Near-threshold photoionization mass spectra of combustion-generated high-molecular-weight soot precursors

Journal of Aerosol Science 58 (2013) 86–102

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Near-threshold photoionization mass spectra of combustion-generated high-molecular-weight soot precursors Scott A. Skeen a, Hope A. Michelsen a, Kevin R. Wilson b, Denisia M. Popolan b, Angela Violi c, Nils Hansen a,n a

Combustion Research Facility, Sandia National Laboratories, Livermore, CA 94550, USA Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA c Department of Mechanical Engineering, University of Michigan, Ann Arbor, MI 48109, USA b

a r t i c l e in f o

abstract

Article history: Received 4 October 2012 Received in revised form 29 December 2012 Accepted 30 December 2012 Available online 18 January 2013

In this work, we present mass spectra showing organic species with mass-to-charge ratios between 15 and 900 sampled from near-atmospheric pressure, non-premixed, opposed-flow flames of acetylene, ethylene, and propane using an aerosol mass spectrometer with flash vaporization. Near-threshold photoionization was achieved by synchrotron-generated tunable vacuum-ultraviolet (VUV) light. Among the three different fuels, we observed variation in the mass progression, peak intensities, and isomeric content identifiable in photoionization-efficiency curves. The results indicate that different pathways contribute to the molecular growth of soot precursors and that the significance of these mechanisms is likely to depend on the fuel structure and/or flame conditions. Previous work has highlighted thermodynamic propensities for precursor formation; however, our results suggest that kinetic mechanisms play a role in determining the partitioning of soot precursor isomers under the conditions investigated here. Evidence for aliphatic-bridged and oxygenated species was also observed. Such species have been proposed as a possible precursor to particle inception following cluster formation but have never been confirmed. & 2013 Elsevier Ltd. All rights reserved.

Keywords: Soot Particle Flame Combustion Mass spectrometry PAH

1. Introduction Despite extensive evidence suggesting that polycyclic aromatic hydrocarbons (PAHs) are the molecular precursors of soot particles (Bockhorn et al., 2009; Richter and Howard, 2000; Wang, 2011), there is a paucity of information about the specific species involved in soot formation and the gas-phase nucleation (particle-inception) processes. To exacerbate the problem, the chemistry of molecular growth from small molecules to large PAHs is poorly understood. The first step towards soot formation from non-aromatic fuels is generally accepted to be the formation of the first aromatic ring, normally benzene or phenyl (Richter and Howard, 2000). Recent work in premixed flames has identified fuel-structurespecific benzene-formation pathways and has solidified the role of resonantly stabilized radicals (e.g., propargyl) as key benzene/phenyl precursors (Hansen et al., 2011, 2012). The subsequent formation and growth of PAHs beyond benzene is

n Correspondence to: Combustion Research Facility, Sandia National Laboratories, Livermore, P. O. Box 969, MS 9055, CA 94550, USA. Tel.: þ 1 925 294 6272; fax: þ1 925 294 2276. E-mail address: [email protected] (N. Hansen).

0021-8502/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jaerosci.2012.12.008

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modeled as a repetitive reaction sequence of hydrogen abstraction followed by acetylene (C2H2) addition, the so-called HACA (H-abstraction-C2H2-addition) mechanism (Frenklach, 2002; Frenklach and Wang, 1991). However, experimental results and theoretical considerations indicate that other PAH growth pathways may be equally or even more important ¨ than the HACA mechanism under some conditions (Bockhorn et al., 2009; Bohm et al., 1998; Keller et al., 2000; McKinnon and Howard, 1992; Siegmann et al., 1995; Wang, 2011). For example, Siegmann et al. (1995) found C20H12 and C28H14 species at positions lower in laminar-diffusion flames than would be expected based on a successive-growth process, such as the HACA mechanism describes. They concluded that these species are formed low in the flame through reactive dimerization of smaller PAHs. In another example, Keller et al. (2000) showed that large open-shell PAHs exist as p-radicals rather than the s-type required by the HACA mechanism. These p-radicals are resonantly stabilized (having a delocalized unpaired electron) and are therefore no more reactive than large closed-shell PAH species. Keller et al. (2000) concluded that unimolecular reactions of large PAHs result in structural rearrangements yielding species with armchair (e.g., phenanthrene) and bay (e.g., benzo[c]phenanthrene) sites that are more reactive to the addition of smaller unsaturated hydrocarbons. These examples highlight the need for new experimental benchmarks that can aid in the further development and optimization of a combustion-chemistry model that accurately describes the molecular-weight growth of soot-precursor species. As more information about the large gas-phase soot-precursor species is acquired, progress toward understanding and accurately describing the elusive soot-particle-inception process can also be made. Efforts to close the gap between gas-phase chemistry and particle inception by probing the chemical structure of sootprecursor particles began with Dobbins et al. (1995, 1998). The existence of young soot or soot-precursor particles having diameters as small as 1.5–2 nm was first observed by Wersborg et al. (1973) via molecular beam sampling and followed by D’Alessio et al. (1992) using non-intrusive spectroscopic techniques. Additional evidence for the existence of  2–5 nm precursor particles in flames has been provided by differential-mobility analysis (Sgro et al., 2007; Siegmann et al., 2002; Zhao et al., 2007, 2003a,b, 2005), small-angle X-ray scattering measurements (di Stasio et al., 2006 and Hessler et al., 2001, 2002), and flame-sampling photoionization mass spectrometry experiments (Grotheer et al. 2004, 2007, 2011). Nevertheless, these experiments contained only limited chemical information. Dobbins et al. (1995, 1998) used rapid insertion thermophoretic sampling along with laser microprobe mass spectrometry (LMMS) and transmission electron microscopy (TEM) to investigate the structure and composition of soot in non-premixed co-flow flames of ethylene. The region of particle inception along the flame centerline was identified by TEM analysis, which revealed isolated ‘‘liquid-like’’ particles. In the earlier work (Dobbins et al., 1995), laser irradiances of 100 MW/cm2 at a wavelength of 266 nm resulted in ablation and ionization of the samples and the LMMS detected mainly species at 252, 276, and 300 amu. In the later work (Dobbins et al., 1998), the laser irradiance was reduced to 10–20 MW/cm2, and the authors indicated that the samples were partially evaporated and ionized. Under these conditions, the authors reported that masses between 202 and 472 amu were observed with the dominant peaks corresponding to some of the most thermodynamically stable PAH species previously identified by Stein and Fahr (1985). Higher in the flame, where mature-soot aggregates were observed by TEM, PAH species were no longer detected. Moreover, none of the samples revealed any evidence of aliphatic or oxygen-containing species. From these results, one might postulate that the incipient-soot particles consist of stacked PAH clusters between 200 and 472 amu held together by van der Waals forces (Schuetz and Frenklach, 2002; Miller, 2005). Experimental and computational results (Sabbah et al., 2010; Chung and Violi, 2011; Elvati and Violi, 2013; Totton et al., 2012), however, have shown that particle inception through the physical dimerization of species smaller than circumcoronene (C54H18) is very unlikely at flame temperatures; nevertheless, without a suitable alternative, models continue to describe soot particle inception via pyrene dimerization (Eaves et al., 2012). The possibility that the dimerization of larger PAHs (e.g., circumcoronene) (Miller, 1991) constitutes particle inception along the centerline of a non-premixed co-flow flame was presumably ruled out by Siegmann et al. (2002), who showed that the spatial profiles of PAHs with four or more benzenoid rings peaked later, or higher in the flame, than the profiles of soot particles. Siegmann et al. (2002) proposed that the particles are the precursors for PAHs, which grow on the particles’ surfaces and are released into the gas phase once all the chemical bonds are saturated. These findings, however, may not be generally applicable because the composition of soot and the physicochemical process of particle inception may differ significantly along the centerline of a non-premixed co-flow flame when compared to premixed flames or opposed-flow non-premixed flames (more similar to the radial dimension in a co-flow flame). Work by D’Alessio et al. (1992, 1998), D’Anna (2009), and Minutolo et al. (1998) has relied on the ultraviolet absorption and fluorescence spectra of soot-precursor particles in premixed flames and in the high-temperature region of nonpremixed flames to suggest that these particles are not aggregates of large PAHs, but consist of mostly two-ring aromatics connected by aliphatic and potentially oxygen bonding. Bruno et al. (2008) used time-resolved fluorescence polarization anisotropy in a non-premixed opposed-flow flame, and observed two species of particles that could be characterized by the number of constituent aromatic rings. Particles having an average diameter of 1.7 nm were identified as macromolecules built on a framework of 2–3 aromatic ring species. Slightly larger particles with an average size of 2.3 nm were believed to consist of molecules with at least 4 aromatic rings. They noted that their results were consistent with the work of D’Anna and Violi (1998), who suggested that two- and three-ring PAHs connected by aliphatic bonds could be responsible for tar-like soot precursor particles. If the incipient particles in the work of Dobbins et al. (1998) were comprised of aliphatic-bridged or oxygen-bonded PAHs, one might expect the mass spectra to contain significant amounts of PAH radicals resulting from fragmentation with some contribution from the associated aliphatic and/or oxygenated

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species; however, ion signals from potential radical species observed at m/z¼239 and 263 were attributed to the loss of a hydrogen atom from masses 240 and 264 yielding stable cations in the mass spectra. Furthermore, Dobbins et al. (1998) did not observe ion signal from small aliphatics and oxygenated species. Reilly et al. (2000) used a real-time aerosol mass spectrometer to investigate the size, composition, and number density of particles sampled from a non-premixed co-flow acetylene flame. The sampling method involved periodically drawing the entire luminous flame into the suction probe near the flame’s half height; the authors noted that this approach yielded ‘‘a consistent sampling of particles from the entire lateral cross section of the flame’’. In addition to the flame-generated aerosol sample, each suction event drew in dilution air resulting in immediate quenching of the flame. Ablation and ionization of the sample was achieved with a pulsed (20 ns) excimer laser (308 nm) at an irradiance of 200 MW/cm2. The observed mass spectra between 200 and 350 amu resembled those of Dobbins et al. (1998), even though the laser irradiances were reported to be significantly higher and the wavelength was somewhat longer. The mass spectra of Reilly et al. (2000) additionally included low-molecular-weight oxygen-containing species, which may be associated with the carbonization process of young soot or may provide evidence for oxygen bonding. Oxygen bonding has been proposed as the means by which large clusters of PAHs form and survive at flame temperatures and lead to particle inception (D’Alessio et al., 1992, 1998; D’Anna, 2009; Minutolo et al., 1998). ¨ ktem et al. (2005) collected particles from a premixed ethylene flame onto an aluminum probe mounted in the source O region of a time-of-flight mass spectrometer. Semi-volatile matter was desorbed from the probe using an infrared laser pulse with subsequent ionization by 10.5 eV photons. Their observed mass spectra of PAHs between 200 and 500 amu ¨ ktem et al. were significantly more populated than those of Dobbins et al. (1998) and Reilly et al. (2000). Moreover, O (2005) observed ion signal at low m/z, which they attributed to fragments of aliphatic hydrocarbons. The presence of these ions was interpreted as evidence for unsaturated aliphatic hydrocarbon chains in incipient soot. At greater distances above ¨ ktem et al. (2005) saw the amount of aliphatic species increase significantly relative to the PAHs and the burner, O concluded that surface reactions with aliphatics contribute significantly to soot mass growth beyond the nucleation region in premixed flames. ¨ ktem Maricq (2009) sampled from premixed methane, acetylene, ethylene, and ethane flames in a similar fashion to O et al. (2005), but used online laser-ablation/ionization mass spectrometry as in Reilly et al. (2000). Maricq (2009), however, used a 193-nm excimer laser at an irradiance about one order of magnitude lower than that of Reilly et al. (2000). Three distinct regions in the mass spectra were identified. The first region corresponded to molecules with between 1 and 20 carbon atoms. These species were associated with photoionization fragments of three- and four-ring PAHs. A second region was characterized mainly by PAH ions with peak ion counts attributable to m/z ¼202, 228, 252, 276, and 300. This region shows consistency with much of the previous work cited above. The third region included ions as large as 1000 amu and was characterized by peaks corresponding to fullerenes. Several other experimental studies have examined the composition of combustion-generated soot particles and/or precursor species using gas and liquid chromatography, laser-assisted mass-spectrometry techniques, and spectroscopic methods (Anacleto et al., 1992, 1993; Apicella et al., 2007; Baquet et al., 2007; Bente et al., 2009; Blevins et al., 2002; Bouvier et al., 2007; Cain et al., 2010, 2011; Ciajolo et al., 1998; Dobbins, 2007; Dobbins et al., 1995; Faccinetto et al., 2011; Fialkov and Homann, 2001; Grotheer et al., 2004, 2011; Happold et al., 2007; Lafleur et al., 1996; Maricq, 2009; ¨ ktem et al., 2004, 2005; Olten and Senkan, 1999; Reilly et al., 2000; Senkan and Castaldi, 1996; Siegmann et al., 2011; O 1995, 2002; Yamamoto et al., 2007). Although a review of these numerous studies is beyond the scope of the present work, some discussion will be presented below when relevant to the interpretation or presentation of the current data. Despite this large body of literature, uncertainty about the chemical origin of incipient soot particles remains, and inconsistencies between studies and interpretations persist even though flame conditions and experimental techniques were, at times, comparable. Therefore, a major objective of this work involves the pursuit of further insight into the composition of soot precursor species in flames using flame-sampling aerosol mass spectrometry. In this work, we present near-threshold VUV photoionization mass spectra of flame-generated species associated with soot using flame-sampling aerosol mass spectrometry with flash vaporization. The uniqueness of the present work lies in the tunability of our ionization source. We used near-threshold photoionization to minimize dissociative photoionization (fragmentation), and we varied the photon energy to investigate differences in the isomeric composition of samples extracted from flames of three fuels based on comparisons of photoionization efficiency (PIE) curves. 2. Experimental approach The experimental setup consists of a non-premixed, opposed-flow burner enclosed in a vacuum chamber, a quartz microprobe, and an aerosol mass spectrometer. A schematic diagram is shown in Fig. 1. Each side of the opposed-flow burner system has a 14-mm inner diameter (ID) through which the Ar-diluted reactants are supplied. The burner is surrounded by a concentric tube of 20-mm ID through which an Ar sheath flow is provided for stabilizing the flame. A third concentric tube of 50-mm ID, surrounded by a water jacket, can be used as an exhaust or as an additional inlet for low-velocity sheath flow. For the experiments discussed here, the spacing between O2 and fuel outlet was 14 mm. Table 1 summarizes the flame conditions. Calibrated mass-flow controllers regulated the flows of O2, Ar, and fuel (C2H2, C2H4, and C3H8), and a mechanical vacuum pump in conjunction with a butterfly valve and a baratron maintained the chamber pressure at 700 Torr. We chose the flame parameters to yield lightly sooting flames with similar maximum flame

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Fig. 1. Schematic diagram of non-premixed flame chamber, sampling probe, and aerosol mass spectrometer. Table 1 Flame conditions at 700 Torr with 14-mm burner separation. Fuel stream Mole fractions

Oxidizer stream Mass flow 

Mole fractions

Mass flow 

Fuel

xF

xAr

m / g cm  2 s  1

xO2

xAr

m / g cm  2 s  1

C2H2 C2H4 C3H8

0.08 0.16 0.32

0.92 0.84 0.68

3.7  10  2 2.6  10  2 2.8  10  2

0.22 0.18 0.22

0.78 0.82 0.78

2.8  10  2 2.8  10  2 2.8  10  2

temperatures and strain rates. All three flames exhibited a similar faint yellow luminosity and resided on the oxidizer side of the stagnation point under the assumption of near unity Lewis number. Combustion products from within these flames were sampled along the stagnation streamline using a horizontally fixed quartz microprobe constructed from 3-mm outer diameter quartz tubing. The microprobe-sampling tip was tapered at an angle of 301 and the entrance orifice had a 0.15-mm ID. The entire burner assembly can be translated vertically using computer-controlled stepper motors enabling sampling from any flame position with 70.25-mm uncertainty. Temperatures were measured along the stagnation streamline of each flame using a Type-B (Pt-30%Rh/Pt-6%Rh) thermocouple with the microprobe removed. The thermocouple had a bead diameter of 150 mm, and radiation corrections were made as described by Shaddix (1999). The measured temperature profiles for all three flames are shown in Fig. 2. The uncertainty in the temperature measurements was approximately750 K. The chemical composition of the extracted particles and/or condensed species was investigated using an aerosol mass spectrometer, the main components of which have been described in detail elsewhere (Shu et al., 2006). In this setup, the aerosol sample is drawn through a 100-mm-diameter orifice nozzle into an aerodynamic-lens (ADL) system, which focuses the particles through the apertures of a differentially pumped vacuum chamber. Using a similar ADL system Maricq (2009) reported modest particle beam focusing for diameters between 10 to 100 nm, which degraded with decreasing size. The reported angular divergence was approximately 731 (52 mrad). For an ADL system design identical to that used here, Headrick et al. (this issue) measured an angular divergence of 2.6 mrad for flame-generated particles with a median diameter of 73 nm. Inside the low-pressure chamber (  10  8 Torr), the aerosol beam impacts a heated target of variable temperature (up to 400 1C) where semi-volatile species are flash-vaporized. The resulting plume is ionized by tunable-VUV light (7.4–17 eV) generated by quasi-continuous synchrotron radiation at the Advanced Light Source of the Lawrence Berkeley National Laboratory. The mass spectrometer used in these experiments has been upgraded from the original linear time-of-flight system described in Shu et al. (2006). The new orthogonal extraction time-of-flight mass spectrometer is shown in Fig. 1. Ions, produced by photoionization of the gas-phase species vaporized in the heated target, are continuously extracted from the ionization region with 50-eV kinetic energy. The ions are then injected into the

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Fig. 2. Experimental temperature profiles for propane, ethylene, and acetylene flames at 700 Torr. Temperatures were measured using a fine-wire Type-B (Pt-30%Rh/Pt-6%Rh) thermocouple. The bead diameter was 150 mm. Radiation corrections were made according to Shaddix (1999).

time-of-flight region and pulse extracted into the flight tube at a repetition rate of 15 kHz. Ions are detected on a multichannel plate detector, and mass spectra are recorded using a multichannel scaler. The mass resolution of the spectrometer is 2000. In our experiment the particles hitting the target were either formed in the flames prior to extraction or were formed upon cooling in our sampling line. We understand that immediate dilution at the sampling point is necessary to prevent coagulation of nascent soot particles. The present sampling configuration, however, does not allow for immediate dilution. Coagulation and surface adsorption may thus occur in the microprobe and sampling line. The largest mean electricalmobility diameter of the particles entering the ADL, as measured by a Scanning Mobility Particle Sizer, was approximately 40 nm for all three flames. This observation suggests that the aerosol samples were not dominated by mature soot, which is typically characterized by aggregates with electrical mobility diameters of 100 nm or larger. Rather, the species we observe in our mass spectra represent an average over all of the condensable matter, particulate matter (young soot nuclei subject to flash vaporization), and adsorbed species present at each sampling position in the flame. Because the ADL system does not efficiently focus particles of the size of nascent soot, allowing coagulation to occur in the sampling system enhances our sensitivity toward detecting species attributable to these smaller particles; however, coagulation also prevented us from resolving any size specific compositional features of the particles. Regardless of changes due to coagulation or adsorption in the sampling line, the detailed chemical composition of the measured particles or droplets is related to the chemistry and physics of the molecular-growth processes leading to soot. 3. Results and discussion 3.1. General characteristics of mass spectra A typical flame-sampled mass spectrum is shown in Fig. 3. For this measurement from within the acetylene flame, the probe was at a distance from the fuel outlet (DFFO) of 5 mm, the heated target temperature was 370 1C, and the photon energy was 10.0 eV. Under these conditions, we observed ion signal for species with mass-to-charge ratios (m/z) ranging from 15 to 4900, with a peak around m/z¼202 or 226 (C16H10 or C18H10). It is beyond the scope of the present paper to identify all of the species observed in the mass spectrum of Fig. 3 and to unravel their possible formation pathways. In the following analysis of our data, we focus on improving the understanding of the chemical composition of the soot precursors. ¨ ktem et al. (2005). The spectrum The mass spectrum in Fig. 3 is in qualitative agreement with those reported by O includes peaks from low-mass aliphatic species, which are discussed in more detail in the next section, and higher-mass species, which are generally believed to be PAHs. We also suspect contributions from oxygenated species for certain peaks based on their distinct burner profiles, which will also be discussed later. The shape of the spectrum in Fig. 3 is consistent with the exponential decrease in the mole fraction of PAHs up to coronene (C24H12, m/z¼300) as has been observed in premixed flames by Homann (1985) and Keller et al. (2000). However, in the latter paper, the reported mole fractions began to increase after the C24 species with a subsequent peak near the C52 species. In a related experiment, Faccinetto et al. (2011) observed a second peak in their mass spectra near the C40 species when analyzing soot sampled from a smoking low-pressure premixed methane flame on adsorbing borosilicate filters by laser-desorption/laser-ionization mass spectrometry. This second peak was only observed when sampling at flame heights associated with incipient and young soot. At larger heights above the burner, where mature soot was present, the larger PAH species were no longer observed. Faccinetto et al. (2011) concluded that the large PAH growth

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Fig. 3. Flame-sampled mass spectrum from the fuel side of an acetylene opposed-flow diffusion flame. The energy of the ionizing photons was 10.0 eV. The distance of the probe from the fuel outlet was 5 mm, and the heated target was 370 1C.

process is catalyzed at the surface of incipient and young soot. This interpretation is consistent with the conclusions of Siegmann et al. (2002) for non-premixed laminar co-flow flames. These results suggest that large PAHs form after particle inception and therefore cannot be the source of the observed incipient particles through physical dimerization (Siegmann et al., 2002). As an alternative, Minutolo et al. (1998) postulated that incipient soot or soot precursor nanoparticles consist of one- and two-ring aromatic functionalities connected by aliphatic bonds. In the present work, we do not observe a second peak near C40 or beyond. Rather, the ion signal continues to decrease at a constant rate until approximately C44H20 (m/z¼548). After C44H20, the rate of decrease is reduced by a factor of about 5. This significant change in the decay rate of ion signal near C44 may be evidence that the larger species are formed through a different chemical pathway than that associated with PAHs between C16 and C42. It is also possible that we are not sensitive to these species because they do not vaporize into our ionization region from our heated target at temperatures r400 1C. At present, we cannot otherwise explain the lack of a second peak in our mass spectra near C40 as in Faccinetto et al. (2011) or C52 as in Keller et al. (2000). Both of these prior studies, however, investigated low pressure fuel-rich premixed flames and used significantly different diagnostics.

3.2. Low-molecular-weight species We have identified several low-molecular-weight species in our mass spectra by measuring their characteristic photoionization efficiency (PIE) curves. Comparing flame-sampled PIE curves with known PIE curves of individual species allowed us to identify isomers and near-equal-mass species. This technique has been used successfully in many combustion-chemistry studies (Hansen et al., 2009). For example, the signal at m/z¼40 (C3H4) sampled from the acetylene flame is best reproduced by a weighted sum of the reference PIE curves of allene and propyne assuming a Gaussian-shaped energy distribution of the ionizing photon beam with a spectral bandwidth of 0.25 eV (Fig. 4a). Fig. 4b shows that the acetylene flame-sampled signal at m/z ¼42 is reproduced by the reference PIE curve of ketene, whereas in the propane flame (Fig. 4b, inset) a strong signal of propene appears to dominate the PIE curve for this mass channel. The flame-sampled PIE curves for m/z¼44 are shown in Fig. 4c. In the acetylene flame, the ion signal appears near 9.33 eV, the ionization energy of the ethenol (vinyl alcohol), a species recently identified as a combustion intermediate in premixed flames. The isomeric acetaldehyde contributes at photon energies above 10.2 eV. Ethenol appears to be significantly less abundant than acetaldehyde in propane flames (Fig. 4c, inset), which is consistent with the premixed flame results of Taatjes et al. (2005, 2006). These results suggest differences in the partitioning between isomers for the two fuels. To determine if these aliphatic species were associated with particles, we used the temperature of the heated target as a diagnostic tool. For example, when we turned off the heated target, we still observed stable low-molecular-weight aliphatic species, such as allene, propyne, propene, and diacetylene. We concluded that these species were entirely transported to the detection chamber in the gas phase and were not associated with the particles or condensed phase material reaching the temperature-controlled target. We confirmed this observation by slightly changing the flame conditions such that the PAHs were no longer present in the mass spectrum. Under these conditions, the peak-ion signal and spatial profiles associated with the low-molecular-weight stable aliphatics remained relatively unchanged. Also, as demonstrated by the PIE curves of Fig. 4, our use of near-threshold photoionization rules out the dissociation of larger species as the source of the low-molecular-weight stable aliphatics. Our conclusion that the stable low-molecular-weight aliphatic species shown in Fig. 4 are not associated with the ¨ ktem et al. (2005), who identified the particles or condensed matter is different from the premixed flame results of O

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Fig. 4. Photoionization efficiency curves for species sampled from acetylene (squares) and propane (circles) flames at (a) m/z¼40, (b) m/z¼42, and (c) m/z¼ 44 compared with PIE curves of (a) allene and propyne, (b) ketene and propene, and (c) ethenol and acetaldehyde. Species were sampled at a DFFO of 4.5 mm with the heated target at 300 1C. See text for additional details.

low-mass species detected in their mass spectra as fragments generated by the dissociative photoionization of larger ¨ ktem et al. (2005) observed the greatest aliphatic content at larger aliphatic species associated with soot. Furthermore, O heights above the burner (HAB) where the soot was presumably more mature. At lower HAB in their experiment, in the region of particle inception, the peak-ion counts for the aliphatic-like fragments and PAHs were similar. In our experiment, we specified the flame conditions to limit the formation of mature soot, which may also explain the lack of stable low-molecular-weight aliphatic content associated with particles or semi-volatile components in our mass spectra. Nevertheless, as we will discuss later, evidence for aliphatic-bridged or oxygen-bonded aromatics may be apparent in our data at larger m/z. The studies of Maricq (2009, 2011) and Bouvier et al. (2007) did not reveal an aliphatic component in the soot mass spectra.

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We also observed low-molecular-weight radical species, e.g., CH3, C2H5, C3H2, C3H3, C3H5, C4H5, C5H3, C5H5, C6H5, etc., which only appeared when the target temperature was elevated. This observation suggests that these radicals are associated with the sampled particles or condensed matter. The PIE curves shown in Fig. 5 demonstrate consistency among the flame-sampled and reference PIE curves for methyl, propargyl, and allyl, at photon energies near their ionization thresholds, thus indicating that these species are not attributable to fragmentation. If dissociative photoionization were responsible for these species, their ionization onsets would not necessarily appear at the radical’s specific ionization threshold. In contrast, dissociative photoionization does appear to contribute to the observed signal above 9.25 eV at m/z¼39 (Fig. 5b) and 8.8 eV at m/z¼ 41 (Fig. 5c). The ion signal intensity of the small radicals increased at higher target

Fig. 5. Flame-sampled (symbols) and reference (lines) PIE curves for (a) m/z¼ 15 (methyl, CH3), (b) m/z¼39 (propargyl, C3H3), and (c) m/z¼41 (allyl, C3H5). Species were sampled from an acetylene flame at a DFFO of 4.5 mm with the heated target at 400 1C. The resonant structures of propargyl and allyl are also shown. The reference PIE curves were obtained from Taatjes et al. (2008) for methyl, from Savee et al. (2012) for propargyl, and from Robinson et al. (2004) for allyl.

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Fig. 6. Flame-sampled PIE curve of m/z¼ 116 in the acetylene flame at a DFFO of 4.5 mm with the heated target at 300 1C. The data are best reproduced with contributions of phenylallene and the phenyl-propyne isomers.

temperatures and may be facilitated by thermal processes, but a full understanding of the origin of these radicals (and fragments) will require further investigation. These small-radical fragments may arise from side chains bonded to small aromatics and/or PAHs. For example, for the acetylene flame the PIE curve for m/z¼116 (C9H8), which normally is interpreted as signal arising from indene, can be best reproduced by including contributions from C3H3-substituted phenyl-rings. The flame-sampled PIE curve (Fig. 6) exhibits an apparent ionization threshold of 8.2 eV, slightly above the known ionization energy (IE) of indene (IE¼8.14 eV). Fig. 6 shows that the flame-sampled PIE curve for m/z ¼116 from the acetylene flame is best reproduced by including contributions from phenylallene, 1-phenyl-1-propyne, and 3-phenyl-1-propyne. Zhang et al. (2011) identified these species as products of the phenylþallene or phenylþpropargyl reactions, thus highlighting the importance of the phenyl radical. The PIE curve from the acetylene flame deviates from those obtained for the ethylene and propane flames (not shown), which suggests a less pronounced presence of 3-phenyl-1-propyne (IE ¼8.9 eV). The larger contribution of this isomer to the acetylene flame may be explained by an increased importance of the benzylþacetylene reaction compared to the other flames. The observation of substituted aromatic rings suggests new reaction pathways. Through Habstraction reactions of phenylallene, 1-phenyl-1-propyne, and 3-phenyl-1-propyne, phenyl-substituted propargyl radicals are formed that could potentially lead to larger PAHs in reaction schemes similar to the propargylþpropargyl reaction (Stein et al., 1991). For example, the reaction of any of the C9H7 radicals with propargyl is a potential pathway to form biphenyl (m/z¼154); alternatively, a terphenyl (m/z¼ 230) could be formed through a reaction with another C9H7 radical (Richter and Howard, 2000). 3.3. Larger-molecular-weight species Fig. 7 shows a comparison of the flame-sampled mass spectra between m/z¼170 to 310 amu from the (a) propane, (b) ethylene, and (c) acetylene flame at 4.5 mm from the fuel outlet. These spectra were obtained at a photon energy of 9.0 eV with the heated target at 300 1C. The C3H8 flame-sampled mass spectrum differs significantly from the mass spectrum obtained from sampling particles in the C2H2 flame, with the C2H4 flame data showing intermediate features. Although contributions from oxygenated species are present (discussed later), these spectra mainly consist of regular þ progressions of groups of CxHy ions (with x ¼14–24 and y¼10–16). Several authors (Dobbins et al., 1995, 1998; Faccinetto et al., 2011; Maricq, 2011) have argued that the CxHy species with even carbon numbers correspond to the most thermodynamically stable PAHs (i.e., ‘‘stabilomers’’) identified by Stein and Fahr (1985). Concerning the intensities in Fig. 7, we realize that groups of Cx appear to alternate in intensity, with the even carbon numbers being favored. For example, C17H10 and C19H10 are much weaker in signal (or not detected at all), while signal at CxH10 with x¼16, 18, and 20 is significantly larger. Although not shown in Fig. 7, this trend, which is consistent with previous work (Blevins et al., 2002; Dobbins et al., 1995; Apicella et al., 2007; Bouvier et al., 2007; Faccinetto et al., 2011; ¨ ktem et al., 2005), is also visible for other CxHy progressions. For no obvious reasons, ion counts for C19 and C21 species in O the propane flame are large relative to those at C20, when compared to the ethylene and acetylene flame results. In most cases the peak intensities within each Cx group decline with the number of hydrogen atoms; exceptions are the C17, C20, and C23 groups. The relative signal intensities in Fig. 7 vary from one flame to another and we observe that the maximum ion signal for the propane flame is at m/z ¼202 (C16H10) and m/z¼226 (C18H10) for the ethylene and acetylene flames. Most commonly, C16H10, pyrene or an isomer thereof, has been observed as the predominant PAH species associated with soot

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Fig. 7. Flame-sampled aerosol mass spectra of (a) propane, (b) ethylene, and (c) acetylene flames in the range of 170–310 amu. Samples were collected at a DFFO of 4.5 mm with the heated target at 300 1C, and mass spectra were obtained at a photon energy of 9.0 eV.

particles and surface adsorbed species for a variety of fuels, flame configurations, sampling techniques, and diagnostic configurations (Siegmann and Sattler, 2000; Happold et al., 2007; Bouvier et al., 2007; Maricq, 2009; Faccinetto et al., 2011). Exceptions include measured spectra with peak ion counts attributable to C20 and C22 PAHs in Dobbins et al. (1998), ¨ ktem et al. (2005), and C10 and C14 PAHs in Bouvier et al. (2007). C17 PAHs in Reilly et al. (2000), C22 and C24 PAHs in O As noted by Maricq (2009), because ion intensities are not easily calibrated in these studies, it is impossible to identify the origin of intensity differences between different experimental and measurement conditions. Neighboring species with similar quantities of conjugated carbon–carbon double bonds, however, should have similar photoionization cross sections, and small differences in their masses should not cause a significant difference in their responses to the ion optics.

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The occurrence of the mass increments of 24 amu in the mass spectra shown in Fig. 7, e.g., m/z¼178, 202, 226, y or 204, 228, 252, y, etc. suggests that growth of the PAH species can proceed through H-abstraction-C2H2 addition yielding larger PAHs. Starting from benzene, most likely the ‘‘first aromatic ring’’ in many flames, the CxHy series with odd values of the carbon number x cannot be explained by H-abstraction-C2H2 addition alone, but instead are likely to result from

Fig. 8. Spatial profiles of integrated ion counts for species at m/z ¼250, 252, 254, 256, 258, 260, and 262 in the three flames studied here. Mass spectra were obtained with the heated target at 300 1C at a photon energy of 9.5 eV.

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addition of C1, C3, y species or when one of the reactants already contains a five-membered ring (Dobbins et al., 1998; Maricq, 2011; Shukla and Koshi, 2011; Shukla et al., 2010). The observed mass increments of Dm ¼38 can result from H-abstraction/propargyl-addition reactions and from sequential additions of methyl and acetylene. In a similar way, mass increments of Dm ¼76 may result from phenyl addition and/or from two subsequent H-abstraction/propargyl-addition reactions. Masses 215, 239, and 263 amu (indicated in Fig. 7c) represent the radical species C17H11, C19H11, and C21H11, which ¨ ktem et al., 2005). Dobbins et al. have been reported previously (Dobbins et al., 1998; Homann, 1985; Maricq, 2011; O (1998) attributed their occurrence to the loss of an odd number of hydrogen atoms during photoionization of the respective CxHy (x ¼17, 19, 21) PAHs. However, at a photon energy of 7.4 eV (the lowest possible energy in the current experiment), the radical signal is still detectable. At these low photon energies the hydrocarbon ions are formed with only a limited amount of excess energy; therefore, fragmentation is not a likely explanation for the presence of the radical species in our experiment. Only a more detailed, quantitative analysis can reveal the potential role of these radicals in soot-formation processes. Taking mass spectra as a function of distance from the fuel outlet provides spatial profiles that are well suited as validation targets for combustion-chemistry modeling. Examples for the C20Hy species are shown in Fig. 8 for all three flames. In the propane and ethylene flames, the relative ion signal decreases dramatically with increasing number of hydrogen atoms in the molecule (y) for y412, i.e., the signal decreases with increasing bond saturation at any selected location in the flame. The trend is shown in Fig. 8 to last over a range of four additional hydrogen atoms beyond the initial CxHy peak (i.e. Dyr4). In the acetylene flame, the signal decrease with increasing saturation is not as dramatic, and the trend persists over a broader range of y (i.e. Dy¼10); thus comparable signal appears for every m/z ratio. Furthermore, the spatial profiles in Fig. 8 appear to be shifted toward the oxidizer side of the flame for more saturated species. This shift, which is most apparent in the acetylene flame, is likely related to increased presence of H atoms and small hydrocarbon radicals in the region of high temperature. The profiles for the lowest-molecular-weight species in the Cx groups (not shown) do not show any spatial shifting that might be indicative of a delayed sequential growth; a phenomenon that had been discussed by Siegmann et al. (1995) with respect to PAH concentrations along the centerline in laminar non-premixed co-flow flames of methane. 3.4. Evidence of oxygenated species Evidence for the presence of oxygenated species is also found in the burner (spatial) profiles. Fig. 9 displays the spatial profiles of m/z¼202, 218, and 234. These spatial profiles demonstrate a sudden rise in signal for m/z¼234 closer to the oxygen side of the propane and ethylene flames. A similar rise is observed for m/z¼218 in the ethylene flame, whereas the profile for m/z¼218 in the propane flame appears broader than that of m/z¼202. This sudden rise in signal is attributed to oxidized species, whereas the signal farther toward the fuel side is attributed to more saturated hydrocarbons. Because the mass spectra do not provide the chemical structure of these oxygenated species, we currently cannot comment on whether the oxygen is bound as an alcohol, ether, or carbonyl function. Additional sequences starting at m/z¼128, 178, 216, 226, 240, 250, 252, 276, 300, and others are present. The acetylene flame does not demonstrate this behavior (Fig. 9c), and only a small shift to the oxygen side is observed for m/z¼234 relative to m/z ¼218, relative to m/z ¼202. The spatial profiles are significantly narrower when compared to the other two flames. This narrowing presumably leads to more overlap of the hydrocarbon growth and oxidation regions. These pronounced differences in the acetylene flame’s mass spectrum and spatial profiles are likely related to the influence of flame structure (i.e., the relationship between the local temperature and local species concentrations). Recent results indicate that combining fuel dilution with oxygen enrichment in non-premixed flames alters the boundaries of the soot-formation zone, such that the high-activationenergy soot chemistry is restricted to the region near the radical pool, where H and O atoms along with CHn (1rn r3) radicals are readily available (Kumfer et al., 2008; Skeen et al., 2010). Because the acetylene flame had a more dilute fuel stream than the propane and ethylene flames, flame structure effects could potentially contribute to the apparent increase in more saturated aromatic species and/or oxygen-containing species. 3.5. Fuel-specific isomer partitioning By comparing PIE curves obtained from sampling within different flames, we observed differences that may provide some insight into flame-structure-specific soot-formation chemistry. Differences between the PIE curves for m/z ¼116 sampled from the acetylene flame (Fig. 6) or the ethylene and propane flames are discussed above. We also observed differences between the PIE curves of the aerosol mass spectra at other m/z ratios. Fig. 10 shows examples for m/z ¼142 (C11H10 isomers), m/z¼226 (C18H10 isomers), 240 (C19H12 isomers), and 250 (C20H10 isomers). As is the case for m/z¼116, deviations in the PIE spectra among the three flames indicate different isomeric contributions to this mass channel. Although species cannot currently be assigned, ionization thresholds near 8.5 eV and above are not characteristic of the ‘‘stabilomer’’ species and may be representative of aromatic rings joined by aliphatic-linked or oxygen-containing aromatic hydrocarbons. The features shown in Fig. 10 provide evidence for additional, yet unexplored, molecular-growth pathways, and the results indicate that the isomeric distribution of soot components and/or precursor species is subject to kinetic control. Dobbins et al. (1998) concluded that the well-known invariance in soot properties for different fuels and

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Fig. 9. Spatial profiles of normalized integrated ion counts for m/z¼202, 218, and 234 in the three flames studied here. The peaks near 6 mm attributed to the presence of oxygenated species. Mass spectra were obtained with the heated target at 300 1C at a photon energy of 9.5 eV. (a) Propane flame, (b) Ethylene flame and (c) Acetylene flame.

combustion devices could be explained if PAH growth prior to soot carbonization is dominated by the most thermodynamically stable species, i.e., ‘‘stabilomers’’. This argument was based on their observation that the molecular weights of about 80% of the PAHs they detected in precursor particles were identical to the molecular weights of stabilomers. It was noted, however, that isomers could not be resolved with their technique. Our experimental technique, which allows us to differentiate isomers and near-mass species based on their respective photoionization thresholds, provides evidence that the partitioning of PAH isomers and the formation of aliphatic-bridged PAHs and/or oxygencontaining near-mass species (in the mass range of those observed by Dobbins et al. (1998)) may be kinetically controlled.

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Fig. 10. Photoionization efficiency curves for (a) m/z ¼142, (b) m/z¼226, (c) m/z¼ 240, and (d) m/z¼ 250 sampled from the three flames described in the present study.

Aliphatic-bridged, oxygen-containing aromatics, and non-stabilomer PAHs could be preferentially formed if the kinetic mechanisms governing their formation are subject to lower barriers than the kinetic mechanisms associated with the formation of the most thermodynamically stable species.

4. Conclusions We have measured mass spectra of flame-sampled particles and/or condensable species using photoionization aerosol mass spectrometry. The contrasting characteristics of the mass spectra shown here for three different flames suggest that the relative amounts and compositions of soot precursor species are dependent on the chemical structure of the fuel. Our results provide additional evidence that the widely accepted HACA mechanism cannot explain all observed PAH species. The significance of the other mechanisms and the isomeric content of the soot-precursor species are likely to depend on the fuel structure and/or flame conditions. Considering that the formation of the first aromatic ring is widely regarded as a critical step in the soot formation process for non-aromatic fuels, this finding augments an earlier observation that the fuel structure influences the significance of different benzene formation pathways (Hansen et al., 2011). Spatial profiles of the benzenoid PAHs do not indicate delayed sequential growth, as was observed by Siegmann et al. (1995) in non-premixed co-flow flames of methane; however, more saturated hydrocarbon species showed a systematic shift toward the oxidizer side of the flame. Spatial profiles also indicated the presence of oxygen containing species. A comparison of PIE curves among the three flames suggested the presence of species with higher ionization thresholds than would be expected for the most thermodynamically stable PAHs commonly identified in flames (Stein and Fahr, 1985). These species may be related to the aliphatic-bridged or oxygen-bonded aromatics associated. Their presence provides evidence for kinetically, as opposed to thermodynamically, driven soot precursor formation mechanisms; however, their role in the particle inception process needs further investigation.

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Acknowledgments This work is supported by the U.S. Department of Energy (DOE), Office of Basic Energy Sciences (BES) under the Single Investigator Small Group Research (SISGR), Grant no. DE-SC0002619. The measurements are performed at the Advanced Light Source (ALS) of the Lawrence Berkeley National Laboratory. The authors acknowledge the expert technical assistance of Sarah Ferrell and Paul Fugazzi and sage advice from Prof. Fokion Egolfopoulos (USC). The ALS, KRW, and DMP are supported by the Director, DOE Office of Science, BES, under Contract no. DE-AC02-05CH11231. DMP is also grateful to the Alexander von Humboldt Foundation for a Feodor Lynen fellowship. HAM and NH were supported by Division of Chemical Sciences, Geosciences, and Biosciences, DOE BES. Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the National Nuclear Security Administration under Contract no. DE-AC04-94-AL85000. 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