A modeling study of the mechanism of the halogen–ozone–mercury homogeneous reactions in the troposphere during the polar spring

Computer simulations are described for the homogeneous portion of the chemistry of the ozone and mercury depletion observed in the troposphere during the polar spring. Conditions chosen are similar to those of a clear day (17 March) at the location of Barrow, Alaska. Small amounts of reactive trace gases, representative of the Arctic spring, are present initially with typical background levels of Hg (0.24 ppt) and 50 ppb of O3. The simulations show that ozone and gaseous atomic mercury depletion are both strong functions of two important variables: (1) the concentrations of Br2 and BrCl present during the day; and (2) the concentrations of reactive trace gases such as CH2O present in the air mass. The simulated rate of Hg depletion and the expected product distribution are also functions of the size of the rate coefficient for the Hg+BrO reaction (k17). The relatively small extent of O3 and Hg depletion rates that result from Cl–ClO chemistry and the major effects that result from the Br–BrO chemistry are rationalized in terms of the significant differences in the reaction mechanisms for the two halogens.