This problem statement was submitted as part of a research funding application which was awarded by Spark.

Methane is a potent greenhouse gas which, due to its high radiative forcing, relatively short atmospheric lifetime and increasing emissions, will play a critical role in determining our climate over the next few decades and beyond. Methane’s major atmospheric loss route is through reaction with the OH radical, but it also reacts rapidly with Cl atoms. Estimates of the global fraction of methane removed by Cl are highly uncertain due to poor understanding of inorganic chlorine abundances and Cl production routes. Mobilisation of chloride from sea salt aerosol (SSA) is understood to be the major process for Cl production (Wang et al. 2019). However, a photocatalytic mechanism from iron-bearing Mineral Dust-Sea spray Aerosols (MDSA) (van Herpen et al. 2023) has been proposed to represent a significant route for Cl production and potentially for atmospheric methane removal (Gorham et al. 2024). This mechanism remains to be evaluated through direct field measurements of chlorine.

Global modeling studies vary widely in their calculation of the Cl burden and global fraction of methane removed by Cl, with the latter estimated as 0.8 to 3.3% (Li et al. 2023 ; Sherwen et al. 2016 ; Wang et al. 2019). Photolysis of MDSA has been proposed to represent a significant additional route to Cl and the main source over the Atlantic, with the potential to increase global oxidation of methane by ~ 1% (van Herpen et al. 2023). A quantitative understanding of atmospheric Cl and of the MDSA mechanism is however inhibited by a lack of field observations. 

Key goals: 

• Quantitatively evaluate the drivers of Cl production through field observations of speciated chlorine at the Cape Verde Observatory (CVO) 
• Robustly evaluate indirect field evidence for Cl enhancement
• Determine whether the natural Cl sink of methane via the MDSA mechanism is effective
• Improve knowledge of the potential impact of enhanced Cl levels from MDSA chemistry on methane, climate and air quality

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CEMDEM-CVO will provide direct evidence of the importance of the MDSA mechanism for methane lifetime in the present-day atmosphere and provide quantitative observations in the marine boundary layer (MBL) needed for validating its incorporation in models. The dataset and understanding gained will represent a significant advancement in quantifying the true impact of chlorine as a global methane sink and the sensitivities of this chemistry to aerosol and trace gas composition. The knowledge on the impacts from this study will not only be applicable in the Atlantic but will also be used to quantify methane oxidation by chlorine in other parts of the world that are affected by MDSA. The project also has the potential to form the basis of climate intervention studies for increased chlorine emissions leading to methane destruction.