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

• Published in: Proceedings of the National Academy of Sciences
• Main Authors: van Herpen, Maarten M. J. W., Li, Qinyi, Saiz-Lopez, A., Liisberg, Jesper B, Röckmann, Thomas, Cuevas, Carlos A., Fernandez, Rafael P., Mak, John E., Mahowald, Natalie M., Hess, Peter, Meidan, Daphne, Stuut, Jan-Berend W., Johnson, Matthew S.
• Other Authors: National Science Foundation (US), European Commission, #NODATA#
• Format: Article in Journal/Newspaper
• Language: English
• Published: National Academy of Sciences (U.S.) 2023
• Subjects: Aerosol chemistry; Chemistry–climate; Isotope modeling; Methane removal; Tropospheric chlorine; Mak; North Atlantic
• Online Access: http://hdl.handle.net/10261/352570; https://doi.org/10.1073/pnas.2303974120; https://doi.org/10.13039/100000001; https://doi.org/10.13039/501100000780; https://api.elsevier.com/content/abstract/scopus_id/85165662410

8 pags. 4 figs. 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 implementation 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)], 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 13C-depleted biological sources such as agriculture and wetlands. Since 2014, an increase in North African dust emissions has increased the 13C 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. The CESM project is supported primarily by the NSF. Computing resources, support, and data storage were provided by the Climate Simulation Laboratory at NCAR’s Computational and Information Systems Laboratory, sponsored by the NSF. Spark Climate Solutions: M.M.J.W.v.H., A.S.-L., J.B.L., T.R., and M.S.J. ACTRIS-DK: M.S.J. SilverLining: J.B.L., N.M.M., and M.S.J. European Research Council Executive Agency under the ...