The production of ozone-depleting bromocarbon gases in near-shore Antarctic waters

Lead Research Organisation: University of East Anglia
Department Name: Environmental Sciences

Abstract

There is a continual two-way exchange of chemicals between the sea-surface and the atmosphere. The bromocarbons are a group of volatile compounds that are produced naturally in seawater and carry the element, bromine from the ocean reservoir in to the atmosphere. Once in the atmosphere bromine has an important influence on the chemistry taking place there. The major impact of this flux is a reduction in the amount of ozone in the air, which can reduce the potential for the breakdown of harmful greenhouse gases and lead to an increase in the amount of harmful UV light reaching the Earth's surface. Understanding how, when and where these bromocarbon compounds are produced in the marine environment is essential to allow us to predict their impact on the Earth's system. Results from our recent study in the Antarctic show that the bromocarbon compounds bromoform (CHBr3) and dibromomethane (CH2Br2) are produced during a phytoplankton bloom that occurs as the sea-ice breaks up during the summer months (October to May). Blooms such as the one we studied occur all along the area of the Antarctic known as the Western Antractic Peninsula during the summer. If bromocarbon production occurs in all of these blooms, there could be a large sea-to-air flux of bromine during the Antarctic summer which could have an important influence on atmospheric chemistry. In this study, we propose to identify the main biological, chemical and physical processes influencing bromocarbon concentrations in the sea-ice edge phytoplankton blooms and use this knowledge to estimate the potential impact of sea-air bromine flux using mathematical models.

Publications

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Description The bromocarbons such as bromoform and dibromomethane are a group of halogenated organic compounds which are produced in seawater and once across the sea surface can impact atmospheric composition and chemistry including including ozone cycling and cloud formation. This project generated results from field and laboratory studies aimed at investigating bromocarbon production and emission from coastal waters of the western Antarctic Peninsula.



Our time-series measurements at the Rothera Time Series (RaTS) Site allowed us to show that an alteration in the phytoplankton community towards a lesser fraction of diatoms, driven by a change in sea-ice dynamics, resulted in a factor of 3-4 decrease in sea-to-air bromocarbon transfer. This change is likely to have resultant impacts on tropospheric composition and chemistry. This result is particularly important given that we were able to show that outside of ozone depletion events, when inorganic emissions dominate, biogenic bromine can contribute up to 81% of total inorganic bromine in the troposphere. Previous studies had concluded that this source was negligible throughout the year.The rapid climate-induced warming on the WAP means that this area serves as a bell-weather for other ocean regions. Hence, this research allowed us to predict that similar changes in marine trace gas emissions are likely as climate-change progresses.



In addition our laboratory studies allowed us to develop the first detailed schemes describing the microbial control of bromocarbon concentrations in seawater. These schemes help to explain previous bromocarbon concentration distribution measurements and can be used to guide modelling studies aimed at predicting how biogenic bromocarbon production and emissions to the atmosphere will change in the future.