Methoxyphenols are key compounds in BB smoke, since they constitute from 20 % to 40 % of the total identified organic aerosol mass (Hawthorne et al., 1989; Yee et al., 2013). For this reason, these compounds are considered potential markers for wood combustion (Schauer et al., 2001) and have been used as probable biomarkers to determine human exposure to BB emissions (Simpson and Naeher, 2010; Dills et al., 2006). Our analysis of 13 methoxyphenols (Fig. 2; Table S1) showed that guaiacol (molecular weight – MW – of 124 g mol-1) was the major contributor to EFs of the measured methoxyphenols in Moscow peat (33.5±3.3 mg kg-1), Pskov peat (49.7±4.8 mg kg-1), and Malaysian peat (68.6±6.7 mg kg-1). Syringol, another methoxyphenol commonly found in BB emissions (Schauer et al., 2001), had the highest EF for Moscow peat fresh emissions (20.4±2.7 mg kg-1), while for the other fuels, the EF was much lower (0–5.5 mg kg-1). EFs for syringic acids (MW of 198 g mol-1) were in the range of 0.06–37.9 mg kg-1 for all fresh emissions. Syringols are generally not formed during the pyrolysis of coniferous lignin but during the pyrolysis of deciduous lignin, where both guaiacols and syringols are formed (Mazzoleni et al., 2007). Presence of both guaiacol and syringol moieties in fresh emissions indicates that the part of the plant material that was responsible for peat formation was probably from deciduous trees, and this signature of deciduous trees from peat-burning emission is irrespective of the geographical origin of those peats (also shown by Schauer et al., 2001). Acetovanillone, vanillin, and vanillic acid also were observed in fresh emissions with a high abundance (5–50 mg kg-1). For example, vanillin is an abundant methoxyphenol in the fresh emissions from Pskov peat (46.7±5.4 mg kg-1), which contributed 4.4 % of the total mass of the 84 analyzed compounds.

Low-MW methoxyphenols (e.g., guaiacol) are expected to be found in the gas phase (Yatavelli et al., 2017), in close agreement with our results. For example, guaiacol and substituted guaiacols were mostly present in the gas phase (82 %–100 %) for emissions from the combustion of different fuels (Fig. 2; Table S1). With the addition of more oxygenated functional groups to a molecule, and thus with an MW increase, the equilibrium gas–particle partitioning of the compound tends to shift toward the particle phase, which also was confirmed by our results (e.g., for acetovanillone, a keto form of lignin derivative, from Malaysian peat combustion, 33.5 % of its mass was found in the gas phase; for the more oxygenated syringic acid, 99 % of its mass was found in particle-phase emissions from the same fuel).

The highest methoxyphenol EFgroup from the combustion of all fuels was observed in the fresh Pskov peat (Fig. 7a) emissions (239±11 mg kg-1). For Moscow peat, which was sampled close to the geographical region of Pskov peat, the EFgroup of methoxyphenols was 229±10 mg kg-1 (Fig. 7a), very similar to that for Pskov peat. The methoxyphenol EFgroup values for peat samples were in the range of 66 to 239 mg kg-1 (Fig. 7a) for our 13 analyzed compounds. A previous study analyzed for 30 different compounds (Schauer et al., 2001) and consequently found a larger value of EFgroup of up to 1330 mg kg-1, at least partially a result of the larger number of compounds analyzed. The formation of methoxyphenols during biomass combustion is mainly because of the pyrolysis of lignin (e.g., Simoneit et al., 1993). Lignin, an essential biopolymer of wood tissue, is primarily derived from three aromatic alcohols: p-coumaryl, coniferyl, and sinapyl alcohols (Hedges and Ertel, 1982). The lignins of hardwoods (angiosperms) are enriched with products from sinapyl alcohol; softwoods (gymnosperms) instead have a high proportion of products from coniferyl alcohol with a minor contribution from sinapyl alcohol; grasses have mainly products from p-coumaryl alcohol. The relative proportions of these biomonomers vary considerably among the major plant classes (Sarkanen and Ludwig, 1971), reflected in our total emission factors estimate for 13 methoxyphenols.

Dicarboxylic acids play a significant role in the atmospheric organic aerosols budget (Samburova et al., 2013; Yatavelli et al., 2017) via secondary-organic-aerosol formation that either changes radiative forcing directly or indirectly by acting as cloud condensation nuclei (Kawamura and Bikkina, 2016). The EFgroup for dicarboxylic acids (Fig. 7b) varied among the fuels with the highest EF for fresh Pskov peat samples (123±10 mg kg-1) and with the lowest for eucalyptus (1.5±0.1 mg kg-1). This range in EFs can be attributed to the difference in fuel type and burning conditions (smoldering and flaming). We also observed, however, a difference in the EFgroup of dicarboxylic acid between two tropical peats from the same geographical area (EF of Malaysian peat of 35.33±2.9 mg kg-1 and of Malaysian agricultural peat of 12.29±1.02 mg kg-1). The highest EF for individual dicarboxylic acids was observed for azelaic acid. For example, for Pskov peat the EF was 32.1±4.1 mg kg-1, and for Moscow peat it, was 17.6±2.6 mg kg-1. Azelaic acids were mostly found in the particle phase (Fig. 3; Table S1) and their relative abundances in the gas phase varied between 0.77 % (for Pskov peat) and 2.85 % (for eucalyptus). Maleic acid was mostly found in the gas phase (73 %–83 %), since it is a lower-MW compound (MW of 116.0 g mol-1) compared to azelaic (MW of 188.22 g mol-1) and adipic (146.14 g mol-1) acids. Succinic and methylsuccinic acids are found in both the gas and particle phases (Table S1), and their abundance in the particle phase was 19 %–59 % and 53 %–100 %, respectively. For Malaysian peat BB emissions, succinic acid was present only in the particle phase. A distinguishable increase in dicarboxylic-acid mass concentrations was observed for ambient aerosols followed by a biomass-burning event (Cao et al., 2017) compared to normal ambient concentrations. The formation of saturated dicarboxylic acids (e.g., succinic acid) and unsaturated dicarboxylic acids (e.g., maleic acid) also was reported for ambient aerosols collected near a biomass-burning event (Graham et al., 2002; Kundu et al., 2010; Zhu et al., 2018) and in ice core records historically affected by biomass burning (Müller-Tautges et al., 2016).

Monocarboxylic acids can constitute up to 30%–40 % of total identified organic aerosol mass from BB emissions (Oros et al., 2006). In our study, we characterized the range from C6 to C24, where some unsaturated monocarboxylic acids (e.g., oleic acid) also are included. For Alaskan and Malaysian peat fresh emissions (Fig. 4), the highest EF (gas + particle) among all analyzed monocarboxylic acids was for hexadecanoic acid (C16) with EFs of 55.7±6.6 and 51.8±6.2 mg kg-1, respectively. The dominance of hexadecanoic acid among other monocarboxylic acids in combustion emissions also was observed in ambient measurements during biomass-burning events in southeastern Asia (Fang et al., 1999). For Moscow (41.5±6.5 mg kg-1) and Pskov (49.8±7.8 mg kg-1) peats, tetradecanoic acid (C14) had the highest EFs in fresh samples (Fig. 4). For eucalyptus and Malaysian agricultural peat fresh samples, the largest contributor to monocarboxylic acids was tetracosanoic acid (C24; Fig. 4) with EFs of 3.81±0.5 and 42.0±5.1 mg kg-1, respectively. As we expected, low-MW monocarboxylic acids like hexanoic acid (MW of 116 g mol-1) was mostly present in the gas phase, and the gas phase mass fraction varied between 72 % (for Malaysian agricultural peat) and 98 % (for Moscow peat). Similar trends were observed for other low-MW monocarboxylic acids. For example, the relative abundance of octanoic acid (C8) in the gas phase was 93.4 % for Alaskan peat. High-MW monocarboxylic acids (C16>) abundance in the gas phase was <2 % for all analyzed fuels.

The carbon preference index (CPI) is generally used for the source apportionment of organic aerosols (Fang et al., 1999). We also computed the carbon preference index for monocarboxylic acids from all analyzed fuel combustion emissions by taking the ratio of even carbon numbers to odd carbon numbers on the EFs of monocarboxylic-acid ranges from C6 to C24 (Fig. S1). For fresh emission samples, the CPI values ranged from 1.28 (for Moscow peat) to 4.53 (for eucalyptus). The CPI values are higher for fresh emissions of tropical peats (for example, 2.78 for Malaysian peat) than for emissions from peats from high latitudes (for example, 1.74 for Pskov peat). An average CPI index of 3.7 for monocarboxylic acids was reported for combustion emissions from sedimentary bogs (Freimuth et al., 2019), in the range of our reported values (CPI of 1.3–4.5).

The sums of the EFs for 25 monocarboxylic acids are shown in Fig. 7c. The EFgroup was in the range of 5 to 515 mg kg-1 for all fuels. This range is comparable to the EFgroup reported previously for grass (tundra, cotton, Pampas, and ryegrass) combustion (32–250 mg kg-1; Oros et al., 2006). Overall, the trend of low EFs associated with the flaming combustion of eucalyptus (16±0.7 mg kg-1) compared to smoldering-peat combustion is also evident for this compound category. The combustion of peat fuels from tropical regions (e.g., Malaysian agricultural peat) resulted in monocarboxylic-acid EFs of 212±9.6 mg kg-1 compared to higher EFs from Alaskan peat combustion (505±23 mg kg-1). The origin of monocarboxylic acid is mostly plant wax and oils (Simoneit, 2002). The relative proportion of plant wax and oils can vary widely among vegetation taxa, and also their concentrations in peat depend on biogeochemical processes involved in peat formation. The differences in the relative abundance of waxes and plant oils in living vegetation and the differences between biogeochemical processes involved in peat formation for arctic and tropical regions may be responsible for diverse EFs for monocarboxylic acids.

Aromatic acids from BB emissions can contribute up to 20 %–35 % of total identified organic mass (Wan et al., 2019). In our study, the aromatic acids (e.g., p-hydroxy benzoic acid) excluded methoxyphenol derivatives and resin acids. For most of the fuels, low-MW aromatic acids (MW<150 g mol-1; e.g., benzoic acid and o-, m-, and p-toluic acids) contributed more (almost 40 % of total aromatic-acid emissions) toward total fresh emissions, compared to high-MW aromatic acids (MW>150 g mol-1). For example, the benzoic-acid EF for Malaysian agricultural peat fresh emission (Fig. 5) is 9.98±1 mg kg-1, and the EF for the same benzoic acid is 6.36±0.6 mg kg-1 for Pskov peat (Fig. 5). Although, o-toluic and p-toluic acids are found in the gas phase with 50 %–100 % abundance in Alaskan and Moscow peat, benzoic acid is found only in the particle phase. Benzoic acid was found in high concentrations in the XAD blanks that introduced a substantial uncertainty to the quantification of this compound. One of the most abundant aromatic acids in fresh peat emissions was 2,3-dimethoxybenzoic acid. For example, for Moscow peat the EF was 18.6±4.7 mg kg-1. The acid was mostly found in the gas phase (91 %–100 %) for all fuels (Fig. 5; Table S1). 2,3-dimethoxybenzoic acid is potentially derived from the combustion of lignin moieties of biomass, and the emission of this compound is more than an order of magnitude lower in emissions from flaming-combustion samples (EF of 0.38±0.09 mg kg-1) than in emissions from smoldering-combustion samples (EF of 5.44±1.36 mg kg-1). Emissions of 3,4 dimethoxybenzoic acid were only observed for peats from tropical regions (EF of 5.64±0.8 mg kg-1) with 80 %–100 % abundance in the particle phase. This compound can be used for the source apportionment of aerosols emitted from the burning of tropical peat and also can potentially help one to distinguish between emissions from tropical and high-latitude peatland fires.

The EFgroup for aromatic acids in fresh-combustion emissions from eucalyptus fuel is extremely low (0.71±0.05 mg kg-1) compared to that for peat fuels (13–69 mg kg-1). Among all peat samples, the Alaskan peat fresh EF was the lowest EF (13.5±0.9 mg kg-1), whereas Moscow peat fresh emissions yielded the highest EF (69±4.4 mg kg-1). The difference in total aromatic-acid emissions can be attributed to the variation in the lignin content of the fuels and burning conditions (Simoneit, 2002).

We quantitatively analyzed combustion emissions for isomers in six resin acids (Table S1). The most abundant resin acid (78 % of total resin acid emission) is dehydroabietic acid (C20) that does not have isomers. The preponderance of this acid over other resin acids in emissions from oak and pine biomass burning was reported by Simoneit et al. (1993). We found that dehydroabietic acid (C20) content in fresh emissions is 15–30 times higher in fuels from high-latitude peatlands than in those of tropical origin. For example, the EF for dehydroabietic acid in fresh Alaskan peat emissions is 92.2±14 mg kg-1 (Fig. 6), whereas the same in fresh Malaysian peat emissions is 3.44±0.5 mg kg-1 (Fig. 6). Resin acids are supposed to be found mostly in the particle phase based on their MW and functional groups (Asher et al., 2002; Karlberg et al., 1988; Pankow and Asher, 2008), confirmed by our results (80 %–100 % in the particle phase) with the exception of Malaysian peat emissions, where 56.6 % abundance of dehydroabietic acid was found in the gas phase. Although a distinct peak of dehydroabietic acid was observed at the desired retention time for this sample during GC-MS analysis, we believe this result can be attributed to some unknown interference from our analysis procedure.

The EFgroup for seven resin acids are presented in Fig. 7e. A high EFgroup was observed for Alaskan (117±15 mg kg-1), Pskov (89±12 mg kg-1), and Moscow (59±7.7 mg kg-1) peats representing midlatitude and arctic peats. Resin acids (e.g., pimaric acid) are biosynthesized mainly by conifers (gymnosperms) in temperate regions. In previous work, Iinuma et al. (2007) gave a range of resin-acid EFs from 0 to 110 mg kg-1, in agreement with our results. Very low resin-acid EFs were found for peat from tropical regions (e.g., 4 mg kg-1 for Malaysian peat fresh samples). As deciduous trees in tropical zones are not prolific resin and mucilage (gum) producers, compositional data on smoke from such sources should not be expected to show moderate concentrations of resin acids. This is supported by earlier work by Iinuma et al. (2007), where resin acids were not even detectable for emissions from Indonesian peat combustion.

Levoglucosan can be found mostly in the particle phase (Simoneit, 2002). We report levoglucosan EFs from our analysis of the quartz filter using the IC-PAD technique (no gas-phase EFs reported). The EFs of levoglucosan (Fig. 7f) were found to be 20.9±0.68 and 485±11.8 mg kg-1 for eucalyptus and Malaysian peat, respectively, and their carbon content is approximately 1.8 % and 2.5 % of the total organic carbon mass characterized by the thermo-optical technique. Fine et al. (2002) reported 9 % to 16 % contribution of levoglucosan to total OC from residential wood combustion, a relatively higher percentage than values obtained in our study. Anhydrosugars (e.g., levoglucosan and its isomers) are found in great abundance and have been widely used as a BB tracer because of their atmospheric stability, as summarized by Bhattarai et al. (2019). We found that levoglucosan constituted 36 % and 51 % of GC-MS characterized polar (listed in our method) organic aerosol mass for eucalyptus and Malaysian peat, respectively, which also is consistent with the previous BB literature assembled in the recent review article by Bhattarai et al. (2019).

Methoxyphenols can undergo gas-phase oxidation reactions via either aromatic-ring fragmentation or opening to form short-chain ketones, acids, esters, and double bonds in conjugations with all functional groups or the further hydroxylation of aromatic rings to form multiple substituted aromatic compounds (Yee et al., 2013). In either case, a decrease in methoxyphenols after oxidation was expected. In our study, a decrease in methoxyphenol's EFgroup with OFR oxidations was observed for all fuels (e.g., for Pskov peat from 239±11 to 95±4 mg kg-1) except for Alaskan peat, where an insignificant increase from 66±3 to 70±3 mg kg-1 after OFR oxidation was observed.

A significant increase (2.5–8.5 times) in the EFgroup of dicarboxylic acids was observed for OFR-aged samples. For example, the EFgroup of dicarboxylic acids increased from 35±3 to 301±25 mg kg-1 for Malaysian peat and from 56±5 to 326±27 mg kg-1 for Alaskan peat. The oxidation of aerosols potentially produces more oxygenated functional groups (Jimenez et al., 2009), demonstrated by an increase in O:C ratios (from 0.45 to 0.65) in the recent laboratory oxidation of BB emissions by Bertrand et al. (2018), where TAG-AMS was used to identify the fate of organic compounds. In this work, however, the number of identifiable compounds with highly functional groups is constrained by the elution technique used in the TAG method. Our results on the fate of BB organic aerosols with 18 dicarboxylic acids can provide a better mechanistic understanding about the processes inside the OFR.

We observed a decrease in the monocarboxylic-acid EFgroup from OFR aging for all fuels. For example, the EFgroup for monocarboxylic acids from Alaskan peat combustion decreased from 514±23 mg kg-1 (fresh) to 298±14 mg kg-1 (aged). A relatively small decrease compared to Alaskan peat was observed for Malaysian agricultural peat (from 216±10 mg kg-1 – fresh – to 179±8 mg kg-1 – aged) and Malaysian peat (from 298±14 mg kg-1 – fresh – to 240±11 mg kg-1 – aged) too. This is probably because of the formation of low-MW monocarboxylic acids (e.g., hexanoic acids; MW of 116 g mol-1) after OFR oxidation demonstrated in Fig. 10 and Table S2c and will be discussed further in Sect. 3.3. Monocarboxylic acids can be oxidized in the atmosphere (Charbouillot et al., 2012), leading to the formation of dicarboxylic acids from C2 to C6 (Ervens et al., 2004). This is consistent with our results (Fig. 7b, c; Table S2b). Moreover, monocarboxylic acids, during their atmospheric transformations, can produce a potential precursor for the formation of high-MW compounds, such as a humic-like substance (HULIS; Carlton et al., 2007; Tan et al., 2012).

Levels of aromatic acids from peat burning increased for Alaskan peat, Malaysian peat, and Malaysian agricultural peat (e.g., from 29±2 to 68±4 mg kg-1 for Malaysian peat) by OFR aging (Fig. 7e; Table S2e). This increase could be from the oxidation of phenols and methoxyphenols in the OFR chamber (Akagi et al., 2011; Legrand et al., 2016). For eucalyptus and Moscow and Pskov peats, it was an insignificantly small decrease (e.g., from 69±4 to 57±3 mg kg-1 for Moscow peat and from 34±2 to 33±2 mg kg-1 for Pskov peat). This small decrease in the EFgroup of monocarboxylic acids is insignificant. The oxidation processes occurring in the OFR are complex, especially in the case of multicomponent BB emissions. The decrease observed for aromatic acids after the OFR may be attributed, however, to multiple generations of oxidation leading to the breaking of aromatic rings and formation of low-MW organic compounds via fragmentation (Jimenez et al., 2009).

Resin acids can be oxidized to corresponding oxo acids (e.g., 7-oxodehydroabietic acid; Karlberg et al., 1988), and they are considered to be stronger contact allergens than the resin acids themselves (Sadhra et al., 1998). Our data showed a small decrease in 7-oxodehydroabietic-acid levels after the OFR (e.g., from 8.2±0.8 to 4.8±0.5 mg kg-1 for Pskov peat). We noted a significant decrease in the EFgroup of resin acids from 117±15 mg kg-1 (fresh) and 63±8 mg kg-1 (aged) for Alaskan peat after OFR oxidation, mostly because of some individual compounds like dehydroabietic acid. Although resin acids are considered to be stable atmospheric tracers for biomass burning (Simoneit et al., 1993), we observed a decrease in the dehydroabietic-acid (most abundant) EF after OFR oxidation of emissions from all fuels (Fig. S2b). For example, for Alaskan peat (Fig. S2b), the decrease was from 92±14 to 57±8 mg kg-1 after OFR oxidation. The fate of resin acids during OFR aging, however, was beyond the scope of this work and may be the subject of future investigations.

Levoglucosan is one of the most popular tracers of BB emissions, since it has been considered a stable compound in the atmosphere (Oros et al., 2006; Simoneit, 2002; Simoneit et al., 1999). Several laboratory studies, however, have demonstrated the degradation of levoglucosan in the presence of OH radicals (Hennigan et al., 2010). Here we observed a decrease of 30 % in levoglucosan levels following OFR oxidation. For example, Malaysian peat decreased from 485±12 to 327±8 mg kg-1. For eucalyptus, the decrease was from 20±0.7 to 14±0.6 mg kg-1. This decrease also can be attributed to the degradation process during OH oxidation (Hoffmann et al., 2010). Levoglucosan oxidation should be studied more so that it can be adequately used as a tracer of BB emissions.

Overall, we found that abundances for methoxyphenol derivatives rapidly decreased upon OFR oxidation (Fig. 8; Table S2a). Some compounds – vanillic acid, acetovanillone, and syringic acids – demonstrated both increasing and decreasing trends. For example, for Pskov peat, the aged-to-fresh ratio of guaiacol was 0.04±0.01 reflecting a significant decrease during OFR oxidation. For Pskov peat, we also observed a ratio of less than one for vanillin (0.44±0.05), indicating that vanillin also decreased during OFR oxidation for the same fuel but not to the extent of guaiacol. At the same time and for the same fuel, a slight increase (aged-to-fresh ratio >1) in vanillic acid was observed (1.30±0.13) in the OFR-oxidized sample. This increase in vanillic-acid concentration can be attributed to the oxidation of vanillin, one of the abundant methoxyphenol compounds in the fresh emissions from Pskov peat (Fig. 8; Table S2a). For the combustion of other peats, vanillic-acid concentrations also decreased (e.g., aged-to-fresh ratios were 0.74±0.08 and 0.67±0.07 for Alaskan peat and Malaysian agricultural peat, respectively). Acetovanillone increased by a factor of 3 during OFR oxidation for Alaskan peat and around 15 % for Malaysian agricultural peat (aged-to-fresh ratio of 1.15±0.13), but the increase for Malaysian agricultural peat was not significant. For other fuels, acetovanillone decreased during OFR oxidation. For example, for Moscow peat, the aged-to-fresh ratio for acetovanillone was 0.30±0.03. We still need to investigate the reason why both acetovanillone and vanillic acid increased for some fuels and decreased for others. The reduction of acetovanillone and vanillic acid was because of a photochemical decomposition process in the OFR with the formation of lower MW products, such as succinic acid and maleic acid (Schnitzler and Abbatt, 2018).

In the case of dicarboxylic acids, we observed an increase of 2–20 times in EFs, but the degree of enhancement of low-MW dicarboxylic-acid EFs was higher than for high-MW dicarboxylic-acid EFs. For example, a 20-fold increase in maleic acid, a low-MW dicarboxylic acid (MW of 116.07 g mol-1), was observed during OFR oxidation of Pskov peat emissions (aged-to-fresh ratio of 9.6±2.8), whereas for 1,11-undecanedicarboxylic acid, a high-MW dicarboxylic acid (MW of 244.33 g mol-1), the EF increased 2.6 times (aged-to-fresh ratio of 2.60±0.01) for the same fuel. Similarly, the concentration of succinic acid, a low-MW dicarboxylic acid (MW of 118.09 g mol-1), increased almost by 5 times after OFR oxidation (aged-to-fresh ratio of 5.07±0.62), whereas that of undecanedioic acid, a high-MW dicarboxylic acid (MW of 230.30 g mol-1), increased 2.5 times (aged-to-fresh ratio of 2.46±0.30) for Moscow peat. This trend was in accordance with results from ambient observations after BB events (Cao et al., 2017; Kawamura and Bikkina, 2016).

Our analysis of OFR-aged samples showed that concentrations of monocarboxylic acids with different MWs changed during OFR oxidation, but the changes varied from one fuel to another. For example, the EF of hexanoic acid (C6) was reduced for eucalyptus (aged-to-fresh ratio of 0.5±0.05) and fuels from other high-latitude peatlands like Alaskan (aged-to-fresh ratio of 0.89±0.09), Moscow (aged-to-fresh ratio of 0.88±0.09) and Pskov peat (aged-to-fresh ratio of 0.33±0.03). The reduction of hexadecenoic acid was insignificant for Alaskan and Moscow peat. The peats from tropical regions showed exactly the opposite change. Hexanoic acid increased for both Malaysian (aged-to-fresh ratio of 1.47±0.14) and Malaysian agricultural peat (aged-to-fresh ratio of 1.40±0.14). We observed a similar trend for heptanoic acid (C7) during OFR oxidation. The tropical peats clearly demonstrated increases (for example, aged-to-fresh ratio of 1.92±0.22 for Malaysian peat) in heptanoic-acid concentration. For Moscow peat, even though hexanoic-acid concentrations were insignificantly decreased, heptanoic-acid concentrations increased significantly (aged-to-fresh ratio of 2.37±0.27). This contrast between changes in hexanoic and heptanoic acid can be explained by a decrease in CPI indices during OFR oxidations (for example, from 2.78 to 1.7 for Malaysian peat). The reduction of CPI indices indicated that during oxidation more monocarboxylic acids with odd carbon numbers were formed than monocarboxylic acids with even carbon numbers. The abundance of hexadecenoic acid (C16) was reduced during OFR oxidation for all fuels (for example, aged-to-fresh ratio of 0.63±0.08 for Alaskan peat) except for the Moscow and Pskov peat aged-to-fresh ratio of 1.05±0.13 for peat), and we believe this small increase is insignificant. Similarly, tetracosanoic acid (C20) was reduced for all fuels (for example, aged-to-fresh ratio of 0.61±0.07 for Malaysian peat) except for Pskov peat (aged-to-fresh ratio of 1.24±0.15). The small increase in tetracosanoic acid (C20) concentration was again insignificant. Even though our results indicated the possibility of the fragmentation of high-MW monocarboxylic acids and formation of low-MW monocarboxylic acids, this was because of the complexity of the OFR oxidation environment. We are not able to hypothesize what is the main reactive mechanism.