Aerosols (small liquid or solid particles in the air of either natural or anthropogenic origin) play a major role in Arctic climate, both locally in terms of air pollution and fog at settlements and regionally with respect to their effect on radiative transfer and
heating. Early explorers often commented on the pristine environment and exceptional visibility associated with low atmospheric humidity. However, industry has degraded Arctic air quality. Recognition of the existence of reduced slant visibility by aircraft operating in the Arctic in late winter and spring, gave rise to the term "Arctic haze" by Mitchell (1957) and opened a new chapter in Arctic meteorology. Subsequent aircraft research programs and surface-based measurements have vastly increased our knowledge of Arctic air chemistry and the types and sources of aerosols (Barrie, 1986).
The constituents of Arctic haze were first measured at Barrow, Alaska (Rahn et al., 1977). Thirty percent were sulfates, but 60% that were unidentified were thought to be of organic origin. A three-year observational record conducted in Alaska during January through April shows non-sea salt sulfate concentrations increasing northward from Homer to Poker Flat and being greatest at Barrow (Quinn et al., 2002). Seven-day back trajectories for Barrow show that for October-January, the eastern and western sectors of the Arctic Basin each account for 27% of the source areas, with Siberia east of 90° E a further 18% and North America 16%. The dominant source areas for March-June are: the western Arctic Basin (25%), the North Pacific (23%), the eastern Arctic Basin (18%) and North America (17%). The corresponding sources for July-September are the western Arctic Basin (31%), the North Pacific (22%), the eastern Arctic Basin (17%) and Siberia (15%).
Measurements of optical depth (t , a unitless measure of the cumulative depletion of radiation through the atmosphere, a larger value means more depletion) at Kold-ewey (79° N, 12° E) for 1991-9 show average values in winter of 0.053 and in spring of 0.089. The latter compares with a spring background value of 0.067 and a doubling (0.122) during haze events, which have a 40% frequency in the season (Herber et al., 2002). Figure 2.21 illustrates the relationship between trajectories for such haze events in March-May 1999 and industrial source regions. Industrial plants in northern Russia located in the Kola Peninsula, Norilsk (nickel-copper smelting) and oil and gas exploitation in the Tyumen area, are all chemically identifiable sources within the Arctic Circle (Harris and Kahl, 1994).
While haze particles in winter and spring are mostly of anthropogenic origin, those in summer are primarily from natural sources. In summer, aerosols over the central Arctic Ocean have a direct source for growth up to 50 nm (nanometers, 1 nm = 10-9 m) and a dimethyl sulfide source that promotes growth of particles to larger cloud condensation nuclei (CCN) size after oxidation (Bigg and Leck, 2001). As the name suggests, CNN represent nuclei associated with cloud formation. The direct source may involve fragments of bacteria and microalgae, or organic compounds from bubbles bursting on the surfaces of leads in the sea ice cover. For wind speeds of 512 m s-1, sulfur-containing particles (mostly sea salts) dominate the CCN populations and mass (Leck et al., 2002). Mean optical depths in summer are about 0.06 at Barrow and 0.08 at Alert (Bokoye et al., 2002).
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