mol. When the atmospheric importance of these data is assessed, two conclusions are reached. In the stratosphere, under aerosol conditions observed soon after the Mt. Pinatubo volcanic eruption, the rates of HCl activation via the HOBr/HCl heterogeneous reaction are comparable with the rate of activation via gas-phase reaction with OH at relatively warm temperatures (205−220 K), where other HCl-activating heterogeneous reactions occur slowly. In the high Arctic boundary layer, it is possible that significant HCl activation could occur when elevated levels of photochemically active bromine are present.

Reactivity of BrONO2 and HOBr on sulfuric acid solutions at low temperatures

“…For simplicity in deriving the rate coefficient for in bulk solution, it is assumed that the contribution of the surface reaction is negligible. The measurements of by Waschewsky and Abbatt [1999] are consistent with this assumption. The lines are fits according to The second relation comes from and shows the relation between the parameter a and H HCl * , D l , H HOBr , and k II .…”

“…This is not surprising as it was found that H HOCl [ Donaldson et al , 1997] also does not vary strongly with sulfuric acid content over this range (from 1.7 × 10 3 to 1 × 10 3 M atm −1 ). The independence of H HOBr with acid content was also reported by Waschewsky and Abbatt [1999]. However, extrapolation of their data to 250 K (they report measurements from 208 to 238 K) results in a value for H HOBr of 3 × 10 4 M atm −1 , much lower than the average of the values shown in Figure 6 of 1.4 × 10 5 M atm −1 .…”

“…This indicates that secondary reactions transforming HOBr into unmonitored Br compounds may have taken place. Earlier researchers also report [ Waschewsky and Abbatt , 1999] erratic behavior in their solubility measurements when pHOBr was relatively high (∼3 × 10 −9 atm or higher.) One experiment here where an acid sample had come into contact with a metal tweezers resulted in a value of H HOBr that was about twice the other experiments.…”

“…H HOBr was determined by measuring the amount of HOBr taken up by a known volume of solution as described by Donaldson et al [1997] and Hanson and Lovejoy [1996] who measured the solubilities of HOCl and HCl. The results of earlier studies of H HOBr [ Hanson and Ravishankara , 1995; Abbatt , 1995; Waschewsky and Abbatt , 1999] depend on the detailed time dependence of the uptake coefficient and an estimated value for the diffusion coefficient of HOBr in solution. The method used here does not depend on these factors; it involves measuring the amount of reactant taken up relative to its equilibrium partial pressure.…”

Halogen‐driven low‐altitude O3 and hydrocarbon losses in spring at northern high latitudes

“…The Henry's law constants were measured at bulk liquid conditions, which is very different from the quasi‐liquid surface layer of snow and aerosols where the reactions take place [e.g., Koop et al , 2000]. Additional uncertainties could come from the mass accommodation coefficients and their temperature dependence [e.g., Waschewsky and Abbatt , 1999]. Similar kinetic uncertainties also exist for HOBr uptake.…”