Particle Inception Research Articles

Single-pulse ultrafast real-time simultaneous planar imaging of femtosecond laser-nanoparticle dynamics in flames

The creation of carbonaceous nanoparticles and their dynamics in hydrocarbon flames are still debated in environmental, combustion, and material sciences. In this study, we introduce single-pulse femtosecond laser sheet-compressed ultrafast photography (fsLS-CUP), an ultrafast imaging technique specifically designed to shed light on and capture ultrafast dynamics stemming from interactions between femtosecond lasers and nanoparticles in flames in a single-shot. fsLS-CUP enables the first-time real-time billion frames-per-second (Gfps) simultaneous two-dimensional (2D) imaging of laser-induced fluorescence (LIF) and laser-induced heating (LIH) that are originated from polycyclic aromatic hydrocarbons (PAHs) and soot particles, respectively. Furthermore, fsLS-CUP provides the real-time spatiotemporal map of femtosecond laser-soot interaction as elastic light scattering (ELS) at an astonishing 250 Gfps. In contrast to existing single-shot ultrafast imaging approaches, which are limited to millions of frames per second only and require multiple laser pulses, our method employs only a single pulse and captures the entire dynamics of laser-induced signals at hundreds of Gfps. Using a single pulse does not change the optical properties of nanoparticles for a following pulse, thus allowing reliable spatiotemporal mapping. Moreover, we found that particle inception and growth are derived from precursors. In essence, as an imaging modality, fsLS-CUP offers ultrafast 2D diagnostics, contributing to the fundamental understanding of nanoparticle’s inception and broader applications across different fields, such as material science and biomedical engineering.

Effect of horizontal insertion of metal wire mesh into the flame on the formation and suppression of soot in acetylene diffusion flame

A simple and efficient strategy to suppress soot from acetylene diffusion flames is reported for the first time. Horizontally insertion of metal wire mesh into the acetylene diffusion flame is found to greatly suppress the soot emission. The effects of experimental parameters such as acetylene flow rate, wire mesh height above nozzle (WHAN), material of wire mesh, mesh number, and inner diameter of the burner nozzle on soot suppression in acetylene diffusion flames are investigated. In addition, the collected soot samples are characterized by TEM, HRTEM, XRD, BET and Raman. The soot suppression efficiency can reach 100 % by horizontal insertion of the metal wire mesh in the acetylene diffusion flame. The WHAN has a great influence on the effect of soot suppression. In the first 210 s of horizontally inserting the metal wire mesh into the flame, the total mass of collected soot first decreases and then increases with the increase of WHAN. The minimum total mass of soot is achieved when the normalized WHAN (obtained by dividing the wire mesh height above nozzle by flame height without insertion of the wire mesh) is at 0.1. This position corresponds to the soot particle inception zone of the acetylene diffusion flame. The flame is divided into two parts when the metal wire mesh is horizontally inserted into the flame. The temperature of the flame below the metal wire mesh decreases from 1770 K to 1480 K. The temperature of the flame above the metal wire mesh increases from 1590 K to 1780 K. The soot collected on the metal wire mesh grows into a volcano-shaped carbon (VSC) structure. The results of the schlieren imaging experiments show that this structure can promote the mixing of air and fuel in the lower region of the flame thereby suppressing the formation of soot particles. The soot analysis shows that the horizontal insertion of metal wire mesh into the flame leads to high amorphous feature and low degree of structural order of the soot, which means that the oxidation reactivity of the soot increases.

Numerical Analysis of the Overtopping Failure of the Tailings Dam Model Based on Inception Similarity Optimization

The analysis of overtopping dam break caused by extreme rainstorms and other special circumstances is very important in the feasibility analysis of new construction or expansion projects of tailings reservoirs. Reduced-scale physical model tests can directly reflect the topography and dam-break influence range, but the reasonable selection of model dam material is the key to ensure the model’s similarity. Based on the similarity optimization of the limit state of scour inception of sediment particles, a new method for the model material of tailings dams can be proposed, but it needs to be verified by a similar overtopping model test. In this paper, the modeling and numerical calculation analysis of a prototype tailings dam and a similar reduced-scale model are carried out by using FLOW-3D v11.2 numerical software. The calculation results show that the model test scheme optimized by inception similarity can well reproduce the overtopping failure process of the prototype dam.

Large eddy simulation of iron(III) oxide nanoparticle synthesis in spray flames

The SpraySyn burner has been developed to investigate nanoparticle synthesis from spray flames by various diagnostic methods and simulations from research groups within the DFG (German Research Foundation) priority program SPP1980. We investigate the formation of iron oxide nanoparticles from iron nitrate dissolved in a mixture of ethanol and ethyl hexanoic acid. In the SpraySyn experiment, the nanoparticle precursor solution is injected and atomized via an air-blast nozzle, surrounded by a pilot flame. The solvent itself is combustible, such that the droplets evaporate at high heating rates in a highly reactive environment. We investigate the synthesis flame in large eddy simulations. The liquid droplets are described by Lagrangian particles, and gas-phase combustion is modeled by the flamelet-generated manifold approach with adaptations for particle inception. The nanoparticle dynamics are predicted by three models: a monodisperse model, a bimodal model, and a sectional model. The monodisperse and bimodal models account in terms of number-, surface-, and volume-concentration from inception, coagulation, and sintering processes, while the sectional model accounts for inception and coagulation and provides the particle size distribution. The comparison of nanoparticle sizes with the in-situ measurements shows that the bimodal model can be a suitable alternative approach to the computationally expensive sectional model.

Relevance of C/O ratios in the gas-phase synthesis of freestanding few-layer graphene

In microwave-plasma synthesis of few-layer graphene from hydrocarbon precursors, the carbon-to-oxygen and the carbon-to-hydrogen ratios are known to influence the product ratio of graphene to amorphous soot-like particles. While the role of oxygenated hydrocarbons and water as oxygen-supplying species has been studied before, in this paper, we compare the effect of carbon dioxide, molecular oxygen, and nitrous oxide mixed with ethylene to systematically change the C/O ratio while keeping the hydrogen supply constant. Ex situ powder analysis, emission spectroscopy, gas chromatography, and simple reaction kinetics simulations are employed to evaluate and describe the synthesis process. Additionally, thermophoretically sampled nanomaterials are collected close after the first particle inception and analyzed ex situ. The results show that molecular oxygen and nitrous oxide increase the graphene fraction with decreasing C/O ratios and pure graphene is reached at 2:1.5. The decrease in C/O ratio results in an overall decrease in solid carbon yield. With carbon dioxide, pure graphene cannot be generated at a C/O ration of 2:1.5, although a similar reduction of the particle yield is observed. Thermophoretic sampling showed that the specific mixture of carbon allotropes is already defined a few cm downstream to the plasma zone. Emission spectroscopy shows that carbon dioxide forms carbon species during its decomposition in the plasma, we hypothesize that these released carbon species might influence the environment for local nucleation of solid carbon. Thus, the C/O ratio and available carbon fraction for growth cannot always be used to tailor the carbon microstructure. Moreover, the source of oxygen atoms also seems to have an effect on the resultant microstructure.

Investigation of resonance-stabilized radicals associated with soot particle inception using advanced electron paramagnetic resonance techniques

In order to tackle the climate emergency, it is imperative to advance cleaner technologies to reduce pollutant emission as soot particles. However, there is still a lack of complete understanding of the mechanisms responsible for their formation. In this work, we performed an investigation devoted to the study of persistent radicals potentially involved in the formation of soot particles, by continuous wave and pulsed electron paramagnetic resonance. This work provides experimental evidence of the presence in nascent soot of highly branched, resonance-stabilized aromatic radicals bearing aliphatic groups, linked together by short carbon chains, and reinforced by non-covalent π-π interactions. These radicals appear to be highly specific of nascent soot and quickly disappear with the increasing soot maturity. Their presence in nascent soot could represent an underestimated health risk factor in addition to the already well documented effect of the high specific surface and the presence of harmful adsorbates.

The impact of ammonia addition on soot formation in ethylene flames

The present work investigates computationally the impact of blending ammonia with ethylene on soot formation in four laminar diffusion flames and the corresponding turbulent partially premixed flames. The conditions conform to experimental investigations (Bennett et al., Combust. Flame, 2020, and Boyette et al., Combust. Flame, 2021) with both sets of flames featuring identical fuel blends. A mass and number density preserving sectional method was applied to all cases with the soot surface growth and oxidation models updated to include reactions with oxides of nitrogen. The gas phase chemistry was extended to include a comprehensively validated ammonia sub-mechanism. For the laminar flames, the inception of soot particles is based on detailed chemistry with pyrene treated as representative of the PAH pool. The accuracy of fitting a global acetylene-based inception step is evaluated for the different fuel blends and subsequently applied to the turbulent flames using a fully coupled transported joint probability density function method featuring an 84-dimensional joint-scalar space. Results obtained for laminar flames show that detailed chemistry coupled with the sectional soot model provides excellent agreement with the measured suppression of the soot volume fraction with increased use of ammonia. Computed particle size distributions (PSDs) show an increase in smaller soot particles under such conditions, consistent with experimental observations. The experimentally observed reduced impact in turbulent flames is also reproduced computationally. The suppression of soot is principally caused by changes in the radical pool leading to reduced soot surface growth and, to a lesser extent, soot inception. The contribution of oxides of nitrogen to soot oxidation is modest. Computed PSDs in laminar and turbulent flames highlight the importance of differences in flame structures and flowfield timescales.

On the Mechanism of Soot Nucleation. IV. Molecular Growth of the Flattened E-Bridge.

Rotationally excited dimerization of aromatic moieties, a mechanism proposed recently to explain the initial steps of soot particle inception in combustion and pyrolysis of hydrocarbons, produces a molecular structure, termed E-bridge, combining the two aromatics via five-membered aromatic rings sharing a common bond. The present study investigates a hydrogen-mediated addition of acetylene to the fused five-membered ring part of the E-bridge forming a seven-membered ring. The carried out quantum-mechanical and rate theoretical calculations indicate the plausibility of such capping reactions, and kinetic Monte Carlo simulations demonstrate their frequent occurrence. The capping frequency, however, is limited by "splitting" the fused five-membered bridge due to five-membered ring migration. A similar migration of edge seven-membered rings is shown to be also rapid but short, as their encounter with five-membered rings converts them both into six-membered rings.

Optical properties of incipient soot

The exact knowledge of the optical properties of soot nanoparticles is fundamental for several aspects including the correct determination of the soot concentration in combustion environments using optical diagnostics and the correct estimation of the environmental impact of the emitted particles. Although extensive researches over the years have led to a substantial agreement on the optical properties of mature soot particles, the optical properties of the incipient soot nanoparticles are still uncertain. From the particle inception point to the formation of large and more mature soot particles, the evolution of the optical properties must account for variations due to the size and the physicochemical transformation of the investigated particles. This work aims to determine the refractive index and optical properties of inception particles formed in lightly sooting flames. A previous determination based on in-situ light absorption and scattering measurements is revisited taking advantage of particle size measurements by differential mobility analysis. The spectral dependencies of the optical properties are derived by the Kramers-Krӧnig analysis of the ex-situ VUV-NIR light absorption measurements. Results confirm the strong decrease in the absorptivity in the vis-NIR region of inception particles with unimodal size distribution and d63∼3 nm, and confirm a strong size dependency of soot optical properties. The thermal-optical analysis of the sampled particles shows that the particle mass absorption coefficient also correlates with organic carbon content.

Effects of ammonia addition on soot formation in ethylene laminar diffusion flames. Part 3. The morphology and nanostructure of soot particles

Ammonia (NH3) fuel has been widely considered a competitive alternative fuel and hydrogen carrier due to its “non-carbon and hydrogen-rich” nature. In this study, the thermophoretic sampling combined with transmission electron microscopy (TEM) analysis was conducted to investigate the effects of NH3 addition with varying ratios (between 0% and 60% by volume fraction while the remainder is argon) on the evolution of morphology and nanostructures of soot particles along the flame axis in the ethylene (40% by volume fraction) co-flow diffusion flames. Results showed that the flame visible height increased with the addition of NH3, and the flame temperature decreased at the height above burner (HAB) below 30 mm while increased at HAB > 30 mm. With NH3 addition, the size of soot (primary particles and aggregates) and the number of soot primary particles contained in the soot aggregates decreased significantly. In addition, the investigation of soot nanostructures showed that the carbon layers with shorter fringe length (Fl) and higher tortuosity (Ft) would be oxidized before it arrived to the flame tip in the NH3-doped flame, resulting in the longer Fl and higher Ft. In general, the inter-fringe spacing (Fd) increased with NH3 addition. The evolution of soot morphology and nanostructures indicated that NH3 would delay the process of particle inception and surface growth.

Experimental and numerical investigation of the transition from non sooting to sooting premixed n-butane flames, encompassing the nucleation flame conditions

In a small range of rich equivalence ratios, premixed flat flames, so-called nucleation flames, were shown to produce very small-size (2–4 nm) soot particles, which undergo negligible growth with the residence time. In such flames, the contribution of the soot nucleation step with respect to the soot growth process is larger than in standard sooting flames, making nucleation flames ideal target flames to better understand the inception process. In order to scrutinize the transition from non sooting to sooting flames, three premixed atmospheric n-butane/oxygen/nitrogen flames surrounding the nucleation flame conditions were investigated: the flame at ϕ=1.6 is a fuel-rich non-sooting flame, the flame at ϕ=1.75 is a nucleation flame and the flame at ϕ=1.95 is a lightly sooting flame. The soot volume faction (SVF) profiles were previously measured by laser-induced incandescence as well as the soot size distributions. In the present study, mole fractions of stable species up to benzene were measured by gas chromatography while those of naphthalene and pyrene were obtained by jet-cooled laser-induced fluorescence. It is found that acetylene, propyne, benzene, naphthalene and pyrene species show the highest sensitivity with the equivalence ratio, while a two orders of magnitude increase of SVF is observed. The chemical flame structure of these flames was modeled using three kinetic mechanisms of the literature to which a recently developed soot code, based on a sectional approach, was coupled. The chosen model is representative of the current state-of-the-art of models describing all steps of soot formation and growth and fully coupled with the gas phase. The ability of such a model to predict soot volume fraction with reasonable predictions of gas-phase species is validated here based on the new experimental database presented in this work. It also highlights the improvement required on particle inception modeling to correctly predict particle size distributions.

Measurement of Sub-23 nm Particles Emitted from PFI/DI SI Engine Fueled with Oxygenated Fuels: A Comparison between Conventional and Novel Methodologies

This study focuses on the measurement of sub-23 nm particles emitted from a small DI/PFI spark ignition engine through conventional techniques and innovative systems. Measurements were performed with well-known systems, such as the EEPS coupled to a PMP-compliant sample conditioning device. Moreover, a novel instrument developed within the European project Sureal-23, the advanced HM-DMA, capable of operating with a simplified conditioning setup was used. The engine was fueled with ethanol, both pure and in blend at 30% v/v. The effects of fuel on the particle emissions were analyzed at different operating conditions. The results highlighted that a larger fraction of emissions consists of particles smaller than 23 nm, and their number changes according to the fuel, injection strategy and operating condition. A significant effect of the sampling system conditions was observed reveling the inception of nucleation mode particles or the condensation of the volatiles onto existing particles depending on the combination fuel/injection strategy. Different trends were noted at certain operating conditions between the results from the EEPS and the advanced HM-DMA ascribable to the different measurement principle and to the dilution system.

Promotion of particle formation by resonance-stabilized radicals during hydrocarbon pyrolysis

We measured aerosol mass spectra and mobility size distributions of particles formed during the pyrolysis of ethylene, indene, and a mixture of ethylene with a small amount of indene. Measurements were recorded with a scanning mobility particle sizer and an aerosol mass spectrometer employing vacuum ultraviolet photoionization using tunable radiation from the Advanced Light Source at Lawrence Berkeley National Laboratory. The results demonstrate that particle formation occurs at a temperature of ∼1123±50 K for ethylene alone, at ∼923±50 K for indene alone, and at a comparable temperature to that of indene for ethylene seeded with a small amount of indene. Our results demonstrate that indene and indenyl, a resonance-stabilized radical (RSR) formed from indene pyrolysis, promote particle formation at lower temperatures and in the absence of acetylene, supporting the hypothesis that RSRs promote soot-particle inception. Even a small amount of indene added to ethylene promotes particle formation while circumventing the traditional pathways for mass growth by acetylene-addition reactions. Further support is provided by the appearance in the aerosol mass spectra of prominent peaks for masses corresponding to other RSRs from particles formed at lower pyrolysis temperatures. In addition, odd-numbered carbon species, associated with RSRs, appear in higher concentrations at low temperatures and are closely tied to particle inception. In indene pyrolysis, mass signals indicative of covalently bound indenyl dimers and trimers are prominent at pyrolysis temperatures near soot onset. Photoionization efficiency (PIE) curves demonstrate that, while the isomeric composition for some of these peaks may differ between ethylene and indene, the addition of indene to ethylene does not notably alter the isomers formed at these peaks between ethylene and the ethylene-indene mixture. PIE curves also show that m/z 202, which is commonly assumed to be pyrene, is composed principally of fluoranthene under these conditions.

On the Mechanism of Soot Nucleation. III. The Fate and Facility of the E-Bridge

Rotationally excited dimerization of aromatic moieties is a mechanism proposed recently to explain the initial steps of soot particle inception in combustion and pyrolysis of hydrocarbons. The product of such dimerization, termed E-bridge, is an angled molecular structure composed of two aromatic rings sharing a common bond. The present study explores the immediate fate of the E-bridge. The performed theoretical analysis indicates that abstraction of a bridge H atom by a gaseous H leads to a rapid transformation of the angled to planar structure. The implications of this result is that the collisionally activated E-bridge formation followed by its flattening effectively increases the size of "planar" aromatic precursors by combining two aromatic moieties with essentially collisional rates, instead of a slower "atom-by-atom" buildup. The faster growth speeds up PAH reaching a size when physical dimerization takes over. The dimerization can be further assisted by the biradicaloid valence structure of the flattened E-bridge.

Fuel molecular structure effect on soot mobility size in premixed C6 hydrocarbon flames

Soot formation in premixed laminar flames is examined for a canonical set of flames burning C6 hydrocarbon fuels. Particle mobility size and flame temperature measurements are complemented by flame structure calculations using detailed flame chemistry. Specifically, the evolution of the detailed soot particle size distribution (PSDF) is compared for n-hexane, n-hexene, 2-methylpentane, cyclohexane and benzene at a carbon-to-oxygen ratio of 0.69 and maximum flame temperature of 1800 K. Under this constraint, the overall sooting process is comparable as evidenced by similar time resolved bimodal PSDF. However, the first inception of particles and the persistence of nucleation-sized particles with time are depend upon the structure of the parent fuel. For the given conditions, the fastest onset of soot is observed in cyclohexane and benzene flames and the observed evolution of the PSDF also shows that nucleation-sized particles disappear sooner in cyclohexane and benzene flames. Flame structure computations incorporating detailed chemistry show a clear connection between the early onset of soot particles as fuel specific routes to PAH formation are predicted in the pre-flame region of the cyclohexane and benzene flames. These observations illustrate the impact of alkane, alkene, cycloalkane and aromatic fuel structure on soot formation in premixed flames. Analysis of soot particle morphology by atomic force microscopy indicates that most of size distribution is composed of aggregates. Simple aggregate mobility diameter analysis shows the spherical assumption taken to interpret the mobility diameter does not impact the PSDF number density result but the inferred volume fraction for aggregates deviates by up to an order of magnitude depending on the morphology assumptions adopted.

Soot and combustion models for direct-injection natural gas engines

This paper summarizes the validation of a modified multi-step phenomenological soot model and an enhanced combustion model used for direct-injection natural gas engines. In this study, a modified phenomenological soot model including the key steps for soot formation, such as particle inception and surface growth, was developed in KIVA-3V to replace the empirical model for use in a glow plug assisted natural gas direct-injection engine. The soot model was integrated with a CANTERA based kinetic model, which employs a recently developed low temperature natural gas mechanism to predict the reactions of some important gaseous species involved in the soot formation, such as acetylene and hydroxyl. The simulated in-cylinder flame propagation process induced by a glow plug was compared to the experimental optical images obtained in an engine-like environment. In addition, both the kinetic model and modified soot model were compared with the experimental emission data to validate their reliability for predicting natural gas engine emission characteristics. The engine combustion efficiencies obtained in simulations and experiments were compared as well. The matched results suggest that the computational models can well predict the natural gas combustion and emission characteristics, and will be suitable for investigating the direct-injection natural gas engine technologies.

Nucleation of soot: experimental assessment of the role of polycyclic aromatic hydrocarbon (PAH) dimers

Abstract The irreversible dimerization of polycyclic aromatic hydrocarbons (PAHs) – typically pyrene (C16H10) dimerization – is widely used in combustion chemistry models to describe the soot particle inception step. This paper concerns itself with the detection and identification of dimers of flame-synthesized PAH radicals and closed-shell molecules and an experimental assessment of the role of these PAH dimers for the nucleation of soot. To this end, flame-generated species were extracted from an inverse co-flow flame of ethylene at atmospheric pressure and immediately diluted with excess nitrogen before the mixture was analyzed using flame-sampling tandem mass spectrometry with collision-induced fragmentation. Signal at m/z = 404.157 (C32H20) and m/z = 452.157 (C36H20) were detected and identified as dimers of closed-shell C16H10 and C18H10 monomers, respectively. A complex between a C13H9 radical and a C24H12 closed-shell PAH was observed at m/z = 465.164 (C37H21). However, a rigorous analysis of the flame-sampled mass spectra as a function of the dilution ratio, defined as the ratio of the flow rates of the diluent nitrogen to the sampled gases, indicates that the observed dimers are not flame-born, but are produced in the sampling line. In agreement with theoretical considerations, this paper provides experimental evidence that pyrene dimers cannot be a key intermediate in particle inception at elevated flame temperatures.

Impact of Biofuel Blends on Black Carbon Emissions from a Gas Turbine Engine.

Presented here is an overview of non-volatile particulate matter (nvPM) emissions, i.e. "soot" as assessed by TEM analyses of samples collected after the exhaust of a J-85 turbojet fueled with Jet-A as well as with blends of Jet-A and Camelina biofuel. A unifying explanation is provided to illustrate the combustion dynamics of biofuel and Jet-A fuel. The variation of primary particle size, aggregate size and nanostructure are analyzed as a function of biofuel blend across a range of engine thrust levels. The postulate is based on where fuels start along the soot formation pathway. Increasing biofuel content lowers aromatic concentration while placing increasing dependence upon fuel pyrolysis reactions to form the requisite concentration of aromatics for particle inception and growth. The required "kinetic" time for pyrolysis reactions to produce benzene and multi-ring PAHs allows increased fuel-air mixing by turbulence, diluting the fuel-rich soot-forming regions, effectively lowering their equivalence ratio. With a lower precursor concentration, particle inception is slowed, the resulting concentration of primary particles is lowered and smaller aggregates were measured. The lower equivalence ratio also results in smaller primary particles because of the lower concentration of growth species.

On the mechanism of soot nucleation.

The mechanism of carbon particulate (soot) inception has been a subject of numerous studies and debates. The article begins with a critical review of prior proposals, proceeds to the analysis of factors enabling the development of a meaningful nucleation flux, and then introduces new ideas that lead to the fulfillment of these requirements. In the new proposal, a rotationally-activated dimer is formed in the collision of an aromatic molecule and an aromatic radical; the two react during the lifetime of the dimer to form a stable, doubly-bonded bridge between them, with the reaction rooted in a five-member ring present on the molecule edge. Several such reactions were examined theoretically and the most promising one generated a measurable nucleation flux. The consistency of the proposed model with known aspects of soot particle nanostructure is discussed. The foundation of the new model is fundamentally the H-Abstraction-Carbon-Addition (HACA) mechanism with the reaction affinity enhanced by rotational excitation.

Optical band gap analysis of soot and organic carbon in premixed ethylene flames: Comparison of in-situ and ex-situ absorption measurements

The similarity of in-situ and ex-situ absorption/extinction properties was found by comparing data sets obtained in premixed flat ethylene flames (C/O = 0.77); both ex-situ absorption measurements at CNR of Naples and in-situ laser extinction/absorption measurements at Lund University. The optical band gap analysis was performed as a method to separate/evaluate the contributions to the UV–Visible absorption from organic carbon and soot by selecting two spectral regions; above 685 nm where only soot is assumed to absorb radiation and below 685 nm, where both soot and organic carbons absorb. Optical band gap for soot was analyzed separately from organic carbon, obtaining their individual contribution to the spectral absorption from inception, throughout the growth region up to soot aggregation. While the optical band gap of soot strongly decreased along the flame from 1 to 0.1 eV, the band gap for organic carbons remained constant at ∼1.7–1.8 eV. By using the relationship of the optical band gap with nanostructural parameters (layer length and aromatic rings number), the average molecular weight of organic carbon around 400 u resulted rather far away from the molecular weight of 1000–2000 mass units evaluated for incipient soot consistently with the view of PAH species sticking together for particle inception.