Modelling the Enhancement of Methane Oxidation through the Addition of Chlorine

As a potent greenhouse gas that has increased in abundance in recent years, methane is an important target for mitigation. Enhancing the chemical loss of methane by adding chlorine to the atmosphere has been proposed by some as a potential method for atmospheric methane removal. Chlorine is known to have a wide range of impacts in the atmosphere, therefore a rigorous assessment of the potential unintended impacts that could arise from atmospheric chlorine addition is required.

Here, the Frontier Research System for Global Change version of the University of California Irvine Chemical Transport Model (FRSGC/UCI CTM) was used to investigate the efficacy and unintended consequences of atmospheric chlorine addition for methane removal. A range of scenarios were designed with varying chlorine emissions magnitudes and spatial distributions to assess how much chlorine is needed, where the most efficient areas to release the chlorine could be, and how the unintended impacts vary with these amounts and locations.

Using an idealised distribution with chlorine emissions (as Cl₂) evenly distributed over the global oceans, we find that atmospheric chlorine addition produces a complex response on the methane lifetime. In broad agreement with previous modelling work, we find a global emission magnitude that is below a threshold of ~100 Tg Cl₂/year would increase the methane lifetime and thus not be effective. Below this threshold, the additional chlorine results in enough tropospheric ozone destruction to reduce methane loss via hydroxyl radicals more than the increased loss via chlorine radicals. Above ~100Tg Cl₂/year, the additional chlorine begins to decrease the methane lifetime, and chlorine becomes a more important sink of methane.

Other scenarios tested include concentrating chlorine emissions over specific ocean basins (e.g. Pacific, Atlantic) and in different latitude bands, such as the tropics. The response of methane under these and other scenarios, along with the unintended impacts of chlorine addition on air quality (e.g. ground level ozone) and the wider environment, will be discussed.