DOI: 10.5061/dryad.sbcc2fr6h
title: Global distribution of oxygenated polycyclic aromatic hydrocarbons in mineral topsoils

description: 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 &lt;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 &lt;3) to the
 Σ7OPAHs concentrations. We identified mean annual precipitation
 (Spearman-r = 0.33, p &lt;0.001, n = 143) and clay concentrations (r =
 0.55, p &lt;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 &gt;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.
description: The concentrations of 16 EPA-PAHs and up to 15 OPAHs were taken
 from the literature as listed in Table 1 or specifically analyzed for this
 study using the method briefly described below. In the various cited
 studies, from which the data originated and in our new sampling in Germany
 and Switzerland, topsoils were sampled in a way representative for the A
 horizon (Argentina, China, Slovakia), at 0-0.05 m depth (Switzerland [two
 sites], Thailand), at 0-0.1 m depth (Brazil, Germany, Uzbekistan,
 Switzerland [one site]), or 0-0.15 m depth (Switzerland [one site]). The
 soil samples were frozen in field-fresh state or air/freeze-dried, sieved
 &lt;2 mm, and stored at ‑20°C before analysis. pH was measured with a
 glass electrode in H<sub>2</sub>O, 0.01 M
 CaCl<sub>2</sub>, or 1 M KCl at a soil:solution ratio of
 1:2.5. Clay concentrations were determined with the pipet method after
 removal of carbonates with HCl, organic matter with
 H<sub>2</sub>O<sub>2</sub>, and Fe oxides with
 dithionite-citrate-hydrogencarbonate, and wet sieving &lt;50 μm. An
 aliquot of each soil sample was milled and the total carbon (TC) and total
 nitrogen (TN) concentrations were determined with an elemental analyzer
 (vario EL cube, Elementar Analysensysteme GmbH, Langenselbold, Germany).
 The inorganic C (IC) concentration of each sample was also determined from
 an aliquot of soil after combusting soil organic carbon
 (C<sub>org</sub>) in a muffle oven (550°C, 2 h). The
 C<sub>org</sub> concentration was quantified as the difference
 between the TC and IC concentrations. All available soil properties are
 summarized in Table S2. Although our data set did not cover all
 continents, we consider it as global because it covers a wide range of
 latitudes and longitudes (Fig. S1). At all study sites,
 at least the concentrations of the 16 EPA-PAHs, which we used here, and of
 up to 15 OPAHs (see Table S1) in topsoils, European Reference Material
 (ERM-CC013a), and procedural blanks (inert sorbent, Isolute HM-N, Biotage,
 Sweden or diatomaceous earth, Dionex, Sunnyvale, CA, USA) were determined
 using previously published methods (Bandowe & Wilcke, 2010;
 Bandowe et al., 2010; Lundstedt et al., 2014). In brief, soils (10-20 g)
 were mixed with Isolute HM-N or diatomaceous earth and transferred into
 33-mL accelerated solvent extractor (ASE) cells. Each sample was spiked
 with a mixture of deuterated PAHs (six to 11 compounds) and two deuterated
 OPAHs. Each sample was extracted twice by pressurized liquid extraction
 with an ASE (ASE 200, Dionex, Sunnvale, CA, USA). Dichloromethane was used
 for the first extraction followed by
 acetone:dichloromethane:trifluoroacetic acid (1%) [250:125:1 v:v:v] for
 the second extraction. An acidified solvent mixture was used in the second
 step to improv the extraction of hydroxyl-PAHs and carboxyl-PAHs which
 were part of the target compound list in some of the previous studies
 (Bandowe & Wilcke, 2010; Bandowe et al., 2010; Bandowe et al.,
 2011), but were not considered here. The instrumental conditions for the
 ASE were the same as in Bandowe & Wilcke (2010), were more details
 of the used method are explained Extracts from each
 sample were combined, passed through
 Na<sub>2</sub>SO<sub>4</sub>, spiked with hexane
 and rotary evaporated until &lt;1 mL remained. Extracts were
 transferred to 3 g silica gel (10% deactivated) in an 8-mL glass column.
 Each sample was eluted with (a) hexane:dichloromethane (5:1 v:v) and (b)
 dichloromethane followed by acetone. Fractions a and b, which contained
 PAHs and OPAHs, respectively, were collected in separate flasks. Each
 flask was spiked with drops of toluene, and then rotary evaporated to
 &lt;1 mL before being transferred to 2-mL vials to determine the
 concentrations of target PACs. PAHs and OPAHs were determined in two
 different runs with a gas chromatograph-mass spectrometer (Agilent 7890 A
 GC coupled to Agilent 5975 C mass spectrometer, Agilent, Santa Clara, CA,
 U.S.A.). The GC-MS was operated in the electron ionization mode with
 selected ion monitoring of target PACs. As a check of the accuracy of our
 analytical procedure, we simultaneously analyzed aliquots of the European
 Reference Material ERM-CCO13a (Polycyclic Aromatic Hydrocarbons in Soil)
 from the Federal Institute of Materials Research and Testing (BAM),
 Berlin, Germany. Procedural blanks (inert bulk sorbent: Isolute HM-N or
 diatomaceous earth) were also extracted and analyzed with the same methods
 as the samples and reference materials. Concentrations of target compounds
 were determined by the internal standard procedure. The average mass of
 target compounds in blanks was deducted from that in the samples before
 calculating the final concentrations per dry mass of extracted soil.
 Further details of quality control procedures are specified in previous
 papers (Bandowe & Wilcke, 2010; Bandowe et al., 2010; 2019;
 Lundstedt et al., 2014). Table S1 lists the names and abbreviations of all
 analyzed compounds. Results of our quality control procedures are reported
 in the Supporting Information and Table S2 reports the OPAH and PAH
 concentrations in all considered individual soil samples along with
 important soil properties. The sum of the
 concentrations of 16 PAHs defined by the U.S. Environmental Protection
 Agency (EPA) as priority pollutants is Ʃ16EPA-PAHs and the sums of the
 concentrations of the seven OPAHs (i.e., 1-indanone, 1,4-naphthoquinone,
 1-naphthaldehyde, 2-biphenyldicarboxaldehyde, 9-fluorenone,
 1,2-acenaphthenequinone, and 9,10-anthracenedione), which have been
 determined in all samples, and 15 OPAHs (Table S1) are ∑7OPAHs and
 ∑15OPAHs, respectively. To roughly convert all pH values measured in
 H<sub>2</sub>O or 0.01 M CaCl<sub>2</sub> to the
 pH in 1 M KCl, we lowered the pH (H<sub>2</sub>O) by 1 unit
 and the pH (0.01 M CaCl<sub>2</sub>) by 0.5 units accounting
 for the different ionic strength of the various commonly used equilibrium
 solutions. Because the data were usually not normally distributed and
 could not be brought to normal distribution by square root or ln(x+1)
 transformations, we used the nonparametric Spearman correlation analysis
 in the software package STATISTICA (Statsoft Inc., Tulsa, OK, U.S.A.).
 Octanol water partition coefficients (K<sub>OW</sub>) for
 OPAHs (Table S1) were estimated according to the US EPA KOWWINTM module of
 Estimation Programs Interface Suite™ for Microsoft® Windows, v 4.11. U.S.
 EPA, Washington, DC, USA.
 https://www.epa.gov/tsca-screening-tools/epi-suitetm-estimation-program-interface, verified on 04/03/2019.
description: If cells are empty, data is not available.