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. 2026 Apr 3;11(15):22560–22570. doi: 10.1021/acsomega.5c08838

Estimation of the Emission Factors of the Oxygenated Organic Compounds Generated via Pyrolysis of Eucalyptus and Pine Biomass

Xinyao Xie 1, Michiya Fujita 1,*, Kenichi Tonokura 1
PMCID: PMC13103849  PMID: 42040423

Abstract

Wood pyrolysis during eucalyptus and pine forest fires causes considerable environmental concern because of the associated release of oxygenated organic compounds (OOCs), such as acetic acid, acetol, and aldehydes. Both estimation of the emission factors (EFs) and mechanistic attribution for these OOCs necessitate pyrolysis conducted under rigorously controlled conditions, specifically, low-temperature regimes and gradual rates in heating protocols, temperature-resolved sampling in pyrolysis gas capturing, and independent contribution evaluation of components-derived OOCs emissionutilizing eucalyptus and pine biomass as substrates. The generation of these OOCs was quantified by means of pyrolysis gas chromatography/mass spectrometry (Py–GC/MS) with dedicated gas detector tubes. As species comparison, the EFs at 260–380 °C for acetic acid, acetol, formaldehyde, and acetaldehyde generated via eucalyptus pyrolysis were determined to be 17.0, 7.5, 14.9, and 3.9 g/kg dry biomass, respectively. The corresponding values for pine pyrolysis were 1.6, 0.6, 11.2, and 3.0 g/kg dry biomass, respectively. According to emissions of independent components, the substantial contribution of acetylated pyranoses within hemicellulose to acetic acid formation was evident, and the elevated acetic acid yield in eucalyptus11-fold higher than that in pinebeing attributed primarily to the high reactivity of its acetylated hemicellulose, with a minor influence from the abundance of acetylated substituents in hemicellulose. Glucose reagent is the exclusive source of acetol, and its formation during eucalyptus pyrolysis differed from the well-known retro–aldol pathway, particularly in the rate-limiting step of enediol–ketone isomerization, which may be modulated by interactions with components specific to eucalyptus, although the identity and its mechanistic factor in the level of elementary reaction remains unknown. Furthermore, aldehydes can be derived from both glucose and xylose reagents in (hemi)­cellulose, and the distinct aldehyde production profiles were attributed to such components of eucalyptus and pine, which exhibit different decomposition behaviors in the pyrolysis stage. These conclusions, serving as baseline reference data under anaerobic pyrolysis conditions, provide valuable insights into analyzing pollutant emissions from intact wildfires in future works.

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1. Introduction

Because of the frequent occurrence of forest fires and the use of household fuel in developing countries, biomass combustion remains a major source of air pollutants. In particular, the annual emission of organic matter exceeds 2.5 × 107 tonnes. Notably, oxygenated organic compounds (OOCs), such as aldehydes and acetic acid, are known irritants with considerable carcinogenic effects in humans. Formaldehyde and acetaldehyde, in particular, exemplify compounds with acute and chronic inhalation risks at concentrations as low as parts-per-billion (ppb). These health implications underscore the imperative for comprehensive investigations into both the emission dynamics and the mechanistic pathways underlying the formation of these hazardous species.

Eucalyptus and pine, which are widely recognized as hardwoods and softwoods, respectively, , are major sources of organic pollutants. Eucalyptus exists predominantly in subtropical and Mediterranean climates, especially in Australia, whereas pine is common in temperate and polar regions of the Northern Hemisphere, including sparsely inhabited regions in Canada and Siberia. Both species are highly flammable because their bark contains volatile terpenes. , During forest fires in Australia from October 2019 to January 2020, an estimated 5.66 × 108 tonnes of eucalyptus burned. , Similarly, high quantities of pine have burned in forest fires across North America. ,

Biomass burning is characterized by a multistage progression involving heterogeneous thermal reactions among its constituents. This sequence is conventionally segmented into four distinct phases: preignition, pyrolysis, oxidative combustion, and smoldering. Of these, the preignition and smoldering phases, primarily characterized by carbonization, dehydration, and the oxidation of charred residues, exhibit minimal contribution to the formation of the OOCs. In contrast, the pyrolysis and oxidative combustion stages are pivotal, playing a central role in the generation and transformation dynamics of the OOCs.

In the context of forest fires, biomass does not undergo direct combustion in a singular step; rather, volatile and flammable gases are initially liberated via pyrolysis, which subsequently reacts with atmospheric oxygen to sustain combustion. During pyrolysis, the core biomass is typically insulated from ambient oxygen by the outer bark, resulting in thermal decomposition under oxygen-deficient conditions. Under these circumstances, the three primary biopolymerscellulose, hemicellulose, and ligninundergo depolymerization into monomeric intermediates and volatile organic compounds within the C1–C8 range, encompassing a variety of OOCs.

Conversely, oxidative combustion transpires external to the bark layer, predominantly degrading pyrolysis-derived gaseous products through partial or complete oxidation, thereby transforming or eliminating the oxidation of the OOCs. These processes are frequently spatially segregated, with pyrolysis occurring within the interior biomass layers and combustion localized to the exterior. The formation and degradation of the OOCs span both stages, necessitating their discrete evaluation to accurately characterize emission behavior and attribute the formation mechanism.

However, prevailing experimental methodologies often conflate these stages by conducting measurements under aerobic conditions, thereby obscuring the distinct mechanistic pathways and yielding emission factors (EFs) that vary by a factor of 5 to 10 across studies. ,− Consequently, the precise identification and quantification of pyrolysis products under strictly anaerobic conditions are imperative for the derivation of reliable EFs and for elucidating the mechanistic underpinnings of the formation of the OOCs.

The pyrolysis technique has been extensively adopted to examine the thermochemical behaviors of eucalyptus and pine, and over 40 distinct pyrolysis-derived products have been identified under a range of heating conditions. − Although elevated temperature regimes exceeding 500 °C and rapid heating rates on the order of ∼1000 °C/s are frequently utilized in laboratory settings, such parameters diverge markedly from those encountered in natural scenarios, such as forest fires. This discrepancy diminishes the applicability of the resulting EFs and mechanistic interpretations, particularly with respect to the formation of hazardous OOCs.

Empirical evidence indicates that the pyrolysis of lignocellulosic biomassincluding eucalyptus and pinepredominantly occurs within the temperature range of 250–400 °C. , Given that the principal thermochemical reactions within this interval are distinct from those occurring in the elevated temperature range, experimental protocols employing elevated temperatures (>500 °C) and rapid heating rates (∼1000 °C/s) fail to accurately replicate the nuanced thermal behavior and product distribution characteristic of pyrolysis under natural conditions, such as those observed during forest fires.

Moreover, biomass decomposition is confined to a relatively narrow thermal window, wherein incremental temperature changes exert pronounced effects on the distribution of pyrolysis products. Consequently, the implementation of rigorously controlled heating regimes and sampling protocols, specifically, sub-400 °C temperature thresholds, slow heating rates (<10 °C/min), and temperature-resolved sampling, is imperative for achieving high-resolution characterization of OOCs generation across the temperature gradient. Although such methodological refinements are critical for enhancing the accuracy of EFs estimations and elucidating temperature-dependent mechanistic pathways of OOCs formation, current research remains insufficiently developed in this domain and warrants further investigation.

Additionally, an even greater challenge in analyzing the mechanisms of OOCs generation is that the intrinsic heterogeneity in the chemical composition of woody biomass renders the generation of OOCs inherently dependent on species-specific compositional attributes. This species-specific sensitivity of pyrolysis pathways is consistent with observations from copyrolysis studies of compositionally distinct biomasses. For example, Fardi et al. demonstrated that differences in lignocellulosic vs macroalgal composition, particularly ash-forming minerals, protein content, and polysaccharide structure, produce markedly different distributions of pyrolysis volatiles and alter the formation of oxygenated compounds and acids. Their findings highlight that feedstock composition fundamentally governs volatile evolution and oxygenate formation, emphasizing the need to account for species-dependent compositional factors when interpreting the formation of the OOCs in eucalyptus and pine. Therefore, it is essential to delineate the chemical divergences between eucalyptus and pine, particularly with respect to the relative concentrations and structural profiles of cellulose, hemicellulose, and lignin, and to assess the individual contributions of these constituents through the use of structurally analogous model compounds. The development of a comprehensive analytical framework that integrates these compositional and thermal parameters will significantly advance the mechanistic understanding of the formation of OOCs, particularly with respect to the distinct chemical signatures associated with different biomass species.

The primary objectives of this study were to attribute the mechanism of OOCs formation during the pyrolysis of eucalyptus and pine through EFs estimation under rigorously controlled conditions, characterized by low-temperature regimes, slow heating rates, temperature-resolved sampling protocols, and individual component analysis. The chemical characteristics of eucalyptus and pine were analyzed through compositional analysis and Fourier transform infrared (FTIR) spectroscopy, consistent with previous studies. , The generated pyrolysis gas was analyzed via pyrolysis gas chromatography/mass spectrometry (Py–GC/MS), as described previously, with dedicated gas detector tubes. Mechanism attribution was achieved by analyzing the differential thermochemical behavior of eucalyptus and pine at different sampling temperatures as well as the contribution of natural biomass or its separated components (cellulose, hemicellulose, and lignin) to OOCs formation. Through this approach, baseline reference data under anaerobic pyrolysis conditions are anticipated to provide valuable insights into analyzing pollutant emissions from intact wildfires.

2. Materials and Methods

2.1. Sample Preparation

Eucalyptus gunnii (from Ireland; hereafter referred to as eucalyptus) and Pinus densiflora (from Japan; hereafter referred to as pine) wood were obtained from commercial suppliers. To prepare samples for subsequent analysis, the wood was mechanically processed into particles with a maximum size of 3 mm. These wood particles were then dried to reduce the influence of the inherent moisture on the analytical results. Specifically, the particles were placed in a constant temperature oven at 80 °C for at least 12 h. During drying, the weights of the eucalyptus and pine samples decreased by approximately 9 and 6%, respectively. Drying was critical to ensure that the experimental results accurately reflected the pyrolysis characteristics of the biomass components rather than variations induced by changes in the water content.

Additionally, standard glucose (Wako), xylose (TCI), and dealkalined lignin (TCI) reagents were obtained as the representatives of the cellulose, hemicellulose, and lignin components. Their respective contributions to the formation of OOCs were evaluated. Given the substantial presence of acetyl substituents in hemicellulose, acetylated xylose (1,2,3,4-Tetra-O-acetyl-β-D-xylopyranose, TCI) was employed to specifically assess the influence of acetyl functional groups on the generation of OOCs.

Given that the lignin reagent contained 10%–20% inorganic impurities, the lignin reagent was washed with distilled water and subsequently dried in an oven at 80 °C for 12 h. In contrast, the glucose, xylose, and acetylated xylose reagents exhibited high chemical purity (>98%) but were susceptible to structural alteration upon dissolution; therefore, only drying pretreatment was applied prior to pyrolysis experimentation.

2.2. Analytical Methods

2.2.1. Transmission FTIR Spectroscopy

The microstructural characteristics of the eucalyptus and pine wood samples were analyzed by using transmission FTIR spectroscopy (FT/IR 6600, Jasco, Japan). The samples were prepared as potassium bromide (KBr) pellets, as described in standard protocols. Specifically, 0.8 mg of finely powdered eucalyptus or pine wood was blended with 15.2 mg of spectroscopy-grade KBr, and the mixture was pressed into thin pellets to create uniform films suitable for analysis.

To obtain accurate and reliable spectral data, the asymmetric least-squares method was applied for baseline correction and peak fitting. This method effectively mitigates baseline drift to increase the precision of peak identification. Each sample was analyzed in quintuplicate. The analytical parameters are listed in Table S1.

2.2.2. Py–GC/MS

The pyrolytic composition of the wood samples or isolated standard reagents (e.g., glucose, etc.) were determined via thermal decomposition in a quartz tubular furnace (length: 200 mm, inner diameter: 9 mm) under a controlled nitrogen atmosphere. The wood was heated at a rate of 5 °C/min, and pyrolysis gas samples were collected at regular intervals to enable precise and continuous data collection. Specifically, the pyrolysis gases were extracted from the sampling port by using a gastight syringe and then injected into a gas chromatography/mass spectrometry (GC/MS) system (GC-2010/GCMS-QP2010Plus, Shimadzu; column: InertCap 5MS/NP, 0.25 mm × 30 m, 0.25 μm). A schematic of the instrumental setup is shown in Figure S1.

Each sample was analyzed in triplicate. Details of the heating protocol and GC/MS analytical program are provided in Table S2.

2.2.3. Gas Detector Tube Test

Small molecules, such as formaldehyde and acetaldehyde, are prone to analytical interference in GC/MS analysis because of their low molecular weight and high volatility. To determine the concentrations of these aldehydes in the pyrolysis gases, dedicated gas detector tubes (Gastec 91/92 series) that allowed direct extraction of the pyrolysis gases from the sampling port were utilized, as illustrated in Figure S1.

As in the GC/MS analysis, each sample was analyzed in triplicate. The heating protocol parameters and operational settings are listed in Table S3.

3. Results and Discussion

3.1. Chemical Characteristics of Eucalyptus and Pine

3.1.1. Relative Proportions of Cellulose, Hemicellulose, Lignin, and Minor Constituents

Woody biomass is primarily composed of three principal constituents: cellulose, hemicellulose, and lignin. In this study, compositional analysis of eucalyptus and pine biomass was performed by an external laboratory (Tokai Techno Co., Ltd.) using Technical Association for the Pulp, Paper, and Converting Industry tests (TAPPI) or the Wood Science Experiment Manual. , The detailed experimental procedure and results are presented in Section S1 and Table S4.

In eucalyptus biomass, the normalized weight fractions of holocellulose, cellulose, and lignin were determined to be 70.7  ±  0.5%, 39.4  ±  0.1%, and 26.3  ±  0.3%, respectively. These values are consistent with compositional ranges reported across 12 eucalyptus species, wherein holocellulose, glucose, and lignin contents span 55.4–70.1%, 37.3–44.1%, and 24.0–30.8%, respectively. For pine biomass, the corresponding normalized weight fractions were 63.0  ±  0.4% for holocellulose, 44.0  ±  0.2% for cellulose, and 26.5  ±  0.1% for lignin, aligning well with previously reported ranges for six pine species: holocellulose (62.9–73.5%), cellulose (41.9–49.9%), and lignin (25.8–28.8%). Accordingly, the compositional profiles of eucalyptus and pine utilized in this study are deemed to be representative of their respective species in natural environments.

Further, comparative analysis reveals that eucalyptus and pine exhibit nearly equivalent proportions of cellulose and lignin, with deviations of only 4.6% and 0.2%, respectively. However, the hemicellulose content in eucalyptus is approximately 1.6 times greater than that in pine. These results suggested that hemicellulose plays a crucial role in defining the chemical characteristics of these two wood species, which, in turn, may influence the thermal decomposition behavior and product distribution of the woods.

3.1.2. Microstructures of the Constituents

In addition to the differences in the relative proportions of the constituents in hardwoods and softwoods, the microstructural characteristics of hemicellulose and lignin also vary significantly, , greatly affecting thermal decomposition and the resulting pyrolysis products. To investigate these structural characteristics, infrared (IR) absorption spectra of the eucalyptus and pine samples were obtained, and the results are provided in the Supporting Information file Experimental raw data.xlsx.

The obtained IR absorption spectra and relevant peak assignments are presented in Figure and Table . The cellulose-related bands (c, d, e, and i) exhibited similar absorbance intensities and peak positions, suggesting negligible differences between the two species. The lignin-related bands (b, f, i, and j) exhibited comparable absorbance intensities but different peak positions; this was particularly notable for bands b and f, which were attributed to the benzene ring skeleton and methoxy groups within the syringyl and guaiacyl moieties of lignin, indicating variations in the syringyl-to-guaiacyl ratio, while the overall abundance of functional groups remained similar in both biomass samples.

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IR absorption spectra of eucalyptus and pine. Known absorption bands are indicated using a color band.

1. Absorption Bands Attributed to Known Chemical Structures.
band number peak position (cm–1) functional groups source components
a-1 780–790 C–H in pinene other
a-2 770–800 uronic acid hemicellulose
b 800–850 benzene skeleton lignin
c 896 C–O–C (glycosidic bond) cellulose
d 1100–1150 C–O bond in – CH2–O– , cellulose and hemicellulose
e 1150–1170 C–O bond in – CH–O– cellulose and hemicellulose
f-1 1200–1260 C–O in C–OH ,, lignin
f-2 C–O in C–O–CH3 ,, lignin
g 1300–1335 C–O–C in acetate hemicellulose
h-1 1370–1380 C–H in – CHOH cellulose
h-2 C–H in acetate hemicellulose
i-1 1400–1450 C–H in – CH2– cellulose
i-2 C–H in – CH2– hemicellulose
i-3 C–H in – OCH3 , lignin
j 1450–1600 –CC– lignin
k 1628 –C(O)–O– in uronic acid hemicellulose
l 1700–1770 CO in acetate ,, hemicellulose
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Conversely, the hemicellulose-related bands (a, g, k, and l), which were attributed to the acetyl and 4-O-methylglucuronic acid (OMeGAc) moieties, exhibited significant differences in absorbance intensity, indicating that the acetyl and OMeGAc contents in eucalyptus were approximately twice and 4–7 times those in pine. Furthermore, although not shown directly by these results, the ratio of pentose to hexose units in hardwood hemicellulose was substantially greater than that in softwood hemicellulose, contributing to the differences in thermal stability observed during heating. Notably, these characteristics of the hemicelluloses in eucalyptus and pine, which are hardwood and softwood, respectively, influenced the primary decomposition of these materials and the subsequent production of the resulting OOCs.

Nevertheless, the impact of these different chemical characteristics on thermal decomposition and production of OOCs warrants further investigation, as described in Figure .

3.2. Pyrolysis Products Obtained from Eucalyptus and Pine Biomass

The composition of the pyrolysis gases derived from the eucalyptus and pine samples was systematically evaluated with a focus on the thermal decomposition of (hemi)­cellulose and lignin, leading to the formation of OOCs. Particular attention was given to temperature-dependent characteristics and the independent contributions of these components. The aim of this approach was to establish a concise mechanistic framework connecting the (hemi)­cellulose and lignin in the biomass samples with the resulting OOC products.

The pyrolysis products were analyzed via GC/MS, and the peak intensities for each compound were quantified by summing the three largest ionization fragments in the mass spectra. The detailed raw data are provided in the Supporting Information file, Experimental raw data.xlsx.

3.2.1. Devolatilization Ratio

To confirm that the differences in products obtained via eucalyptus and pine pyrolysis were not caused by variations in the devolatilization ratios of these species, we measured the masses of the samples before and after pyrolysis were measured. The devolatilization ratios for eucalyptus and pine were similar, 66.7 ± 2.2% and 70.3 ± 1.5%, respectively (Table S5), indicating that the observed differences in the thermal decomposition of (hemi)­cellulose and lignin and the production of OOCs could be attributed to the intrinsic chemical characteristics of the biomass samples rather than to variations in the devolatilization ratio.

3.2.2. Primary Decomposition of (hemi)­Cellulose and Lignin

The IR absorption spectra (Section ) showed distinct chemical properties for eucalyptus and pine; specifically, pentose units and acetyl and OMeGAc substituent groups were abundant in hemicellulose and the syringyl-to-guaiacyl ratios in lignin differed. These distinct chemical features influenced the formation of OOCs via both primary and secondary decomposition.

Although more than 40 peaks were detected in the GC/MS results, prior studies indicate that several compounds are characteristic pyrolysis products of (hemi)­cellulose such as levoglucosan and other pyranoses. These species can further convert into anhydropyranone, 2-furanmethanol, and 5-hydroxymethylfufural via secondary dehydration and rearrangement reactions. After excluding compounds contributing less than 1% of peak intensity, the remaining major products were classified into pyranones (P-1 and P-2), pyranoses (P-3, P-4, and P-5), furanones (F-3 and F-4), and 5-hydroxymethylfufural (P-8), with their temperature-dependent evolution shown in Figure . Mechanistic insights from Fardi et al. corroborate these assignments: cellulose first depolymerizes into active oligomers (e.g., cellobiose, cellotriose), which degrade to levoglucosan and subsequently to various anhydro-monosaccharides that can form furan derivatives or undergo decarbonylation/decarboxylation to yield CO and CO2, while char forms through polymerization. Hemicellulose decomposes via dehydration and glycosidic bond cleavage to generate intermediates such as dianhydro-xylopyranose, which further fragment into acids, ketones (including acetone), and furans before condensing into char. Collectively, these pathways support the observed distribution of pyranose and furanone species in the pyrolysis products.

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Peak intensity of pyrans and furans in Py–GC/MS results for (a) eucalyptus and (b) pine samples.

The formation patterns of these compounds varied significantly between eucalyptus and pine. In the eucalyptus samples, pyranose production was significant at lower temperatures, indicating that the primary decomposition of (hemi)­cellulose into pyranose occurred in the early stage. However, the pyranose production decreased rapidly with an increasing temperature, proving an intense secondary decomposition of pyranose itself. Notably, although secondary decomposition of pyranose was very significant in the range of 340–380 °C, there is no significant increase for components of pyranone, furanone, and HMFU. Conversely, in pine samples, we observed stable pyranose production across the entire temperature range, while HMFU steadily increased with increasing temperature, indicating that cellulose decomposition in pine is more moderate in both primary and secondary processes.

These differences in primary decomposition behavior were attributed to component: in particular, the (hemi)­cellulose content between eucalyptus and pine species. First, eucalyptus biomass exhibits a higher hemicellulose content compared to pine. Among the three principal constituents, cellulose, hemicellulose, and lignin, hemicellulose is the most thermally labile, undergoing decomposition at temperatures of 200–350 °C and tending to crack into small-molecule compounds. Rather, cellulose is more plausible to decompose into products like HMFU at the temperature until 400 °C. , Second, within the hemicellulose fraction, the predominant monomeric units differ between species: eucalyptus is primarily composed of pentose units, whereas pine contains a greater proportion of hexose units. Notably, pentose units exhibit lower thermodynamic stability under thermal stress relative to hexose counterparts, strengthening the distinctive thermochemical properties of eucalyptus and pine species. Therefore, it can be found that the decomposition process of eucalyptus carbohydrates closely resembles that of hemicellulose, characterized by primary and secondary decomposition at temperatures lower than 350 °C, with the secondary decomposition tending to break down into smaller molecules. Conversely, the decomposition process of pine carbohydrates exhibits stronger cellulose characteristics, namely, a more gradual primary and secondary decomposition until 400 °C, with the secondary decomposition tending to form larger molecules, including HMFU. Undoubtedly, this characteristic of eucalyptus leads to higher levels of OOCs formation, a point that will be discussed later.

In contrast to the decomposition products of (hemi)­cellulose, in the past decades, aromatic compounds have been identified in GC/MS results and can be attributed to the decomposition products of lignin. , However, because of lignin’s intricate structure, no aromatic compound was predominant, unlike the pyranoses that were generated predominantly via (hemi)­cellulose decomposition. Thus, as discussions of individual aromatic compounds would be both impractical and unlikely to yield meaningful insights, the lignin pyrolysis products were analyzed collectively, and variations in the total peak intensities are discussed qualitatively.

Figure shows the intensities of the peaks attributed to the aromatic compounds in the pyrolysis gases generated from eucalyptus and pine biomass. These data indicated that eucalyptus pyrolysis produced more aromatic compounds than pine pyrolysis. This difference was attributed to differences in the syringyl-to-guaiacyl ratio or variations in the linkages between lignin monomers. However, the temperature dependence of this aromatic compound production was less pronounced than that of pyranose production during (hemi)­cellulose decomposition. This phenomenon is consistent with the conclusions in the previous studies, that the activity of the chemical linkages in lignin varies, leading to its decomposition at a wide range of temperatures ranging from 100 to 900 °C. ,

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Peak intensity of aromatics in Py–GC/MS results of eucalyptus and pine samples.

Therefore, although previous studies have reported the possibility of the formation of OOCs from lignin, the decomposition temperature range is too wide to match the temperature range used in this study. Thus, further quantitative investigations are necessary to elucidate the precise role of lignin in the formation of OOCs.

3.3. Production of OOCs

As previously emphasized, harmful OOCs warrant detailed investigation. Among such compounds, acetic acid and acetol were identified through Py–GC/MS analysis. The corresponding peak intensities are shown in Figures and . Moreover, aldehyde production was quantified by using dedicated gas detector tubes, and the results are presented in Figure .

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Peak intensity for acetic acid in Py–GC/MS results of (a) eucalyptus and (b) pine samples. The error estimates are derived from the results of three replicates.

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Peak intensity for acetol in Py–GC/MS results of (a) eucalyptus and (b) pine samples. The error estimates are derived from the results of three replicates.

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Quantification of aldehyde generation in 100 mL of sampling gas extracted from the pyrolysis of (a) 0.1 g of eucalyptus and (b) 0.1 g of pine samples in a gas detection tube test. The error estimates are derived from the results of three replicates.

This quantification was performed as follows. GC/MS calibration curves for acetic acid and acetol were plotted with data obtained with toluene or dichloromethane solvents at varying concentrations (Figure S2a,b). The EFs of acetic acid, acetol, and aldehydes were calculated using eq . The mass produced at each temperature T(i), denoted as mass T(i), was quantified at 260, 300, 340, 360, and 380 °C, respectively (indicated by i = 1, 2, 3, 4, and 5, respectively). The extraction volume (V extraction) and sample mass (m sample) were 100 μL and 0.6 g for acetol and 100 mL and 0.1 g for aldehydes, and the respective heating protocols are described in Tables S2 and S3. The calculated EFs, including estimated errors, are listed in Table . All errors remained within a 25% tolerance range, confirming the reliability and reproducibility of the experimental results.

EFs=∑i=14(massT(i)+massT(i+1)2)×(Ti+1−Ti5C°/min)×(50mL/minVextraction)msample 1

2. EFs of Acetic Acid, Acetol, and Aldehydes .

  emission factors (g/kg)
sample type acetic acid acetol formaldehyde acetaldehyde
eucalyptus 17.0 ± 2.6 7.5 ± 1.9 14.9 ± 3.0 3.9 ± 0.2
pine 1.6 ± 0.2 0.6 ± 0.1 11.2 ± 1.1 3.0 ± 0.3
glucose reagent 1.8 ± 0.4 0.9 ± 0.2 8.1 ± 1.6 3.7 ± 0.2
lignin reagent n.d. n.d. 2.2 ± 0.4 1.6 ± 0.0
xylose reagent 0.9 ± 0.2 n.d. 12.0 ± 3.7 3.6 ± 0.7
acetylated xylose reagent 7.2 ± 2.1 n.d. 0.4 ± 0.1 1.8 ± 0.4
cellulose 2.0 2.0 no result no result
glucose 3.2 5.9 4.1 6.1
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a

Errors represent the standard deviations from three replicates. The temperature range for the calculations was 260–380 °C.

b

From ref. , constant temperature of 350 °C for 10 min

c

From ref. , fast pyrolysis from room temperature to 500 °C.

Significant acetic acid production in the eucalyptus samples began at 300 °C, reaching a level that was not observed in pine throughout the investigated temperature range (Figure ). The quantity of acetic acid generated via eucalyptus pyrolysis was 11 times that generated via pine pyrolysis.

When glucose and lignin reagents were used, negligible acetic acid was produced, indicating that cellulose and lignin components played minimal roles in acetic acid formation. Instead, a comparative analysis of xylose and acetylated xylose (Figure S3a) underscores the significant contribution of acetyl substituents to acetic acid generation. Accordingly, the observed differences in EFs were attributed to variations in both the abundance and reactivity of acetylated pyranose units within the hemicellulose fractions of eucalyptus and pine. Notably, the IR absorbance data (Figure ) indicated a 2-fold difference in the abundance of acetylated compounds, contrasting sharply with the 10-fold difference observed in EFs, suggesting that the reaction rate was a more critical factor in acetic acid production.

Several factors contributed to this phenomenon. First, more pyranose decomposition occurred in eucalyptus than in pine. This trend is evident in the thermal degradation profiles of (hemi)­cellulose presented in Figure and is further corroborated by existing literature on hardwood and softwood species. Second, the softwood-specific arabinose units in pine hemicellulose facilitated acetic acid migration and then released at elevated temperatures. Consequently, acetic acid production at lower temperatures was hindered in pine. These effects together accounted for the lower acetic acid formation observed in pine.

The temperature dependence and quantities of acetol production are listed in Figure and Table . Similar to acetic acid production, significant acetol production in eucalyptus began at 340 °C, reaching a level not observed in pine. The quantity of acetol produced via eucalyptus pyrolysis was 13 times that generated via pine pyrolysis.

The experimental results (Figure S3b) indicated that neither xylose, acetylated xylose, nor lignin contributed significantly to acetol formation. Instead, glucose was identified as the exclusive source. This does not contradict the literature on the formation of hemicellulose-derived acetol, since glucose units are also present in hemicellulose.

To further explain the detailed mechanism of glucose–acetol conversion, the retro–aldol pathway has been proposed as the primary most kinetically favorable mechanism for acetol formation because of its relatively low-energy barrier (Figure ). − The retro–aldol pathway included two stages: (a) dehydration, ring-opening, and C–C scission of hexose, forming an enediol fragment, and (b) enediol–ketone isomerization of the enediol fragment into acetol, which was the rate-determining step with an energy barrier of 240 kJ/mol. Therefore, a lower ratio of glucose–acetol conversion under a given temperature range is resulted from the barrier of enediol–ketone isomerization, a point also supported by the acetol yields from glucose pyrolysis at comparable temperatures. ,

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Graphical representation of the retro–aldol pathway in acetol formation from hexose. Enediol-type fragmentation products generated in stage (a) further isomerize to acetol in the stage (b).

However, the situation is significantly different when acetol production is considered in eucalyptus and pine samples, respectively. Acetol production in pine samples is consistent with the expected glucose component, but the 13–fold greater acetol production in eucalyptus appeared anomalous. Therefore, it is necessary to identify at which step in the retro–aldol pathway the difference occurs. Fortunately, maltol, a pyranone compound, was discovered in the pyrolysis gas. This compound has also been identified in literature on other biomass pyrolysis gases. , Since this compound requires a precursor of terminal-methyl substances that existed in the retro–aldol pathway, it can be considered an indicator of the progress at different steps in the retro–aldol pathway.

Therefore, the temperature-resolved generation of pyranose–maltol–acetol in eucalyptus and pine samples is shown in Figure . It can be seen that the difference in acetol yield between the two species is significantly greater than that of maltol, indicating that the difference in acetol generation originates from the final step of enediol–ketone isomerization. In other words, the enediol–ketone isomerization in eucalyptus is significantly accelerated.

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Peak intensity for pyranose–maltol–acetol in Py–GC/MS results of (a) eucalyptus and (b) pine samples. The result of pyranose is assigned to the left axis, and the results of maltol and acetol are assigned to the right axis.

However, the underlying causes of the acceleration in eucalyptus remain unresolved. One potential factor is that the steric hindrance of sp2 hybrid orbit is weakened by other groups (Figure S4). These groups encompass minor components specific to eucalyptus. Although the weight percentage of these minor components in eucalyptus was measured, their detailed composition, including fatty acids, terpenes, flavonoids, and various metal ions in unknown forms, remains unclear. This complexity makes it difficult to characterize individual species and assess their catalytic influence, limiting the conclusions of this study.

To address this issue, two future research directions are necessary. First, the possible forms of these minor inorganic and organic components must be identified, and their catalytic effects on cellulose pyrolysis need systematic evaluation. Quantitative effects of AAEM (alkali and alkaline-earth metal) chlorides, carbonates, and oxalates on cellulose pyrolysis products have been reported for inorganic components, ,, whereas trace organic components may contain dozens of molecules with short carbon chains and structural isomers. Understanding their catalytic roles will require an extensive literature review or targeted experiments.

Second, previous studies involve highly inconsistent reaction conditions (reactor type, pressure, temperature, duration, etc.), resulting in diverse catalytic behaviors. This indicates a complex, multivariable data set, making it difficult even to isolate factors influencing specific products such as acetol. In such cases, computational tools, particularly machine learning, may offer an effective solution. Recent work by Chen et al. showed that ML can successfully model nonlinear thermochemical processes by integrating diverse catalyst properties (e.g., BET surface area, crystallinity index) and operating parameters to predict multiple product distributions with high accuracy (R = 0.73–0.95). Their gradient boosting model also identified key descriptors and enabled multiobjective optimization across 479 reaction cases. These results strongly suggest that ML-based frameworks could be applied to the study of trace inorganic and organic components in cellulose pyrolysis, where similarly complex and multiparametric data sets are expected.

Less difference was observed in aldehyde production between eucalyptus and pine than in acetic acid or acetol production. Aldehydes were generated slightly earlier in eucalyptus than in pine, with EFs for eucalyptus being approximately 1.3 times greater than those for pine (Figure ). These results aligned with the production levels observed in other biomass-derived samples and were within acceptable limits. ,

The results of experiments using standard reagents indicated that aldehyde production correlated strongly with glucose and xylose rather than acetylated xylose or lignin (Figure S3c,d), suggesting that the distinct production profiles originated from primary or secondary decomposition of (hemi)­cellulose.

It is noteworthy that existing isotopic analyses of glucose thermal decomposition products indicate that the methyl carbons in acetol and acetaldehyde, as well as carbon in formaldehyde, are all predominantly originated from the C-6 position of glucose in a ratio of 34.2, 32.5, and 70.5%, respectively. , Furthermore, acetol and aldehydes exhibit similar temperature-resolved formations in all eucalyptus, pine, and glucose samples. Therefore, there is strong consistency with acetol and aldehydes, e.g., they may be products of parallel branching reactions in cellulose pyrolysis or probably play the role of the reactant and product separately in secondary reactions. The former hypothesis corresponds to a branch of the retro–aldol reaction that generates acetol, while the latter is assumed to refer to the homologous pathway of hydroxyacetaldehyde to formaldehyde (Figure ).

9.

9

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Proposed pathway for aldehyde production via acetol. Hydrogen atoms are abstracted by radicals.

However, previous reports lack definitive kinetic evidence to determine which hypothesis is more consistent with reality. For the branching reaction pathway of cellulose directly generating acetaldehyde, its detailed reaction kinetics require pathways designing and rate calculating using computational chemistry methods as that in reference to acetol. On the other hand, for the pathway of acetol generating acetaldehyde via H-abstraction, although its kinetic results under an oxidizing atmosphere have been given, it remains unknown whether it is comparable to the direct cellulose-to-aldehydes pathway during the pyrolysis stage. In any case, finding the optimal pathway for the generation of aldehydes from glucose or xylose units is challenging under the methodology of this study and requires further study in the future, and methods that include isotopic labeling experiments, quantum chemistry computation, or lower-cost machine learning approximations are expected to play a stronger role. ,

Finally, although this study points out the differences in the formation of OOCs and mechanistic attribution between eucalyptus and pine during pyrolysis, this still differs from emissions under real wildfire conditions. For example, the OOCs generated during pyrolysis can convert to more complex pollutants, such as methane, carbon monoxide, polycyclic aromatic hydrocarbons, and particulates, under the high-temperature and high-speed heating protocol of the oxidative combustion stage. However, there is no doubt that the differences in the composition of eucalyptus and pine as well as the differences in pyrolysis products lead to the diversity of pollutant emissions during the oxidative combustion phase. Therefore, the conclusions of this study regarding the pyrolysis stage are expected to be further extended to the oxidative combustion stage in future work, in order to construct a comprehensive study of wildfire pollutant emissions.

4. Conclusions

In this study, the production of OOCs during eucalyptus and pine pyrolysis was quantified under rigorously controlled conditions characterized by low temperatures, slow heating rates, and temperature-resolved sampling. Acetic acid, acetol, formaldehyde, and acetaldehyde were identified in the pyrolysis gases, and their respective EFs were determined to be 17.0, 7.5, 14.9, and 3.9 g/kg dry biomass for eucalyptus and 1.6, 0.6, 11.2, and 3.0 g/kg dry biomass for pine.

The mechanistic factors resulting in the distinct production profiles of the OOCs were elucidated through detailed individual experiments of isolated biomass constituents. Acetic acid generation was strongly linked to acetylated pyranose units in hemicellulose, with the elevated yield in eucalyptus was attributed primarily to its high chemical reactivity, with a minor contribution from the abundance of acetylated moieties in hemicellulose. Glucose reagent is the exclusive source of acetol, and its production in eucalyptus diverged from the expected enediol–ketone isomerization in the retro–aldol pathway, suggesting a possible interaction of component specific in eucalyptus, although the identity and its mechanistic factor in the level of elementary reaction remains unknown. Furthermore, aldehydes can be derived by both glucose and xylose reagents in (hemi)­cellulose, and the distinct aldehyde production profiles were attributed to the distinctive decomposition behavior of such components in eucalyptus and pine species. A more detailed understanding of the kinetic mechanism precise to the level of elementary reactions is expected to be further investigated in the future by using computational methods.

Finally, differences in the composition of eucalyptus and pine, as well as the differences in pyrolysis products, lead to the diversity of pollutant emissions (e.g., methane, carbon monoxide, polycyclic aromatic hydrocarbons, particulates, etc.) during the oxidative combustion phase. Future work that links pyrolysis behavior with oxidative combustion experiments will be necessary to more accurately quantify species-dependent differences in total fire emissions.

Supplementary Material

ao5c08838_si_001.pdf (837.9KB, pdf)
ao5c08838_si_002.xlsx (2.8MB, xlsx)

Acknowledgments

The authors thank Associate Professor Akizuki for providing the sample preparation instruments in T-FTIR experiment. This work was supported by JST SPRING, Grant Number JPMJSP2108. This study was partly supported by JSPS KAKENHI (grant number 23KK0165).

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c08838.

  • Table S1: Instrumental parameters of transmission-FTIR; Figure S1: Instrumental scheme; Table S2: Instrumental parameters of Py–GC/MS; Table S3: Instrumental parameters of gas detector tube; Section S1: Experimental procedures of compositional analysis; Table S4: Compositional analysis of samples; Table S5: Residue mass after pyrolysis; Figure S2: Calibration curve of acetic acid and acetol; Figure S3: GC/MS result of isolated constituents; Figure S4: Role of third bodies in enediol–ketone isomerization (PDF)

  • Experimental raw data: (i) Raw transmission FTIR data; (ii) Raw Py–GC/MS data; (iii) Raw Gas detector tube data (XLSX)

X.X.: Analyzed data, wrote the paper. M.F.: Analyzed data, reviewed the paper. K.T.: provide instrument, reviewed the paper. CRediT X.X. contributed to conceptualization, data curation, formal analysis, validation, investigation, methodology, project administration, writingoriginal draft, and visualization. M.F. contributed to formal analysis and writingreview and editing. K.T. contributed to methodology, resources, supervision, and writingreview and editing.

During the preparation of this work the author(s) used Copilot in order to Grammar Correction. After using this tool/service, the author(s) reviewed and edited the content as needed and take(s) full responsibility for the content of the publication.

The authors declare no competing financial interest.

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