Understanding chlorine chemistry in polluted and clean atmospheres

About the Project
Over the last decade, tropospheric chlorine chemistry has been shown to play an important role in both the clean and polluted atmosphere. In this project, you will explore the role of chlorine chemistry in creating and removing air pollutants by making observations at Bermuda and in the UK, and using numerical models, as part of the joint UK-US BLEACH project.

Chlorine chemistry plays a pivotal role in stratospheric ozone depletion. However, until recently it was not thought to be important in the troposphere. Over the last 10 years though, this perspective has changed. In polluted regions, the nighttime generation of ClNO2 leads to the generation of highly reactive Cl radicals which can drive the production of ozone and particulate matter (https://www.nature.com/articles/ngeo177?proof=t) and chlorine itself can make up a significant fraction of particulate matter in some locations (https://www.nature.com/articles/s41561-020-00677-x). In clean regions, chlorine radicals can remove methane and other hydrocarbons from the atmosphere and destroys ozone (https://acp.copernicus.org/articles/19/3981/2019/). Chlorine is emitted into the atmosphere in a number of ways: sea-salt dominants but emissions of natural and man-made chloro-carbons, combustion (notably of coal) and industrial processes. Increases in the anthropogenic sources have led to a significant increase in the mass of chlorine in the troposphere since the preindustrial (https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2021GL093808).

Despite this previous work, there are still some "big questions" about chlorine chemistry: Does the chlorine radical play a globally significant role in the oxidation of hydrocarbons and methane? How does this impact methane isotopic ratios? Do we understand the fraction of chlorine emitted as sea salt that becomes gas-phase chlorine species? How does chlorine chemistry impact regional oxidation? What is the role of chlorine in anthropogenic aerosol? What are the anthropogenic emissions of chlorine compounds?

This project will explore some of these processes primarily through the use of numerical models but also by collecting new observations as part of international and national field campaigns.

The GEOS-Chem model (https://www.geos-chem.org) provides a computational representation of the chemistry of the atmosphere. An open-source model, it is used by a large number of groups around the world (Harvard, NASA, MIT, PKU, CalTech etc). The Evans group has long experience of working with this modelling tool to deliver high-quality science. The model describes our best estimate for the chemical, physical and biological processes controlling the composition of the atmosphere, but it needs to be evaluated against observations to ensure its robustness, to explore failures, to make new process level developments and to explore the impact of these processes on air pollution and climate change. As well as developing out theoretical representation of chlorine chemistry with the GEOS-Chem model, the student will also be involved in going into the field to collect observations in both clean and polluted environments.