Fine particle toxicity and soot formation

Atmospheric particles can be classified as fine particles or coarse particles. The fine particles are typically formed by chemical processes such as soot formation, whereas the coarse particulates are formed by physical processes such as grinding of soil during road construction. Numerous studies over the past decade, in both the United States and Europe, have concluded that exposure to fine particles increases mortality due to heart and lung disease. Since these studies involve real-world subjects they can’t be completely definitive. However, the results are reproducible and have withstood considerable scrutiny. Furthermore , the magnitude is potentially enormous: the toxicity of fine particles may cause up to 60, 000 deaths per year in the United States which is more than either homicide or traffic accidents (about 15, 000 and 40, 000 annual deaths). Overall, these considerations have been persuasive enough to cause regulatory action. For example, the United States Environmental Protection Agency has adopted a new health effects-based Ambient Quality Standard that limits the maximum allowable ambient concentrations of fine particles. It estimates that meeting these standards will cost more than 30 billion US dollars per year and require the development of the new control technology.

Soot formation is a complex process that involves many chemical and physical steps. They include: 1) the decomposition of the hydrocarbons in the fuel; 2) the formation of small aromatic hydrocarbons from decomposition products; 3) the growth of the small aromatics to the compounds containing larger numbers of rings; 4) the inception of the small soot particles from the large aromatic hydrocarbons and 5) the growth of the small particles to the particles with larger masses. In this review, ‘fuel decomposition’ refers to chemical consumption of the fuel by any chemical process, including either unimolecular or bimolecular reactions. The dividing line between the small and large aromatics is ~ 3 benzenoid rings. We have chosen this value so that the small aromatics are the compounds formed by the addition of the first new benzenoid ring to the aliphatic, single-ring and two-ring hydrocarbons that constitute the bulk of combustion fuels, while the large aromatics are the compounds formed by subsequent growth steps. This division is significant because the addition of the first new ring is typically the rate-controlling step in soot production. Furthermore, the mechanisms of the first ring addition are much more sensitive to the chemical structure of the fuel than subsequent growth steps. All of these steps are important research topics. Indeed too much literature exists on each one to cover them all in a single review. Instead this review will focus on fuel decomposition and small aromatics formation.

Although fuel decomposition and small aromatics formation constitute only a small part of the overall soot formation process, they are frequently the rate-controlling steps and thus they strongly influence the soot concentration in flames.

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Fullerene production	\|	Fine particle toxicity and soot formation

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