Heterogeneous interactions of BrO and ClO: Evidence for BrO surface recombination and reaction with HSO3-/SO32-

The uptake coefficients (gamma) for loss of BrO and ClO radicals on surfaces characteristic of particulate material in the atmosphere have been measured using a coated-wall flow tube coupled to a resonance fluorescence detector. Neither radical is lost efficiently at 213 K on sulfuric acid (60 to 70 wt%), pyrex or ice surfaces, with BrO uptake coefficients ranging from 5 x 10(-4) to 1 x 10(-3) on these surfaces and with ClO uptake coefficients smaller than 1x10(-4). Based on the observation by mass spectrometry of Br-2 formation coincident with BrO loss on ice surfaces, it is believed that radical self-reaction is occurring on the surface. Finally, for BrO it was observed that the uptake coefficient on 23 wt% aqueous solutions of NaCl at 253 K was small (<3x10(-3)) but that it could be significantly enhanced when sulfur(IV) species were added to the solution, either by exposure to gas phase SO2 or by the addition of Na2SO3. Aside from the BrO/S(IV) reactions which may promote condensed-phase free-radical chemistry, these findings imply that heterogeneous interactions of BrO and ClO in the atmosphere will be insignificant with respect to gas-phase processes.