84 



MERRILL AND DUCE 



of the salt to the island surface was also very wind speed 

 dependent but increased with wind speed much more 

 rapidly than the sea salt concentration itself (Table 3). 



TABLE 3 



Atmospheric Sea Salt Concentrations 

 and Deposition at Enewetak' 



Wind Atmospheric Atmospiicric 



speed, salt concentration, salt deposition. 



Mg m 



ng cm 



3.4t 

 6.5 

 10 



3 

 16 

 29 



170 

 410 



'Measured 18 m above sea level (McDonald 

 et al., 1982). 



fS 4 ms ' data obtained from Pigeon Key, 

 Florida 



These authors pointed out that this is because larger salt 

 particles are present in the atmosphere at higher wind 

 speeds, and these larger particles have a much higher 

 deposition, or settling, velocity than the smaller particles. 

 With their much shorter atmospheric residence times, a 

 relatively low concentration of larger particles can dom- 

 inate the flux of the entire particle population. For exam- 

 ple, at a wind speed of 2.4 ms ', particles with a radius 

 (at 80% relative humidity) of 4.5 Mm or smaller account 

 for 60% of the total salt mass but less than 10% of the 

 salt deposition, while salt particles with radii greater than 

 10 ^lm account for only 13% of the salt mass but 70% of 

 the depKjsition. 



Asian Soil Dust 



The geographical distribution of some mineral com- 

 ponents, such as quartz and illite, in North Pacific marine 

 sediments suggests that the atmosphere may be a very 

 important transport path for mineral matter, or soil dust, 

 to mid-latitude (30° to 40°N) areas of the North Pacific. 

 There are, however, few data available on the dust con- 

 centration in the atmosphere over the North Pacific and no 

 direct information on the atmospheric input rate of this 

 material to the ocean surface. During the SEAREX experi- 

 ments, air filter samples were collected for dust analysis. 

 The atmospheric concentration of aluminum was used as 

 an indicator of continental dust in these samples, with the 

 dust containing about 6.5% Al in the aluminosilicate 

 matrix. The observed concentrations of dust and salt in the 

 atmosphere at Enewetak are shown in Fig. 7. While the 

 atmospheric salt concentration remained relatively con- 

 stant, the dust concentration dropped by a factor of ~-100 

 from mid-April to early August 1979 (Duce et al., 1980). 



The high concentrations of soil dust observed at 

 Enewetak were unexpected, especially since Enewetak lies 

 well within the easterly tradewind regime, and the nearest 



APRIL I MAY I JUNE I JULY I AUG I SEP I 



SAMPLE COLLECTION DATE, 1979 



Fig. 7 Atmospheric concentrations of dust and sea salt at 

 Enewetak l)€tween April and August, 1979. 



continental land mass, Asia, is about 5000 km to the 

 northwest. The dramatic decrease in dust over the 

 5-month period was also unexpected, but both these obser- 

 vations can be explained on the basis of the seasonal 

 changes in the large-scale wind patterns over the North 

 Pacific and the seasonal character of dust storm activity in 

 the Takia Makan, Gobi, and Ordos Desert regions of 

 China. Dust storm activity is apparently greatest in the 

 spring in China due to the combined effects of low rainfall, 

 the increased occurrence of high surface winds associated 

 with strong cold fronts, and soil freshly plowed for plant- 

 ing. The mean surface winds from March through May are 

 strong easterlies over the western North Pacific between 

 30°N and the equator; north of 30°N, the surface winds 

 are weak, with a tendency toward being westerly. How- 

 ever, at 700 mbar (about 3000 m) there is very strong 

 westerly flow north of about 20°N extending from well 

 within Asia to the central North Pacific. Thus dust raised 

 over China could easily be transported by the mean winds 

 at this level to the region north of Enewetak. During June 

 through August, however, conditions are not favorable for 

 the transport of dust to the central North Pacific. Surface 

 winds are easterly from Enewetak northward to about 

 40°N. At 700 mbar the northern boundary of the easter- 

 lies is located at about 30°N. Persistent westerlies appear 

 at 700 mbar only north of 40°N, and they are very weak. 

 Thus we would generally expect much higher atmospheric 

 soil dust concentrations and deposition rates to the ocean 

 at Enewetak in the late winter and spring than the rest of 

 the year. In corroboration of this, Ing (1972) documented 

 an April 1969 dust storm over China, and satellite photos 

 showed that dust cloud moving well out over the East 

 China Sea. 



