Source Characterization of Polycyclic Aromatic Hydrocarbons by Using Their Molecular Indices: An Overview of Possibilities - Reviews of Environmental Contamination and Toxicology - page 114

6.1 What Are the Most Important PAH Sources in the Aquatic

Environment and Which PAH Indicators Can Be Used
to Unequivocally Identify Them?

Thermal, source and degradation PAH indicators produce fingerprint physicochem-
ical gradients that are developed during pyrogenic and petrogenic PAH formation,
transport, and deposition. One of the most widely applicable and robust approaches
to PAH source characterization is to apply different discriminant analysis tech-
niques (e.g., ratios, PMF) to the PAH pattern of parent versus alkylated (e.g.,
reflected in PI, PPI), stable versus unstable isomer, and HMW versus LMW forms.
Mathematically, it is not possible to discriminate more sources (times their tem-
poral, climatic and geographical variations) than the known variables (PAH ana-
lytes). Therefore, to limit the number of possible suspected sources, knowledge of
current and historic records of contamination is helpful. PAHs are classified accord-
ing to their origin or temperature of formation into three classes: pyrogenic, petro-
genic, and biogenic/natural. The multitude of PAH sources in the aquatic
environment, their variability and the differential fate of PAHs have resulted in a
plethora of PAH indicators for the purposes of PAH source assessment.
Each class of indicator is based upon a physicochemical aspect of the contami-
nant (source, degradation, and thermodynamic) and has been designated to solve a
particular problem, but also indicates an existing gradient in the PAH chemistry. For
example, the sulfur gradient (dibenzothiophenes) was initially applied for petroleum
sources, but it can also be used to discriminate pyrogenic ones. There are extensive
databases for PAHs and their sources. Table 4 gives an overview of the PAH indices
used to characterize or apportion the pyrogenic or petrogenic nature of a sample.
Of special importance is the spatial extent of the PAH analysis, which is ascer-
tained by distinguishing between point and non-point sources and measuring back-
ground concentrations. Background PAH concentrations always make it necessary
to proceed with caution when using PAH ratios, because some degraded PAH source
fingerprints (e.g., coal tar, creosote, etc.) are similar to urban background finger-
prints. PAH indices such as N0/F0, C2/C0, FP0/(C0 + BaA + BaP + BjF + ghi + IP + D
A) have been used to discriminate different sources that contribute to urban back-
ground. If urban background concentrations are high, it is possible to break down
the composite urban background into individual sources by examining the inventory
of sources contributing to it (e.g., asphalt, automobile soot, etc.).
PAH ratios are a fast and simple means to overview and understand the origin of
pollution, particularly when coupled with a discriminant analysis approach. In gen-
eral, two- to three-ringed PAHs (including alkylated) are good for distinguishing
petrogenic contamination, whereas four- to six-ringed PAHs are appropriate for dis-
criminating between pyrogenic sources. One of the most successful approaches for
deducing the pyrogenic or petrogenic character of sedimentary contamination is the
apportionment of PAHs to sources (e.g., the PI). The PPI and the FL0/PY0 ratio are
robust and simple to use.
When petrogenic contamination is suspected, chrysenes, PAHs that are lighter
than C0, and definitely alkylated PAHs are useful. For example, the D n /P n ratios are