Asian desert dust over Alaska: Anatomy of an Arctic haze episode

A series of haze bands over Barrow, Alaska, in April and May 1976 was found to consist of crustal dust. The bulk elemental composition of the particles was crustal or nearcrustal; the particles had angular shapes, some of which were characteristic of micas; most of their mass was in the giant-particle range, a characteristic of airborne soils; most of the larger particles were rich in silicon and aluminum. The mass-median radius of the particles (2 μm) indicated that they had probably traveled more than 5,000 km from their source, thereby effectively eliminating Alaska itself as a source. Trajectory analysis showed that the hazecontaining air had passed over the arid and semi-arid regions of eastern Asia a few days earlier, when intense dust storms had occurred there. One of the strongest of these storms, the cloud of which could be traced (by dust reports) from China past Japan and Korea and out over the Pacific Ocean, has been tentatively identified as the source of the Alaskan haze. The Asian dust layers, however, did not seem to increase the local atmospheric turbidity at Barrow. Rather, the turbidity decreased somewhat during the episode and sharply thereafter. The decrease in turbidity accompanied a temporary migration of the jet stream to near the North Pole, which allowed clean Pacific air from far to the south to sweep over Barrow. The high prehaze turbidities, which are characteristic of Barrow air during winter and spring, were associated with Arctic air from the north, which contained much higher concentrations of pollutants than did the southern air after the haze bands. Recent evidence strongly suggests that anthropogenic aerosol is responsible for much of this Barrow haze. It thus seems that distant natural andanthropogenic aerosol sources may affect the radiation balance of the Arctic.