Sources and Impacts of Short-Lived Anthropogenic Chlorine (SISLAC)

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.

Impact

The results of SISLAC will provide important information for future international assessments e.g. WMO/UNEP and IPCC reports.

Publications and outputs

• Chipperfield, M.P., S. Dhomse, R. Hossaini, W. Feng, M.L. Santee, M.Weber, J.P. Burrows, J.D. Wild, D.Loyola and M. Coldewey-Egbers, On the cause of recent variations in lower stratospheric ozone, Geophys. Res. Lett., 45, 5718-5726, doi:10.1029/2018GL078071, 2018.
• Harrison, J., M.P. Chipperfield, R. Hossaini, C.D. Boone, S. Dhomse, W. Feng and P.F. Bernath, Phosgene in the upper troposphere and lower stratosphere: a marker for product gas injection due to chlorine-containing very short-lived substances, Geophys. Res. Lett., 46, 1032-1039, doi:10.1029/2018GL079784, 2019.
• Hossaini, R., E. Atlas, S.S. Dhomse, M.P. Chipperfield, P.F. Bernath, A.M. Fernando, J. Muhle, A.A. Leeson, S.A. Montzka, W. Feng, J.J. Harrison, P. Krummel, M.K. Vollmer, S. Reimann, S. O'Doherty, D. Young, M. Maione, J. Arduini, C.R. Lunder, Recent trends in stratospheric chlorine from very short-lived substances J. Geophys. Res., 124, 2318-2335, doi:10.1029/2018JD029400, 2019.