Rate constants for free radical reactions in the liquid phase will be measured in the laboratory to determine the ionic strength, pH, and temperature dependence of several important reaction systems over a range of variables relevant to the atmosphere. Earlier measurements do not extend to the very high electrolyte concentrations that can occur in atmospheric aerosols. The focus of this work will be on the reactions of the nitrate radical with halogen species which are important to understand "halogen activation". This is a poorly understood process by which halides dissolved in aerosols are oxidized to yield photochemically active gas phase halogen-containing species. These can be photolyzed by sunlight and produce free halogen atoms, which attack gas phase ozone and other trace atmospheric constituents. These processes have been implicated in observed tropospheric ozone depletion events shortly after polar sunrise in the Arctic. Free radical reactions in the aqueous phase are initiated by photodissociation of precursors and by the solvation of gas phase free radicals. The measurements will be performed by using the laser flash photolysis/time-resolved absorption method. The experimental conditions are controlled in order to isolate the effects of ionic strength, pH, temperature, and any other variables desired. The rate constants which result from this work can be used directly in atmospheric models.
