88 



MERRILL AND DUCE 



residence time for the vapor phase. Studies of Hg specia- 

 tion in the atmosphere at Enewctak indicate that the gas 

 phase is principally inorganic mercury, of which elemental 

 Hg is probably the major component. Mercury in rain at 

 Enewetak was found to have a concentration of 2 ng 1 

 Apparently this concentration is derived primarily from the 

 washout of the particulate Hg rather than the vapwr phase. 



are reported in Table 11. It is apparent that the vapor 

 phase dominates the concentration of these compounds, at 

 least from n-C2i to n-C^Q, and probably for the lower car- 

 bon number alkanes as well. Duce and Gagosian (1982) 

 used the concentration distribution in Table 11 to model 

 the input of particulate nalkanes (n-Ci5 to n-C3o) and 

 vapor phase nalkanes (n-Cjo to n-Cao) from the atmo- 



Organic Carbon 



The organic carbon concentration of atmospheric f)arti- 

 cles at Enewetak was ~0.9 ^g m~^ (Chesselet et al., 

 1981) (Table 10). This is typical of marine regions, where 

 the concentration generally ranges between 0.2 and 1.2 /xg 

 m""^. Eighty to 85% of the mass of this organic carbon at 

 Enewetak is found on particles with radii less than 1 nm. 



TABLE 10 

 Atmospheric Organic Carbon at Enewetak 



Concentration 



Mg m 



mg 



Reference 



Particulate 89 + 017 



Rain 



Chesselet et al., 

 1981 

 0.64 ±0 48 Gagosian et al., 

 1981b 



Carbon isotope studies by Chesselet et al. (1981) have 

 suggested strongly that the small particle (<1 ^m) organic 

 carbon does not have a marine origin. By measuring both 

 C and C, one can calculate 5 C as follows: 



5'3c = 



\ ^/ *^/sample 



/13/-;12, 



C/ CJstandard 



-1 



X 1000 



5 C values calculated for the smallest particles (r < 1 /xm) 

 are -26°/oo to -28°/oo. Chesselet et al. (1981) point 

 out that this range is similar to b^^C values for continental 

 vegetation, coal, and the products of jsetroleum combus- 

 tion, — 26 ± 2°/oo, suggesting the small particle carbon is 

 of continental origin. The d^'^C values for the larger parti- 

 cles (r > l^im) are -187oo to -22° I ^. This is similar to 

 the 6 C value for marine organic carbon, which is gen- 

 erally -21 ± 2°/oo in low latitude regions (40°S to 

 50°N), suggesting the large particle carbon in the 

 Enewetak marine atmosphere is of marine origin. 



The organic carbon content of rain at Enewetak aver- 

 aged 1.2 mg l~^ during April and May and 0.3 mg 1 

 during July and August 1979 (Gagosian et al., 1981b). 



Organic Lipid Class Compounds 



Particulate and vapor phase heavy n-alkanes were 

 measured independently in the atmosphere at Enewetak by 

 two research groups in 1979. The observed concentrations 



TABLE 11 

 Atmospheric N-Alkancs at Enewetak 



'Gagosian et al , 1981b, 1982. 

 fAtJas and Giam, personal communication, 

 1981. 



sphere to the ocean. Consideration was given to rain 

 scavenging of both aerosol and vapor phase n-alkanes, dry 

 depKDsition of aerosol n-alkanes, and direct gas exchange 

 with the ocean of vap>or phase n-alkanes. Estimates of the 

 atmospheric input of n-alkanes to the ocean at Enewetak 

 are given in Table 12. Note that rain scavenging of n- 

 alkanes on particles appears to be the primary method of 

 n-alkane removal from the atmosphere. 



It can be seen from Table 11 that the odd carbon 

 number n-alkanes on aerosols have higher concentrations 

 than the adjacent even carbon number n-alkanes. This is 

 observed for higher n-alkanes up to n-Cae as well (Gago- 

 sian et al., 1980). The odd-to-even carbon preference 

 index and the fact that the major alkanes are n-C27. n-C29, 

 and n-Csi strongly suggest that the source of these heavier 

 n-alkanes present on aerosols is vascular plant waxes, 

 probably of Asian origin (Gagosian et al., 1981a). 



Concentrations of fatty alcohols, fatty acid esters, and 

 fatty acid salts were also measured in the Enewetak atmo- 

 sphere and are presented in Table 13 (Gagcsian et al.. 



