Source Characterization of Polycyclic Aromatic Hydrocarbons by Using Their Molecular Indices: An Overview of Possibilities - Reviews of Environmental Contamination and Toxicology - page 64

2.2 Pyrogenic

Pyrogenic substances are defined as organic substances produced from oxygen-
depleted, high-temperature combustion of fossil fuels and biomass (e.g., incomplete
combustion, pyrolysis, cracking, and destructive distillation) (Saber et al. 2006 ).
Pyrogenic PAHs are released in the form of exhaust and solid residues, and are
largely prevalent in aquatic environments (De Luca et al. 2004 ; Zakaria et al. 2002 ).
Only in a limited number of locations do petrogenic PAHs dominate over pyrogenic
ones (Guo et al. 2007 ; He and Balasubramanian 2010 ; Wickramasinghe et al. 2011 ;
Zakaria et al. 2002 ). The HMW PAHs of pyrolytic origin reach aquatic environ-
ments by direct atmospheric deposition or via contaminated soil (Budzinski et al.
1997 ; Morillo et al. 2008b ). LMW pyrogenic PAHs are mainly introduced to aquatic
environments by rain washout (Ollivon et al. 1999 ).
Hailwood et al. ( 2001 ) list the main industrial processes that produce significant
amounts of PAHs. Power stations may contribute less than 5% to the PAH emissions
of a large city (Masclet et al. 1987 ). It is the mobile PAH emission sources that have
sharply increased in the environment during recent decades (Hwang et al. 2003 ). The
main pyrogenic sources in urban waterways include fuel combustion products, and
discharges from aluminum smelters and manufacturing gas plant (MGP) sites (Stout
et al. 2001b ). Sources such as municipal and industrial waste discharges, and runoff
(e.g., from farms and farmland) contain a mixture of pyrogenic, petrogenic, and
natural PAHs (Van Metre and Mahler 2010 ; Zeng and Vista 1997 ). The distribution
of PAHs from such sources is similar to that in pyrogenic sources (Stout et al. 2001b ).
In pyrogenic PAH patterns, unsubstituted compounds predominate over their
alkylated homologues. As the alkylation level increases, the PAH homologues
become less abundant (i.e., a skewed pattern), whereas the HMW four- to six-ringed
PAHs are more abundant than LMW two- to three-ringed PAHs (e.g., Boll et al.
2008 ; Ou et al. 2004 ; Page et al. 2006 ; Stout 2007 ; Stout et al. 2004 ; Wang et al.
1999a ). Furthermore, the abundance of alkyl PAHs relative to parent PAHs, and also
the abundance of LMW PAHs relative to HMW ones in combustion products,
decrease with increasing combustion temperature (Laflamme and Hites 1978 ;
Sporstol et al. 1983 ; Takada et al. 1990 ; Tobiszewski and Namiesnik 2012 ; Zeng
and Vista 1997 ). Some researchers (Budzinski et al. 1997 ; Sicre et al. 1987 ) have
noted that catacondensed PAHs (wherein no more than two rings have a carbon
atom in common) are abundant in pyrolytic PAHs.
Combusting two petrogenic products (Fig. 4 -kerosene and diesel) in an open
flame does not create a significant amount of important five-ringed PAHs (Douglas
et al. 2007a ). Open-flame combustion significantly alters the distribution of naph-
thalene (N0) and its homologues. Simultaneously, the relative abundance of phen-
anthrene, fluorene, pyrene and their alkylated homologues increases. Combustion in
a closed system (such as diesel engines), however, characteristically creates the
pyrogenic signature of LMW and HMW PAHs and their alkylated homologues
(Douglas et al. 2007a ).
Wang et al. ( 1999b ) found that emissions, residues, and soots of combusted
PAH-containing liquid fuels (e.g., diesel, crude) were likely to inherit the petrogenic