Iron(III)-Induced Activation of Chloride and Bromide from Modeled Salt Pans

The photochemistry of halides in sea spray aerosol, on salt pans, and on other salty surfaces leads to a formation of reactive halogen species. We investigated the photochemical formation of atomic chlorine (Cl) and bromine (Br) in the gas phase in the presence of laboratory-modeled salt pans consisting of sodium chloride doped with iron­(III) chloride hexahydrate (0.5 and 2 wt %). The samples were spread on a Teflon sheet and exposed to simulated sunlight in a Teflon smog chamber in purified, humidified air in the presence of a test mixture of hydrocarbons at the ppb level to determine Cl, Br, and OH formation by the radical clock method. Driven by the photolytic reduction of Fe­(III) to Fe­(II), the production rates of the Fe­(III)-doped NaCl salt samples (up to10⁷ atoms cm^–3 s^–1) exceeded the release of Cl above a pure NaCl sample by more than an order of magnitude in an initially O₃-free environment at low NO_X. In bromide-doped samples (0.5 wt % NaBr), a part of the Cl release was replaced by Br when Fe­(III) was present. Additions of sodium sulfate, sodium oxalate, oxalic acid, and catechol to NaCl/FeCl₃ samples were found to restrain the activation of chloride.