Source Characterization of Polycyclic Aromatic Hydrocarbons by Using Their Molecular Indices: An Overview of Possibilities - Reviews of Environmental Contamination and Toxicology - page 99

et al. 2008 ; Page et al. 1999 ; Stout 2007 ; Stout et al. 2006 ; Wang et al. 1999a ). Page
et al. ( 1996 , 1999 ) applied a two-component mixing model that used both D n /P n
ratios and apportioned petroleum and background sources. However, this model can-
not discriminate a source contribution of less than 3-8%. Similarly, Stout et al. ( 2006 )
apportioned different historic oil inputs (diesels of different blends) in non-aqueous
phase liquids. Douglas et al. ( 2007b ) used the D n /P n ratios in combination with the
Nordtest method, to rule out spilled fuel oils in tarballs and mousse samples.
The success of the D n /P n ratios as diagnostic tools for heavy fuel oils that con-
tain LMW or HMW PAHs is based on the fact that these ratios are not a strong
function of TPAH (Douglas et al. 2007b ). However, if all sources have the same
sulfur content, or their upper boiling point (bp) is below those of D2, D3, P2, P3
(e.g., kerosene), then the D n /P n ratios are not applicable (Stout et al. 2006 ).
Both D n /P n ratios, especially the trimethyl one, are not significantly affected by
heavy (bio) degradation-up to 98% depletion of total PAHs-for a range of petro-
genic products (Douglas et al. 1996 ; Uhler and Emsbo-Mattingly 2006 ). Wang et al.
( 1999b ) noted that both combustion and heavy degradation decrease the D n /P n .
Natural weathering, long-term biodegradation and photo-oxidation will increase the
D n /P n ratios, but evaporation is thought to slightly decrease them, rendering them
inapplicable for source characterization (Hegazi and Andersson 2007 ; Kim et al.
2008 ; Page et al. 1999 ; Stout et al. 2006 ; Uhler and Emsbo-Mattingly 2006 ; Wang
et al. 1999a , b ).
Costa et al. ( 2004 ) used a version of the D n /P n ratio slightly modified by the
addition of the respective anthracenic homologues to the denominator to distinguish
between background and petroleum contamination. The D3/PA3 and D2/PA2
(alkyldibenzothiophenes to the sum of alkylphenanthrenes and alkylanthracenes, all
with the same alkylation degree) are slightly affected by weathering and biodegra-
dation of coal tars in the laboratory, although in field samples, degradation can sig-
nificantly alter such ratios (Douglas et al. 2007a ; Uhler and Emsbo-Mattingly 2006 ).

5.5.2 Other Dibenzothiophenic Ratios

The D3/P3 ratio in conjunction with the D n /C n ( n = 2, 3) ratios has been used to
describe oil depletion and identify sources in subtidal sediment data (Hegazi and
Andersson 2007 ; Stout et al. 2002 ; Wang and Fingas 2003 ; Wang et al. 1999a ).
Furthermore, the double ratio plot of P2/P3 versus the D2/D3 has been used as a
weathering index for oil residues from the Exxon Valdez oil spill (Hegazi and
Andersson 2007 and references therein).

5.5.3 Alkyldibenzothiophene and Benzonaphthothiophene Distributions

The distribution of methyldibenzothiophenes (1-D1, 2-D1, 3-D1, 4-D1) is charac-
teristic for particular crudes, and the methyldibenzothiophene ratios exhibit excel-
lent consistency with weathering (so they can be used as weathering ratios), even for