The project developed 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), provided the answer. Critically, this enabled 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 provided powerful new insight into poorly understood, yet fundamental processes important for both climate change and air quality - pressing environmental concerns of today.

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.