Global distribution of oxygenated polycyclic aromatic hydrocarbons in mineral topsoils

Hazardous oxygenated polycyclic aromatic hydrocarbons (OPAHs) originate from combustion (primary sources) or postemission conversion of polycyclic aromatic hydrocarbons (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-PAH concentrations with the Σ7OPAH/Σ16EPA-PAH concentration ratios (a measure of the variable OPAH sources) could be described by a power function with a negative exponent <1, leveling off at a Σ16EPA-PAH concentration of approximately 400 ng g^–1. We suggest that below this value, secondary sources contributed more to the OPAH burden in soil than above this value, where primary sources dominated the OPAH mixture. This was supported by a negative correlation of the Σ16EPA-PAH concentrations with the contribution of the more readily biologically produced highly polar OPAHs (log octanol-water partition coefficient <3) to the Σ7OPAH concentrations. We identified mean annual precipitation (Spearman ρ =.33, p <.001, n = 143) and clay concentrations (ρ =.55, p <.001, n = 33) as important drivers of the Σ7OPAH/Σ16EPA-PAH concentration ratios. Our results indicate that at low PAH contamination levels, secondary sources contribute considerably and to a variable extent to total OPAH concentrations, whereas at Σ16EPA-PAH contamination levels >400 ng g^–1, there was a nearly constant Σ7OPAH/Σ16EPA-PAH ratio (0.08 ± 0.005 [SE], n = 80) determined by their combustion sources.