Reactive chlorine in the marine boundary layer in the outflow of polluted continental air: A model study

Results from a numerical one-dimensional model show that continental pollution that is advected over the ocean leads to substantial acid displacement from newly emitted sea salt aerosol. Oxidation of HCl by OH and cycling between the gas and aerosol phase lead to a build-up of nighttime Cl2 mixing ratios of up to 90 pmol mol−1 in our base scenario and 125 pmol mol−1 in a sensitivity study with high wind speeds. Therefore the recirculation of continental airmasses over the ocean and back to the coast might explain previous measurements of high nighttime Cl2 in onshore winds. Furthermore, the model runs suggest a substantial contribution of daytime Cl-radical chemistry to the oxidation of volatile organic compounds under these conditions.