Photochemical transformation of polycyclic aromatic compounds sorbed on coal stack ash in the absence and presence of coadsorbates

The combustion of coal and other fossil fuels releases into the atmosphere particles and potentially toxic organic compounds. A particular class of these compounds, polycyclic aromatic hydrocarbons and their derivatives, is known to be associated with combustion-generated particles in the environment. This association may prolong the atmospheric lifetimes of these compounds, may facilitate their transport to remote environments, and provides an avenue by which humans may be exposed to them. The work described herein attempts to better understand the nature and the consequences of the interactions of PAH and related compounds with atmospheric particles.

A typical coal stack ash was fractionated according to particle size, and then according to chemical composition, specifically into fractions enriched in ferromagnetic, carbonaceous, and mineral content. 1-Nitropyrene, a representative polycyclic aromatic compound, was deposited from the vapor phase onto these adsorbents and onto a model adsorbent, then exposed to UV/visible light. Its photochemical behavior was compared to that of its parent, pyrene. It was found that 1-nitropyrene is more strongly associated with the carbonaceous component of coal stack ash and is quite resistant to photodegradation on that fraction and on the unfractionated stack ash. It was concluded that nitro substituted polycyclic aromatic compounds are unlikely to be photolyzed on atmospheric fly ash.

The photochemical behavior of pyrene in the presence of coadsorbates was then compared to the photochemical behavior of sorbed pyrene in the absence of coadsorbates. Pyrene was deposited from the vapor phase onto model and coal stack ash adsorbents containing a coadsorbate, succinic acid. In other studies, n-decane was deposited onto adsorbents containing pyrene. It was found that the photolysis of pyrene was enhanced when the coadsorbate was deposited first, and inhibited when pyrene was deposited first. Observed behaviors were tentatively attributed to the ability of a coadsorbate to shield from, or to facilitate the exposure of sorbed pyrene to, incident light.