Understanding chlorine and its impact on methane lifetime in remote marine environments through measurements and modelling

Over the last decade, tropospheric chlorine chemistry has been shown to play an important role in both the clean and polluted atmosphere. The high reactivity of atomic chlorine makes it a potent atmospheric oxidant and it has recently been proposed as a potential target for climate geoengineering interventions, but its production mechanisms are still poorly understood. In this project, you will explore the production and fate of tropospheric chlorine by making measurements at the Cape Verde Atmospheric Observatory (https://amof.ac.uk/observatory/cape-verde-atmospheric-observatory-cvao/) and using these, along with other observations, in a chemical box-model to challenge and improve our understanding of chlorine chemistry in remote regions.

It is well known that chlorine chemistry plays a pivotal role in stratospheric ozone depletion, but it was generally considered to not be important in the troposphere. However, over the last 15 years this perspective has changed as the presence of significant levels of reactive chlorine in the troposphere has been established. Tropospheric oxidants are responsible for the degradation of pollutants emitted into the atmosphere and so determine Earth system processes including: the lifetime of climate gases such as methane; the production of secondary pollutants such as ozone and particles that impact on air quality; and the deposition of chemicals to ecosystems. The ¹³C isotopic ratio of methane, which has been used to provide constraints on the various contributions to the methane budget, is also highly sensitive to the presence of chlorine.  Chlorine atoms however are the least understood of the major atmospheric oxidants, with current estimates of concentrations varying by 3 orders of magnitude.  This uncertainty represents a major limitation in our understanding of these important atmospheric processes.

Chlorine is emitted into the atmosphere from both natural and anthropogenic sources, often involving complex heterogeneous chemistry on atmospheric aerosols (e.g. https://www.nature.com/articles/s41467-022-28383-9?fromPaywallRec=false). In polluted regions, the nighttime generation of nitryl chloride leads to the generation of atomic chlorine which can drive the production of ozone and particulate matter (https://www.nature.com/articles/ngeo177?proof=t). In clean regions the production of atomic chlorine is less well understood, but once formed it can remove methane and other hydrocarbons from the atmosphere and destroy ozone (https://acp.copernicus.org/articles/19/3981/2019/).  It has recently been proposed that increasing chlorine production in the remote marine atmosphere could be a possible approach to reducing global methane levels and their associated climate warming (https://www.nature.com/articles/s41467-023-39794-7). However, the large gaps in our understanding of the production and cycling of chlorine mean the true impacts of such interventions are unknown.

This project will make field observations of a range of chlorine containing compounds in the gas and aerosol phase with techniques including chemical ionisation mass spectrometry (CIMS) and tunable infrared laser direct absorption spectroscopy (TILDAS).   You will develop a chemical box-model to investigate the production and cycling of atomic chlorine in remote marine environments, evaluated using these measurements alongside existing observations.   Once created, you will use this model to study of some of the “big questions” remaining about chlorine chemistry, such as: Does the chlorine radical play a globally significant role in the oxidation of hydrocarbons and methane? How does this impact methane isotopic ratios? How does chlorine chemistry impact regional oxidation? The project will involve significant amounts of data analysis and computational modelling and will also provide the opportunity to operate cutting edge analytical equipment to gather the field observations needed.