ES&T OUTLOOK Air pollutant baselines from glacial studies Modern ice, snow, and air sampling and analytical techniques could determine natural backgrounds and human contributions of about two dozen anions and cations, and maybe settle the controversy over lead concentrations in millennia-old ice Differentiating between air pollutants originating from human activity and those stemming from natural phenomena in order to determine a chemical baseline is a tall order. One way to do this is to find a place where there is a record of airborne impurities going back from the present to many thousands of years in the past. The Greenland Ice Sheet is such a place. It is remote; the ambient air is virtually pristine; and any atmospheric impurities found in the glacial ice must have been immobilized there since the ice was formed, since it never melted over the millennia. Nevertheless, although the ice may not have melted, it has always been moving in a flowing manner. Meteorologist Austin Hogan of the State University of New York at Albany suggests that while this flow might not affect the chemical integrity of cores taken from glacial ice, it may present some problems with the interpretation of chemical data obtained from such cores, and should be taken into account. Hogan also mentioned another complication: On the southern portions of the Greenland Ice Sheet, generally up to the Arctic Circle (66°30'N), summer temperatures can become high enough to cause precipitation in the form of rain. This situation can also lead to superficial ice/snow melt in those regions during the summer months, thereby exacerbating existing problems in chemical sampling of the snow and ice. Adding to the scientific and technical difficulties of this task are the rigorous conditions and inhospitable climate on the ice cap. But one must go there to address this question: What were airborne concentrations of sulfate 0013-936X/82/0916-0437A$01.25/0
Glacier: a very old repository of chemical records in Greenland
© 1982 American Chemical Society
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( S 0 4 - 2 ) and nitrate (N0 3 ~) over the last, say, 200-300 years? Answers might provide clues to variations in acid precipitation through that time period, and the extent, if any, to which human activity may be blamed for more recent occurrences of this phenomenon. Similar information is sought concerning airborne lead and 15 other metals, as well as chloride, fluoride, and phosphate. Such past records, especially those reflecting times during which atmospheric emissions from human activities were insignificant, could provide the desired baseline for these impurities. Analyses of glacial ice, along with deep and surface snow, newly fallen snow, and ambient air, might show the relative magnitudes of industrial developments during ancient and medieval times, and how those developments affected the atmosphere and environment. Perhaps on the basis of past records and trends, plausible projections for the future could be made. With funding for such efforts, the National Science Foundation (NSF) is betting that these goals can be achieved. An undisturbed record Since whatever impurities once deposited with the snows that formed the ice sheet are essentially immobilized there, records of them, as well as pH, can go back as far as the onset of the Wisconsin glaciation, up to about 105 years. If one knows the depth or "horizon" of the portion of the glacier from which impurities are recovered and analyzed, one could key their occurrence to various historic or prehistoric events. For instance, one could tie SC>4~2 to a volcanic eruption, or increases in lead deposition to certain human activities. Thus, for example, an SO4 - 2 concentration of more than 2200 ng/g of ice is ascribed to sulfuric acid originating from SO* hurled into the air by the 1783 eruption of the Laki Volcano in Iceland. Michael Herron of the State University of New York at Buffalo (Amherst, N.Y.) notes that "normal" sulfate concentrations found in ice at Milcent site in central Greenland (70°N) average 40 ng/g. Likewise, chloride "excursions" traceable to the volcano reached 100 ng/g, as compared to the normal 20 ng/g, Herron said. He added that this event record is a "strong" one, detectable in all existing Greenland ice cores covering that time. Herron also says that the prehistoric glacial record shows events that brought "much more continental dust to the atmosphere than one presently 438A
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encounters under normal conditions." These early events are believed to have occurred 10 000-25 000 years ago, during the last third of the Wisconsin glaciation. This dust contains elevated concentrations of aluminum, calcium, silicon, and microparticles.
are collected in polystyrene cups that are precleaned without the use of acid. One of the Greenland study sites at which cores and air samples were and are taken is called "Dye 3." This is a U.S. Air Force station on the ice sheet
SO4 2 and CI concentration peaks reflect volcanic eruption of 1783 SO*"2 (ng/g) 0
20
40
60
Cl~ (ng/g) 80
100
0
30
60
90
120 150
Source: "Glaciochemical Dating Techniques," by Michael M. Herrc-n.
about halfway between the east and west coasts, at latitude 65°N. During 1979-1981, Willi Dansgaard and his group from the University of Copenhagen (Denmark) took ice cores at that site all the way down to bedrock, or 2037 m. This depth could represent the full 105-year age of the ice sheet. It took them three 3-month seasons to drill down that far. Ice coring While the cores themselves were of Sampling the ice sheet in such a excellent quality, there could be conmanner to assure not only the chemical tamination problems for any sort of but also the historical integrity of the chemical studies. The reason is that the sample is a difficult job. The depth of drill fluid was a combination of Arctic the core has to be verifiable, and it diesel fuel and perchloroethylene, Karl must in no way be contaminated by Kuivinen, director of the.University of air, tools, or fuel being used. Not even Nebraska's Polar Ice Coring Office the slightest melting that could remo- (PICO, Lincoln, Neb.) told ES& T. bilize analytes may be allowed to take Kuivinen said that under NSF place during the coring operation. sponsorship, PICO is developing an One way to core the ice, Herron electromechanical drill that will go said, is by means of a "dry-core" pro- down to 300 m. The drill itself will cedure that removes surface contami- hang on a cable, which in turn is atnation, and involves rinsing and melt- tached to an electrically operated ing exterior surfaces with ultrapure winch. The complete system will weigh water. A 7.6-cm diameter ice corer, less than 1000 lb. If for chemical originally developed for South Pole analysis, for example, pristine condiuse, contains an inner core 2.5 cm in tions are needed, the drill system could diameter. The corer is built of pre- be operated with power from photocleaned stainless steel. Exposed ends of voltaic cells instead of gasoline-fueled core sections are shaved away with generators, Kuivinen added. precleaned stainless steel chisels. Work Clair Patterson of the California is done in a room at temperatures of Institute of Technology (Caltech, — 15 "Cor colder. Surface samples are Pasadena) expressed disagreement collected in snow pits under ultraclean with the sampling methods described conditions. For anion analysis, samples by Herron and Kuivinen, especially Also, there were dust concentration and isotopic oxygen changes that were both "fairly sudden" 10 000 years ago, Herron observed. Temporal and quantitative similarities among Greenland and Antarctic ice cores for those times "suggest a common global phenomonon for this time horizon," he said.
Electromechanical drill for ice coring 3
Solar photovoltaic cell panels ^,
Winch with slip rings Electric drive
Antitorque skates Electric drive
Outer nonrotating barrel Inner rotating barrel Cutter bits •
The design of this drill aims at minimizing or eliminating any possibility of contamination. Powef couid even be derived from solar photovoltaic ceil panels, when weather permits, if "pristine" chemical conditions are desired during coring. Source: Communication from Karl Kuivtrten, Director, PICO, University of Nebraska.
with resepect to porous firn (porous portions of older snows being formed into ice. "[Claude] Boutron [of the Laboratory of Glaciology, Grenoble, France] has recently shown in my laboratory that these coring methods provide unacceptably lead-contaminated samples of porous firn in the Antarctic." Core analyses Working in a clean room specially outfitted and exclusively used for gla-
ciochemical research, Herron and fiis group measure aluminum, for example, by graphite furnace atomic absorption (AA) spectrometry. Sodium, chloride, nitrate, and sulfate are measured by ion chromatography (IC). Correlation of core analysis results to given natural events, such as eruptions, help to establish remote-area environmental chemical baselines. For instance, when a core section, corresponding to the time of an eruption, is carefully melted, sampled, and analyzed, it could show a low pH, high specific conductance, and elevated concentrations of SCM - 2 , chloride, and fluoride. Thus, an "unmistakable" glaciochemical horizon throughout Greenland's ice sheet permits the analyst to pinpoint a natural event such as the Laki (Iceland) eruption of 1783. Contaminant deposition To understand fully ice core study results, knowledge of how SCv - 2 , N O 3 - , and other contaminants entered the ice is needed. All of these contaminants were in the atmosphere at one time. The two basic ways in which contaminants can reach the ice sheet are wet and dry deposition. Wet deposition involves processes that occur during cloud formation or precipitation: snowflakes nucleating on contaminant particles, gas adsorption on cloud droplets, and scavenging of atmospheric particles by falling snowflakes below clouds are examples. Dry deposition occurs between and during snowstorms. Examples include diffusion of particles and gases from the atmosphere onto the snow surface, and particle settling under the influence of gravity. Is wet or dry deposition the more important means by which air pollutants are carried to the ice sheet? That question must be answered if one is to interpret chemical data from ice cores correctly. For instance, if wet deposition is the dominating factor, then transport of contaminants to the ice sheet is brought about through mechanisms of cloud and snow formation and movement. But if dry depostion is more important, the main vehicle by which pollutants come to the ice may be found in atmospheric flow patterns between snowstorms. Studies of how airborne particles and gases reach the Greenland Ice Sheet are being conducted by a team of researchers from Carnegie-Mellon University (CMU, Pittsburgh). CMU professor Cliff Davidson explains that these studies call for measuring concentrations of SO4 -2 , NO3 - , and other
species in air and in surface snow at the Dye 3 site. By sampling falling snow, fresh surface snow, and older surface snow (between storms), one can obtain information on in-cloud processes and on particle settling onto the ice sheet. Particle and gas movement within the top few millimeters of snow and the bottom few millimeters of the atmosphere can also be studied.
Davidson: research on pollutant deposition mechanisms The type of air and snow sampling being done requires specially devised equipment and techniques. For instance, air filter holders constructed entirely of Teflon and polyethylene, and developed at CMU, are fitted with acid-washed cellulose acetate and Nuclepore filters (for flameless AA and neutron activation analysis), and specially treated quartz fiber filters (for IC analysis). These filters are connected to a 5-hp vacuum pump through 100 ft of tubing. Hi-vol sampling is carried out only when winds are "in sector"—that is, from such a direction that no-source of contamination exists between the sampler and upwind points on the Greenland coast. The wind sector controller is modeled after that used by Robert Duce and Ken Rahn at the University of Rhode Island (URI, Kingston). After exposure, the filters are bagged in three layers of specially cleaned polyethylene, and shipped to the clean laboratory at CMU. Pieces of the filters are digested in air-tight Teflon vessels by techniques much like those developed by Caltech's Clair Patterson. They are then analyzed by flameless AA and IC. Other filter pieces are sent to URI for neutron activation analysis. With the aid of these techniques, concentration data for some 20-30 species can be obtained. Snow sampling techniques are also strictly contamination-controlled. Procedures are based upon those developed by Claude Boutron. Environ. Sci. Technol., Vol. 16, No. 8, 1982
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On ice: taking air samples near the Dye 3 site Samples are collected by personnel wearing clean-room garb at a site ap proximately 10 km upwind of Dye 3. One-liter polyethylene bottles, washed in concentrated acids for AA analysis, or in distilled/deionized water for IC analysis, are used to scoop up the snow. The samples are bagged in three layers of specially cleaned polyethylene and kept frozen until they are ready for digestion in CMU's clean laboratory. Teflon and quartz digestion vessels are used; samples are then analyzed by flameless AA, IC, and neutron acti vation. According to Davidson, results so far indicate that wet deposition is far more important for S 0 4 ~ 2 and lead, for example, originating from human activity. On the other hand, both wet and dry deposition may be significant for sustances naturally derived from the earth's crust. Davidson points out that crustal particles are produced by wind erosion and other mechanical abrasion pro cesses that form relatively large (greater than 1 μπι in diameter) par 440A
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ticles that could settle on the ice sheet. Man-made particles, by contrast, are generally produced by combustion, and may be too small to be affected much by dry deposition; in-cloud pro cesses may be more efficient for the deposition of these submicron parti cles, Davidson said. Mercury and lead disagreements Snow studies have revealed changes that have occurred with time in the rates of deposition of trace metals onto the Greenland Ice Sheet. In 1969, Patterson and his associates, studying the glacial record, found that the nat ural background level of lead in an cient ice was slightly less than 1 picogram (pg)/g, and showed that the concentration of lead in Greenland snow increased more than 200-fold between 800 B.C. and 1965. They ob served a greater than 30-fold increase in the last two centuries alone, and placed the blame on aerosols origi nating from leaded gasoline and other industrial emissions. Later, other in vestigators reported analogous in
creases in sulfur, mercury, copper, and zinc, and pointed the finger at indus trial sources. In 1977, Herron's group reported natural background levels of lead in old layers of Greenland snow at 50-80 pg/g—more than 50 times higher than those found by Patterson's group—and said that levels of lead, zinc, and sulfur in recent Greenland snow layers were elevated by factors of only 2-3 above those much higher natural background levels. They also found no evidence of man-made inputs of cadmium and vanadium. Herron and other investigators also reported high levels of mercury in both old and recent snows, with no increase with time. They and others ascribed such high background levels of these metals in old snow to natural sources, such as volcano fumes. These later data, being more recent and numerous, were thought by many investigators to be more valid. This spurred new searches for natural sources of these high levels of heavy metals in the at mosphere, such as metal-enriched microlayers concentrated in sea spray, organometallic gas emissions from plants, and metal gas emissions from rocks, in addition to metal fume emissions from volcanoes. In 1978, Appelquist and his col leagues in the Danish Academy of Technical Sciences showed, by careful contamination control and neutron activation analysis, that all previously reported measurements of mercury in both old and recent Greenland snow were erroneously high by factors of 10-100, because of contamination during snow collection and analysis. Their data showed that if concentra tions of mercury did increase at these newly determined low levels, with time, in Greenland snows, the change was by a factor of not more than 2, though sporadic perturbations by large volcanic eruptions probably affected the record. Murozumi and his co workers at the Muroran Institute of Technology (Japan) confirmed the validity of these low levels of mercury by using ultraclean analytical proce dures to find somewhat lower concen trations of mercury in Antarctic snow. Samples from pre-Roman times Last year, Amy Ng and Patterson at Caltech (Ng is now with Dow Chem ical Co.) used improved ultraclean mass spectrometric (MS) analytical techniques to study lead in small samples of ice cores, and confirmed the existence of ultralow natural back ground concentrations of lead of 1 pg/g in the centers of cores from an-
cient Greenland ice. During the 1960s, Patterson's group needed 50-100 kg of ice to measure such levels by older analytical techniques; for samples, they cut large blocks of ice out of the sides of deep tunnels with ultraclean saws and chisels, and melted them down in large ultraclean plastic drums. By comparison, the new analytical methods required only several hundred grams of ice, which allowed the Patterson team to determine lead concentrations in a series of veneer layers of ice chiseled in sequence from the outside toward the centers of 15005500-year-old cores drilled in the Greenland and Antarctic ice sheets. The Greenland cores chosen were more than 2700 years old, so that they may antedate lead smelting activities by the ancient Romans. These cores were analogs of ice core samples studied earlier by Herron and others who had reported high natural background lead levels in them. Ng and Patterson found that lead concentrations decreased continuously by factors of 10 4 -10 6 from the exteriors to the interiors of these cores, and concluded that contamination which had intruded to the core centers gave interior upper limit concentrations of 1.4 pg/g in the Greenland cores, and 1.2 pg/g in the Antarctic cores. These findings gave a broader geographical significance to the 200-fold increase in lead concentrations in the Greenland Ice Sheet over the past 3000 years, that had first been observed during the late 1960s. New quantitative measurements of lead emissions from volcanic plumes by Buat-Ménard and his associates at the French National Scientific Research Center (Gif-sur-Yvette), and by Patterson's group in 1978 and 1981J showed that in prehistoric times, volcanic fumes and silicate dusts contributed about equally to low natural lead levels in the atmosphere, and to the 1-pg/g natural background level of lead in ancient Greenland ice. Patterson's team also measured atmospheric lead contributions from sea spray and found them to be less than those from dusts and volcano fumes. The Caltech scientists thus attribute virtually all of the 200-fold excess lead above natural levels in Greenland snow to industrial emissions for these reasons: • The historic increase of lead in snow strata coincides with the historic increase in industrial production and emission of lead to the atmosphere. • Mass inventories of industrial lead emissions can account for the observed excess of lead in Greenland
snow. • Quantitative measurements of natural lead emissions to the atmosphere from volcanoes and sea spray show that such sources cannot account for 99% of the lead in excess above silicate dust contributions presently observed in the atmosphere. • The historic increase of lead in snow strata is paralleled by analogous increases in excess lead, shown by isotopic tracers to be of industrial origin, observed in water-laid sediments in a remote continental region. • Atmospheric inputs of lead to the oceans have been shown, by comparison with authigenic lead in sediments, and its isotopic relationships with lead in terrestrial fluvial drainage basins, to have increased 10-100-fold during the past two centuries. The geographic pattern in variations of this increase is regulated by the geographic pattern of variations of land-based industrial lead emissions to the atmosphere. The geographic pattern of variations of lead concentrations in sea water coincides with that for atmospheric imputs of lead. Some other projects Because of the utility of remote area work for developing pollutant baselines, several projects are under way, or contemplated, in addition to those in Greenland and Antarctica. One involves work at Point Barrow, Alaska, in which meteorological and climatological modalities of the transport of trace element contaminants to the Arctic are being studied. Still another covers a densely populated area in a mountain valley in Nepal where there are neither industries nor roads; lead concentrations in air and blood are being studied. Remote-area projects may also take place in the Himalayan or Swiss Alpine ice sheets, large U.S. national parks, and other localities. They could, as CMU's Davidson wryly put it, be ideal for the workaholic scientist who enjoys roughing it. Perhaps the work such a scientist does will not only enhance the understanding of baselines, but also the manner in which contaminant aerosols, both of natural and human origin, migrate to and settle in the faraway places they are found. —Julian Josephson Additional reading Herron, M. M. "Glaciochemical Dating Techniques"; ACS Symp. Ser., American Chemical Society: Washington, D.C., 1982. Ng, Α.; Patterson, C. Geochim. Cosmochim. Acta 1981,45,2109-2121. Davidson, C. et al. Atmos. Environ. 1981,15($), 1429-1437.
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