Fire condensates and charcoals: Chemical composition and fuel source identification

Organic Geochemistry 130 (2019) 43–50

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Fire condensates and charcoals: Chemical composition and fuel source identification Arne Kappenberg a,⇑, Melanie Braun a, Wulf Amelung a, Eva Lehndorff a,b a b

Institute of Crop Science and Resource Conservation – Soil Science and Soil Ecology, University of Bonn, Nussallee 13, 53115 Bonn, Germany Soil Ecology, Dr.-Hans-Frisch-Str. 1-3, University of Bayreuth, 95448 Bayreuth, Germany

a r t i c l e

i n f o

Article history: Received 14 May 2018 Received in revised form 11 December 2018 Accepted 17 January 2019 Available online 19 January 2019 Keywords: Fire residues Pyrogenic carbon Polycyclic aromatic hydrocarbons Dimethylphenanthrenes

a b s t r a c t Fire condensates from vegetation and household burnings occur ubiquitously in the environment. Until now, however, it was difficult to estimate their origin and any relation to pyrogenic carbon deposited as charcoal. Our aim here was: (i) to differentiate the chemical composition of fire condensates from charcoal particles, and (ii) to relate this to fuel origins from grass, softwood or hardwood. We analysed d13C and d15N isotope composition, lignin-derived phenols, benzene polycarboxylic acids (BPCA) and polycyclic aromatic hydrocarbons (PAH), in lab-produced charcoals and condensates at combustion temperatures of 300, 350, 400, 450, 500 and 600 °C. We found that the BPCA and PAH composition of condensates differed significantly from that of charcoals. Condensates exhibited larger portions of benzene penta- to hexacarboxylic acids (B5CA to B6CA), phenanthrene (p < 0.01) and four-ring PAH (fluoranthene, pyrene, chrysene and benz[a]anthracene, p < 0.01). PAH ratios of indeno[1,2,3-cd]pyrene to benzo [ghi]perylene (IP/(IP + B[ghi]P) and fluoranthene to pyrene (Flua/(Flua + Py) were diagnostic for condensates, but independent from fuel type. Composition of the 1,2-, 1,7- and 2,6|3,5 dimethylphenanthrenes (DMP) was fuel specific, with the ratio of the (1,7 + 2,6|3,5)/(1,2 + 1,7 + 2,6|3,5) isomers separating hardwood (0.2–0.6), from grass (0.6–0.9), and softwood (<0.9), thus enabling the identification of both condensate and charcoal fuel sources. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Pyrogenic organic matter (PyOM) is known to affect the environment as part of the stable carbon pool (Brodowski et al., 2007; Czimczik and Masiello, 2007; Santín et al., 2016), as fertilizer supplement (e.g., Steiner et al., 2007; Jeffery et al., 2011; Schulz et al., 2013) and as an adsorbant of pollutants and elements (Braida et al., 2003; Kookana et al., 2011), while, when released into air, it may impair air quality, climate, and public health (Mauderly and Chow, 2008; Pöschl and Shiraiwa, 2015). Furthermore, PyOM is ubiquitously found in the environment (e.g. Goldberg, 1985; Crutzen and Andreae, 1990; Jaffé et al., 2013) and stable against degradation (Goldberg, 1985; Glaser et al., 2000; Brodowski et al., 2007), affecting soil C storage both positively (Schmidt and Noack, 2000; Rodionov et al., 2010) and possibly also negatively (Wardle et al., 2004). Until now, however, analyses of PyOM mainly concentrated on rather undefined mixtures of fire condensates and charcoal particles (e.g. Oros and

⇑ Corresponding author. E-mail address: [email protected] (A. Kappenberg). https://doi.org/10.1016/j.orggeochem.2019.01.009 0146-6380/Ó 2019 Elsevier Ltd. All rights reserved.

Simoneit, 2001a,b; Simoneit, 2002; Théry-Parisot et al., 2010), but hardly addressed their individual source assignment. Charcoals are formed by incomplete combustion and often retain the physical structure of the parent fuel (Goldberg, 1985; Hedges et al., 2000), while condensates are formed from the gas phase and consist of solid and liquid nano- and micro-particles (Schmidt et al., 2004; Keiluweit et al., 2010). The condensates usually exhibit small particle sizes (1–100 mm) so they may be transported over global distances in the atmosphere (e.g. Bin Abas and Simoneit, 1996; Walter et al., 2016; Diapouli et al., 2014). For instance, condensates produced in Asia can be transported to locations as far as North America, and substantially contribute to the local level of air pollution (Cooper et al., 2015; Lin et al., 2012; Verstraeten et al., 2015). For charcoal, the deposition of a large portion usually takes place in the direct vicinity of the fire, even though erosion may significantly contribute to its dispersal (Czimczik and Masiello, 2007; Ravindra et al., 2008; Abney and Berhe, 2018). Fossil C sources can be quantified based on the absence of radiocarbon (e.g., Currie et al., 2002; Rethemeyer et al., 2005; Lehndorff et al., 2015a), but this method is expensive, and thus less suited for routine analyses and it fails to distinguish fire remains from differ-

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ent plant origins. To achieve the latter goal, different geochemical markers have been tested for the determination and characterization of PyOM (e.g. Simoneit, 2002; Hammes et al., 2007; McBeath et al., 2011), such as the analyses of polycyclic aromatic hydrocarbons (PAH) (e.g. Yunker et al., 2002, 2015; Lima et al., 2005) and of pyrogenic carbon (PyC) via the so-called benzenepolycarboxylic acid (BPCA) method (e.g. Lehndorff et al., 2015b; Wiedemeier et al., 2015). Environmental PAH typically are low molecular aromatic compounds comprising 2–7 aromatic rings, which occur in free forms and are directly extractable. In contrast, the BPCA method quantifies polycyclic PyC structures that are produced upon hot acid digestion from structures with more than 3 aromatic rings (Ziolkowski et al., 2011; Wiedemeier et al., 2015). However, both low and high molecular aromatic compounds are part of charcoals and condensates, rendering their source assignment on the basis of these methods, difficult. Nevertheless, it has been possible to deduce the chemical composition of rather undefined mixtures of PyOM from the BPCA patterns (e.g. Schneider et al., 2010; Wolf et al., 2013; Wiedemeier et al., 2015). These studies assigned the larger degree of aromatic condensation to larger shares of benzene with 6 carboxyl groups (B6CA) (Schneider et al., 2010, 2013; Wolf et al., 2013), but this method was unable to reconstruct fuel origin. To succeed with the latter, individual PAH and diagnostic PAH ratios are likely more powerful tracers for different sources of PyOM (e.g. Oros and Simoneit, 2001a,b; Lima et al., 2005; Kado et al., 2005), also when deposited in soil and sediments (Bandowe et al., 2014; Yunker et al., 2015; Khan et al., 2015). Wiedemeier et al. (2015) thus used both PAH and BPCA to characterize PyOM from rye and maize charcoal. They found that the combination of both methods can lead to a better source apportionment of PyOM in soils and sediments and illuminate the pathways and fluxes of differently sized PyOM in the environment. We used both methods here for source assignment of fire condensates. Additionally, we analysed the stable C and N isotope composition and lignin-derived phenols as alternative markers to specify sources of plant remains in charcoals (e.g. Krull et al., 2003; Schmidt et al., 2004; Pyle et al., 2015). With this study we aimed at: (i) differentiating charcoals from fire condensates and (ii) identifying their sources by utilising the analyses of stable isotopes, lignin, BPCA and PAH composition. We produced charcoals and condensates from apple wood (Malus domestica), beech wood (Fagus sylvatica), pine wood (Pinus sylvestris), larch wood (Larix decidua), emmer grass (Triticum dicoccum), and einkorn grass (Triticum monococcum) under controlled laboratory conditions. The robustness of fuel, condensate and charcoal specific parameters against variation in combustion temperature was tested by employing a thermosequence of charcoals and condensates.

2. Materials and methods 2.1. Charcoal and condensates production We produced charcoals and condensates from apple, beech, pine and larch wood and from emmer and einkorn grass (Wolf et al., 2013). Samples were dried at 40 °C and cut to about 10 cm length. Charcoals and condensates were produced at 300, 350, 400, 450, 500 and 600 °C (10 °C/min, heating duration 3 h) using a split tube furnace with external control (Carbolite, UberstadtWeiher, Germany), equipped with a constant synthetic air flow (5.0 hydrocarbon free, purity 99.999%; 20.5% O2 in N2) of 60 ml/min. Charcoal residues were collected after passive cooling to ca. 50 °C. Condensates were trapped by an impinger filled with distilled water. In addition, the condensate at the furnace tube (glass) was collected by rinsing with dichloromethane. Both

condensates were combined to one sample and dried for two weeks in an oven at 40 °C to almost dryness. All sample preparation was done in duplicate, and each replicate was analysed twice with the methods outlined below. 2.2. Elemental analysis The total content of organic carbon (OC) was determined by an elemental analyzer (Vario MicroCube, Elementar, Langenselbold, Germany, DIN ISO10694). The measurement precision (standard error of the mean) of the standard (ISE 918, WEPAL, Wageningen, Netherlands) was <0.02%. All samples were analysed in duplicate with a precision of <1% of the mean. 2.3. d15N and d13C analyses Stable carbon and nitrogen isotope analysis was performed using an elemental analyzer coupled to an isotope ratio mass spectrometer (Flash EA 1112, coupled to a DeltaV Advantage, ThermoFisher Scientific, Bremen, Germany). Stable carbon isotope composition was reported against Vienna Pee Dee Belemnite (VPDB; International Atomic Energy Agency, Vienna, Austria). Stable N isotope composition was calibrated against atmospheric d15N. Additional calibration of both isotope ratios was performed using acetanilide, cellulose, sucrose and urea and yielded a measurement precision of ±0.02‰ (standard error of the mean). All samples were analysed in duplicate with a precision of <0.5% of the mean. 2.4. Lignin-derived phenols Lignin was tested in charcoals and condensates, as it is known not to be stable at high temperatures. Hence, this method allowed us to verify the general view that the condensates produced here consisted predominantly of newly condensed carbon forms. Lignin analysis was carried out for charcoals and condensates after alkaline CuO oxidation according to the method developed by Hedges and Ertel (1982), as modified by Kögel (1986), and Amelung et al. (1997). We quantified eight phenolic oxidation products (vanillin, acetovanillone, vanillic acid (R = V-lignin), syringaldehyde, acetosyringone, syringic acid (R = S-lignin), p-coumaric acid, ferulic acid (R = C-lignin)). All samples were analysed with a gas chromatograph (GC 6890, Agilent, Böblingen, Germany) equipped with a flame ionisation detector and an Optima 5 capillary column (30 m  0.32 mm i.d., 0.25 mm film thickness, Macherey-Nagel, Düren, Germany). The GC oven program was: 100 °C for 3 min, increasing to 130 °C at 10 °C/min, kept constant for 15 min, increased at 1 °C/min to 170 °C, and at 15 °C/min, to 205 °C, kept constant for 5 min, increased at 50 °C/min to 300 °C, and kept constant for 5 min. Ethylvanillin was added as an internal standard (surrogate standard) for the quantification of phenols. Derivatisation was done with N,O-Bis(trimethylsilyl)trifluoroacetamide (BSTFA) and pyridine (2:1 v/v). Duplicate analyses (whole procedure) had a deviation <12%. Phenyl-acetic acid was added as second standard for quantification of the recovery of the first internal standard. Recovery was always >74%. 2.5. Pyrogenic carbon analysis (via benzene polycarboxylic acids, BPCA) All samples were prepared according to Glaser et al. (1998) and Brodowski et al. (2005). The threshold of 5 mg OC sample concentration was strictly maintained and PyC quantity was based on the yields of BPCAs with five to six carboxyl groups (benzene pentacarboxylic acid and mellitic acid) (Kappenberg et al., 2016). Samples were analysed with a gas chromatograph (GC 6890, Agilent,

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Böblingen, Germany), equipped with a flame ionisation detector (FID) and an Optima 5 capillary column (30 m  0.32 mm i.d., 0.25 mm film thickness, Macherey-Nagel, Düren, Germany). Precision from duplicate analysis of real replicates was <11% and recovery of the internal standard (citric acid) was always >80%. 2.6. PAH analysis Lipid extraction was done via accelerated solvent extraction (ASE 350, Dionex; 120 °C, heating time 6 min, static time, 20 min, 3 extraction cycles) using dichloromethane:methanol (9:1, v/v). Extracts were spiked with 10 ng/m [2H10]phenanthrene as a surrogate standard. Extracts were purified via solid phase extraction (SPE; 2 g deactivated silica gel 0.063–0.2 mm, Macherey-Nagel, Düren, Germany). Filled SPE columns were conditioned with n-hexane. The extract was consecutively eluted with 4.5 ml n-hexane and 4 ml dichloromethane:n-hexane (2:1, v/v; the latter fraction yielding aromatic compounds). Samples were analysed using a gas chromatograph equipped with a mass selective detector (GC 6890 with 5973 MSD, Agilent, Böblingen, Germany) and an Optima 5 MS column (30 m  0.32 mm i.d., 0.25 mm film thickness, Macherey-Nagel, Düren, Germany). Helium (He) was used as carrier gas at constant flow (1.2 ml/min). The oven temperature program was: 70 °C (held 2 min), to 225 °C at 10 °C/min, then heated to 325 °C at 7 °C/min. The routine limit of quantification was 0.01 mg/ml. Precision from duplicate analysis of real replicates was <10% and recovery was always >81%. 2.7. Statistics Statistical data distribution was tested via Shapiro–Wilk test (for all tests a = 0.05 was selected). If the data set was normally distributed and showed an equal variance, a t-test was applied. In the case of normal distribution and no equal variance, a Welch test was conducted. If data sets showed non-normal distribution, a Mann Whitney Rank Sum test was applied for comparison (Toutenburg, 2002). Statistical calculations used SPSS 22 (IBM). 3. Results and discussion The stable C isotope composition of charcoals has been recommended to be an indicator for paleoecological conditions due to its stability against thermal alteration (Pyle et al., 2015). Furthermore, Pyle et al., (2015) found that 15N was enriched in soils after fire events. Hatton et al. (2016) found that the d15N and d13C values in lab produced charcoals (at 200, 300, 450 and 600 °C) from Pinus banksania and Acer rubrum remained stable during combustion, and thus maintained their fuel-specific isotope composition. Also, our results did not provide hints on systematic significant isotope fractionation during the combustion process for either d15N or d13C. However, these analyses did not help to either differentiate the produced charcoals from the respective condensates or to differentiate fuel sources from hardwood, softwood and straw (Table A.1 and A.2; Supplementary Material).

ing the general view that they consist predominantly of newly condensed carbon forms. Lignin structures could not be detected for combustion residues produced at temperatures >300 °C. However, aromatic structures (e.g. BPCA and PAH) could be detected (Table A.1 and A.2, Supplementary Material), agreeing with observations by Keiluweit et al. (2010) and Bird and Ascough (2012). The PyC contents in charcoals ranged from 37.8 to 65.1 g PyC per kg OC (Table A.1, Supplementary Material); in condensates these PyC contents were larger (13.7–140.8 g PyC per kg OC; Table A.2). However, there was no increase in PyC concentration parallel to increasing combustion temperature, supporting findings from Schneider et al. (2013) and Wolf et al. (2013). Nevertheless, its composition altered towards a higher degree of condensation of aromatic structures (McBeath et al., 2011; Schneider et al., 2013; Wolf et al., 2013), e.g., the relative contributions of the B6CA molecular marker strongly increased with charring temperature, as reflected by increasing B5CA/B6CA ratios with temperature (Fig. 1, see also Schneider et al., 2010, 2013; Wolf et al., 2013). In these studies a reconstruction of mean fire temperature via B5CA/B6CA ratios of charcoal could be achieved, despite the temperature variation of natural fires (Schneider et al., 2013; Wolf et al., 2014; Lehndorff et al., 2015b). Notably, this temperature-dependent composition of PyC differed significantly between charcoals (B5CA/B6CA ratio = 0.81– 1.83) and condensates (B5CA/B6CA ratio = 2.43–4.76; p < 0.01; Fig. 1). The respective regression lines ran almost parallel: for both, condensates and charcoals, an increase in combustion temperature by 100 °C upon charring resulted in a decline of the B5CA/B6CA ratio by 0.25 (R2 = 0.89; p < 0.01; Fig. 1). Hence, temperature reconstruction potentially also works for condensates. Due to their B5CA/B6CA ratio being systematically different from charcoals, however, our findings imply that a temperature assignment of fire residues will be erratic if the contribution from condensates to total PyC is unknown. Here we focused on fire condensate production from simulated vegetation burnings. These condensates might be different from those from fossil fuel combustion. For example, the composition of PyC in diesel particulate matter collected at the exhaust pipe of a diesel vehicle was characterized by a low B5CA/B6CA ratio around 0.2 (Roth et al., 2012), which was interpreted as the result of highly condensed, large polyaromatic soot molecules (Currie

Condensate

3.1. Differentiation of fire condensates from charcoals There may be residual organic matter in charcoal (Schmidt and Noack, 2000; Masiello, 2004; Keiluweit et al., 2010), confirmed here by the presence of lignin until combustion temperatures of 300 °C, but not above this temperature (Table A.1 and A.2, Supplementary Material). Likely, this lignin was degraded (Czimczik et al., 2002). This implies that alteration of feedstock is complete at >300 °C. The fire condensates did not contain any lignin, support-

Fig. 1. Pentacarboxylic (B5CA) to mellitic acid (B6CA) ratios of charcoals and fire aerosols at combustion temperatures of 300, 350, 400, 450, 500 and 600 °C (dashed line: charcoal data from Wolf et al., 2013). B5CA/B6CA ratios of charcoals and condensates differentiate charcoals from fire condensates, irrespective of fire temperature. The asterisks *** indicate significant difference between B5CA/B6CA ratios and combustion temperatures at p < 0.001 level of probability, respectively.

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et al., 2002; Gustafsson et al., 2001). In the fire condensates studied here, however, we found a very wide B5CA/B6CA ratio (Fig. 1), thus pointing to a low degree of aromatic condensation (Currie et al., 2002; Gustafsson et al., 2001) and, accordingly, to a small number of aromatic rings within given PyC fragments (McBeath et al., 2011). Nevertheless, we have to keep in mind that combustion products of different fossil fuels can exhibit a larger biochemical variability than those investigated here under controlled lab conditions. The PAH concentration in charcoals ranged from 5.4 to 200 mg per kg OC (Table A.1, Supplementary Material). In condensates, PAH concentrations reached from 95 to 7159 mg per kg OC (Table A.2). The total PAH concentrations of condensates were 18 to 36 times larger than those of the respective charcoals and were also independent of fuel type (p < 0.01; Table 1). For charcoals, three ring PAH (R acenaphthylene, acenaphthene, phenanthrene and anthracene) were dominant (40–87%, Welch test, p < 0.01; Table A.1, Supplementary Material), followed by four ring (fluoranthene, pyrene, benz[a]anthracene and chrysene, 8–47%) five ring (benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[e]pyrene and benzo[a]pyrene, 1–16%), and six ring PAH (indeno[1,2,3-cd]pyrene and benzo[ghi]perylene, 0.2–10%). These coincide with portions of parent PAH classes found for domestic heating (wood burning) by, e.g. Oros and Simoneit (2001a,b), Lima et al. (2005) and Bläsing et al. (2016). The condensates showed significantly larger portions of four ring PAH compared to charcoals (Welch test, p < 0.01) (Fig. 2a; Tables 1, A.1 and A.2, Supplementary Material), consistent with reports for exhausts of coal power stations and road traffic, especially heavy-duty diesel and road diesel (Sjögren et al., 1996; McDonald et al., 2004; Kado et al., 2005). Furthermore, the concentration of five ring PAH in condensates changed significantly with combustion temperature (R2 = 0.94; t-test, p < 0.01; Fig. 3, Supplementary Material), as did, therefore, also bulk PAH concentrations (Table 1). Our findings are thus in line with the general observation that larger molecules have a higher tendency to be incorporated into soot particles (Lima et al., 2005). At the individual PAH level, phenanthrene mainly explained the differences in total PAH concentrations between charcoals and condensates (Tables 1; A.1 and A.2, Supplementary Material). The concentrations of phenanthrene ranged in charcoals from 0.8 to 28 mg per kg OC and in condensates from 5.6 to 1273 mg per kg OC. The mean proportion of phenanthrene relative to the sum of PAH was 2.4–24% for condensates and 5.8–17% for charcoals. Besides phenanthrene, individual PAH ratios also discriminated the chemical composition of fire condensates from that of respective charcoal remains, as tested by, e.g., McDonald et al. (2004), Wei et al. (2015) and Yunker et al. (2014, 2015). These

Fig. 2. PAH ratios of unburned material and burned apple wood, beech wood, pine wood, larch wood, einkorn straw and emmer straw at 300, 350, 400, 450, 500, 600 °C. (a) Percentage of 3-ring versus 4-ring PAH in charcoals and fire condensates and (b) PAH ratios of fluoranthene to pyrene (Flua/(Flua + Py) versus indeno[1,2,3cd] pyrene to benzo[ghi]perylene (IP/(IP + B[ghi]P) in lab produced charcoals and condensates.

authors found that PAH ratios of fluoranthene to pyrene Flua/(Flua + Py) and indeno[1,2,3-cd]pyrene to benzo[ghi]perylene IP/(IP + B[ghi]P can help to elucidate source and composition of PAH, as they differ between fire residues from biomass, coal, liquid fossil, petroleum and fuel combustion. Indeed, also in our study, the ratios of IP/(IP + B[ghi]P) and Flua/(Flua + Py), for instance,

Table 1 Significance between selected compounds and ratios versus aerosols, charcoals and fuel type (t-test of paired differences to the p < 0.05 level of significance) as well as significant linear relationships of these parameters to combustion temperature (linear correlation analyses).

d15N isotopes d13C isotopes Lignin-derived phenols Phenanthrene 4-ring PAHs 5-ring PAHs Flua/(Flua + Py) IP/(IP + B[ghi]P) 1,7/1,2-DMP (1,7 + 2,6|3,5)/(1,2 + 1,7 + 2,6| 3,5)-DMP B5CA/B6CA

Aerosols vs. charcoals

Hardwood vs. softwood

Hardwood vs. straw

Softwood vs. straw

Temperature (aerosols)

Temperature (charcoals)

n.s. n.s. n.t. p < 0.01 p < 0.01 n.s. p < 0.01 p < 0.01 n.s. n.s.

n.s. n.s. p < 0.01 n.s. n.s. n.s. n.s. n.s. p < 0.01 p < 0.01

n.s. n.s. p < 0.01 n.s. n.s. n.s. n.s. n.s. n.s. p < 0.01

n.s. n.s. p < 0.01 n.s. n.s. n.s. n.s. n.s. p < 0.01 p < 0.01

n.s. n.s. n.t. n.s. n.s. p < 0.01; R2 = 0.94 n.s. n.s. n.s. n.s.

n.s. n.s. n.t. n.s. n.s. n.s. n.s. n.s. n.s. n.s.

p < 0.01

n.s.

n.s.

n.s.

p < 0.01; R2 0.89

p < 0.01; R2 0.88

n.s. = not significant; n.t. = not tested, due to lacking detection of lignin in charcoals samples >300 °C and aerosols.

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a

aerosols

apple beech einkorn emmer pine larch

30

5- rings PAHs [%]

25 20 15

R2 = 0.84 p < 0.001

10 5 0 300

350

400

450

500

550

600

650

combustion temperature [°C] 35

charcoals

b

apple beech einkorn emmer pine larch

30

5- rings PAHs [%]

25 20 15 10 5 0 300

350

400

450

500

550

600

650

combustion temperature [°C] Fig. 3. Percentage of 5-ring polycyclic aromatic hydrocarbons (PAH) in a: condensates and b: charcoals.

differed significantly between charcoals and condensates (Tables A.1 and A.2; Supplementary Material). The ratio of IP/(IP + B[ghi] P) ranged from 0.29 to 0.69 for charcoals and from 0.34 to 0.96 for all condensates. The Flua/(Flua + Py) ratio ranged from 0.14 to 0.61 for charcoals and from 0.40 to 0.66 for condensates. If these two ratios were combined, a differentiation of charcoals versus condensates was achieved (p < 0.05; Fig. 2b). In summary, the combined application of BPCA and PAH analyses enabled us to distinguish between lab produced fire condensates and charcoal. We thus support the results of Wiedemeier et al. (2015) that the combination of both methods leads to a better source apportionment of PyOM in soils and sediments and potentially helps illuminate the pathways and fluxes of differently sized and pyrolyzed PyOM through environmental compartments. However, the analytical separation of such mixtures of charcoal and condensates in real-world samples remains an open research challenge.

matter may contain low molecular weight compounds that can be related to fuel type, such as BPCA and PAH (e.g. Benner et al., 1995; Christensen et al., 1997; Wiedemeier et al., 2015). Lignin residues were found in low temperature charcoals (max. 300 °C combustion temperature; Fig. 4; Table A.1, Supplementary Material), with concentrations ranging from 2.2 to 16.7 g per kg OC. In these charcoals, we were able to differentiate hardwoods, softwoods and straw-based on their lignin-derived oxidation products, i.e. the C/V and S/V ratios as published by Hedges and Mann (1979) for unburned biomass (Fig. 4). This could offer the potential to reconstruct fuels of low-temperature fires which occur frequently in grasslands and forests (forest ground fire). Noteworthy, lignin was absent in charcoals produced at temperatures >300 °C, and also fire condensates were free of lignin (Tables A.1, A.2, Supplementary Material). It has been shown earlier that the aromatic composition of charcoalss, as indicated by their BPCA pattern, is insensitive to different types of fuels (Schneider et al., 2010, 2013; Wolf et al., 2013), which was also confirmed in our study (Fig. 1, Tables A.1 and A.2, Supplementary Material). However, PAH have been used successfully as tracers for diverse combustion sources (Bandowe et al., 2014; Khan et al., 2015), and alkylated PAHs such as the dimethylphenanthrenes (DMP) hold particular promise for distinguishing fuel sources from softwood and hardwood (Benner and Gordon, 1989; Benner et al., 1995). The latter authors found that softwood can be differentiated by the content of 1,7- and 1,8DMP. However, it has to be mentioned that in the work of Benner and Gordon (1989) and Benner et al. (1995) the coelution of 3,5- with 2,6-dimethylphenanthrene is missing and the peak assigned to 1,8-dimethylphenanthrene should be 1,2dimethylphenanthrene. The currently accepted isomer assignments we used can be found in e.g. Huang et al. (2004) and Yunker et al. (2015). We found that PyC after softwood combustion can be specified due to outstanding contents of 1,7-DMP, while hardwood combustion products were dominated by 1,2-DMP, and residues of grass combustion had almost equal contributions of these two DMP isomers (see Fig. 5, Supplementary Material). We therefore derived characteristic ratios of the (1,7 + 2,6|3,5)/(1,2 + 1,7 + 2,6|3,5) isomers, which ranged from 0.2 to 0.6 for hardwood, from 0.6 to 0.9 for grasses, and were >0.9 for softwoods (p < 0.05; Fig. 6), with

7 apple beech einkorn emmer pine larch

6 5

A

4

S/V

35

3 2

a

1 0

g

G

3.2. Differentiation of fuels 0,0

A characterization of the fuel of combustion residues enables allocation of sources and reconstruction of past fire processes. If residual organic matter in charcoals still exists, this may preserve characteristic structural remains of the fuel, such as stable carbon, nitrogen isotope and lignin composition (Czimczik et al., 2002; Brodowski et al., 2005). Also, thermally altered, pyrogenic organic

0,2

0,4

0,6

0,8

C/V Fig. 4. Proxies for lignin origin: Syringyl and vanillyl derived lignin phenols (S/V) versus cinnamyl and vanillyl derived lignin phenols (C/V) of charcoals (produced by 300 °C) from apple wood, beech wood, pine wood, larch wood, einkorn grass and emmer straw with ranges for gymnosperm (G), non-woody gymnosperm (g), angiosperm (A) and non-woody angiosperm (a) after Hedges and Mann (1979).

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Fig. 5. Selected ion chromatograms of dimethylphenanthrenes (DMP) in extracts of lab-produced charcoals at 400 °C of (a) pine, (b) larch, (c) apple, (d) beech, (e) einkorn straw and (f) emmer straw. Peak assignments (Huang et al., 2004; Yunker et al., 2015) are: (1) 3-ethylphenanthrene, (2) 3,6-dimethylphenanthrene|9|2|1-ethylphenanthrene, (3) 2,6|3,5-dimethylphenanthrene, (4) 2,7-dimethylphenanthrene, (5) 1,3|3,9|2,10|3,10-dimethylphenanthrene, (6) 2,5|2,9|1,6-dimethylphenanthrene, (7) 1,7dimethylphenanthrene, (8) 2,3-dimethylphenanthrene, (9) 1,9|4,9|4,10-dimethylphenantrene, (10) 1,8-dimethylphenantrene and (11) 1,2-dimethylphenanthrene.

cum) and einkorn grass (Triticum monococcum) in lab produced charcoals and condensates, via PAH and BPCA analyses. Hence, we can provide indicators for different burning conditions. Transferring our findings from lab-produced condensates to those formed under open environmental fires now warrants further attention. Acknowledgments The authors acknowledge the Deutsche Forschungsgemeinschaft, Germany (German Research Council; DFG), Collaborative Research Centre 806 ‘‘Our Way to Europe: Culture-Environment Interaction and Human Mobility in the Late Quaternary” for their financial support. Appendix A. Supplementary material Fig. 6. Biplot of 1,7/1,2 and (1,7 + 2,6|3,5)/(1,2 + 1,7 + 2,6|3,5) dimethylphenanthrenes (DMPs) in lab produced charcoals and condensates.

additional differentiation in Y direction showing the 1,7/1,2 DMP isomer ratios (Fig. 6).

Supplementary data to this article can be found online at https://doi.org/10.1016/j.orggeochem.2019.01.009.

Associate Editor—Mark Bernard Yunker References

4. Conclusions Our findings show, that it is possible to distinguish between: (i) condensates and charcoals and (ii) between the fuels apple wood (Malus domestica), beech wood (Fagus sylvatica), pine wood (Pinus sylvestris), larch wood (Larix decidua), emmer grass (Triticum dicoc-

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