METEOROLOGY AND ATMOSPHERIC CHEMISTRY 



85 



The observed mass median radii for the Asian dust in 

 the SEAREX study at Enewetak ranged from 0.7 to 

 1.0 /im, considerably smaller than the atmospheric salt 

 particles. Eighty to 85% of the mass of the dust was 

 present on particles with radii between 0.2 and 2 ^m. This 

 is consistent with a very long atmospheric transport path. 



Removal of dust to the ocean by rain and dry deposi- 

 tion was estimated at Enewetak through the analysis of 

 rain samples and samples obtained by the exposure of flat 

 plates on top of the tower. The total (wet and dry) deposi- 

 tion of dust during May 1979 was estimated as about 

 4 ^g cm - Assuming this deposition was applicable for 3 

 to 5 months during the spring and early summer, with 

 somewhat lower deposition the rest of the year, leads to 

 an estimated annual atmospheric dust deposition to the 

 ocean near Enewetak of 15 to 30 ^g cm~^ (Duce ct al., 

 1980). Settle and Patterson (1982) report dust in rain and 

 dry deposition at Enewetak which converts to a yearly flux 

 of about 13 and 1 /:ig cm~^ respectively, the latter being 

 recycled in sea spray and not contributing to net input. 

 These inputs can be compared with an estimate of the 

 annual nonbiological marine sedimentation rate to the 

 ocean floor in that region of about 50 ng cm^^ 

 (M. Leinen, personal communication). Within the uncertain- 

 ties in both estimates, it is clear that the atmosphere is a 

 significant transport path for the nonbiological material 

 found in marine sediments near Enewetak. It is also clear 

 that the transport of Asian derived substances to the 

 Enewetak region is seasonal. 



Lead-210 was also measured in the atmosphere at 

 Enewetak. Lead-210 is a radioactive nuclide produced in 

 the atmosphere by the decay of gaseous Rn, which in 

 turn is derived from continental soils. Atmospheric ^'"Pb 

 was found to decrease over the April to August 1979 

 period in a manner similar to the atmospheric Al concen- 

 tration. Lead-210 in air ranged from about 4 dpm per 

 1000 m^ in April to 0.8 to 1.0 dpm per 1000 m^ in late 

 July and August (Turekian and Cochran, 1981a, b). Using 

 ^b as an indicator of Asian dust transport, Turekian and 

 Cochran (1981a, b) calculated a dust deposition of about 

 10 /ig cm~ yr~ to the ocean at Enewetak. 



Trace Metals 



A number of trace metals were investigated on parti- 

 cles in the atmosphere at Enewetak. Some of these trace 

 metals, e.g., Na, Mg, K, and Ca, were clearly derived from 

 the ocean as part of the atmospheric sea salt. Interelemen- 

 tal concentration ratios among this group were the same 

 as found in sea water. Another group of metals was clearly 

 associated with the mineral aerosol or Asian dust. This 

 was determined by using the Al content of the particles as 

 a reference element for crustal weathering products and 

 comparing the metal/Al ratio on the aerosols to the aver- 

 age metal/Al ratio in the earth's crust. An enrichment fac- 

 tor relative to the crust, EFj_i,,, can be defined as follows: 



where (X/AO^,, and (X/AO^^,,, refer to the mass ratio of 

 metal X to aluminum in the Enewetak aerosols and the 

 earth's crust, respectively. Values of EF^rust near 1 for any 

 metal suggests that crustal weathering is likely its source in 

 the particles (Duce et al., 1975; Rahn, 1976). EF<^, 

 values for samples collected at Enewetak are given in 

 Table 4. From this table it is clear that such elements as 

 Al, Ta, Sc, Mn, Fe, Eu, Ni, Co, V, Hf, Cr, Th, Cu, and Rb 

 are primarily found associated with mineral or soil aerosol 

 particles at Enewetak. Metals with an EF,-^^ value higher 

 than 4, e.g., Zn, Cs, Sb, Ag, Pb, Cd, and Se, apparently 

 have some source other than continental weathering. 



TABLE 4 



Geometric Mean EFct,,,, Values for 

 Atmospheric Trace Metals at Enewetak* 



Metal 



EF 



Metal 



EF„ 



E'cnist 



(X/Al), 

 (X/A1)„ 



•From Duceet al., 1981. 

 fGeometric standard deviation. 



On the basis of the measurements made at Enewetak 

 in 1979, Table 5 presents the expected mean atmospheric 

 concentrations for a number of trace metals during the 

 March to June (high Asian dust) period and during the rest 

 of the year (Duce et al., 1981). Concentration units are 

 ng (10^^ g) and pg (10"'^ g) per cubic meter of air. Note 

 that the concentration of all the metals is higher in the 

 spring than the rest of the year, although the increase in 

 concentration for many metals during the spring is not as 

 great as for the metals clearly associated with the dust. 

 For example, while the mean dust associated metals are 

 —25 times higher in the spring, the difference for Pb is 

 less than a factor of 2, Se is about 2, Cd is 5, etc. We 

 assume the source of these "enriched" metals is also pri- 

 marily continental regions. Metals associated primarily 

 with the desert dust have considerably higher 

 concentrations during the spring and early summer due to 

 both stronger source functions (i.e., more frequent dust 

 storms) and wind fields which are conducive to effective 

 long-range transport to Enewetak during that period. 

 However, the enriched elements may have continental 

 sources which are not so seasonal in nature but which are 



