Experimental Determination of the Photooxidation of Aqueous I^– as a Source of Atmospheric I₂

The chemistry of iodine plays an important role in the oxidizing capacity of the global marine atmosphere. In this study, we experimentally determine the photooxidation parameters of iodide ions in aqueous phase (I^–_(aq)) and estimate the subsequent emission of gaseous iodine molecules (I_2(g)) into the atmosphere. The values of the molar absorption coefficient (ε_iodide(λ)) and the photooxidative quantum yields (Φ_iodide(λ)) of I^–_(aq) in the range of 290–500 nm were determined. The influence of pH and dissolved oxygen (DO) on the values of Φ_iodide(λ) was also investigated. The emission of I_2(g) into the atmosphere following the photooxidation of I^–_(aq) in deionized water solution (pH 5.6, DO 7.8 mg L^–1) and artificial seawater solution (pH 8.0, DO 7.0 mg L^–1) was estimated to be (2.2 × 10^–8 × [I^–_(aq)]_sea) and (1.8 × 10^–8 × [I^–_(aq)]_sea) mol L^–1 s^–1, respectively. Using a global chemistry-climate model, we estimated that the photooxidation of I^–_(aq) can increase the atmospheric iodine budget by up to ∼8% over some oceanic regions.