Global distribution of oxygenated polycyclic aromatic hydrocarbons in mineral topsoils

The hazardous oxygenated polycyclic aromatic hydrocarbons (OPAHs)
originate from combustion (primary sources) or post-emission conversion of
PAHs (secondary sources). We evaluated the global distribution of up to 15
OPAHs in 195 mineral topsoils from 33 study sites (covering 52°N-47°S,
71°W-118°E), to identify indications of primary or secondary sources of
OPAHs. The sums of the (frequently measured 7 and 15) OPAH concentrations
correlated with those of the Σ16EPA-PAHs. The relationship of the
Σ16EPA-PAHs concentrations with the Σ7OPAHs/Σ16EPA-PAHs concentration
ratio (a measure of the variable OPAH sources) could be described by a
power function with a negative exponent <1, leveling off at a
Σ16EPA-PAHs concentration of ca. 400 ng g-1. We suggest that below this
value, secondary sources contributed more to the OPAHs burden in soil than
above, where primary sources dominated the OPAHs mixture. This was
supported by a negative correlation of the Σ16EPA-PAHs concentrations with
the contribution of the more readily biologically produced highly polar
OPAHs (octanol-water partition coefficient, log KOW <3) to the
Σ7OPAHs concentrations. We identified mean annual precipitation
(Spearman-r = 0.33, p <0.001, n = 143) and clay concentrations (r =
0.55, p <0.001, n = 33) as important drivers of the
Σ7OPAHs/Σ16EPA-PAHs concentration ratios. Our results indicate that at low
PAH contamination levels, secondary sources contribute considerably and to
a variable extent to total OPAH concentrations, while at Σ16EPA-PAHs
contamination levels >400 ng g-1, there was a nearly constant ratio
of Σ7OPAHs/Σ16EPA-PAHs (0.08±standard error 0.005, n = 80) determined by
their combustion sources.