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

Methane (CH4) oxidation by OH radicals result into an atmospheric CH4 lifetime of 11.2 yr dominating its total lifetime of 9.25 yr (IPCC, 2021). The reactivity of Cl atoms with CH4 is an order of magnitude higher than for OH. Ghosh et al. (2015) calculated that the combined effect of increased Cl atom concentration and air temperature can reduce the total CH4 lifetime from 9.4 yr to 9 yr. This high reduction potential puts the Cl atom into context of geoengineering ideas that aim to reduce the impact of the greenhouse gas CH4. The challenging aspect is to increase the formation of Cl atoms. Those are primarily formed from particulate chlorine. The complex activation depends on the levels of HOx(HO + HO2), NOx(NO + NO2) and non-methane volatile compounds (NMVOCs) as well as aerosol composition and acidity, see Fig. 1. Currently, there is no model framework describing the activation and further Cl atom oxidation processes in sufficient detail to assess proposed geoengineering approaches.

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A core problem for the geoengineering ideas is the complex non-linear Cl atom activation and its impact on methane oxidation (cp. Fig. 1). Processes are strongly impacted by the gas-phase and aerosol composition. Dominant precursors are Cl2 and ClNO2, for which different formation pathways exist in contrasting (anthropogenic or pristine) environments. These processes are currently not implemented and understood in necessary detail for implementation into Earth system models (ESM) to investigate and evaluate geoengineering concepts. Key goals of the proposed project are (i) development of an explicit multiphase halogen module considering the latest findings on Cl atom activation and cycling, (ii) performing detailed multiphase box model simulations to understand Cl atom activation, (iii) evaluation of the model developments with CVAO field data and (iv) deviation of an advanced parameterization for application in ESMs.

The current explicit multiphase mechanism CAPRAM–HM3.0 (Hoffmann et al., 2019) will be further advanced for its detailed Cl atom activation and adjunct model simulations will result into improved Cl atom activation parameterizations for ESMs. The improved understanding of atmospheric Cl atom activation enables a better evaluation of proposed geoengineering approaches dealing an enhanced Cl atom driven CH4 removal and its potential environmental side effects. Following benefits can be achieved:

• Understanding how modulated aerosol compositions influences Cl atom activation
• Identification of feasible geoengineering approaches dealing CH4 removal
• Improved understanding of increased Cl atom concentrations on atmospheric oxidation capacity
• Targeted Cl atom related multiphase chemistry pathways of other trace gases important for modeling Earth’s climate, such as dimethyl sulfide
• Quantification of unwanted side-effects on air quality, human health and the environment.