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

Abstract

Active chlorine in the atmosphere is poorly constrained and so is its role in the oxidationof the potent greenhouse gas methane, causing uncertainty in global methane budgets.We propose a photocatalytic mechanism for chlorine atom production that occurs whenSahara 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 13C depletion in CO in air samples from Barbados [J.E. Mak,G. Kra, T. Sandomenico, P. Bergamaschi, J. Geophys. Res. Atmos. 108 (2003)], whichremained unexplained for decades. The production of Cl can also explain the anomalyin 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. Lawleret al., Atmos. Chem. Phys. 11, 7617–7628 (2011)]. Our model finds that 3.8 Tg(Cl) y−1is produced over the North Atlantic, making it the dominant source of chlorine in theregion; globally, chlorine production increases by 41%. The shift in the methane sinkbudget 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 13C-­depleted biologicalsources such as agriculture and wetlands. Since 2014, an increase in North African dustemissions has increased the 13C isotope of atmospheric CH4, thereby partially maskinga much greater decline in this isotope, which has implications for the interpretation ofthe drivers behind the recent increase of methane in the atmosphere.