METEOROLOGY AND ATMOSPHERIC CHEMISTRY 



87 



these metals had resulted from pollution sources on the 

 continents, since the marine sedimentation rates for these 

 metals are mean rates applicable to approximately the past 

 15,000 years. Input of p)ollution-derived trace metals, 

 which has developed significantly only in the past 50 years 

 or so, would not be reflected in the measured marine sedi- 

 mentation rates. Schaule and Patterson (1982) proposed 

 that a shift may have occurred from principally fluvial 

 inputs of lead to the oceans in earlier times to primarily 

 atmospheric input in recent times. Second, these results 

 would also be expected if a significant fraction of the 

 atmospheric deposition of these trace metals came from 

 their recycling from the ocean surface into the atmosphere 

 and back to the ocean. Recent studies (Weisel, 1981) sug- 

 gest that recycling of marine-derived metals probably does 

 not account for more than a few percent of the mass of 

 these metals in the atmosphere at Enewetak. However, 

 since these ocean-derived metals would be found on the 

 large sea salt particles, their dry dep>osition back to the 

 ocean surface could be rather high (Duce, 1982). Thus, 

 while it is believed that most of the mass of the enriched 

 trace metals in the atmosphere at Enewetak is derived 

 from the continents and very possibly from pollution 

 sources, a significant fraction of the gross dry deposition of 

 these metals into the ocean from the atmosphere may be 

 due to recycled metals from the ocean surface, as 

 mentioned above. 



Lead isotope ratios reported by Settle and Patterson 

 (1982) confirm that, during the high dust period in April 

 1979, the pollution-derived Pb had an Asian origin (Tables 

 7a and b). However, as the Asian dust decreased, the 



TABLE 7a 



^Pb/^'Pb Ratios in 

 Filtered Air Sample at Enewetak' 



Collection 

 date, 1979 



*Pb/^Pb 



4/22 to 5/09 

 5/09 to 5/15 

 7/12 to 8/10 



1.170 

 1.196 

 1 205 



^°^Pb/^''^Pb ratio increased and became similar to that for 

 pollution-derived Pb from North America. Thus some, if 

 not most, of the small particle pollution-derived Pb found 

 at Enewetak in the summer may have been transported 

 from North America to Enewetak. On the basis of "Tb 

 and stable lead measurements, Settle et al. (1982) and Set- 

 tle and Patterson (1982) calculated a net atmospheric 

 stable lead deposition rate of 4 to 10 ng cm~ yr at 

 Enewetak. This agrees well with the value of 8 to 12 ng 

 cm~^ yr~^ given in Table 6. 



The mass median radii (MMR) for the particles contain- 

 ing the various trace metals are presented in Table 8. 

 Note that the sea salt metals (Na, Mg, K, and Ca) have 

 MMRs near 3.5 /zm while the crustally derived metals have 

 MMRs of 0.75 to 1.0 nm. The enriched metals (Zn, Se, 

 Sb, and Pb) have MMRs of <0.5 ^m, consistent with a 

 possible pollution source for these elements. 



TABLE 8 



Mean Particle Mass Median Radii (MMR) 

 for Trace Metals at Enewetak* 



'From Duce et al., 1981. 



Gaseous and particulate mercury were also investigated 

 at Enewetak (Fitzgerald et al., 1981). The concentrations 

 observed are given in Table 9. It is apparent that mercury 

 exists almost entirely as a gas at Enewetak. The relatively 

 small temporal variation in gaseous Hg concentration (and 

 the fact that similar concentrations are found at other 

 marine areas) suggests a relatively long atmospheric 



TABLE 7b 



^Pb/^'Pb Expected from 

 Major Continental Sources* 



Region 



*Pb/^Pb ratio 



Asia/ Japan 1.153 to 1.165 



USA. 1.190 (1974) to 1.230 (1978) 



Mexico 1.187 



'From Settle and Patterson, 1982. 



TABLE 9 

 Atmospheric Mercury at Enewetak' 



Collection 



period, 



1979 



Gaseous Hg, Particulate Hg, 



ng m 



ns m 



4/27 to 5/21 

 6/28 to 8/6 



1.6 ±0.6 

 1.7±0.5 



0.0005 

 00012 



•From Fitzgerald et al., 1981. 



