Extraction and Analysis of Halocarbons and Other Trace Gases in Greenland Ice Cores

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0221480SaltzmanThe gases trapped in polar ice cores are a unique and invaluable archive for investigating paleoatmospheric composition and variability. To date, efforts have focused on carbon dioxide, oxygen, methane, and nitrous oxide, which occur at mixing ratios in air of parts-per-billion and above. There are many more trace gases present at lower concentrations in ice cores that contain a wealth of information about biogeochemical cycles and atmospheric chemistry. In particular, ice cores potentially contain a record of natural variability of halocarbons such as methyl chloride (CH3Cl) and methyl bromide (CH3Br) that are major contributors to the atmospheric halogen burden and to stratospheric ozone loss. Ice cores also contain a record of paleoatmospheric carbonyl sulfide (OCS), the most abundant sulfur gas in the atmosphere and a precursor to the stratospheric sulfate layer. The Principal Investigator will measure CH3Cl, CH3Br, and a number of other trace gases on ice core samples from shallow and deep Greenland ice cores. Trace gases will be extracted by mechanically shredding ice core samples, with analysis by gas chromatography with mass spectrometric detection. Preliminary ice core measurements indicate that: 1) paleoatmospheric levels of CH3Cl and OCS can be obtained from ice core samples using dry extraction with analysis by gas chromatography/mass spectrometric detection, and 2) the interpretation of ice core CH3Br measurements is more complex, as this gas is contained both in gas bubbles and in the ice matrix. Experiments will be carried out to differentiate between gas stored in the air and in the ice matrix, and to understand the factors controlling the partitioning of this gas between these phases. The objective is to provide new baseline data for the concentration of halocarbons and other gases in pre-industrial northern hemisphere air. These data will provide strong constraints on the anthropogenic contribution to their atmospheric budgets. This study will also provide insight into the relationship between the atmospheric levels of halocarbons and climate variability, over periods of widely varying climate conditions and rapid climate change. In terms of broader impacts on society, this research will help to provide a stronger scientific basis for policy decisions regulating the production and use of ozone-depleting and climate-active gases.