Opportunity Understanding the indirect climate impacts of halogens in the troposphere

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Halogen (i.e. chlorine, bromine & iodine) gases are ubiquitous in Earth's atmosphere and since the 1970sare recognised as the principal driver of stratospheric ozone layer depletion. While the impact of halogensin the stratosphere is well understood, it is now increasingly recognised that halogens play a far wider roleand may have profound, but overlooked, impacts on the chemical composition of the troposphere - thelowest region of Earth's atmosphere[1,2]. At the forefront, halogens alter the troposphere's oxidising power(or "self-cleansing" capacity) and therefore its ability rid itself of pollutants[3-5]. From a climate perspective,this has important implications as halogens may (i) alter the time greenhouse gases (e.g. methane) remainin the atmosphere, thereby influencing their warming potential, and (ii) modify the production of aerosol(microscopic particles suspended in the atmosphere) which effect clouds and cool the climate. Tropospherichalogens thus possess leverage to influence climate indirectly, though these effects are poorly constrainedas most climate models have yet to consider them.Working with scientists at the Met Office and University of Leeds, the student will develop and apply a state-of-the-art Earth System Model to quantify the impacts of halogens on the composition of Earth'stroposphere. As understanding of tropospheric halogens is poor (e.g. little is known of how natural halogensources have changed over time) there is strong potential for high impact research in this rapidly evolvingfield. The project will tackle the following key research questions: What is the tropospheric abundance/distribution of reactive halogens and how has this changedduring the Anthropocene? What are their key natural/anthropogenic sources?How do halogens affect the troposphere's regional/global oxidising power and what are theimplications for climate-relevant gases (e.g. methane, volatile organic compounds, troposphericozone) and aerosol?Do tropospheric halogens affect estimates of pre-industrial to present-day and future climateradiative forcing?