Very short-lived chlorine, bromine and iodine-containing halocarbons (VSLH): Understanding the role of the oceans in ozone layer chemistry
The Montreal Protocol is widely regarded as the world’s most successful environmental treaty. It has resulted in striking reductions in many long-lived anthropogenic halocarbons (e.g. chlorofluorocarbons, CFCs) and more recently in an upturn in stratospheric ozone in some regions, adding confidence to the projections of ozone layer recovery sometime around mid-century. In addition, as some halocarbons are potent greenhouse gases, the Protocol has made substantial contributions to climate change mitigation.
However, there is increasing evidence that very short-lived halocarbons (VSLH), which are not controlled by the Montreal Protocol, are leading to additional stratospheric ozone depletion. Therefore, it is important to understand the emissions, transport and atmospheric chemistry of these VSLH, which have both natural and anthropogenic sources, in order to correctly attribute trends in stratospheric halogens and ozone. For example, recent years have seen an unexplained and persistent downward trend in northern hemisphere 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 abundant 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 VSLH (Carpenter et al., 2012 doi: 10.1039/C2CS35121H). Increases in these emissions as a result of climate and environmental change could counteract the effect of halocarbon 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 work with and continue the long-term time series of high quality GC-ToF-MS atmospheric halocarbon measurements at the Cape Verde Atmospheric Observatory (CVAO), with training and support from 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 be trained in data analysis and study the long-term VSLH data record, for example examining trends, seasonal patterns, and ocean emission characteristics. You will also have access to a nearly year-long data set of biweekly seawater measurements of VSLH upwind of the CVAO, which will allow seasonal sea-air emissions to be calculated. Analysis of the relationships of the ocean emissions with oceanic biogeochemical variables 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 use the TOMCAT 3-D atmospheric model, which contains a detailed description of VSLH chemistry. You will implement model updates 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.
You will join a large and active atmospheric research group in York, and have strong links to the modelling group in nearby Leeds. These groups will provide a vibrant research community and provide extensive support. There will be regular opportunities to present your work at international conferences.
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.