A Century-long Record of Trace Gases in the Northern Hemisphere from the NEEM Ice Core Drilling Project in Greenland

An international effort is underway to drill a new ice core in Greenland: the North Greenland Eemian Ice Drilling project (NEEM). This presents an exciting and rare opportunity to obtain 'firn' air and ice cores from a Northern Hemispheric (NH) location. 'Firn air' is air trapped in the porous snow overlying solid ice. This air can be pumped out with a special drilling and pumping arrangement. As one moves deeper in to the snow, the older the air is: eventually representing the composition of air in the NH in the first half of the 20th century. Below the firn, even older air is trapped in individual bubbles in solid ice. We will also extract this air and analyse it, but the measurement is much more difficult and not as many chemicals can be analysed as in the firn. We nevertheless should be able to study the rise of numerous gaseous pollutants (carbon monoxide (CO) and its isotopic composition, hydrocarbons, halocarbons, organic nitrates, etc.) over the last century or more. Deep drilling will begin in the summer of 2008, with a second borehole drilled at that time to extract firn air. Large firn air volumes will be retrieved to enable an extensive suite of trace gases to be measured, and also to permit comprehensive analysis of the isotopes ratios of CO (delta13C, delta18O, and Delta17O). Measurements will concentrate on trace gases with anthropogenic sources, for which we expect to observe significant increases over the last century. We will further concentrate on gases that are relatively short-lived in the atmosphere (lifetimes of weeks to a few years), since prior Antarctic studies have only partly / or not at all - delivered trends of such gases which are emitted largely in the NH. The target gases include those of importance to stratospheric ozone, and those which are precursors of tropospheric ozone, or are proxies of other species (e.g. alkyl nitrates as proxies of reactive nitrogen, NOx: an important driver of global-scale chemistry). The trend and stable isotopic composition of CO will be determined to probe both the past sources of CO (e.g. biomass burning versus methane oxidation) from measurement of delta13C(CO), and past levels of hydroxyl radical (OH) by studying delta18O(CO) and Delta17O(CO). Hydroxyl radical is a crucial component of atmospheric chemistry: often referred to as the 'detergent' of the atmosphere. There is presently little consensus on whether OH has changed over the last hundred years as a result of increased pollution. Any significant decrease would be a cause for concern for future levels of this important molecule.