Edited by Barbara J. Finlayson-Pitts, University of California, Irvine, CA, and approved June 7, 2016 (received for review March 22, 2016)
Furans and related large oxygenated organic carbon species (OC) are highly toxic pollutants. Their integration into soot particles may greatly enhance soot’s hygroscopicity, leading to regional and global climate change. We show that furans are the primary oxygenated functional group on soot formed in hydrocarbon combustion and report a reaction scheme that elucidates the interplay between nonoxygenated and oxygenated hydrocarbons. We expect this reaction pathway to be important in many hydrocarbon oxidation systems spanning geosciences, astrophysics, and energy research. We discovered ∼100 oxygenated species previously unaccounted for in hydrocarbon models. This study advances the understanding of the oxidation chemistry of OC, which is critical to many processes, from controlling emissions of toxic combustion by-products to reducing anthropogenic climate change.
Many oxygenated hydrocarbon species formed during combustion, such as furans, are highly toxic and detrimental to human health and the environment. These species may also increase the hygroscopicity of soot and strongly influence the effects of soot on regional and global climate. However, large furans and associated oxygenated species have not previously been observed in flames, and their formation mechanism and interplay with polycyclic aromatic hydrocarbons (PAHs) are poorly understood. We report on a synergistic computational and experimental effort that elucidates the formation of oxygen-embedded compounds, such as furans and other oxygenated hydrocarbons, during the combustion of hydrocarbon fuels. We used ab initio and probabilistic computational techniques to identify low-barrier reaction mechanisms for the formation of large furans and other oxygenated hydrocarbons. We used vacuum-UV photoionization aerosol mass spectrometry and X-ray photoelectron spectroscopy to confirm these predictions. We show that furans are produced in the high-temperature regions of hydrocarbon flames, where they remarkably survive and become the main functional group of oxygenates that incorporate into incipient soot. In controlled flame studies, we discovered ∼100 oxygenated species previously unaccounted for. We found that large alcohols and enols act as precursors to furans, leading to incorporation of oxygen into the carbon skeletons of PAHs. Our results depart dramatically from the crude chemistry of carbon- and oxygen-containing molecules previously considered in hydrocarbon formation and oxidation models and spearhead the emerging understanding of the oxidation chemistry that is critical, for example, to control emissions of toxic and carcinogenic combustion by-products, which also greatly affect global warming.
Oxygenated hydrocarbons produced during combustion can have a wide range of detrimental effects on human health, air quality, and regional and global climate. Furans, for example, are compounds that contain five-membered rings with four carbon atoms and one oxygen atom. They are frequently observed in the exhaust plumes and nearby environment of combustion sources. Many studies have shown that they are toxic, whether ingested or inhaled, and thus pose a considerable threat to human health (1⇓⇓–4). The simplest of these compounds (i.e., unsubstituted furan, C4H4O) is a cyclic, dienic ether with a low molecular weight, high volatility, and high lipophilicity. Studies on rats and mice have shown a dose-dependent increase in hepatocellular adenomas and carcinomas, indicating that furan is carcinogenic (4), and furan is marked as a high-priority substance and a carcinogenic risk by the World Health Organization (5).
Combustion sources of furans include biomass burning (6⇓⇓–9), cigarette and pipe smoke (10, 11), waste incineration (12), electronic waste recycling (13, 14), and volcanic activity (15). The polychlorinated dibenzofurans (PCDFs) are among the most notorious environmental pollutants, and the main source of PCDFs is biomass burning (6⇓⇓–9, 16). Nonchlorinated furans and PCDFs have been shown to be kinetically linked (17, 18). Furans released during combustion are often partitioned into particles and are found in ash from peat (9) and wood (6) burning, in primary organic aerosols from meat cooking (19), and in secondary organic aerosols from hydrocarbon oxidation (20, 21). Wood burning for heating and cooking constitute a major human exposure to airborne particulate PCDFs in some parts of the world (22, 23).
Previous work has suggested that oxygenated species can be attached to surfaces of soot particles of varying maturity emitted from flames and diesel engines, even before atmospheric processing (24⇓⇓⇓⇓⇓⇓⇓–32). Functional groups that have been identified include alcohols/enols, carbonyls, peroxies, and ethers. Oxygen atoms bound to organic species on the particle surface have been shown to greatly affect soot hygroscopicity (28) and the ability of soot particles to adsorb atmospheric water vapor and act as cloud-condensation or ice nuclei. Soot particles emitted from combustors, such as diesel engines, are generally hydrophobic, and enhancements in hygroscopic particle emissions could have substantial indirect climate effects via their influence on cloud formation (33). The effect of soot emissions on cloud-nucleation properties is a major uncertainty in climate predictions (34⇓–36).
Despite the impact of large oxygenated hydrocarbons on combustion chemistry, the environment, and human health, very little is known about their formation mechanisms and emissions. In this paper we present evidence of the formation of oxygenated compounds, including furans, during the combustion of hydrocarbon fuels. Via a synergistic approach that includes ab initio methods and a stochastic model in conjunction with experimental measurements, we identify reaction pathways leading to formation of oxygenated compounds during the combustion of ethylene. We recorded aerosol mass spectra sampled from premixed and diffusion flames, using synchrotron-generated vacuum-UV (VUV) radiation for ionization, for comparison with masses of the predicted chemical compositions. The mass spectra show masses of oxygenated species that agree with the atomic compositions predicted by the simulations. Both experiments and simulations demonstrate that ∼50% of the mass peaks observed at some flame heights in the mass range 140–350 u (unified atomic mass units) contain signal from oxygenated species. We also recorded X-ray photoelectron spectroscopy (XPS) spectra of soot samples extracted from these flames for further validation of these mechanisms by comparison with functional groups of the predicted oxygenated species incorporated into particles. The XPS measurements confirm formation of furan precursors, hydroxyl groups, early in the soot-formation process and evolution of furan signatures, ether groups, as the combustion and particles evolve.
The present study represents an important step toward the development of predictive models for the oxidation of hydrocarbons, which will require that the presence and reactivity of these oxygenated compounds are taken into account. Understanding the chemistry related to high-temperature hydrocarbon oxidation may provide a key to controlling emissions of harmful combustion byproducts, such as soot, nonchlorinated furans, and PCDFs, leading to multiple environmental and health benefits.
We investigated new reaction pathways for the formation and decomposition of oxygenated species using ab initio electronic-structure calculations to elucidate how oxygen becomes incorporated into the carbon framework of organic carbon (OC) species and soot. We found a generic furan-formation route with low reaction barriers involving species that are abundant in hydrocarbon flames, such as acetylene. The reaction pathway is likely to end with a unimolecular ring closure, suggesting that there should be a high fraction of furans present among oxygenated OC formed during combustion. Fig. 1 illustrates the barriers (TS1 and TS2) of the unimolecular furan-ring closure by showing the formation of benzofuran from phenoxy-acetylene. The energy barriers are relatively low compared with the average temperature fluctuations in combustion environments.
Potential-energy diagram for the formation of benzofuran. The energies are in kilocalories per mole and are referenced to the reactant species. The ring-closing reaction is exothermic, and the closed-ring structure is thus favored over the open-ring structure. The energy barriers are low compared with the average temperature fluctuations in flames. For example, at 1,500 K, TS1 is 3kBT and TS2 is −1kBT with respect to the reactant species.
Reaction rates for different oxygen chemistry pathways were calculated and incorporated into the Stochastic NAnoParticle Simulator (SNAPS) (37) along with kinetic rates from the literature that accounted for different oxygen-adsorption reactions and ring-closure reactions. SNAPS is a probabilistic code that predicts the formation of different soot-precursor molecules and other OC species in flames based on individual reaction trajectories of initial seed molecules as they evolve in the flame.
A premixed C2H4/O2 flame, in which fuel and oxygen are mixed before ignition, was chosen for the SNAPS simulations. SNAPS yielded six main classes of oxygenated groups: alcohol/enol, peroxy acid/radical, ketene, pyran, noncyclic ether, and furan. Alcohols/enols and peroxy acids and radicals are formed via reactions of hydrocarbons with small oxygen-containing molecules, mainly OH and O2, and serve as precursors to the other four oxygenated groups. Enols containing 6–14 carbon atoms (Fig. S1) account for ∼20% of the oxygenated structures close to the burner, where oxygen-containing radicals are present, demonstrating that large enols are important for the chemistry of oxygenated hydrocarbons, and extending the studies of small enols by Taatjes et al. (38) (Important Enol Intermediates Predicted by SNAPS). We found that, in addition to the isomerization reaction, removal of enols may proceed via reactions with flame radicals, particularly H and OH, producing oxyradicals. Production of large enols can proceed through radical attacks on the carbon–carbon double bond of enols, followed by alkyl radical addition.
Enols predicted by the SNAPS simulations. Enols predicted to be important to the OC chemistry at small DFFOs in premixed combustion are (A) 94 u (phenol), (B) 108 u (C7H8O), (C) 118 u (C8H6O), (D) 168 u (C12H8O), and (E) 192 u (C14H8O).
The SNAPS simulations showed that ketenes are formed as the result of oxyradicals present on the terminal carbon of aliphatic chains. Noncyclic ethers are produced when hydrocarbons add to oxyradical sites, following three evolution pathways: (i) decomposition, (ii) continued hydrocarbon addition, or (iii) ring-closure reactions to form furan/pyran groups. Pyran groups are rare, forming when propargyl or methyl and acetylene add to oxyradicals on free-edge sites of aromatic rings, or when acetylene adds to oxyradicals on zigzag sites (see Fig. S2 for site definition). The results also showed potential precursor molecules for formation of dioxins, species with six-membered rings including two oxygen atoms. However, the code does not include any elementary reactions that close a ring containing two oxygen atoms.
Definition of a zigzag site and a free-edge site.
Our simulation results demonstrate that oxygen addition to polycyclic aromatic hydrocarbons (PAHs) occurs mainly through consecutive bimolecular reactions resulting in H-abstraction followed by addition of OH or O2 onto PAH edge sites (Comments on the SNAPS Modeling). The most probable pathway for embedding oxygen into the hydrocarbon molecules is via ethers formed when H is abstracted from hydroxyl groups or OH/O is abstracted from peroxy acid/radical groups, followed by hydrocarbon addition to the oxyradical and furan-ring closure. Acetylene is the most frequently added hydrocarbon, and Fig. 2 shows the most probable reaction sequence leading to formation of a furan group. We expect this scheme to be important to a wide range of hydrocarbon oxidation processes and virtually any hydrocarbon-combustion system because of high acetylene concentrations and low reaction barriers. Thus, apart from the toxic potential of furan species, furans may also play a dominant role in determining growth and oxidation sites, radiative forcing potential, and hygroscopic properties of the OC formed during combustion. As these oxygenated OC species evolve in the flame, they can become large enough to condense onto incipient soot particles, leading to incorporation of oxygen onto the surface of soot. Oxygen embedded in the particle surface is expected to contribute to gaseous oxidation products and affect the further growth and oxidation of the particle (17).
Most probable reaction sequence leading to formation of a furan group. (Left to Right) H-abstraction followed by OH addition to the radical free-edge site on an aromatic ring; H-abstraction from the OH group, followed by acetylene addition, forming an ether group; H-elimination during ring closure to form a furan group. The left side of the molecule has been left attached to an indeterminate PAH backbone to illustrate an arbitrary molecular size (see Comments on the SNAPS Modeling for details on OH addition).
Fig. 3 shows some of the most common structures obtained from SNAPS at 160, 168, 194, and 220 u low in the premixed flame. Many of the ether structures formed are furan precursors. Competing reactions to furan-ring closure in the final step in Fig. 2 includes addition of a second hydrocarbon species (Fig. 3 A and B) or H-addition forming an R–O–CH=CH2 group (Fig. 3 D and F) or acetylene removal. The most common oxygenated structure at 192 u is the 194-u structure shown in Fig. 3D with a furan ring instead of the –OCHCH2 chain; the most common structure at 158 u is the furan formed with the oxygen attached to the phenyl ring in Fig. 3B (Information Related to Fig. 3). The species in Fig. 3B has a ketene group at the terminal carbon of the aliphatic branch, showing that, if acetylene adds to a radical site and OH or O2 adds to the acetylene chain, a ketene may form instead of a furan and that aliphatic side chains provide sites for the formation of carbonyl bonds.
Frequently predicted oxygen-containing structures of selected masses low in a premixed flame. Red atoms, oxygen; gray, carbon; white, hydrogen. (A and B) Ether and ether/ketene, 160 u. (C) Furan, 168 u. (D and E) Ether and furan, 194 u. (F) Ether, 220 u.
The evolution of the structure in Fig. 3F frequently proceeds via acetylene loss from the oxygen atom or formation of a pyran ring with the zigzag-site carbon, yielding a structure with mass 218 u. Many of the furans formed are potential PCDF precursors (e.g., the substituted dibenzofuran in Fig. 3E), although naphtho[2,1-b]furan is more common in the simulations than dibenzofuran at 168 u (Fig. 3C). The majority of the species predicted by SNAPS experienced molecular growth as they evolved in the flame, and furans constituted the largest group of oxygenated species at large distance from the fuel outlet (DFFO), where the flame temperature was around 1,750 K. An important furan-destruction pathway identified in the simulations is CO reactions: CO can open furan rings and abstract the oxygen atom to form CO2, leaving the original furan ring as an aliphatic radical side chain. This mechanism may explain the observed aliphatics on the surface of soot particles under some conditions (25, 27, 31, 39).
To support and validate our modeling results, we studied two McKenna-type premixed C2H4/O2 flames with equivalence ratios matching the flame studied using SNAPS and a counterflow diffusion (CF) flame (Burners and Gas Flows). In premixed combustion the oxidizer is mixed with the fuel before reaching the flame front, and the concentration of volatile oxygenated hydrocarbons is expected to peak at small DFFOs where OH and O2 are present. In the CF flame, fuel and oxidizer are fed separately via counterpropagating flows; mixing of fuel and oxidizer takes place across the gas-stagnation plane formed by the counterpropagating flows. The CF flame is an excellent flame for comparison with premixed combustion because there is no oxygen available at small DFFOs, and large oxygenated structures cannot be present at small DFFOs. Hydrocarbon growth initially takes place under pyrolytic conditions, and particles are horizontally convected away from the flame parallel to the gas-stagnation plane, where the vertical velocity component vanishes.
We performed synchrotron-coupled VUV aerosol mass spectrometry (AMS) of soot particles drawn from the different flames and injected directly into the mass spectrometer using an aerodynamic lens system (ADLS) (see AMS for more details). Large gas-phase species can condense onto particles in the sampling line between the flame and the ADLS, which leads to physical growth of the particles while the chemistry is frozen, allowing small particles to reach sizes that can be efficiently focused by the ADLS. The particle growth also permits us to detect large gas-phase OC species that are prone to condense onto soot particles at atmospheric temperatures but not at flame temperatures. Species condensed onto soot particles are vaporized in the ionization region of the mass spectrometer without changing species internal composition and mass. Fig. 4 shows example mass spectra of species drawn from three DFFOs in a premixed flame. The results are consistent with the SNAPS simulations and demonstrate that a remarkably large fraction (∼50%) of the mass peaks is associated with oxygen inclusion (Determination of Precise Masses and Atomic Constituents, Table S1, Nominal Masses of Oxygen-Containing Species Determined from the Aerosol Mass Spectra, and Table S2). Mass peaks identified as being associated with oxygenated species are highlighted in red in Fig. 4. The masses of these species (Table S2) agree with the masses of oxygenated OC species predicted by SNAPS, that is, the experimentally observed and predicted species have the same atomic compositions (Table S1). Our AMS studies revealed that these oxygenated species are present under a very wide range of combustion conditions, suggesting generic formation mechanisms.
AMS spectra from a premixed flame. Mass spectra are shown for particles extracted from selected heights in the flame; that is, DFFOs of (A) 3.5, (B) 5.0, and (C) 7.5 mm. Red peaks contain signal from oxygenated species. The arrows indicate the peaks at 160, 194, and 220 u for comparison with Fig. 5A; see Fig. 3 for main predicted structures at a DFFO of ∼3.5 mm.
Analysis of some mass peaks
Nominal masses of oxygen-containing molecules observed in the flames
Fig. 4 shows that the signal from oxygenated OC gets weaker at larger DFFOs in premixed flames, which indicates that they are only formed at low DFFOs in premixed combustion (i.e., where small oxygen-containing species, such as OH and O2, are present). The SNAPS simulations showed that oxygen addition to PAHs occurred predominantly below a DFFO of 2 mm (Fig. S3), where the concentrations of reactive oxygen (e.g., OH and O2) are significant in these flames. The soot particles are, however, too small below a DFFO of 3.4 mm to yield signal in the mass spectrometer (see AMS for details). Hence, the mass spectrum in Fig. 4A was recorded at a DFFO of ∼3.5 mm. The SNAPS simulations predicted that the oxygenated species continue to grow as they evolve in the flame, which is consistent with the signal decrease for the red peaks in Fig. 4 with increasing DFFO. As their sizes increase and they become less volatile, they are likely to condense onto incipient soot particles and become integrated into the soot, which further reduces their volatility and prevents them from being vaporized in the ionization region of the AMS instrument. Hence, large oxygenated species are expected to avoid detection at large DFFOs.
Predicted H and OH mole fractions at low flame heights in the Ar-diluted premixed flame using the Appel–Bockhorn–Frenklach (47) mechanism. The ratio between the H and OH mole fractions is also shown. The vertical dashed lines are the edges of the region where the majority of the oxygenation occurs, according to the SNAPS simulations.
Fig. 5A shows the ion-signal DFFO dependence for 160, 194, and 220 u for a premixed flame (Fig. 3 displays major predicted structures for these masses). These three masses are indicated by arrows in Fig. 4. Their spatial profiles peak at the lowest DFFO sampled, and we are unable to experimentally establish the location of maximum concentrations because of lack of AMS signal at lower DFFOs. Profiles inferred from mass peaks identified by the SNAPS simulations to stem from pure (nonoxygenated) hydrocarbons, for example C18H10 isomers (226 u) and C20H10 isomers (250 u), have low intensities at small DFFOs for this flame. Fig. 5B shows that the same trend with reactive oxygen was obtained from the CF flame. In the CF flame the growth of hydrocarbon species initially takes place on the fuel side of the flame under pyrolytic conditions, and the reaction scheme in Fig. 2 cannot occur at small DFFOs. We identified many of the same oxygenated masses in the CF flame as in the premixed flames (e.g., 160, 194, and 220 u) (Table S2), and, as expected, they all appear skewed toward the oxygen side of the flame compared with masses identified as pure hydrocarbon masses (e.g., 226 and 250 u), as shown in Fig. 5B.
Normalized signal profiles at selected masses for (A) a premixed flame and (B) the CF flame. Solid lines refer to masses that include oxygenated species; dashed lines show profiles of nonoxygenated hydrocarbons. For clarity, every third symbol is shown in B.
The profiles at 194 and 220 u reach minima at ∼6.5 mm DFFO in Fig. 5A and then increase with DFFO. The intensity of the 160-u mass peak does not increase at large DFFOs. Analysis of the exact masses of the 194- and 220-u peaks shows that these two peaks shift slightly toward higher masses with increasing DFFO. These shifts in mass-peak locations indicate that pure hydrocarbons are formed at these two nominal masses at large DFFOs, because the combined mass of one carbon and four hydrogen atoms is slightly larger than the mass of one oxygen atom. These nonoxygenated hydrocarbons increase with increasing DFFO; this observation is consistent with previous gas-phase PAH measurements in sooting premixed flames (e.g., ref. 40). A similar result is obtained in the CF flame (Fig. 5B), where the profile at 160 u has a larger shift toward the oxygen side than the profiles at 194 and 220 u, indicating that the signal at 160 u stems entirely from oxygenated species whereas pure hydrocarbon structures contribute to the signals at 194 and 220 u.
The mass spectra in Fig. 4 are composed of clusters of peaks. The width of each cluster is a crude measure of the spread in hydrogenation among species containing the same number of carbon atoms, because each cluster is largely composed of peaks corresponding to OC species containing a specific number of carbon atoms. The low-mass side of the clusters contain peaks stemming from less-saturated (more aromatic) species without oxygen content. Highly saturated species and species with oxygen atoms appear on the high-mass side of the clusters because an oxygen atom weighs more than the most saturated hydrocarbon group for which it can substitute (i.e., CH3). The high-mass side of a cluster may also contain signal from species of very low saturation (e.g., odd-carbon-numbered clusters may include signal from the polyyne species containing one additional carbon atom). Hence, on the high-mass side of each cluster are peaks stemming from species that may contain a different number of carbon atoms than the species yielding peaks in the center and on the low-mass side of the cluster. Some oxygenated OC species have nominal masses identical to those of large polyynes and are likely to be misidentified in soot-sample aerosol mass spectra.
The AMS study confirmed the atomic constituents of the oxygenated OC species predicted by the SNAPS simulations. To validate the predicted OC structures, information on how the oxygen is bonded in the OC molecules is needed. We thus performed XPS studies on soot samples extracted from the premixed flames, as shown in Fig. 6 (XPS). The XPS data verified the SNAPS prediction that alcohol/enol groups are formed low in the flame, and their concentrations decrease with increasing DFFO. The abundance of ethers included in soot particles increases with increasing DFFO, suggesting that alcohols/enols are precursors to ethers. The fractions of C-OH, C-O-C, and C=O species at different DFFOs were determined by analysis of the O 1s spectra (30, 41) and are shown in Fig. 6D. The low fraction of C=O species suggests that our soot samples had not been significantly affected by atmospheric oxygen between flame sampling and XPS measurements (42). The strong XPS signal from oxygen-containing functional groups at large DFFOs suggests that the reduced signal from the mass peaks correlated with oxygen inclusion in the mass spectra recorded at DFFOs of 5.0 mm and 7.5 mm (Fig. 4 B and C) compared with the mass spectrum recorded at a DFFO of 3.5 mm (Fig. 4A) is not due to lack of oxygenated OC in the soot. This observation is consistent with incorporation of oxygenated OC in the soot at larger DFFOs, as predicted based on the SNAPS simulations and the AMS studies.
XPS O 1s spectra of soot sampled from a premixed flame. Spectra are shown for particles extracted from (A) a low flame height (DFFO of 3.5 mm), (B) an intermediate flame height (DFFO of 5.0 mm), and (C) high in the flame (DFFO of 7.5 mm). (D) Fractional contributions of oxygenated functional groups were inferred from fits to these and similar XPS O 1s spectra at other DFFOs and plotted as a function of DFFO. Error bars represent ± 1 SD of the uncertainties associated with the peak fits.
Despite the distinctly different flame conditions for premixed and diffusion-controlled combustion, we observed consistent masses and behavior regarding formation of oxygenated OC, supporting the generality of large-oxygenate formation, as described in Fig. 2. These reaction pathways are expected to have substantial impact on soot properties, such as hygroscopicity and toxicity. The apparent generality of these chemical mechanisms also suggests that they are important to hydrocarbon oxidation systems other than combustion and may be important in geophysics, astrophysics, and energy research.
We discovered ∼100 oxygenated species previously unaccounted for in hydrocarbon-mass-growth and oxidation models. Under the present configurations, oxygen addition to PAHs occurs mainly via reactions with OH and O2. Enols and alcohols that result from these reactions act as precursors to ethers, such as furans and pyrans. Furans are produced in the high-temperature regions of hydrocarbon flames, and they remarkably survive and become the main functional group of oxygenates in the OC on soot particles. Cyclic ethers therefore play a more important role than carbonyl and hydroxyl groups during some phases of organic carbon oxidation and may require inclusion in many hydrocarbon oxidation mechanisms and in models aimed at predicting toxicity and hygroscopicity of soot particles and related OC.
The oxygenated OC species discovered in the present study seem to readily become partitioned into incipient soot particles, the lighter ones at atmospheric temperatures and the heavier ones also at flame temperatures. These species should therefore be integral to the molecular-growth pathways leading to soot formation as well as to OC coating of soot particles in the combustor and in the atmosphere. The emission of oxygenated OC and soot can be minimized by an oxygen-rich postcombustion zone at an elevated temperature that should be kept below the threshold for NOx formation. Furans can be removed through CO reactions as identified by the SNAPS simulations.
Further studies are needed to improve the knowledge of carbon–oxygen interactions in the oxidation and growth chemistry discussed here. Understanding how soot forms and interacts with OC can lead to more efficient combustion technologies that reduce soot emissions, which would have many benefits (e.g., reduced global warming and anthropogenic hydrological impact and reduced concentrations of toxic and carcinogenic environmental pollutants). Flame studies of soot and related OC are, however, also important because they can provide benchmarks for hydrocarbon growth and oxidation models important in other research fields, such as astrophysics (43) and fuel cells (44).
Chemical kinetics simulations were performed using SNAPS (37, 45) (Comments on the SNAPS Modeling). Briefly, SNAPS is Monte Carlo-based and simulates trajectories of reversible reactions for given seed molecules. General insights into the growth/consumption processes require analysis of a large number of trajectories. SNAPS necessitates information on temperature and gas-phase species concentrations. These inputs were computed by solving the gas-energy equation using the PREMIX program in CHEMKIN (46) with the gas-phase mechanism from Appel et al. (47) because it best represented an average of the speciation profiles generated when using four well-validated gas-phase mechanisms [Appel et al. (47), Miller and Melius (48), Richter et al. (49), and Raj et al. (50)]. Benzene and toluene were used as seed molecules because formation of the first aromatic ring is considered to be the first step in soot formation (51). Molecules formed in the simulations were classified by carbon configuration using social permutation invariant topological coordinates (52); see ref. 37 for more details.
Electronic-structure calculations were carried out using the CBS-QB3 method (53) as implemented in Gaussian 09 (54). Singlet, doublet, and triplet spin multiplicities were tested for the reactant, and the lowest energy (doublet) was determined to be the ground state and thus used for all subsequent calculations. Intrinsic reaction-coordinate calculations were carried out at the B3LYP/CBS level to ensure that each optimized transition state connected the expected reactant and product.
Information about the burners and the gas flows are available in Burners and Gas Flows. Briefly, the premixed burner is a McKenna design with a 38.1-mm sintered-brass plug surrounded by a shroud-gas sintered-brass ring. The counterflow diffusion burner consists of two vertically mounted, central flow tubes, facing each other, 12 mm apart. They have a 12.7-mm inner diameter and are surrounded by outer flow tubes that help shield the flame from the surroundings. Fuel and oxidizer were fed separately via counterpropagating flows; mixing of fuel and oxidizer took place across the gas-stagnation plane formed by the counterpropagating flows. Hydrocarbon growth initially takes place under pyrolytic conditions, and particles are horizontally convected away from the flame parallel to the gas-stagnation plane, where the vertical velocity component vanishes.
Soot particles were sampled along the vertical centerline of the flames using a quartz probe with a tapered tip. The probe assembly is fixed, and, to sample from different DFFOs, the burner assembly is translated vertically. In the AMS measurements, the sample enters an ADLS (55, 56), which focuses particles larger than ∼50 nm diameter into a beam. Condensation in the sampling line leads to particle growth, which enables some particles that are originally smaller than 50 nm to be focused by the ADLS onto a target heated to ∼570 K inside a vacuum chamber (∼7 × 10−7 Torr). Species that vaporize from the heated target are photoionized using VUV radiation at 9.6 eV, generated at the Advanced Light Source (ALS) synchrotron facility at Lawrence Berkeley National Laboratory. The molecular ions generated are pulse-extracted into a time-of-flight mass spectrometer at a rate of 15 kHz, and mass spectra are recorded using a multichannel scaler. Additional information about the aerosol mass spectrometry investigation is available in AMS.
Soot samples for the XPS measurements were collected by extracting particles from the N2-diluted premixed flame using quartz microprobes from the same batch of probes that was used for the AMS measurements. The particles were drawn from different points along the vertical centerline of the flame at the same flow rate as was used during the AMS studies. The particles were passed through an aerosol neutralizer (TSI 3087) and collected on clean disks of pure aluminum (99.5 pure, 12.5-mm diameter; Goodfellow) using a nanometer aerosol sampler (TSI 3089).
The XPS measurements were performed under ultra-high vacuum conditions (residual pressure <1 × 10−9 Torr) using an Omicron DAR400 Mg K-alpha X-ray source and Physical Electronics 10–360 electron energy analyzer. The spectra shown have not been corrected or processed apart from a background subtraction. The calibration of our instrument was confirmed by measuring a clean sample of highly oriented pyrolytic graphite, which gives a sharp C 1s peak centered at 284.4 eV and FWHM of 1.2 eV, in good agreement with the literature (30, 41). The fitting process, quantitative analysis, and error estimation were performed using the CasaXPS software. All XPS peaks were baseline-corrected using a Shirley background, and a mixed Gaussian–Lorentzian (70%/30%) line shape was used to fit the spectra. The O 1s photoemission spectra were deconvolved using three components corresponding to the oxygen functional groups: C-OH (533.2 eV), C-O-C (531.9 eV), and C=O (530.2 eV), according to Müller et al. (30).
The premixed burner was a McKenna design with a 38.1-mm sintered-brass plug surrounded by a shroud-gas sintered-brass ring. Two different water-cooled stabilization plates were used: a square aluminum plate with 100-mm side length and a stainless-steel cylinder with 38.1-mm diameter. The aluminum plate was located 12 mm above the sintered-brass plug and was maintained at ∼335 K. The stainless-steel cylinder was mounted 13.3 mm above the plug, and the surface temperature of the cylinder was kept at ∼400 K. The gas-flow rates through the sintered-brass plug used in conjunction with the aluminum plate were 0.75 L/min (lpm) C2H4, 1.10 lpm O2, and 3.40 lpm Ar. All gas flows are referenced to 273 K and 1.01325 × 105 Pa. The shroud-gas flow rate was 14 lpm Ar. The corresponding gas-flow rates used in combination with the stainless-steel cylinder were 0.67 lpm C2H4, 0.96 lpm O2, and 3.62 lpm N2 through the porous plug. The shroud-gas flow rate was 19 lpm N2.
The counterflow diffusion burner consists of two vertically mounted, central flow tubes, facing each other, 12 mm apart. They have a 12.7-mm inner diameter and a maximum outer diameter of 15.875 mm. The outer walls have a 9-mm taper to the edge at the outlets. Fuel and oxidizer were fed separately via counterpropagating flows; mixing of fuel and oxidizer took place across the gas-stagnation plane formed by the counterpropagating flows. Hydrocarbon growth initially took place under pyrolytic conditions, and particles were horizontally convected away from the flame parallel to the gas-stagnation plane, where the vertical velocity component vanished. The fuel-gas mixture was supplied through the lower flow tube and consisted of 0.23 lpm C2H4 and 1.10 lpm Ar. Thus, oxygenated structures could not form at small DFFOs because there was no oxygen available, which was verified experimentally. The flow through the upper tube was for the oxidizer, and it consisted of 0.25 lpm O2 diluted in 1.20 lpm Ar. Outer flow tubes with inner diameter 18.364 mm surround the central flow tubes. The outer flow tubes were used for Ar flows, 2.30 lpm through the lower one and 3.00 lpm through the upper one.
Sample gas and soot particles were extracted from the flames using a quartz microprobe manufactured from a straight quartz tube with an o.d. of 2.9 mm and an i.d. of 2 mm. The last ∼5 mm of the probe end facing the flame was tapered down to a tip opening of ∼300 μm, and the probe drew sample from the flame at a rate of ∼70 standard cubic centimeters per minute (sccm). The relatively large opening diameter of the probe was necessary to avoid the soot particles generated in the present flames from clogging the probe during the measurements. The probe was slightly longer than 40 mm, because the outer radius of the premixed burner is 38.1 mm. The quartz probe was connected to an aluminum probe block using a Swagelok connection. The temperature of the probe block was close to ambient. The probe block is ∼25 mm long, and the i.d. of the sample channel inside the block is ∼8.5 mm. The probe block was connected to a ∼50-cm-long, 6.35-mm o.d., 4.57-mm i.d. stainless steel tube that was connected to the inlet nozzle of the ADLS on the AMS. The total residence time in the sampling system leading to the ADLS was ∼8 s. The ADLS has been characterized by Zhang et al. (55) and Headrick et al. (56). The pressure behind the inlet nozzle feeding the ADLS is about 2 Torr. The outlet of the ADLS is in a vacuum chamber (4.5-inch spherical cube from Kimball Physics) with an operating pressure that is lower than 1 mTorr.
The ADLS acts as a filter against gas-phase molecules and small particles; it focuses particles larger than ∼50 nm into a beam whereas very small particles and gas-phase molecules follow Brownian motion and are pumped away by the turbomolecular vacuum pumps. Heat transfer is a diffusive process and the sample temperature drops to room temperature within 3 cm in the sampling line. The chemistry can be considered to be frozen following extraction from the flame. Large hydrocarbon species that were in the gas phase in the flame condense onto soot particles in the sampling line, making the soot particles grow. This growth does not alter the atomic compositions of the different species, because no covalent bonds are formed or broken.
Because the ADLS acts like a particle filter, particle growth through condensation in the sampling line is favorable; it allows small particles to grow to sizes that are more efficiently focused by the ADLS, and it allows the detection of gas-phase species that are prone to condense onto particles at temperatures lower than in the flame. For example, molecules as small as naphthalene (C10H8, mass 128 u) are often solids at room temperature, but they are in gas phase at flame temperatures. Naphthalene is, however, too volatile to remain condensed on soot particles at the low pressures of the AMS and vaporizes and is pumped away before reaching the ionization region. Nevertheless, for several species with masses as low as ∼135 u (Fig. 4), a fraction of the molecules remain condensed all of the way to the ionization region and are detected by the instrument.
A majority of the present AMS spectra was recorded using an ionizing-photon energy of 9.6 eV. There are several gas-phase species with ionization thresholds below 9.6 eV (e.g., benzene, toluene, and naphthalene), and these should completely dominate the signal from the AMS if gas-phase molecular concentrations were not suppressed by several orders of magnitude. The fact that there is virtually no signal at these masses in the AMS supports the conclusion that volatile species were pumped out of the aerosol mass spectrometer after being guided through the ADLS.
The first vacuum chamber, which houses the ADLS, leads to the second vacuum chamber (4.5-inch spherical cube from Kimball Physics), which has an operating pressure in the 10−5 Torr range. The third chamber is the main chamber (custom 10-inch-diameter cylindrical vessel with multiple conflat ports), and it has an operating pressure of ∼7 × 10−7 Torr. The particles that were focused by the ADLS hit an aerosol target heated to ∼570 K and located in the center of the main chamber. The VUV beam passed ∼1.9 mm in front of the aerosol target and ionized species that had vaporized from the soot on the aerosol target. We have investigated the effect on the signal as a function of different aerosol target temperatures. Increasing the temperature beyond 570 K did not yield any notable new signal peaks on the high-mass side of the spectra but could potentially increase fragmentation.
The O2 concentrations were very low throughout the regions sampled in the three flames, which was verified experimentally and by our gas-phase calculations for the SNAPS simulations. The combination of low O2 concentrations in the flame locations probed and efficient removal of gas-phase molecules in the different pumping stages of the AMS meant that the O2 concentration in the ionization region was negligibly small and insufficient for any surface-catalyzed hydrocarbon–O2 reactions to influence the detected signals. Gas-phase OH was not a concern at the heated target because it was consumed in and around the quartz probe used for sampling and was therefore not even detected in later gas-phase molecular beam mass spectrometry studies performed in the CF flame.
The energy distribution of the ionizing photons used for the AMS studies was relatively broad (∼0.25 eV FWHM), which was not a problem for the present investigation. It is virtually impossible to perform definitive isomer assignments based on photoionization efficiency (PIE) scans (i.e., how the signals of individual mass peaks respond to the tuning of the photoionization energy) for species as large as those under consideration in the present study (57). The reasons are many: the number of isomers grows with the molecular size, the isomers of potential importance have small differences in heats of formation, and similar chemical structures lead to similar ionization energies and PIE curves (58). We have, however, performed PIE scans in the present flames in the range 7.6–10.5 eV. We recorded the signal response to the photon energy for all masses simultaneously, because the instrument used was a time-of-flight mass spectrometer. The PIE curves of the signals at different masses are generally very similar and the ionization thresholds are often below 7.6 eV, which is the lower photon-energy limit at the beamline where we performed the present investigation.
In addition to recording highly averaged signals in the O 1s energy range, as shown in Fig. 6 A–C, we also acquired complete XPS scans between 0 eV and 1,200 eV. These broad scans were performed to verify that the signals that we obtained stemmed only from the hydrocarbon soot sample and that the samples were clean from impurities of other elements.
The XPS results showed that OH and ether groups are almost equally common in the nascent soot particles drawn at small DFFOs. As these particles evolved through the flame, oxygen atoms that were initially bound in OH groups became embedded into ether structures and the fraction of ether functional groups increased and the fraction of OH functional groups decreased with increasing DFFO. It is conceivable from the observed trend that OH functional groups are more abundant than ether groups at very low flame heights, strongly supporting the SNAPS model, which predicted that small oxygen-containing radicals such as OH attached to hydrocarbon structures at low flame heights and then evolved into ether groups (mainly furans).
The presently used version of SNAPS includes hydrocarbon growth through reactions of methyl, methane, acetylene, vinyl, ethylene, ethane, ethyl, propargyl, butatriene, cyclopentadienyl, cyclopentadiene, phenyl, and benzene. Oxygen chemistry is included through reactions with O, O2, OH, HO2, H2O2, H2O, CO, and CO2. For details concerning earlier versions of the model and methods used in the SNAPS code, consult ref. 37. New implementations to SNAPS were focused on the inclusion of pathways and kinetic rates describing oxygenation of PAHs, including but not limited to addition of hydrocarbons to oxyradicals and furan- and pyran-ring formations. We accounted for key kinetic pathways leading to oxygen incorporation into hydrocarbons. These pathways were oxygen adsorption reactions and ring-closure reactions, together accounting for the plurality of oxygen-embedment pathways.
The new reaction pathways for furan formation were incorporated into SNAPS via elementary steps. In the code, reaction sites are specified in terms of properties including the atom type (e.g., aromatic, aliphatic, sp3 hybridized), the bond type, and molecular connectivity [e.g., membership in ring(s) or a specific aliphatic group]. Reactions are defined in terms of these reaction sites and corresponding kinetic-rate constants. SNAPS assumes identical reaction-rate constants for furan-ring closure regardless of the overall molecular size and structure attached to the reactant aromatic ring, as long as the O‒CH=CH· chain is attached to a free-edge site (Fig. S2) of the aromatic ring.
Approximately 70 oxygenation/deoxygenation reactions were implemented in the SNAPS mechanism. Most of these reactions (80%) fall into three categories: bimolecular addition/removal of oxygenated groups to the target molecule, ether formation/destruction, and oxygenated ring closures/openings. The majority of the kinetic reaction rates were sourced from existing literature, and the remaining were estimated based on similarities to analogous reactions with known rates. The high frequency with which we observed furans in the initial simulations demonstrated the significance of the furan formation pathways. Therefore, we used ab initio techniques to validate the kinetic reaction rates of the furan-group formation. The CBS-QB3 method (53) used for the electronic structure calculations involves a sequence of five calculations: (i) geometry optimization at the B3LYP level and (ii) frequency calculation followed by single-point energy calculations at (iii) CCSD(T), (iv) MP4SDQ, and (v) MP2 levels. The extrapolation method unique to CBS then gives the final zero-point corrected energies (59). Intrinsic reaction coordinate calculations were carried out at the B3LYP/CBS level to ensure that each optimized transition state connected the expected reactant and product. Subsequent analysis revealed the importance of the intermediate enol and ether chemistry to oxygenated compounds observed at a DFFO of ∼3.5 mm.
The SNAPS code does not simulate nucleation, particle formation, and particle chemistry. The species suggested by SNAPS can therefore be compared with large gas-phase species in the flames and species that have condensed onto particle surfaces but not participated in surface chemistry on the particles. In addition to precursors to the PCDFs we also observed potential precursor molecules for formation of dioxins, species with six-membered rings including two oxygen atoms. Presently the SNAPS mechanism does not include any elementary reactions that close a ring containing two oxygen atoms. We ran several SNAPS simulations to assess the effect of gas-phase species profiles that had been shifted downstream by a few microprobe orifices. The qualitative SNAPS results remained the same when using nonshifted and shifted gas-phase species profiles.
In Fig. 2, an H atom is abstracted/eliminated from the aromatic ring before OH addition, because, at high temperatures, the prereaction complex formed from direct OH addition to an aromatic ring is unstable; above 350 K the equilibrium highly favors decomposition back to the reactants (60, 61). In addition, electronic structure calculations have shown that benzene + OH reactions do not lead primarily to oxygen addition to benzene at high temperatures. The potential energy barrier for the benzene + OH → C6H5OH + H reaction proceeding via the C6H6OH prereaction complex is 9–82 times greater than the reaction barrier for benzene + OH → phenyl + H2O (60, 62). Hence, H-abstraction reactions by OH are much more favorable than direct OH addition to the benzene ring (60, 62, 63). The substitution reaction (PAH + OH → PAH-OH + H) accounts for less than 10% of the H-atom abstraction rate, and the formation of C6H6OH is negligible (62). Electronic structure calculations have also shown that the products following H abstraction (phenyl + H2O) are 24–40 kJ/mol more stable than the products following oxygenation (phenol + H) (60, 62).
In addition, the mole fraction of H is roughly 8–15 times higher than the OH mole fraction at the flame height (DFFO) where most of the oxygenation occurs, that is, between 0.5 mm and 2 mm, as shown in Fig. S3. Hence, PAHs are predominantly activated by H abstraction. An OH molecule can then add to the radical site on the aromatic ring. This OH group then readily and rapidly decomposes to an oxyradical + H at high temperatures (64). This rapid decomposition leads to an abundance of active oxygen sites attached to aromatic rings, which is why the frequency of acetylene addition to the oxyradical is so high in our SNAPS simulations. At lower temperatures, however, the alcohols are more stable in the SNAPS simulations. We established a step-by-step elementary approach to the oxygenation pathways, but, because the rates are very fast for some reactions (e.g., H elimination from oxygen at high temperatures), the results are akin to a “global” reaction, which proceeds as PAH + OH → PAH-O + 2H (or H2), where the carbon ring maintains its aromaticity.
The SNAPS modeling results showed the importance of enols in the growth mechanisms of furans. We found that, in addition to the isomerization reaction, removal of enols may proceed via reactions with flame radicals, particularly H and OH, producing oxyradicals. Production of large enols can proceed through radical attacks on the carbon–carbon double bond of enols, followed by alkyl radical addition. Masses identified by SNAPS to contain enol contributions at small DFFOs in the C2H4/O2/Ar flame are 94 u (phenol), 108 u (C7H8O), 118 u (C8H6O), 168 u (C12H8O), and 192 u (C14H8O). The structures are displayed in Fig. S1.
The SNAPS simulations revealed that, between a DFFO of ∼3.5 mm and a DFFO of ∼8.2 mm, the structures in Fig. 3 A and B evolved into larger furan compounds; the structure in Fig. 3D either evolved into furan compounds (70% probability) or became deoxygenated (30%), and the structure in Fig. 3E maintained its dibenzofuran substructure while following traditional HACA growth pathways on the other sites of the molecule. These sites are where chlorine atoms may add to form PCDFs. Hence, the structure in Fig. 3E is a likely PCDF precursor.
Although ethers and ketenes are common in Fig. 3, the majority of the oxygenated compounds predicted by SNAPS at a DFFO of ∼3.5 mm were furans. The fraction of ethers, and furans in particular, among the oxygenated structures increased with increasing DFFO in the simulations. This result was validated by the XPS measurements.
The aerosol mass spectrometer yielded different flight times for species of different masses. The flight times were converted into molecular masses through precise calibrations. The resulting masses were sufficiently precise to distinguish between species of different atomic compositions of carbon, hydrogen, and oxygen based on the differences in mass between these elements. The mass peaks were bell-shaped and we fitted Gaussians to the mass peaks to optimize the accuracy of the actual peak locations. We then compared the measured masses with the masses of potential atomic combinations of carbon, hydrogen, and oxygen. A maximum of four oxygen atoms was allowed in a structure because of the high C/O ratios required for soot formation. No constraints were placed on the number of C and H atoms allowed. The atomic masses (65) used were 12C, 12.000000 u; 1H, 1.007825 u; and 16O, 15.994915 u. Some of the results are summarized in Table S1 along with the results from the SNAPS simulations. The mass peak at 141.0705 u included in Table S1 is not influenced by 13C interference, as can be seen in the mass spectra in Fig. 4 (the signals at 139 and 140 u are weak compared with the signal at 141 u).
Table S2 shows the nominal masses of peaks identified as containing signal from oxygenated structures in the AMS investigation. Ions of different masses appear at the detector with decreasing time separations for increasing molecular masses because of the quadratic relation between molecular mass and molecular-ion-flight time (m = a2t2 + a1t + a0) in the aerosol mass spectrometer. As a consequence, the analysis of the precise mass-peak locations becomes less reliable for increasing molecular masses, and, beyond a mass of ∼250 u, we could not completely rely on the measured masses to determine the atomic compositions. Hence, for species heavier than ∼250 u, we also considered the spatial profiles of the species when we determined whether they contained oxygen or not. Profiles related to heavy species, however, tend to be more difficult to interpret than profiles of lighter molecules, indicating a larger degree of coexistence of pure hydrocarbons and oxygenated hydrocarbons for heavy species. Therefore, we limited our analysis to species weighing less than 350 u.
We thank Prof. Barbara Finlayson-Pitts for providing valuable input on an earlier version of our manuscript and to Dr. Paolo Elvati for insightful discussions. This work was funded by the US Department of Energy (DOE) Office of Basic Energy Sciences (BES), Single Investigator Small Group Research Grant DE-SC0002619 (to A.V., T.D., and K.O.J.), and an Alexander von Humboldt Foundation Feodor Lynen Fellowship (to D.M.P.-V.). Experimental expenses, including burner design and construction, were funded under DOE BES, the Division of Chemical Sciences, Geosciences, and Biosciences (M.F.C., P.E.S., and H.A.M.). Aerosol mass spectrometry measurements were performed at the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory. The ALS, N.K.R.-H., and K.R.W. were supported by the Director, DOE BES, under Contract DE-AC02-05CH11231. Experimental preparations and XPS measurements were performed at Sandia National Laboratories, which is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the DOE’s National Nuclear Security Administration under Contract DE-AC04-94-AL85000.
Author contributions: A.V. and H.A.M. designed research; K.O.J., T.D., M.M., F.E.G., M.F.C., P.E.S., D.M.P.-V., N.K.R.-H., K.R.W., A.V., and H.A.M. performed research; K.O.J., T.D., M.M., and F.E.G. analyzed data; and K.O.J., T.D., A.V., and H.A.M. wrote the paper.
The authors declare no conflict of interest.
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