Sources and Impacts of Short-Lived Anthropogenic Chlorine

Lead Research Organisation: University of Leeds
Department Name: School of Earth and Environment


Depletion of stratospheric ozone allows larger doses of harmful solar UV radiation to reach the surface leading to increases in skin cancer and cataracts in humans and other impacts, such as crop damage. Ozone also affects the Earth's radiation balance and, in particular, ozone depletion in the lower stratosphere (LS) exerts an important climate forcing. While most long-lived ozone-depleting substances (e.g. CFCs) are now controlled by the United Nations Montreal Protocol and their abundances are slowly declining, there remains significant uncertainty surrounding the rate of ozone layer recovery. Changes in the LS may cause delayed ozone recovery or even additional depletion, and can also have important effects on climate. One key uncertainty, highlighted in the WMO/UNEP 2014 Assessment of Stratospheric Ozone Depletion, is the increasing importance of uncontrolled chlorine-containing very short-lived substances (VSLS) which can reach the LS and cause ozone depletion.

While significant amounts of brominated VSLS are known to be emitted naturally from the oceans, recent publications also show a rapid, unexpected and unexplained increase in anthropogenic chlorinated VSLS (Cl-VSLS), especially in E and SE Asia. Some of these Cl-VSLS will reach the stratosphere via deep convection in the tropics (through the tropical tropopause layer) or via the Asian Summer Monsoon (ASM) or the E Asian Winter Monsoon.

The Montreal Protocol is arguably the world's most successful environmental agreement. By controlling the production and emission of long-lived ODSs, it has set the ozone layer on the road to recovery. However, short-lived halogenated compounds (lifetimes <6 months) have so far not been included, based on the belief that they would not be abundant or persistent enough to have an impact. Recent observations suggest otherwise; calculations in this proposal suggest that Cl-VSLS may delay the recovery of the Antarctic Ozone Hole (to 1980 levels) by up to 30 years. Fortunately, the Montreal Protocol has a regular review process which allows amendments to deal with new threats to the ozone layer and climate, e.g. the recent 2016 success of including limits to the production of hydrofluorocarbons (HFCs).

This proposal takes advantage of UEA's heritage in atmospheric halocarbon measurements to obtain novel observations of chlorine compounds in the key E/SE Asia region and in the global mid-upper troposphere. Surface observations will be targeted in the key winter periods when we know that we will be able to detect polluted emissions from China, a likely major emitter of Cl-VSLS globally. We will extend the suite of gases currently measured by the CARIBIC in-service global passenger aircraft to include several newly-identified VSLS. This will allow us to investigate the distribution of these VSLS over a much wider geographical area, to identify source regions and to assess longer term changes in their atmospheric abundance.

Our observations will be combined with detailed 3-D modelling at Leeds and Lancaster, who have world-leading expertise and tools for the study of atmospheric chlorine. One model will be used in an 'inverse' mode to trace back the observations of anthropogenic VSLS to their source regions. Overall, the models will be used to quantify the flux of halogenated ozone-depleting gases to the stratosphere and to determine their ozone and climate impact. We will calculate metrics for ozone depletion and climate change and feed these through to the policy-making process (Montreal Protocol) with the collaboration of expert partners. The results of SISLAC will provide important information for future international assessments e.g. WMO/UNEP and IPCC reports.

Planned Impact

We anticipate that the halocarbon dataset generated from the proposed work will feature heavily in future WMO/UNEP Stratospheric Ozone Assessments, particularly when assessing the impact of the Asian region on global emissions and when evaluating the role of very short-lived substances (VSLS) on stratospheric ozone. This will be a continuation of the impact of the halocarbon research programmes at UEA and ULeeds over the past 20 years, who have been regular contributors to international assessments, including the last five of the quadrennial WMO/UNEP Stratospheric Ozone Assessments and various reports to the IPCC. In addition to having our data and model calculations regularly included in these assessments, we have further contributed as lead authors, co-authors, contributors and reviewers. Chipperfield (PI) is now on the steering committee for the 2018 WMO/UNEP Assessment. One of the case studies in the UEA 2014 submission to REF was focussed on this activity and the School of Environmental Sciences came 1st for Impact in the Earth Sciences category.

Although written primarily by scientists, the WMO and IPCC reports are aimed very much at policy makers and, to a lesser extent, a more general audience, through the co-publication of additional material such as the Summary for Policy Makers. The reports are also made available online.

We believe there will also be a strong interest in our findings from the chlorocarbon production industry, particularly in view of the potential for chlorinated VSLS to be included in the Montreal Protocol, which would have a very large impact on the sector. With this in mind we have made arrangements with representatives of the halocarbon producers and policy advisors to exchange information on current industrial practice and interests surrounding the issue of chlorinated VSLS and the Montreal Protocol. These include David Sherry, who is a consultant with expertise in the chlorocarbon industry, and Stephen Anderson, who is a long-term advisor to the Parties of the Montreal Protocol (see Letters of Support). In return we will provide industry with current tropospheric abundance data and estimates of "top-down" regional and global emissions. These can be compared with industry calculated "bottom-up" emissions which are based on production and sales figures. Furthermore, we will calculate new ozone depletion potentials (ODPs) for chlorinated VSLS which are used directly to formulate policy. (Calculation of ODPs for short-lived species is not straightforward and requires realistic understanding of their sources and atmospheric fate which we will obtain in SISLAC). We anticipate that there will be pressure on policymakers over the coming years to consider adding compounds such as dichloromethane to the list of compounds restricted by the Montreal Protocol. Through the above means, SISLAC will contribute towards evidence-based policy making at both national and international levels.

SISLAC as a whole will lead to an improved representation of VSLS emissions in the Asian monsoon region in global chemistry/climate and earth system models. This in turn will improve current understanding as to the significance of the monsoon in topics including stratospheric ozone depletion and future ozone recovery, and associated future climate impacts/prediction (issues which have a large impact on society). This will benefit parties involved in climate prediction.


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Chipperfield M (2020) Renewed and emerging concerns over the production and emission of ozone-depleting substances in Nature Reviews Earth & Environment

Description That so far short-lived chlorine species have not caused major stratospheric ozone depletion, although they have the potential to in the future.

The recently discovered, illegal production of CFC-11 has the potential to delay ozone layer recovery. However, if swift action is taken, this delay could be small (small compared to natural variability). Recent data shows that the CFC-11 emissions decreased strongly after 2018.
Exploitation Route To support the Montreal Protocol process - to strengthen current implementation and to include new species.
Sectors Chemicals,Environment

Description Results fed into the 2018 and 2022 WMO/UNEP Stratospheric Ozone Assessments. This is the process which supports the Montreal Protocol to avoid emissions of gases that deplete the ozone layer. Results were also used for the 2021 Special Report into CFC-11 Emissions.
First Year Of Impact 2018
Sector Environment
Impact Types Policy & public services

Description Lead Author WMO/UNEP Ozone Assessment 2022
Geographic Reach Multiple continents/international 
Policy Influence Type Participation in a guidance/advisory committee
Impact Input to the Montreal Protocol on substances which protect the ozone layer.
Description Montreal Protocol Scientific Assessment Steering Committee
Geographic Reach Multiple continents/international 
Policy Influence Type Membership of a guideline committee
Impact Reduction in stratospheric ozone depletion and in surface climate change
Description Why is Lower Stratospheric Ozone Not Recovering?
Amount £649,267 (GBP)
Funding ID NE/V011863/1 
Organisation Natural Environment Research Council 
Sector Public
Country United Kingdom
Start 01/2022 
End 06/2024