Climate and Air Quality Impact of Airborne Halogens

Bromine, chlorine & iodine (halogens) are chemical elements which play a fundamental role in Earth's atmosphere and are implicated in a range of environmental issues. Since the 1970s, scientists have known that halogens (mostly chlorine) damage the ozone layer in Earth's stratosphere (located between 11 & 50 km above the surface) and are responsible for the infamous Antarctic 'Ozone Hole', first observed in the 1980s. As ozone shields Earth's surface from harmful solar radiation, production of many halogen compounds is prohibited under international law. However, it is now increasingly recognised that halogens also exert a large influence on the lowest region of Earth's atmosphere (the troposphere) in ways important for both climate and air quality. Only recently have field measurements revealed that halogens are virtually ubiquitous throughout the troposphere, though there is much debate as to their source. Unlike the stratosphere, we think most tropospheric halogens come from the biosphere (e.g. the ocean) and other natural sources (e.g. sea-ice, volcanoes), though these sources are poorly characterised. In addition, human activities related to the rapidly growing aquaculture sector (e.g. commercial seaweed farms) and other industries are increasing the amount of halogens entering the troposphere.

Why is this important? Halogens do a number of things, but fundamentally they alter the troposphere's "oxidising power"; that is, its ability to "self-cleanse" and rid itself of various chemical compounds. From a climate perspective, this has important implications; it means halogens may (i) alter the length of time greenhouse gases, such as methane, remain in the atmosphere and thus influence their global warming potential and (ii) alter the production rate of aerosol (microscopic particles suspended in the atmosphere) which alter cloud properties and cool Earth's climate. What's more, halogens degrade air quality by promoting surface ozone formation. At ground-level, ozone is a pollutant (& greenhouse gas) and prolonged exposure can lead to respiratory ailments, including asthma, and is damaging to crops. Nitryl chloride, a halogen-containing precursor to adverse air quality events, has been detected in large quantities in coastal and inland regions of the USA. Elevated levels of this compound have also recently been detected in Germany, though no study has comprehensively examined the role of halogens in air pollution over Europe. As an island nation in the vicinity to significant quantities of sea salt (a major halogen source), UK air quality could be particularly susceptible to being compromised by halogens. Ultimately, despite the leverage halogens possess to impact both climate & air quality, they have yet to be considered in most computer model simulations used to study and forecast these phenomena. This Fellowship addresses that omission, seeking to unravel the wider impact that airborne halogens have on our environment.

I will develop the first fully integrated computer model of the biosphere-halogen-climate system which can (i) characterise and quantify tropospheric halogen sources, (ii) determine the atmospheric fate of these gases and (iii) quantify their impacts, on regional to global scales. A key question to tackle is; in the troposphere, how have halogen levels, processes and impacts changed over time? This holistic modelling approach, which accounts for changes to halogen emissions from the biosphere due to evolving environmental factors (e.g. sea surface temperature & sea-ice cover), will provide the answer. Critically, this will enable climate-induced feedbacks on halogen emissions, which could diminish or amplify future climate change, to be assessed for the first time, leading to climate simulations of greater fidelity. This research provides powerful new insight into poorly understood, yet fundamental processes important for both climate change and air quality - pressing environmental concerns of today.

Halogens have a profound impact on atmospheric composition. They interact with climate-relevant gases, altering Earth's radiative balance, and contribute to adverse urban air quality. By developing halogen processes in computer models, this Fellowship will improve prediction of future climate change and air quality. These are public health issues, high on the environmental policy agenda with a strong societal impact. Further, commercial aquaculture (i.e. "seaweed farming") is increasing levels of halogens in the atmosphere. The main beneficiaries of this Fellowship are 1. policymakers and advisory bodies concerned with climate change and air quality, 2. the seaweed farming industry and 3. the general public.

1.) Climate policy decisions are made in the absence of perfect knowledge regarding the future, but with an appreciation of the potentially serious consequences of climate change. This Fellowship will benefit policymakers requiring climate projections that reflect the current state-of-the-art on which to base decisions; e.g. those working under the auspices of the UN Framework Convention on Climate Change. A specific beneficiary will be the Intergovernmental Panel on Climate Change (IPCC) who, while being policy-neutral, inform policymakers and the public on current best estimates of future climate forcing. Climate model projections of greater fidelity will improve the knowledge pool from which the IPCC draws its conclusions.

Air quality is also a pressing environmental issue, impacting human health and the natural world (e.g. vegetation). According to the UK's National Air Quality Strategy, air pollution reduces life expectancy by, on average, 7-8 months and costs up to £20 billion per annum in health care. Informed policy is vital to protect the public from adverse air quality events and for the UK to comply with European standards (e.g. Directive 2008/50/EC on Ambient Air Quality & Cleaner Air for Europe). Through improved understanding of processes degrading air quality, this Fellowship will benefit the Department for Environment, Food and Rural Affairs (Defra). Defra are responsible for air quality policy and are advised by an air quality expert group (AQEG) who will also benefit, as air quality simulations of improved fidelity are synthesised into their recommendations. See letter of support; Prof P. Monks, chair of AQEG.

2.) Commercial seaweed farming for food and pharmaceuticals is a rapidly growing industry (worth US$6.4 billion). The UN Food & Agriculture Organization estimates global farmed seaweed production doubled in the last decade. Despite many sustainable credentials, almost nothing is known of the atmospheric impact of human aquaculture, see letter of support; Dr A. Hughes, who engages with industry stakeholders and sits on a Ministerial Working Group for Sustainable Aquaculture Practice. Increased awareness on the atmospheric impact of halogens (from this Fellowship) will benefit industry stakeholders (e.g. European Netalgae consortium members) by enabling more effective environmental impact assessment of their practice. This will be particularly relevant in coming years as the industry expands into Europe, see letter of support; Dr T. Atack, director of UK's only commercial seaweed hatchery.

3.) The public receives warning of adverse air quality events from the UK Met Office & Defra. By improving the underlying chemistry in models used by these organisations to predict air quality, this Fellowship will benefit the public through better air quality forecasting. This can help reduce exposure to air pollution, contributing significantly to preventative healthcare. Finally, climate science and its portrayal in the media has significant impact on public reasoning on the topical issue of climate change (polarizing to many). Results from this Fellowship will contribute to greater understanding of the climate system and future global change, impacting public perspectives if communicated effectively.