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

Active chlorine in the atmosphere is poorly constrained and so is its role in the oxidation of the potent greenhouse gas methane, causing uncertainty in global methane budgets. We propose a photocatalytic mechanism for chlorine atom production that occurs when Sahara dust mixes with sea spray aerosol. The mechanism is validated by implemen-tation in a global atmospheric model and thereby explaining the episodic, seasonal, and location-dependent C-13 depletion in CO in air samples from Barbados [J.E. Mak, G. Kra, T. Sandomenico, P. Bergamaschi, J. Geophys. Res. Atmos. 108 (2003)], which remained unexplained for decades. The production of Cl can also explain the anomaly in the CO:ethane ratio found at Cape Verde [K. A. Read et al., J. Geophys. Res. Atmos. 114 (2009)], in addition to explaining the observation of elevated HOCl [M. J. Lawler et al., Atmos. Chem. Phys. 11, 7617-7628 (2011)]. Our model finds that 3.8 Tg(Cl) y(-1) is produced over the North Atlantic, making it the dominant source of chlorine in the region; globally, chlorine production increases by 41%. The shift in the methane sink budget due to the increased role of Cl means that isotope-constrained top-down models fail to allocate 12 Tg y(-1) (2% of total methane emissions) to C-13-depleted biological sources such as agriculture and wetlands. Since 2014, an increase in North African dust emissions has increased the C-13 isotope of atmospheric CH4, thereby partially masking a much greater decline in this isotope, which has implications for the interpretation of the drivers behind the recent increase of methane in the atmosphere. Significance 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.