Climate and Air Quality Impact of Airborne Halogens

Lead Research Organisation: Lancaster University
Department Name: Lancaster Environment Centre


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.

Planned Impact

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.


10 25 50
Description A large body of work has been performed looking at the effects of halogens (chlorine, bromine and iodine) on the chemical composition of Earth's troposphere and stratosphere. This research has involved development and application of global atmospheric models and the analysis of large volumes of atmospheric measurement data. Our research has highlighted the significance of both naturally-occurring halogen compounds that are produced and emitted from the ocean, and also halogenated gases that are emitted by human industrial activities. A key finding of this work is that so-called halogenated very short-lived substances (VSLS, e.g. CH2Cl2, CHCl3) of industrial origin have increased markedly in the atmosphere, primarily as a result of increasing emissions from Asia. We have shown that despite short atmospheric lifetimes (<6 months), some of these compounds can be transported from Earth's surface to the stratosphere where they contribute to ozone layer depletion. We have also shown through numerical modelling that reactive halogen compounds likely have a significant impact on the troposphere's oxidising capacity (or "self-cleaning" capacity), making them relevant to understanding regional air quality and also climate (e.g. through atmospheric oxidants affecting the lifetime of short-lived climate forcers). In consequence, it may be important to account for reactive halogens in chemistry-climate model simulations going forward.
Exploitation Route Our results have highlighted previously unrecognised or poorly known environmental impacts around certain halogenated gases that may need to be considered within environmental impact assessment frameworks.
Sectors Chemicals,Environment

Description Work examining the impact of short-lived chlorocarbon emissions on the stratospheric ozone layer has contributed significantly to the current debate on use of these compounds. There have been numerous articles in the media reporting on these findings and the work has been cited in policy documents and reviews.
First Year Of Impact 2017
Sector Environment
Impact Types Policy & public services

Description Citation in EPA's review of DCM
Geographic Reach North America 
Policy Influence Type Citation in other policy documents
Description Multiple citations in WMO/UNEP Stratospheric Ozone Assessment (geared at decision-makers concerned with the Montreal Protocol)
Geographic Reach Multiple continents/international 
Policy Influence Type Citation in other policy documents
Description Internal PhD studentships offered by Lancaster Environment Centre
Amount £75,000 (GBP)
Funding ID
Organisation Lancaster University 
Sector Academic/University
Country United Kingdom
Start 09/2016 
End 03/2020
Description Pilot study funded through the EPSRC REsearch on Changes of Variability and Environmental Risk (RECoVER) network
Amount £24,854 (GBP)
Funding ID RFFLP027 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 07/2017 
End 01/2018
Description Sources and Impacts of Short-Lived Anthropogenic Chlorine
Amount £586,957 (GBP)
Funding ID NE/R001782/1 
Organisation Natural Environment Research Council 
Sector Public
Country United Kingdom
Start 09/2017 
End 09/2020
Description Understanding and Attributing Extreme Air Pollution Events in a Changing Climate
Amount £9,000 (GBP)
Funding ID 2353903 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 08/2019 
End 03/2023
Description Why is Lower Stratospheric Ozone Not Recovering?
Amount £799,870 (GBP)
Funding ID NE/V011863/1 
Organisation Natural Environment Research Council 
Sector Public
Country United Kingdom
Start 01/2022 
End 06/2024
Description Collaboration with chlorocarbon and fluorocarbon industry consultant 
Organisation Nolan Sherry and Associates Limited
Country United Kingdom 
Sector Private 
PI Contribution Provided expertise and modelling work to assess the atmospheric fate of a range of short-lived chlorocarbons.
Collaborator Contribution Provided expertise and access to data on emissions.
Impact Published review article on renewed and emerging concerns over the production and emission of ozone-depleting substances.
Start Year 2019
Description Numerous interviews for national/international news (print and radio) and magazines 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Media (as a channel to the public)
Results and Impact I gave numerous interviews to national and international media following my 2017 publication in Nature Communications on ozone layer recovery. This culminated in print articles in the Times and Guardian newspapers, and a feature on the research in New Scientist magazine. Combined, the readership of these publications is of the order of several hundred thousand. Additionally, I have provided further commentary for the media on subsequent and related research in 2018. The articles have served to communicate research findings to a general audience and have promoted constructive debate, for example as evidenced by (a) follow up opinion pieces by individuals not involved with the research, and (b) directed questions from the public to me.
Year(s) Of Engagement Activity 2017,2018
Description RMetSoc Atmospheric Chemistry Special Interest Group National Meetings (open to public) 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Other audiences
Results and Impact I have co-organised 'Atmospheric Chemistry Special Interest Group' meetings through the Royal Meteorological Society in 2018, 2019 and 2021. Some of these events have been sponsored by business. These annual meetings are advertised at the national level and have attracted interested parties (around 50 participants) from academia, business and the public. The theme of the 2018 meeting was concerned with innovation in the field of atmospheric composition monitoring. The theme of the 2019 meeting was on improving air quality predictions in the developing world. The 2021 meeting dealt with the impact of the COVID-19 pandemic on air quality. Each meeting sparked debate and discussion on the challenges and opportunities ahead and proved useful networking events for the delegates.
Year(s) Of Engagement Activity 2018,2019,2021