Arctic haze An ambitious research effort has shown it to be surprisingly dense and pervasive Early this spring, a group of 18 scientists flew repeatedly over the Arctic and took the first airborne measurements of Arctic haze that had ever been made from a research aircraft over extended flight paths. The scientists also measured specific properties of Arctic haze that had never been investigated before, such as the extent to which the various layers separately absorb solar radiation. Using a WP-3D Orion research aircraft owned by the National Oceanic and Atmospheric Administration (NOAA), they flew out of Anchorage (Alaska), Thule (Greenland), and Bod9 (Norway), with one flight passing directly over the North Pole. The approximate flight paths are shown on the map (Figure 1). The flights were made during late March and early April because haze occurs primarily during the winter in the Arctic, especially the late winter, and tends to disappear by the end of April. Continuous measurements were made of the sooty carbon component of the haze, radiation flux (above, below, and within haze and cloud layers), and many other physical and chemical characteristics of the haze (see table). At the same time several other haze monitoring expeditions took place. Two West German planes and one Norwegian plane flew out of Spitzbergen, Norway, and a plane owned by the University of Washington flew out of Point Barrow, Alaska. Before this spring, nearly all measurements of Arctic haze had been performed at ground level by a network of sampling stations.
Unexpected results Most of the scientists on board the NOAA plane found the haze to be much denser and more extensive than they anticipated. Glenn E. Shaw (University of Alaska) observed, “Indeed, the pollution was not only thick, but amazingly thick.” The director of 232A
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the project, Russell Schnell (University of Colorado), commented that when they looked down over the ice “it looked brownish-orange” and that in the high Arctic, “We didn’t even have to look down; just looking out, we saw brownish haze.” He also said that the recent flights suggest that Arctic haze covers the entire ice cap at this time of year and that in some places, it extends upward as high as the plane could fly (28 000 ft). These flights showed that haze exists at all latitudes in the northern polar region. This means that in the late winter and early spring, Arctic haze covers an area with a typical dimension of about 3000 miles, a region almost comparable in size to the North American continent. In 1980 Shaw and Knut Stamnes (University of Alaska) wrote an article predicting that Arctic haze absorbs a significant amount of solar energy. Preliminary new information from the flights confirms their prediction for nearly every location observed in the Arctic. Francisco P. J. Vallero of NASA-Ames Research Center (Mountain View, Calif.), who performed these measurements, said that at some locations where the haze was
very dense, at least 10%of the incident energy over the ice cap was being absorbed. Black carbon in the haze is the component believed to be responsible for most of the energy absorption. The haze was found to contain larger concentrations of black carbon than the scientists had expected and, surprisingly, the largest concentrations occurred at certain altitudes in the air rather than at ground level. These recent flights revealed that Arctic haze is usually continuous up to about 10 000 ft. Above this height, it consists of discontinuous horizontal layers that produce a banded visual effect. Schnell said the scientists “were surprised to see that the layers held their integrity for so many miles.” Traditionally, the Arctic, which lies far from most major sources of pollution, has been considered a remote region where the air and water are still clean and pure. Each year, in the late spring and summer, it does appear pristine. Nearly all pollutants are scavenged from the atmosphere by precipitation, and the air is clear. But beginning in late October and November, the weather patterns change and the northern polar region is cut off from sources of water vapor. Rapidly moving, dry air mass systems are free to carry emissions from industrial processing and fossil fuel combustion across vast distances in the Arctic when deposition processes are radically changed at the onset of the Arctic winter, There are few low-level clouds or precipitation to remove aerosols from the air. Likewise, dry deposition processes are slowed by formation of a strong temperature inversion and, over land areas, by a thin snow layer covering the tundra. While the input supply of pollutant mass is modest in comparison with highly industrialized areas, the air pollution emitted in or advected into the Arctic tends to stay aloft because the removal mechanisms are inefficient. The aerosol density
0013-936X/83/0916-0232A$01.50/0
@ 1983 American Chemical Society
from ground-level measurements is observed to be greatest in the mid- to late winter. One possible explanation for this observation is that the influx of pollutants into the Arctic air masses is greater than the attenuated removal processes at this time of year. A recent phenomenon Before the 1950s there were no reports of haze in the Arctic. During the I950s, a number of sighting5 of Arctic haze were made from aircraft, and in 1957, J. Murray Mitchell published an article that summarized existing knowledge. After this, Arctic haze was forgotten for 15 years until Shaw rediscovered it in 1972 while making routine measurements of atmospheric turbidity near Point Barrow, Alaska. Subsequent flights over the pack ice near Point Barrow revealed layers of haze that appeared brownish-yellow in color when viewed horizontally. The haze was not measured systematically until April-May 1976. At that time, Kenneth Rahn (University of Rhode Island), Shaw, and Randolph D. Borys (Colorado State University) used a light aircraft to collect samples from the haze bands over Point Barrow. These indicated that most of the haze consisted of soil particles from the great deserts of eastern China and Mongolia. They concluded that since soil particles seemed to constitute the haze, most of it was probably natural in origin. Beginning the next fall, however, ground-based aerosol sampling revealed something quite different. These samples were dark gray in color and contained much higher concentrations of aluminum, vanadium, and manganese than samples taken the previous spring. The proportions of these elements were similar to those found in urban aerosol samples. Rahn, Shaw, and Borys then began to speculate that anthropogenic pollution is a major component of Artic haze. As it turned out, the previous measurements in the spring of 1976 had been made too late in the year. By this time, the pollution-derived haze had been largely removed from the atmosphere (as happens each spring), and an unusual event had occurred-soil dust from the Asian desert had moved into the northern polar region. A number of techniques have been devised for determining whether an aerosol is natural or man-made in origin. One simple indicator of anthropogenic origin is the presence of scot or black graphitic carbon particles. These have been identified on a molecular
level as crystallites having a graphitic structure, and they can be created only by combustion. Another test is the aerosol-crust enrichment factor of vanadium. According to Rahn, it provides “an extremely sensitive test” of the natural vs. man-made sources of an aerosol. Soil dust supplies most of the aluminum in the atmosphere. In contrast, vanadium comes from two primary sources-soil dust and oil combustion. Less-refined residual oils, which are too viscous to be burned in the Arctic, provide a particularly large amount of vanadium. Soil-derived aerosol has a VAI ratio that is fairly consistent from place to place and is typical of the well-known average ratio for the earth’s crust. Therefore, an aerosol that has a V:AI ratio similar to that of the earth’s crust is thought to be natural in origin. But if the aerosol’s V:AI ratio is higher than the bulk soil ratio, the aerosol is thought to originate partly from anthropogenic sources. Where large amounts of oil are burned, the V:AI ratio can be far higher, sometimes 100 times higher, than the ratio in the crust. The aerosol-crust enrichment factor (EF) is defined as follows: EF (V:AI),,,”~,I/(V:AI).,.,,. When the value of EF exceeds -1.5, the aerosol is said to be derived at least in part from pollution.
After measurements in late 1976 indicated that Arctic haze may be caused by man-made sources, a network of sampling stations was begun (Figure 2). and many samples were collected, primarily at ground level. Though a number of scientists did not believe at first that Arctic haze was caused by human activities, by 1981 a scientific consensus had developed that included the following ideas: A large part of the wintertime haze consists of pollution that has a probable origin in Eurasia. (Other tracers, such as the Mn:V ratio as well as what is known about wind patterns, have been used todetermine the origin of the pollution.) Arctic haze begins in late fall and peaks in March-April. Horizontal visibility in Arctic haze can be restricted to 3-8 km. The component of the haze that is derived from pollution consists primarily of sulfate (with average concentrations of 2 pg/m3). organic carbon ( I pg/m3), and black carbon (0.3-0.5 pg/m3). The haze is well aged in other words, by the time it is dispersed throughout the Arctic, it is not of recent origin because of the time required to traverse long distances in the Arctic. This year’s March-April flights
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were designed to test these concepts as well as to learn about features of the haze that had not yet been investigated. One very important unknown that was measured for the first time at many different locations in the Arctic is the degree to which Arctic haze absorbs solar radiation in situ. Also, very little was understood about the height of the haze or about the vertical distribution and vertical changes in the composition of the haze. In addition, the extent of the haze had never been determined because flights to investigate haze had been made over only short paths in the Arctic. And almost no knowledge existed about sinks for the haze. Climatic effects
Thus far, the effects of Arctic haze on the environment are almost entirely unknown. The possible impact of most concern to scientists is whether the haze is altering, or has the potential to alter, theclimate in the northern polar region and, consequently, global climate to any significant extent. As mentioned earlier, preliminary results indicate that the haze absorbs a substantial portion of incident solar energy during March and early April. If this is true, it could cause heating of the Arctic atmosphere, and this could affect both the Arctic and global climate. Hal Rosen of the Lawrence Berkeley Laboratory said that the effect of the carbon particles on the radiation balance depends on how high the large concentrations of these particles extend in the atmosphere. When assessing this effect, it should be remembered that substantial solar radiation and haze occur simultaneously in the Arctic for only several months each year. But during these months, the impact may be substantial. Black graphitic carbon is the component of the haze that is thought to be responsible for most of the radiation absorption. According to Rosen, who along with Anthony Hansen of the Lawrence Berkeley Laboratory measured black carbon in the haze, the average concentrations were only three or four times smaller than the levels in typical urban areas such as Denver and only IO times smaller than those in New York City. They were orders of magnitude greater than the levels found over the Pacific. Graphitic carbon not only absorbs radiation in the haze, but it may also change the reflectivity of Arctic clouds. According to Rahn and Shaw, this may be one of the most important environmental effects of Arctic haze. Carbon may become part of the cloud 234A
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liquid droplets or ice crystals and cause “darkening” of the clouds, increasing their absorptivity. The carbon may further increase absorption by causing multiple reflections within clouds. In addition to these two effects, black carbon could change the albedo (or reflectivity) of the ice cap. Lightcolored particles would have little effect on the albedo. But black carbon particles that settle on the surface of the ice from the haze would increase the absorption of solar radiation. Graphitic carbon is, of course, deposited throughout the winter haze season, even when the sun is not shining, and remains throughout the summer after the airborne haze has dissipated. Very few measurements have been made of changes in the albedo of the ice cap. This area requires further research.
No observed changes Arctic haze has probably existed for decades. Thus far, it has caused no demonstrable changes in Arctic cli-
mate. If haze is affecting climate, the changes are not easily detected. There are several possible effects, however, that may or may not be related to Arctic haze. For example, some observers have noted less pack ice in the northern polar region. This phenomenon is probably associated with a rise in average global surface temperatures, which can be attributed to several causes other than Arctic haze. These include the greenhouse effect from rising CO2 and other trace gas levels, and variations in the sun’s radiant energy output. But the possibility that Arctic haze is causing alterations in climate cannot be ruled out at this time. Until recently, scientists have not been looking for such changes. They may have been masked by changes produced by other factors. Rising C02 levels and Arctic haze could both cause air temperatures to increase in the northern polar region. Consequently, the effects of Arctic haze could be confused with effects from increasing C02 levels.
FIGURE 2
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Even a t present, there is no integrated program to determine the relation between Arctic haze and global climate. Shaw believes that the dark absorbing haze that exists for several months over white pack ice is going to have some impact on climate. “It is impossible to predict what the eventual climate effects will be,” he said. “We do know enough to say it is important.” Schnell wrote in a similar vein: “Model studies suggest that aerosol-induced heating has the potential of reducing the Arctic ice cap.” No matter how the question of the effects of Arctic haze on climate is finally resolved, study of this phenomenon has underlined the importance of depositional processes for cleansing the air of atmospheric pollutants, even in remote areas. Consequently, caution is required when ascribing entirely to natural sources the atmospheric burden of trace gases and aerosols found in remote areas where the efficiency of removal processes may be low. In addition to effects on climate, Arctic haze may have ecological ef-
fects. Because tundra biosystems are extremely sensitive to chemical perturbations, they may be affected by chemical species in the haze. Sulfate is the most concentrated haze component. During the summer, the ambient sulfate level in the Arctic is comparable to the level in Antarctica and other very remote regions of the world (0.1-0.2 pg/m3). But during late winter and early spring, it reaches mean levels of 2 pg/m3, which is about one-third the mean region-wide concentration found during winter months in polluted areas of Europe and North America (Figure 3). Since there are usually few alkaline ions in the Arctic atmosphere to neutralize the acidity of the sulfate, the tundra is probably subject to acid deposition from anthropogenic sources during the winter and early spring. However, it has not been possible to determine the relative contributions of man-made and natural sources of sulfate deposition onto the tundra nor have the biological effects, if any, of such acid deposition been investigated. Time-series measurements made on the Greenland ice
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cap suggest that anthropogenic sulfate deposition there may be 2’/>times the deposition from natural sources. Many other substances present in Arctic haze may also alter tundra biosystems. Recently, it was calculated tbat more lead enters the Arctic from the atmosphere than from North Atlantic water or from rivers. In addition, chlorinated hydrocarbons (including pesticides), silicones, and a large number of polycyclic aromatic hydrocarbons have been measured in the haze at tbe ground-based stations. It is possible that some of these pollutants may have a significant effect on the highly stressed tundra. A good beginning
The recent flights were very successful. All instruments performed well in the harsh environment, and a tremendous amount of data was collected. Most of it has yet to be analyzed in detail, however. Scientists will be meeting in September 1983 to compare the results they have obtained by then, and in May of 1984, the third symposium on Arctic air chemistry will be hosted by the Canadian Atmospheric Environment Service in Toronto. By this time, it can be expected that analyses of the recently acquired data set will raise many new questions requiring more detailed experimentation. Even though scientists collected a great deal of information during the March-April flights, airborne investigations of Arctic haze are only in the beginning stages. A monitoring flight made across the diameter of the Arctic is analogous to measuring air pollution while flying across the US.one time. No aircraft sampling was performed on the Soviet side of the pole. Moreover, no ground-based sampling data from the Soviet Arctic are available to Western European or North American scientists. So far the Soviets have declined to participate in the cooperative international investigations of this intriguing new phenomenon. The recent NOAA flights were funded by the Environmental Research Laboratories of NOAA, the Office of Naval Research, the Army Research Office, and the National Aeronautics and Space Administration. -Bette Hileman Additional reading Rahn. Kenneth A., Ed. “Arctic Air Chemistry.’ Almos. Enuiron. (special issue). 1981, 15
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Research Review, March 1982, p. 13
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Rahn, Kenneth A.: Shaw. Glenn E. “Sources and Transport of Arctic Pollution Aerosol: A Chronicle of Sir Years of O N R Research.” Nao. Re.$. Reu. March 1982. pp. 3-26.
