Program:                                                                                                                           
15:00-15:15       Coffee & cake in the foyer
15:15-16:00       Scientific talk in Auditorium I

Speaker: Professor Matthew Stanley Johnson, Københavns Universitet

Abstract:
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, which remained unexplained for decades. The production of Cl can also explain the anomaly in the CO:ethane ratio found at Cape Verde, in addition to explaining the observation of elevated HOCl. Our model finds that 3.8 Tg(Cl) per year 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 per year to 13C-depleted biological sources such as agriculture and wetlands. The mechanism is based on photolysis of iron(III) chlorides as demonstrated by laboratory experiments, ab initio calculations and modeling. Experts suggest it is not possible to reach climate targets without rapid reduction in atmospheric methane concentrations and that iron-salt aerosols may be one method to achieve that. The addition of chlorine to the atmosphere has been evaluated as a climate intervention. Today’s shipping will generate chlorine due to iron present as an impurity in shipping fuels. The additional chlorine could increase methane lifetime by suppressing ozone, and thereby OH production, or under low-NOx conditions it could decrease methane lifetime by reacting with methane. This ISAMO project is conducting a global flask sampling campaign in collaboration with Maersk and OceansX to measure the impact of dust and the interactions with anthropogenic pollution and climate change.