Photochemical Activation of Chlorine by Iron and Iron Oxide Aerosol

Abstract:

The photochemical activation of chlorine by dissolved iron in sea-salt aerosol droplets and by highly dispersed Fe₂O₃ aerosol particles (mainly hematite, specific surface > 100 m²/g), exposed to gaseous HCl, was investigated in humidified air in a Teflon simulation chamber. Employing the radical-clock technique, we quantified the production of gaseous atomic Cl. When the artificial sea salt aerosols contained suspended Fe₂O₃ alone at pH 6, no significant Cl production could be observed, even if the dissolution of iron was forced by “weathering” (repeatedly freezing and thawing for five times). Adjusting the pH in the stock suspension to 2.6, 2.2, and 1.9 and equilibrating for one week resulted in a quantifiable amount of dissolved iron (0.03, 0.2, and 0.6 mmol/L, respectively) and in gaseous Cl production rates of ~1.6, 6, and 8 × 10²¹ atoms cm^-2 h^-1, respectively. Exposing the pure Fe₂O₃ aerosol in the absence of salt to various gaseous HCl concentrations resulted in rates ranging from 8 × 10²⁰ Cl atoms cm^-2 h^-1 (at ~4 ppb HCl) to 5 × 10²² Cl atoms cm^-2 h^-1 (at ~350 ppb HCl) and confirmed the uptake and conversion of HCl to atomic Cl (at HCl to Cl conversion yields of 2-5 % mol/mol, depending on the relative humidity). The relevance for environmental processes in the atmosphere will be discussed.