Source Characterization of Polycyclic Aromatic Hydrocarbons by Using Their Molecular Indices: An Overview of Possibilities - Reviews of Environmental Contamination and Toxicology - page 61

The rate of petrogenic PAH degradation decreases with time and as the number
of rings increases, i.e., naphthalenes > fluorenes > phenanthrenes ≈ dibenzothio-
phenes ≈ fluoranthenes/pyrenes > chrysenes (e.g., Hegazi and Andersson 2007 ; Page
et al. 1996 ; Short et al. 2007 ). Hence, the relative ratios of phenanthrenes, dibenzo-
thiophenes, fluorenes and naphthalenes remain very consistent as the weathering
percentages increase, which makes them useful for source identification (Hegazi
and Andersson 2007 ). Nevertheless, oil degradation is a very complex process and
although different first-rate kinetic models have been developed, source apportion-
ment of petroleum products is best dealt with case by case (Wang and Fingas 2003 ).
Crude oil contains significant amounts of PAHs: from 0.2% to more than 7%
total PAHs (Zakaria et al. 2002 ). For certain crudes, only a small portion of the total
PAHs are priority pollutant PAHs (Stout et al. 2007 ). The PAHs in crude oils
decrease in concentration as their molecular weight increases (Stout 2007 ; Stout
et al. 2001b ). The HMW PAHs that may be non-detectable, or only present in min-
ute quantities include acenaphthylene (AY), anthracene (A0), fluoranthene (FL0),
pyrene (PY0), benzo[ ghi ]perylene ( ghi ), indeno[ 1 , 2 , 3 - cd ]pyrene (IP), benzo[ k ]lu-
oranthene (BkF), benzo[ b ]luoranthene (BbF), perylene (PER), dibenz[ ah ]anthra-
cene (DA), benzo[ e ]pyrene (BeP), benzo[ a ]pyrene (BaP) (Fig. 2 and Figs. S1-S4,
Supporting Material). Many crude oils are dominated by alkylnaphthalenes (Figs.
S2-S4, Supporting Material). Stout et al. ( 2007 ) observed variable environmental
weathering for some crudes after their release. In most instances, weathering
resulted in the loss of hydrocarbons that had boiling points below that of n-eicosane
(including naphthalene and alkylated naphthalenes). Weathered crude tends to con-
tain more PAHs than does unweathered crude (Stout et al. 2001b ).
Diesel PAHs (Figs. S5-S7 and S9, Supporting Material) are largely composed of
two- and three-ringed PAHs and their alkylated homologues (Douglas et al. 2007a ;
Wang et al. 2001 ). Diesels contain only extremely small amounts of anthracenes
(A0) and HMW PAHs (four to six rings such as indeno[ 1 , 2 , 3 - cd ]pyrene, dibenz[ ah ]
anthracene and benzo[ ghi ]perylene), because most of these are removed during
refining (Wang et al. 2001 ). Of the five target al kylated PAH series of diesel, the
most abundant (>55%) is alkylated naphthalene and the least abundant (<0.02%) is
chrysene; thus, the absence of chrysene can be used to identify diesel or diesel soot
(Wang et al. 1999a , 2001 ). Conversely, the presence of chrysenes or HMW PAHs
possibly excludes diesel or its soot as a main PAH source (Burns et al. 1997 ).
Figures S8 and S10 (Supporting Material) show that naphthalene is abundant in
jet B fuel (99% of total PAHs is naphthalenes), gasoline (97% of EPA priority
PAHs) and diesel No.2 (86% naphthalenes, 5% phenanthrenic content and no chry-
senes). In contrast, in the bunker C diesel mixture, the alkylphenanthrenes account
for 35% of PAHs and chrysenes for 18% (Karlsson and Viklander 2008 ; Wang and
Fingas 2003 ; Wang et al. 1999a ). In the orimulsion oil (Fig. S11, Supporting
Material), the phenanthrenic and dibenzothiophenic content is much higher (38%
and 22% of total PAHs respectively) (Wang and Fingas 2003 ). PAHs having a MW
greater than that of pyrene are hardly present in light distillates such as jet B fuel or
gasoline, although in heavier products such as bunker C diesel and orimulsion oil
they may be present in significant quantities (Figs. S5-S11, Supporting Material).