Very short lived chlorine, bromine and iodine-containing halocarbons (VSLH): A better understanding of the role of the oceans in ozone layer chemistry
The Montreal Protocol on Substances that Deplete the Ozone Layer is widely acknowledged to have been highly successful, resulting in striking reductions in the total atmospheric burden of ozone depleting substances (ODSs) and more recently in an upturn in stratospheric ozone levels in certain regions. Overall this adds confidence in the projections that ozone will recover sometime around mid-century. In addition, as ODSs are also potent greenhouse gases, the Montreal Protocol and its Amendments and Adjustments has contributed more to climate change mitigation than any other existing international agreement.
However, there is increasing evidence that other halocarbons, which are not controlled by the Montreal Protocol, are leading to stratospheric ozone depletion. Therefore, it is also important to understand the emissions, transport and atmospheric chemistry of these natural and/or unregulated VSLH in order to reconcile observed stratospheric measurements of inorganic or “active” halogens with reported anthropogenic halogen emissions and to correctly attribute trends in stratospheric ozone. For example, recent years have seen an unexplained and persistent downward trend in extra-polar lower stratospheric ozone, a region where ozone changes exert strong radiative effects on climate (Chipperfield and Bekki, 2024 doi:10.5194/acp-24-2783-2024). The surprising recent detection of reactive iodine radicals in the lower stratosphere has demonstrated the potential impact of iodine in this region and indeed it has been implicated as one potential reason for the unexplained ozone trend (Villamayor et al., 2023 doi:10.1038/s41558-023-01671-y). The very uncertain contribution of natural oceanic sources to chlorinated VSLH, especially dichloromethane (CH2Cl2) and chloroform (CHCl3), hampers efforts to establish their anthropogenic emission distributions and trends. Even relatively small oceanic emissions of these compounds have the potential to contribute to stratospheric ozone destruction if emissions occur in the tropics, where rapid transport pathways allow significant amounts of VSLH to enter the stratosphere. In addition, VSLH contribute to tropospheric ozone destruction, and thus exert effects on the atmosphere’s oxidation capacity and climate.
Ocean biology and photochemistry represent the major natural sources of VSL halocarbons (Carpenter et al., 2012 doi: 10.1039/C2CS35121H). Emissions increases as a result of climate and environmental change could counteract the effect of halogen reductions under the Montreal Protocol. However, there is a lack of data to establish the controls and mechanisms which drive ocean emissions of many VSLH and how they respond to climate change.
In this PhD project, you will continue the long term time series of high quality GC-ToF-MS atmospheric halocarbon measurements at the Cape Verde Atmospheric Observatory (CVAO), with the support of local technical staff. The instrument has been running for over 10 years in its current configuration at this unique Global Atmospheric Watch monitoring station located in the tropical northeast Atlantic Ocean. You will lead data analyses and publication of the long term VSLH data record, for example examining trends, seasonal patterns, and ocean emission characteristics. You will also have access to a more-than-year-long data set of biweekly seawater measurements of VSLH upwind of the CVAO, currently being undertaken, which will allow seasonal sea-air emissions to be calculated. Analysis of the relationships of the ocean emissions with oceanic biogeochemical variables such as chlorophyl-a, coloured dissolved organic material (CDOM) and dissolved organic carbon (DOC), as determined from satellite products, will allow improved or completely novel emissions climatologies of chlorine, bromine and iodine-containing VSLH.
As part of the project you will also calculate the impact of derived emissions on the stratospheric ozone layer. In collaboration with the University of Leeds, you will run the TOMCAT 3-D atmospheric model, which contains a detailed description of VSLH chemistry. You will implement a scheme to describe the newly derived emissions and run simulations to quantify the amount of VSLH which reaches the stratosphere, and the location and extent of the subsequent ozone depletion. Model simulations will cover the full period of CVAO observations so that the impact of VSLH on long-term trends can be studied, and you can test the hypothesis that such species are delaying recovery of the ozone layer.
References / Further Reading
Carpenter, L.J., Stephen D. Archer, S.D. and Beale, R., Ocean-atmosphere trace gas exchange, Chem. Soc. Rev., 41, 6473-6506, doi: 10.1039/C2CS35121H, 2012.
Chipperfield, M.P., and S. Bekki, Opinion: Stratospheric ozone – Depletion, recovery and new challenges, Atmos. Chem. Phys., 24, 2783-2802, doi:10.5194/acp-24-2783-2024, 2024.
Villamayor, J.F. Iglesias-Suarez, C.A. Cuevas, R.P. Fernandez, Q. Li, M. Abalos, R. Hossaini, M.P. Chipperfield, D.E. Kinnison, S. Tilmes, J.-F. Lamarque and A. Saiz-Lopez, Very short-lived halogens amplify ozone depletion trends in the tropical lower stratosphere, Nature Climate Change, 13, 554-560, doi:10.1038/s41558-023-01671-y, 2023.