or adsorption onto the smallest particles is proposed to account for the trace element concentration dependence on particle size. The environmental significance of the results is discussed.

The toxic trace elements arsenic, antimony, cadmium, lead, selenium, and thallium were found to be most concentrated in the smallest respirable particles emitted from coal-fired power plants. These elements, or their compounds, are probably volatilized during combustion and preferentially adsorb or condense onto the small particles which can most easily pass through conventional control equipment.

A number of minor and trace elements including Be, C, Ca, Cr, K, Li, Mn, Na, P, Pb, S, Tl, V, and Zn present in coal fly ash are found to be preferentially concentrated on the particle surfaces. Environmentally effective concentrations of these elements are thus much higher than indicated by conventional bulk analyses.

of surface-associated material, which is often highly characteristic of particles derived from a specific source. Thus, automobile exhaust particles have been shown to contain the elements Br, Cl, Cr, Mn, Ni, P, Pb, and T1 on their outer surface (25, 27).In terms of the actual results obtained, the most important conclusions are that lead derived from automobile exhaust particulates can contribute significantly to the total lead present in soils and dusts at a considerable distance from a roadway. Otherwise, it would appear that roadway dusts themselves are likely to contain lead that is almost exclusively derived from automobile exhausts.

Five polycyclic aromatic hydrocarbons (benzo[a]pyrene, pyrene, anthracene, fluoranthene, and phenanthrene) have been adsorbed, from the vapor phase, onto coal fly ash, and the susceptibility of each of the adsorbed compounds to photodecomposition has been studied. Benzo[a]pyrene, pyrene, and anthracene all photolyze efficiently in liquid solution but are highly resistant to photodegradation when adsorbed on fly ash. However, benzo[a]pyrene and anthracene photo-1094 Environmental Science & Technology

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Polycyclic aromatic hydrocarbons adsorbed onto coal fly ash were found to be stabilized against photochemical decomposition. However, a number of adsorbed polycyclic aromatic hydrocarbons will spontaneously oxidize in the absence of light, with those compounds containing a benzylic carbon being particularly susceptible. The decomposition rate appears to be fly ash-dependent.