Uptake of hypobromous acid (HOBr) by aqueous sulfuric acid solutions: low-temperature solubility and reaction

Abstract. Hypobromous acid (HOBr) is a key species linking inorganic bromine to the chlorine and odd hydrogen chemical families. We have measured the solubility of HOBr in 45-70wt% sulfuric acid solutions representative of upper tropospheric and lower stratospheric aerosol composition. Over the temperature range 201-252 K, HOBr is quite soluble in sulfuric acid, with an effective Henry's law coefficient, H^*=10⁴-10⁷mol L^-1atm^-1. H^* is inversely dependent on temperature, with ΔH=-45.0±5.4 kJ mol^-1 and ΔS=-101±24 J mol^-1K^-1 for 55-70wt% H₂SO₄ solutions. Our study includes temperatures which overlap both previous measurements of HOBr solubility. For uptake into 55-70wt% H₂SO₄, the solubility is described by log H^*=(2349±280)/T-(5.27±1.24). At temperatures colder than ~213K, the solubility of HOBr in 45wt% H₂SO₄ is at least a factor of five larger than in 70wt% H₂SO₄, with log H^*=(3665±270)/T-(10.63±1.23). The solubility of HOBr is comparable to that of HBr, indicating that upper tropospheric and lower stratospheric aerosols should contain equilibrium concentrations of HOBr which equal or exceed those of HBr. Upon uptake of HOBr into aqueous sulfuric acid in the presence of other brominated gases, particularly for 70wt% H₂SO₄ solution, our measurements demonstrate chemical reaction of HOBr followed by evolution of gaseous products including Br₂O and Br₂.