Importance of the surface reaction OH + Cl⁻ on sea salt aerosol for the chemistry of the marine boundary layer – a model study

Abstract. The reaction of the hydroxyl radical with chloride on the surface of sea salt aerosol producing gas phase Cl₂ and particulate OH⁻ and its implications for the chemistry of the marine boundary layer under coastal, remote, and very remote conditions have been investigated with a numerical model. This reaction had been suggested by Laskin et al. (2003) to play a major role in the sulfur cycle in the marine boundary layer by increasing the sulfate production in sea salt by O₃ oxidation due to the additional production of alkalinity in the particle. Based on literature data a new "best estimate" for the rate coefficient of the reaction was deduced and applied, showing that the additional initial sulfate production by this reaction is less than 1%, therefore having only a minor impact on sulfate production. Even though the gas phase concentration of Cl₂ increased strongly in the model, the concentration of Cl radicals increased by less than 5% for the "best guess" case. Additional feedbacks between the cycles of chlorine and sulfur in the marine boundary layer are discussed as well as a two-stage acidification of large fresh sea salt aerosol.