Arctic Haze

Arctic haze is a seasonal atmospheric phenomenon affecting the Arctic, peaking in spring, that originates from pollution sources outside the Arctic. When the sun returns after the long polar night, layers of brownish haze are visible above the colorful horizon. Similar to the well-known air pollution phenomena in areas with process industries and large towns (smog), and the murky dust clouds (brown clouds) seen over large regions of tropical Asia, Arctic haze reduces visibility over polar regions to less than 30 km and contributes to contamination of the Arctic environment.

The haze layers consist of small airborne particles and droplets called aerosols. The aerosols are produced both by natural processes like volcanic eruptions and dust storms, and by anthropogenic activities such as the burning of fossil fuels and biomass. Although natural sources of aerosols also contribute to aerosol loading of the atmosphere, Arctic haze is primarily a phenomenon involving the long-range transport of anthropogenic pollutants, particularly sulfate particles. Aerosol particles are transported by winds and atmospheric currents from industrialized areas mainly in Europe and Eurasia to the Arctic. As the winds pass over the chimney plumes of power plants and chemical industries, sulfate aerosols, soot, and other particulate compounds are transported to the Arctic by the large-scale atmospheric circulation. In the Arctic, thousands of kilometers from their source, they have significant effects on climate forcing and represent a source of contamination in Arctic ecosystems and food chains.

During winter and spring, the northward transported particles build up within the cold, dry, and stable air mass over the Arctic basin since winter atmospheric circulation (semipermanent high pressure) over the Arctic and the lack of solar radiation during the long polar night inhibit removal processes. This is because deposition or washing out of aerosol contamination by precipitation, and the photochemical processes that produce hydroxyl radicals (OH) (that modify the chemical and physical properties of the aerosols) are mainly driven by solar radiation.

Aerosol loading of the atmosphere has a significant effect on the regional and global climate by modifying the natural radiation balance of the Earth and atmosphere. The forcing effect works through two different mechanisms. Due to the scattering and absorption properties of the haze particles, they directly influence the amount of solar shortwave radiation that reaches the Earth's surface and the amount of thermal infrared radiation that is radiated back out through the atmosphere. In addition, the particles may modify the micro-physical properties and the amount of clouds, thus indirectly influencing the radiation transfer through the troposphere. However, there are several factors that make estimation of the net climate effect of the aerosols difficult. The radiative effects depend on the chemical composition, size, shape, and spatial distribution, including vertical distribution, of the aerosol layers. The highest concentration of the aerosol layers is found in the lowest few kilometers of the troposphere, but aerosol particles may influence the whole vertical column of the troposphere. The aerosol layers are washed effectively out of the atmosphere by precipitation. All these factors cause a large and inhomo-geneous temporal and spatial variation in the tropospheric distribution of aerosols, and the radiative effects are therefore regional and patchy.

Both natural and anthropogenic sources contribute to the atmospheric content of aerosols. Soil dust is the major natural contributor to atmospheric aerosol loading in the tropics and subtropical regions. However, since the effectiveness of this aerosol source depends on the frequency of strong surface winds as well as the level of human disturbance of the soil, this source of atmospheric aerosols is certainly also modified by anthropogenic activities. This is also true for marine areas such as the Arctic, where sea salt aerosols can be the main contributor to cloud formation and properties as well as the direct scattering of light. This process also depends on the frequency of strong surface winds and is affected indirectly by the anthropogenic forcing of climate through the abundance of strong winds and the fraction of exposed water surfaces, which is believed to increase as the Arctic sea-ice diminishes. Volcanoes also emit a large amount of dust particles and sulfur dioxide (SO2) into the atmosphere. The effect is transient (lasting a few years) and has a significant effect on the upper atmosphere and lower stratosphere. An important biogenic aerosol source is that formed by plant debris, humic matter, and micro-bial particles such as algae and pollen, viruses, and bacteria. These humic aerosols contribute to the absorption of ultraviolet radiation in the atmosphere.

It is becoming increasingly evident that Arctic food chains are contaminated with a number of new substances not previously detected in the Arctic. These are transported to the Arctic by atmospheric and ocean currents. Through precipitation, these substances contribute to the toxification and acidification of the pristine Arctic environment. The burning of biomass and fossil fuel also produces a large amount of so-called carbonaceous compounds, which can be divided into organic and black carbon aerosols. Such aerosols are also formed by atmospheric oxidation of biogenic and anthropogenic volatile organic compounds. Carbonaceous particles from traffic, burning of fossil fuels, and process industries are one of the most important anthropogenic sources of aerosols. These particles, together with sulfate aerosols (formed by the chemical transformation of SO2), perfluorocarbons, and pesticides, are a major threat to the acidification and contamination of the Arctic environment.

The sulfate aerosols are formed mainly by the photochemical transformation of SO2 from anthropogenic emissions and volcanoes as well as dimethyl sulfide (DMS), a trace sulfur-containing gas produced by marine plankton. Ice cores from Arctic ice sheets have shown a marked increase in Arctic air pollution since the 1950s. Other data show, for example, that the summertime visibility in the eastern United States was worst in the 1970s, which coincided with the period of maximum SO2 emission. Since the beginning of the 1990s, the trend has decreased due to reduced SO2 emission in Europe as well as the economic recession and industrial crisis in Russia, however, the growing industries and weaker emission controls, especially in Asia, may lead to further SO2 emission increases. On the other hand, ice cores from the Antarctic show no trend in SO2 emissions since the primary sources in the Southern Hemisphere are of natural origin.

Large uncertainties still exist in the estimation of the regional and global net effects of the aerosols. An important challenge is still to determine the full range of compounds, the chemical properties of the haze particles, and especially how they are transformed, filtered out, and transported into the environment and food chains. The general state of knowledge indicates that most aerosol compounds (sulfate, biomass-burn-ing aerosols, and fossil fuel organic carbon) induce a cooling effect on the climate system due to increased scattering of sunlight, except for black carbon (soot) particles, which constitute a small warming potential due to energy absorption. Other aerosols like mineral and atmospheric dust particles are poorly described and add to the overall uncertainty in the total aerosol climate signal. In the Arctic, surface snow and ice also influence the radiation transfer through the atmosphere due to its high reflective properties (albedo). Some experiments indicate that the aerosol forcing signal is negative and even larger in the Arctic spring, that is, representing a significant cooling effect. Changes in the perennial ice cover of the Arctic Ocean will also affect the source of natural sulfate aerosols by DMS production in marine phytoplankton since the ice cover acts as a lid on the source. The Arctic conditions with the high surface albedo and low sun are complicated, and the uncertainties are too high yet to determine long-term trends in Arctic haze as well as the actual impact of the Arctic aerosols on the global and regional climate system.

Jon B0rre 0rb^k

See also Albedo; Local and Transboundary Pollution

Further Reading

AMAP Assessment Report: Arctic Pollution Issues, Arctic Monitoring and Assessment Programme (AMAP), Oslo, Norway, 1998

Ghan, Steven J., Richard C. Easter, Elaine G. Chapman, Hayder Abdul-Razzak, Yang Zhang, L. Ruby Leung, Niels S. Laulainen, Rick D. Saylor & Rahul A. Zaveri, "A physically based estimate of radiative forcing by anthropogenic sulfate aerosol." Journal of Geophysical Research, 106(D6) (2001): 5297-5293 Haywood, James & Olivier Boucher, "Estimates of the direct and indirect radiative forcing due to tropospheric aerosols: a review." Reviews of Geophysics, 38(4) (2000): 513-543 Houghton, J.T. et al. (editors), Climate Change 2001: The Scientific Basis, Cambridge and New York: Cambridge University Press, 2001 Jacobsen, M.C., H.C. Hansson, K.J. Noone & R.J. Charlson,"Organic atmospheric aerosols: review and state of science." Reviews of Geophysics, 38(2) (2000): 267-294 Rogers, David C., Paul J. DeMott & Sonia M. Kreidenweis, "Airborne measurements of tropospheric ice-nucleating aerosol particles in the Arctic spring." Journal of Geophysical Research, 106(D14) (2001): 15053-15063

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