Photocatalytic chlorine atom production on mineral dust-sea spray aerosols over the North Atlantic

Using a combination of field data and global modeling, we demonstrate a mechanism in which a mix of Sahara dust and sea spray aerosol activated by sunlight produces large amounts of active chlorine. This mechanism resolves a number of unexplained observations and significantly revises our understanding of atmospheric chlorine, reducing uncertainties in the source budget. The chlorine formed by this mechanism impacts two important greenhouse gasses, methane and tropospheric ozone, with an estimated catalytic efficiency of removing ca. 45 methane molecules per iron atom per day. The inclusion of Cl^. made from the photocatalytic oxidation of ocean chloride in models will reduce critical uncertainties in estimates of methane emissions and improve our ability to predict future climate change.