140 TRANS UR AM C ELEMENTS IN THE ENVIRONMENT 



soil rather than rock is the precursor to the crustal aerosol. Some 93% of the earth's 

 continental surface is covered by soils (Kothny, 1973). Many of these soils are in states of 

 loose aggregation which can easily be made airborne by the wind. Chemically, however, 

 Rahn (1976) has found that the composition of the crustal aerosol is not unambiguously 

 that of soil. Elements in natural aerosols with rock-like enrichment factors include silicon, 

 iron, calcium, potassium, and chromium; those with soil-like enrichment factors are 

 titanium and barium. One would expect natural aerosols to be, hke soil, depleted in the 

 more-soluble elements. Except for glacial activity and to a lesser extent in deserts, 

 physical weathering processes, which ultimately produce small particles from boulders, 

 are very slow and are accompanied at all stages by intense chemical weathering. Thus 

 large masses of physically pulverized rock which have not been chemically weathered are 

 not available for aerosol production. 



Rahn (1976) speculates that remote continental aerosols are never as depleted in the 

 soluble elements (e.g., sodium, potassium, calcium, and magnesium) as they should be 

 relative to rock (if natural aerosols were purely soil derived) because of the presence of 

 small amounts of marine aerosol. Soluble elements, especially sodium and magnesium, are 

 abundant in the marine aerosol; thus only small amounts of this aerosol in remote 

 continental areas would noticeably raise the proportions of soluble elements in an aerosol 

 collected there. 



In addition to the similarity in the elemental composition of aerosol and crustal rock, 

 available analytical data are much less numerous and less reliable for soils, especially for 

 several interesting trace elements that are enriched in aerosols. 



For these reasons the majority of investigators who calculate aerosol-crust enrichment 

 factors have chosen one of the several available tables of elemental abundances in average 

 crustal rock. Because the composition of plutonium-bearing particles are compared with 

 data reported by Rahn (1976), the same crustal-rock composition used by him [that 

 reported by Mason (1966)] was selected as the source material composition for this 

 work. Column 2 of Table A.l gives the elemental concentrations in globally averaged 

 crustal rock for those elements found in plutonium-bearing particles. 



Reference Elemen t 



Of the various elements that seem to be reliably crust derived in aerosols, akuiiinum, 

 silicon, and iron are generally considered to be the most suitable reference elements. 

 (When sea salt is the source material, the nearly universal choice is sodium.) An 

 acceptable crustal reference element should have higli concentrations in rock and soil, 

 very low pollution potential, ease of detemiination by a number of analytical techniques, 

 and tYeedom from contamination during sampling. Iron has markedly higher pollution 

 potential than aluminum and so is less suited tor use with urban or rural aerosols. Silicon 

 is probably the most unambiguous elemental indicator of crustal material. Unfortunately, 

 silicon has been detennined in so few aerosol samples that it cann(.)t be used as the 

 reference element where comparisons are to be made. Aluminum is a major element 

 (81,300 ppm in rock), well determined by a variety of analytical techniques, and has a 

 minimum of specific pollution sources. 



Thus f\M- this work enrichment factors for element X in most particles were calculated 

 using 



(X/AI)rock 



