SECT. 2] INSOLUBLES 297 



deserts as a dust source in the Northern Hemisphere, and the smaller South 

 African and Australian deserts in the Southern Hemisphere. But, unlike the 

 Atlantic easterlies, the Pacific equatorial easterlies have no large desert regions 

 as a potential dust source. However, there is a continuous volcanic chain along 

 the eastern margin of the Pacific basin in the zone of prevailing easterlies. For 

 this reason one expects, in the Pacific, a predominantly volcanic fall-out from 

 the easterlies and continental dust from the westerlies. In addition there are 

 continental katabatic winds, such as the Santa Anas, which carry desert silt 

 seaward from the California coast ; these, however, are observed only near the 

 continents. 



The advent of nuclear explosions provided a means of studying the movement 

 of atmospheric dust over long distances. A low yield explosion introduced 

 fission products into the troposphere over Nevada in 1955. The fall-out from 

 this explosion was observed consecutively at Paris, Cairo and Asahikawa, 

 Japan (Miyake, Sugiura and Katsuragi, 1956). The fall-out in Japan occurred 

 as a mud rain two weeks after being introduced into the atmosphere over 

 Nevada. Transport had been in the jet stream. It seems obvious that the jet 

 stream with its enormous speeds should play a major role in global transport 

 of dust, but the first clear-cut example was the Asahikawa fall-out. The fission 

 products were absorbed on a fine buff silt dominantly of 2 to 10 microns size 

 and composed of quartz, feldspar, calcite, mica and clay minerals. Miyake 

 {loc. cit.) noted the chemical similarity of this dust to Manchurian loess and 

 suggested that it is Manchurian loess which has scavenged fission products. 

 Actually this size range of 2 to 10 microns falls into the true aerosol range 

 (Junge, 1957), while loess (Scheidig, 1934; Smith, 1942) is somewhat coarser 

 and falls largely into the size range where gravitational settling predominates. 



Nuclear weapon tests have shown that gigaton explosions are required to 

 lift surface dust into the stratosphere (Libby, 1956, 1956a). For this reason 

 there is little cause to expect transport of continental dust by stratospheric 

 winds. Volcanic eruptions, however, may be of sufficient force to raise dust well 

 into the stratosphere. The most famous example of this was the explosion of 

 Krakatoa which spread volcanic ash into the stratosphere where it circulated 

 in both hemispheres for several years. More recently the advent of high flying 

 aircraft has made possible direct observations of stratospheric dust. The 

 eruption of Mt. Spurr, near Anchorage, Alaska, on July 9, 1953, deposited a 

 blanket of tropospheric fall-out of volcanic ash in Alaska (Wilcox, 1959). The 

 mushroom ash cloud reached an altitude of 70,000 ft. By July 27, ash from this 

 eruption crossed England at an altitude of nearly 50,000 ft, well within the 

 stratosphere (Jacobs, 1954). In 1956 a volcanic eruption in Kamchatka intro- 

 duced ash into the stratosphere which was observed five days later over England 

 at an altitude of about 55,000 ft (Bull and James, 1956). 



The low moisture content of the stratosphere inhibits precipitation scrubbing 

 of dust, and residence times for fission products are known to be much longer 

 than in the troposphere and of the order of a year (Feely, 1960). Fission product 

 studies likewise show that maximum fall-out from the stratosphere comes at 



