Source Characterization of Polycyclic Aromatic Hydrocarbons by Using Their Molecular Indices: An Overview of Possibilities - Reviews of Environmental Contamination and Toxicology - page 105

A probable explanation for the uncertainty of the IP/ ghi is the differential degra-
dation of the two PAHs in the atmosphere. Both isomers degrade at about the same
rate when they are attached to black particles, but IP attached to gray particle
degrades faster than ghi , whereas IP attached to white particles degrades more
slowly than ghi , and IP attached to red particles degrades more variably than ghi
(Behymer and Hites 1988 ). Other studies have also suggested that particle associa-
tion is very important for understanding photodegradation effects on the IP/ ghi ratio
(Tobiszewski and Namiesnik 2012 and references therein).

5.7 Non Isomer Ratios

According to Costa and Sauer ( 2005 ), when the more recalcitrant PAH isomer pairs
(e.g., 4ring/4ring or 5ring/5ring) cannot provide enough resolution in double ratio
plots, then non isomer ratios (e.g., five- over six-ringed PAHs) may be used. It is
also possible to input non isomer ratios into a PCA/PMF model. For such a case,
Christensen and Tomasi ( 2007 ) proposed an external normalization of the ratio (i.e.,
using a reference material).

5.7.1 Ratios of Five- to Six-Ringed PAHs

Some authors (Boitsov et al. 2009 ; Bucheli et al. 2004 ) have found that the BaP/ ghi
ratio is correlated with pyrogenic PAHs, whereas other researchers (Alam et al.
2013 ; Bucheli et al. 2004 ; Jiang et al. 2009 ; Ravindra et al. 2008 ) have investigated
whether traffic sources in soils can be discriminated (e.g., by a BaP/ ghi ratio < 0.6)
from the combustion of coal, wood or oil. Jiang et al. 2009 used both the BaP/ ghi
and the IP/ ghi ratios to identify PAHs from automobile exhausts. Lehndorff and
Schwark ( 2004 ) used the BaP/ ghi ratio together with the FL0/PY0 ratio to investi-
gate biomass burning (indicated by a high ratio). The BaP/ ghi ratio is prone to
photodegradation, as BaP decomposes faster in the atmosphere (e.g., Alves et al.
2009 ). The literature data in Fig. 21 show the low discriminating power of the
BaP/ ghi ratio. For example, the BaP/ghi ratio for traffic sources has intermediate
values that overlap the values of other pyrogenic and petrogenic products.
Nevertheless, the BaP/ ghi ratio may be used in certain situations, for example, when
PAH sources are limited and have BaP/ ghi values in a relatively narrow range (e.g.,
wood combustion, some coals, etc.; see Fig. 21 ).
Another traffic indicator, the ratio of BeP/ ghi , is sometimes studied together with
the IP/ ghi (Alves et al. 2009 ; Tan et al. 2009 ). In a study conducted in Malaysia
(Okuda et al. 2002 ), BeP/ ghi values for diesel vehicles (1.1), gasoline vehicles (0.4),
wood burnings (0.9), smoke hazes (0.3) and coal burnings (2.2) were reported.
Therefore, BeP/ ghi has been used to discriminate traffic sources, specifically gaso-
line, from non-traffic sources such as wood burning (Jensen et al. 1993 ; Larsen and
Baker 2003 ; Okuda et al. 2002 ).