Production of molecular iodine from the heterogeneous reaction of nitrogen dioxide with solid potassium iodide

Abstract

[1] Molecular iodine impacts tropospheric chemistry, especially in coastal regions of the marine boundary layer where iodine bursts correlate with ultrafine particle formation. Although biogenic sources dominate coastal I2 release, inorganic heterogeneous chemistry may also contribute to the global iodine budget. We investigated the heterogeneous chemistry of NO2/N2O4 with solid KI, a component of dehydrated sea salt aerosol, and identified I2(g) as the major gaseous product. Diffuse reflectance infrared spectroscopy and ion chromatography identified nitrite as the major condensed-phase product for the reaction of KI(s) + NO2(g) (5.4 × 1013 to 2.0 × 1015 molecules cm−3) at relative humidities between 2% and 30%. The reaction was second order with respect to NO2, suggesting that N2O4 was the reactive species. Reactive uptake coefficients, calculated with respect to [N2O4], were independent of relative humidity with an average value of γBET = (6.1 ± 0.3) × 10−4. Concurrent gas phase UV-visible spectroscopy indentified I2(g) production with near stoichiometric release corresponding to 2 KI(s) + N2O4(ads) → 2 KNO2(s) + I2(g). Condensed-phase nitrate was also identified at high [N2O4] and low RH, which was attributed to the oxidation of product KNO2 by gaseous NO2/N2O4. The heterogeneous chemistry of ozone with aqueous iodide has previously been considered as a source of molecular iodine in the marine boundary layer. These results suggest that the heterogeneous chemistry between NO2/N2O4 and iodide deserves further consideration as a nonbiogenic source of molecular iodine.