Unravelling mixed sources of polycyclic aromatic hydrocarbons (PAH) in urban soils by visual characterization of anthropogenic substrates and coal particles, 71 PAH and alkylated PAH patterns

The identification of polycyclic aromatic hydrocarbon (PAH) sources in heterogeneous urban soils containing pyrogenic and/or petrogenic anthropogenic substrates is a common task for risk assessment. Here, for the first time, the results of source identification using analysis of 71 PAH, alkylated PAH patterns and PAH Alkylation Index were related to visually identified and quantified anthropogenic substrates in 50 soil samples. Only the combination of chemical methods with visual characterization enabled the deeper understanding of varying alkylated PAH patterns used for source apportionment and their superimposition if multiple sources occur. Pyrogenic substrates show homogenic slope-shape PAH patterns despite large visual variety. Petrogenic substrates (bituminous coals), show prevailingly bell-shape patterns but pyrogenic patterns also occur, probably due to residues from industrial processes and/or sorption of other pyrogenic PAH. Superimposition of both PAH patterns within a sample results in intermediate patterns, which are determined by the abundance of substrates and their individual PAH contents. A discrepancy between the share of petrogenic substrates and petrogenic PAH was observed due to low-medium PAH contents from coals/tailings. This may lead to misinterpretations if only chemical source identification methods are applied. With increasing proportion of petrogenic PAH in the mixture, the intermediate V-shape pattern (later bell-shape) appears in lower molecular weight PAH and moves progressively to higher molecular weight PAH. Sigma 71 PAH contents vary from 1.77 to 326.5 mg/kg (median 26.5 mg/kg). Non-EPA PAH measured include highly toxic Sigma 4 dibenzopyrene isomers (0.045-6.23 mg/kg, median 0.79 mg/kg) and 7H-benzo[c]fluorene (0.008-1.57 mg/kg, median 0.12 mg/kg). Most common anthropogenic substrates are bottom ashes, slags, bituminous coals/tailings and coke/coke ash. The PAH Alkylation Index identifies reliably samples dominated by either petrogenic (<0.4) or pyrogenic (>0.9) PAH, independently of the PAH content. Mixed or primarily pyrogenic PAH sources (0.4-0.9) need further investigations, like the presented combination of methods, which enables a reliable source apportionment.