Uptake and reaction of HOBr on frozen and dry NaCl/NaBr surfaces between 253 and 233 K J. W. Adams1,*, N. S. Holmes1, and J. N. Crowley1 1Max-Planck-Institut für Chemie, Division of Atmospheric Chemistry, Postfach 3060, 55020 Mainz, Germany *Present address: Dept. of Chemistry, University of Cambridge, Lensfield Rd, Cambridge CB2 1EW, UK
Abstract. The uptake and reaction of
HOBr with frozen salt surfaces of variable NaCl / NaBr composition and temperature were investigated with a coated wall flow tube reactor coupled
to a mass spectrometer for gas-phase analysis.
is efficiently taken up onto the frozen
surfaces at temperatures between 253 and 233 K where it reacts to form the
di-halogens BrCl and Br2, which are subsequently released into the gas-phase. The uptake coefficient for
reacting with a frozen, mixed salt surface of similar composition to sea-spray was
<approx> 10-2. The relative concentration of BrCl and Br2 released to the gas-phase was found to be strongly
dependent on the ratio of Cl- to Br - in the solution prior to freezing / drying. For a mixed salt
surface of similar composition to sea-spray the major product at low conversion of surface
reactants (i.e. Br - and Cl-) was Br2.
Variation of the pH of the NaCl /
NaBr solution used to prepare the frozen surfaces was found to have no significant influence on the results. The observations are explained in terms of
initial formation of BrCl in a surface reaction of HOBr with Cl-, and conversion of
BrCl to Br2 via reaction of surface Br -. Experiments on the uptake and reaction of
BrCl with frozen NaCl /
NaBr solutions served to confirm this hypothesis. The kinetics and products of the
interactions of BrCl, Br2 and Cl2 with frozen salt surfaces were also investigated, and lower
limits to the uptake coefficients of > 0.034, >0.025 and >0.028 respectively, were obtained.
The uptake and reaction of
on dry salt surfaces was also investigated and the results
closely resemble those obtained for frozen surfaces. During the course of this study the gas
diffusion coefficients of
in He and H2O were also measured as
(273 ± 1) Torr cm2 s-1 and (51 ± 1) Torr cm2
s-1, respectively, at 255 K. The implications of these results for
modelling the chemistry of the Arctic boundary layer in springtime are discussed.
Citation: Adams, J. W., Holmes, N. S., and Crowley, J. N.: Uptake and reaction of HOBr on frozen and dry NaCl/NaBr surfaces between 253 and 233 K, Atmos. Chem. Phys., 2, 79-91, doi:10.5194/acp-2-79-2002, 2002.