This project would involve field experiments at Barrow, AK, aimed at improving understanding of the oxidation of the sea salt halides, chloride and bromide, into the molecular halogens, Cl2, BrCl, and Br2. In the Arctic at spring time there is a dramatic phenomenon characterized by the complete loss of ozone from the lowest layer of the atmosphere, along with near-complete loss of atmospheric elemental mercury. The elemental mercury is oxidized to short-lived products that then deposit to the surface, where it can enter and damage sensitive ecosystems. These ozone depletion episodes (ODEs) and mercury depletion episodes (MDEs) are known to occur as a result of halogen atom chemistry. However, the details of the halogen reactions that lead to the ODEs and MDEs are still unknown, limiting the ability to develop models that describe these processes, and how they might change in the future. It is thought that the processes that release molecular halogens are related to the presence of seasonal, or first-year (relatively thin and relatively saline), sea ice. The Arctic is currently experiencing a rapid decrease in the extent of multi-year sea ice, with associated increases in the extent of first-year sea ice. Thus the investigator hypothesizes that this halogen chemistry, and the associated potential impacts on atmospheric composition and deposition of mercury, will increase to the extent that the current trend in sea ice loss continues. To improve understanding of the fundamental processes associated with the production of molecular halogens in the Arctic, the investigator proposes an experiment involving exposures of natural snow samples and excised sea-ice samples in a snow chamber to potentially important atmospheric oxidants and measurement of the product molecular halogens. The investigator would also simultaneously conduct measurements of the concentrations of these compounds within the natural snowpack. He would then construct a detailed photochemical model of the chemistry involved with ODEs and MDEs, that could then be incorporated into larger scale Earth System climate and chemistry models to help enable reliable simulations of the future state of the Arctic atmosphere. The project would support a postdoctoral researcher and a graduate student. Numerous outreach activities would be supported through the Purdue Climate Change Research Center, interactions with the local community in Barrow, visits to schools, and through collaboration with adventure and children's book writer Peter Lourie on the web site

Project Report

This project aimed to improve our understanding of halogen chemistry in the atmosphere in the Arctic, and how that chemistry influences the air quality and composition in the Arctic. In most of the Earth's atmosphere, the atmosphere cleans itself through photochemical processes involving one species, the "hydroxyl radical", or OH. However, in polar environments that contain sea ice, there is a different mechanism which was previously poorly understood, involving halogens, i.e. chlorine, bromine, and iodine. It was previously found that ozone and elemental mercury were rapidly consumed in the lower Arctic atmosphere in spring time, and that bromine atoms were likely the most important agent involved in that important process. It was also known that chlorine atom chemistry was simultaneously occurring, and that the chlorine atoms were significantly impacting the atmosphere's cleaning capacity. However, it was not known what the source of those halogen atoms was. In this project, through measurements at Barrow, AK, we showed that the molecular halogens, Cl2, Br2, and I2, were photochemically produced in snowpacks that contain sea salt, and that the molecular halogens produced in the snowpack can be released to and impact the composition and chemistry of the overlying atmosphere. In the atmosphere, the molecular halogens break apart into highly reactive halogen atoms, under the influence of sunlight. We also showed that the production of those species required an acidic pH, which meant that the chemistry does not occur on the surface of fresh sea ice, which has a slightly basic pH. Our discoveries will enable development of new computer models that can better predict how a changing Arctic surface (e.g. with less sea ice) will in turn change the composition and chemistry of the atmosphere. As part of this project, we conducted outreach and education about our science, and why it is important, through our web site, (which relates the science and impact of a changing Arctic through personal video interviews and storytelling), and through our short film "Young Ice", which relates the impact of changing sea ice on chemistry and climate.

National Science Foundation (NSF)
Division of Polar Programs (PLR)
Standard Grant (Standard)
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Program Officer
Henrietta N. Edmonds
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Purdue University
West Lafayette
United States
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