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Global Change and Polar Atmospheric Chemistry
Investigator:
Markus Frey, University of Arizona
The springtime ozone hole in the stratosphere above Antarctica observed since the 1970s is probably one of the most prominent changes of the environment caused by anthropogenic activities. Its discovery triggered the growth of a relatively young scientific discipline: atmospheric chemistry. A part of atmospheric sciences, atmospheric chemistry tries to understand the chemical makeup of the atmosphere, how its many constituents change over time through photochemical or physical processes, how mankind is altering the natural background atmosphere and ultimately how climate eventually changes through the so called chemical climate feedback (e.g. greenhouse effect).
The remote polar regions covered year-round with ice, represent an ideal natural laboratory in which to study fundamental atmospheric chemistry. Remote regions are suitable to measure the very low background levels of atmospheric trace gases due to the minimal interference by anthropogenic pollution sources, vegetation or soil covered surfaces. In addition the polar day and night, each lasting several months, provide two extreme experimental boundary conditions for atmospheric chemistry whose main driver is solar radiation. Results from polar atmospheric chemistry research help to explain many issues, which affect life in general on our planet. Examples include the mechanisms leading to the stratospheric ozone depletion or factors controlling ground level ozone, a major pollutant in big cities.
Longer records of atmospheric chemistry observations exist only at a scarce number of sites around the globe going back at most a few decades. However, ice cores from the polar ice caps provide scientists with the opportunity to extend that record back in time for many water soluble species and gases. Based on reconstructed changes in atmospheric chemistry one hundred, one thousand or even ten thousand years ago, it becomes more feasible to predict the future of the atmosphere currently being altered significantly by anthropogenic emissions.
Two important atmospheric trace gases, hydrogen peroxide and formaldehyde, are water soluble and therefore found in snow and ice. They are both linked to the budget of atmospheric oxidants, which determine the oxidation or “cleansing” capacity of the atmosphere. The oxidation capacity describes how well atmospheric pollutants are oxidized and then removed from the atmosphere and has therefore a key role in controlling the atmospheric build up of climate changing greenhouse gases.
The interpretation of the concentrations of hydrogen peroxide and formaldehyde found in the snow is further complicated by the fact that after a snowfall these gases partially degas from the snow pack back into the atmosphere. This temperature driven physical exchange between snow pack and overlying atmosphere modulates also the concentration of many other atmospheric chemical species. Furthermore, it has been recognized that in the upper centimeters of snow many not fully understood chemical reactions that are driven by sun light, are taking place as in a reaction chamber.
The U.S.ITASE traverse is collecting shallow ice cores over a wide area in West Antarctica in order to reconstruct an environmental record over the past 200 years. An extensive atmospheric chemistry sampling program has been incorporated on this moving platform. The primary scientific objectives include measurements of atmospheric peroxides and formaldehyde in this region and investigation of post-depositional processes of these gases, eventually allowing for the quantitative interpretation of the ice core records in terms of atmospheric chemistry change. Research also includes the interaction between the snow pack and the atmosphere through physical and photochemical processes.
During our 3-day stay at each coring site a variety of experiments are conducted in the field. The Atmospheric Chemistry Shelter houses two custom built atmospheric detectors for the continuous measurement of peroxides and formaldehyde. Air from outside is drawn constantly into the instruments through a heated intake line, where it is passed over a flowing water stream. The respective gases dissolve in the water, a chemical reagent is added and the concentration is determined by measuring the fluorescence of the molecules then produced by that chemical reaction. A simple calculation allows one to derive the original gas concentration in the air above the snow. A second chemistry lab installed in the “Blue Room” is used to produce clean water from melted snow, and to prepare chemical reagents. Additional air filter measurements yield multi-day averages of further trace gases belonging to the family of ketones and aldehydes.
The same atmospheric detectors are also able to measure formaldehyde and hydrogen peroxide in short firn cores which are drilled with a 2” drill, melted and analyzed the same day. The on-site analysis minimizes contamination of the samples and gives also immediate information about snow chemistry and even annual accumulation rates based on the seasonal signal of hydrogen peroxide. The upper 30 cm of snow are sampled with a specially designed snow sampler, with the samples being transported in clean, airtight glass bottles for further analysis back in the Crary Lab in McMurdo.
Since ozone is an important player in the photochemistry of the lower troposphere, it is also monitored using a weather balloon. The balloon is filled with the gas helium, attached to a 1000 m long tether and then raised or lowered using an electric winch, with an ozone sonde, a temperature and relative humidity probe attached to it. The vertical temperature and ozone profiles of the lowest kilometer of the atmosphere give information about the layering of the atmosphere and the distribution of ozone, which are both additional aids for interpreting atmospheric chemistry measurements. Free balloon launches all the way up to the stratospheric ozone layer at more than 20 km altitude are planned this time at site 1 and Hercules Dome with the intention to provide ground truthing for satellite ozone measurements.
Meteorological variables, such as pressure, air and snow temperature at various depths, relative humidity, wind speed and direction and UV radiation are also constantly monitored, in order to better understand the observed changes in trace gas levels.
This year’s traverse, leading us from Byrd Surface Camp in West Antarctica to the South Pole on the East Antarctic Plateau, will be especially exciting from a scientific point of view. We will be able to test our current understanding about atmospheric chemistry and atmosphere-snow transfer in very different environments, going from warm, high accumulation sites at low elevation to very cold, low accumulation sites at more than 3000 m a.m.s.l.
