Primary Soot Particles Research Articles

Variations in diesel soot reactivity along the exhaust after-treatment system, based on the morphology and nanostructure of primary soot particles

The reactivity of soot at different sites of the exhaust after-treatment system of a diesel engine (upstream and downstream of the diesel oxidation catalyst (DOC), downstream of the diesel particulate filter (DPF), as well as inside the DPF) was investigated on the basis of morphology and structure of primary soot particles by high resolution transmission electron microscopy (HRTEM). The results indicate that combustion-formed soot particles are susceptible to further transformations of their morphology within the exhaust system. The same primary soot particles can possess both oxidation-promoting and oxidation-inhibiting morphological features, the particle cores being highly reactive. Most reactivity-promoting features are encountered in pre-DOC and post-DOC primary particles, suggesting that soot can be more easily oxidised before it enters the DPF. The residence time of soot in the DPF contributes to modification of its reactivity by affecting size distribution and nanostructure of primary particles. Partial NO2 oxidation and high temperatures during active regeneration modify the morphology of outer particle shells, thus rendering post-DOC and post-DPF primary soot particles less reactive in this respect. Primary soot particles that pass through the DPF and reach the atmosphere are characterised by the highest graphitisation degree and sizes larger than those entering the DPF. Complementary Near-Edge X-ray Absorption Fine Structure (NEXAFS) analyses proved not as relevant regarding soot reactivity but indicate higher chemical inhomogeneity of pre-DOC than of post-DOC and post-DPF soot and high contents of carboxyl carbon in post-DPF particles.

Microscopic investigation of soot and ash particulate matter derived from biofuel and diesel: implications for the reactivity of soot

Investigation of soot and ash particulate matter deposited in diesel particulate filters (DPFs) operating with biofuel (B100) and diesel (pure diesel: B0 and diesel80/biofuel20 blend: B20) by means of optical microscopy, scanning electron microscopy, and high resolution transmission electron microscopy (HRTEM) reveals the following: the rapeseed methyl ester biofuel used for this study contributes to ash production, mainly of Ca–S– and P-bearing compounds ranging in size between 50 and 300 nm. Smaller ash particles are less common and build aggregates. Ash is deposited on the inlet DPF surface, the inlet channel walls, and in B100-DPF at the plugged ends of inlet channels. The presence of Fe–Cr–Ni fragments, down to tens of nanometers in size within the ash is attributed to engine wear. Pt particles (50–400 nm large) within the ash indicate that the diesel oxidation catalyst (DOC) upstream of the DPF shows aging effects. Radial cracks on the coating layer of the DOC confirm this assumption. The B100-DPF contains significantly less soot than B20 and B0. Based on the generally accepted view that soot reactivity correlates with the nanostructure of its primary particles, the length and curvature of graphene sheets from biofuel- and diesel-derived soot were measured and computed on the basis of HRTEM images. The results show that biofuel-derived soot can be more easily oxidized than diesel soot, not only during early formation but also during and after considerable particle growth. Differences in the graphene sheet separation distance, degree of crystalline order and size of primary soot particles between the two fuel types are in line with this inference.

Nanostructure and oxidative properties of soot from a compression ignition engine: The effect of a homogeneous combustion catalyst

This study examined the influence of an iron-based homogeneous combustion catalyst on the oxidative behaviour and nanostructural characteristics of soot emitted from a single-cylinder compression ignition engine. The catalyst was homogeneously added into a commercial diesel as the reference at ultra low dosage ratios. Smoke opacity was measured using a smoke opacimeter in the engine exhaust stream to indicate the overall soot emission intensity. Soot particles were also sampled while the engine was maintained in steady-state operations and fuelled with the reference diesel and the catalyst treated fuels, respectively. The soot samples were subjected to thermogravimetric analysis (TGA) to study their oxidation reactivities in air, and transmission electron microscopy (TEM) for morphological characterisations. The smoke opacity results showed 7.3–39.5% less soot emissions when the catalyst was applied, depending on the catalyst dosage ratio. The TGA results revealed that soot from the catalyst treated fuels possessed higher oxidative reactivity as indicated by ignition at lower temperatures and faster oxidation rates than those of soot from the reference diesel. This tendency was more remarked as the catalyst dosage ratio increased. Subsequent TEM imaging analyses observed that the irregularly shaped, aggregated soot particles were made of a number of spherical primary particles. Smaller and more narrowly-distributed primary soot particles with the catalyst treated fuels than those of the reference diesel were evident. High-resolution TEM imaging revealed graphitic crystallite structures of the soot samples from both catalyst-treated and -untreated fuels with no obvious variations in the nuclei core areas, suggesting that the internal structure of the soot was not affected by the catalyst. It was evident that iron ions from the catalyst were more involved in the soot oxidation process, rather than in the early soot formation stage, and eventually resulted in smaller and narrowly-distributed primary soot particles.

Experimental Studies on the Influence of Diesel Engine Operating Parameters on Properties of Emitted Soot Particles

Physical and chemical properties of emitted soot particles are influenced by diesel engine operating parameters. In order to find correlations between the in-cylinder processes of combustion and the engine-out properties of soot particles, parameter studies were carried out on a modern light duty production diesel engine (turbo direct injection, TDI) and an optically accessed single cylinder diesel engine (single cylinder). Several techniques have been combined to analyze the combustion process as well as the emitted particles. The size of emitted soot particles of the TDI engine has been determined by a scanning mobility particle sizer (SMPS), and soot samples have been analyzed by a high-resolution transmission electron microscope (HR-TEM) and by thermogravimetry (TG). In addition, the internal combustion in an optically accessed single cylinder engine has been observed by time resolved spectroscopy as well as OH*- and C2-imaging. It turns out that in both cases, increasing injection pressure leads to a decreasing soot particle agglomerate and primary particle size with decreasing oxidation temperature.

Brownian Coagulation of Knudsen Particles on Evaporating/Condensing Aerosol Droplet; Modeling of Capture of Soot Particles by Marine Fog

The theoretical model presented in this work allows quantitative prediction of the rate of Brownian coagulation of pollutant particles on a condensing aerosol droplet. The coupling of the processes of condensation and Brownian coagulation is established through the hydrodynamic-diffusiophoretic force acting upon the Brownian particles in the vicinity of the droplet. The new analytical approach is based on the Grad's moment method of solution to the Fokker Planck equation. The aerosol system is assumed to be isothermal and close to the thermodynamic equilibrium (i.e., the condensation and coagulation fluxes are small). The numerical predictions of the diffusiophoretic enhancement of Brownian coagulation rates are provided for NaCl-water droplets capturing small (primary) soot particles. The results of calculations show substantial enhancement (10 to 200 times) of the capture of soot particles by the condensing marine droplets for typical conditions that occur on the north-eastern coast of the U.S.A.

Aging induced changes on NEXAFS fingerprints in individual combustion particles

Abstract. Soot particles can significantly influence the Earth's climate by absorbing and scattering solar radiation as well as by acting as cloud condensation nuclei. However, despite their environmental (as well as economic and political) importance, the way these properties are affected by atmospheric processing of the combustion exhaust gases is still a subject of discussion. In this work, individual soot particles emitted from two different vehicles, a EURO 2 transporter, a EURO 3 passenger car, and a wood stove were investigated on a single-particle basis. The emitted exhaust, including the particulate and the gas phase, was processed in a smog chamber with artificial solar radiation. Single particle specimens of both unprocessed and aged soot were characterized using near edge X-ray absorption fine structure spectroscopy (NEXAFS) and scanning electron microscopy. Comparison of NEXAFS spectra from the unprocessed particles and those resulting from exhaust photooxidation in the chamber revealed changes in the carbon functional group content. For the wood stove emissions, these changes were minor, related to the relatively mild oxidation conditions. For the EURO 2 transporter emissions, the most apparent change was that of carboxylic carbon from oxidized organic compounds condensing on the primary soot particles. For the EURO 3 car emissions oxidation of primary soot particles upon photochemical aging has likely contributed as well. Overall, the changes in the NEXAFS fingerprints were in qualitative agreement with data from an aerosol mass spectrometer. Furthermore, by taking full advantage of our in situ microreactor concept, we show that the soot particles from all three combustion sources changed their ability to take up water under humid conditions upon photochemical aging of the exhaust. Due to the selectivity and sensitivity of the NEXAFS technique for the water mass, also small amounts of water taken up into the internal voids of agglomerated particles could be detected. Because such small amounts of water uptake do not lead to measurable changes in particle diameter, it may remain beyond the limits of volume growth measurements, especially for larger agglomerated particles.

Effect of fuel on the soot nanostructure and consequences on loading and regeneration of diesel particulate filters

An automotive diesel engine was tested in three representative modes of soot accumulation, active regeneration and spontaneous regeneration of its catalyzed diesel particulate filter (DPF), among the typical driving operation modes. During the engine tests, pressure and temperature along the DPF were measured, and soot samples were taken from the exhaust manifold upstream of the DPF for their thermal, structural and morphological characterization. The collected soot samples were subjected to: Transmission Electron Microscopy (TEM) for morphological analysis, thermal heating under oxidant atmosphere for studying the oxidation kinetics, Raman spectroscopy for describing their nanostructure and X-ray diffraction spectroscopy (XRD) for studying their internal lattice parameters. When the engine was operated in a typical accumulation mode, the pressure drop across the DPF increased up to 80 hPa with diesel fuel, while pressure drop stopped increasing after 4000 s of engine testing with biodiesel. In the regeneration mode, the DPF regenerated more slowly in the biodiesel case as a consequence of lower post-injected fuel energy and thus lower exhaust temperature. In the self-regenerating mode, the DPF was charged more slowly with biodiesel than with diesel fuel and its break even temperature was 40 °C lower with biodiesel fuel. These results provide further evidence that biodiesel soot is more reactive to oxidation. Although thermogravimetric results confirmed this tendency based on the differences on the pre-exponential factor, Raman spectra showed that biodiesel soot reached more ordered graphite-like structures and lower amorphous carbon concentration and XRD analysis showed that biodiesel soot displayed a higher degree of graphitization. The TEM analysis of the agglomerates showed that soot primary particles obtained with biodiesel fuel were significantly smaller and had higher specific active surface than those of diesel soot. From these results, an interpretation of the differences in soot oxidation between both soot samples was made based on the different length scales, from the carbon fringes to the particulate filter.

Soot and char molecular representations generated directly from HRTEM lattice fringe images using Fringe3D

Char and soot are complex carbonaceous structures that are important in traditional energy generation, reactivity, biomass combustion, pollution, climate change, and human health. HRTEM lattice fringe images have significantly improved how we “see” these complex structures. Yet our attempts to use molecular modeling to explore the structure behavior relationships are hampered by the complexity and difficulty in capturing the alignment, graphene to polyaromatic “hydrocarbon” stacks, any symmetry, and more importantly the distribution of structural features. It is the structural detail, which often controls the behavior of soots and chars that are otherwise similar in bulk properties. As a consequence, modeling efforts are often highly simplistic or highly complex with considerable expense being utilized in obtaining structures that do not capture all these structural details. Here we attempt a new approach: Fringe3D to directly generate the lattice fringe images in molecular space from HRTEM lattice fringe images. Image analysis determines the Cartesian coordinates, fringe length, orientation, and more importantly the distributions of those parameters. A Perl script populates the model space with a centroid representing the center of each fringe. From calibration files, either a known molecule or selected number of carbon atoms in the desired catenation style, molecules are constructed for each fringe around each centroid. The molecules are pitched to match fringe orientation. In this manner the: symmetry, stacking, orientation, and structural distributions are retained along with fine-structural details. The approach is demonstrated, with a bituminous coal char and a primary diesel soot particle, to produce initially simplistic atomistic representations of the aromatic carbon structure with greater ease, and far more rapidly than existing approaches.

Characteristics of Atmospheric Elemental Carbon (Char and Soot) in Ultrafine and Fine Particles in a Roadside Environment, Japan

Atmospheric carbonaceous components, particularly char and soot in ultrafine particles (UFPs; Dp < 0.1 μm) and fine particles (FPs; Dp < 2.5 μm), were measured four times during one year in Saitama City, Japan, to observe the concentrations of elemental carbon (EC) and the relationship between the EC concentrations in UFPs and FPs, and to examine the possible emission sources of char and soot that constitute UFPs and FPs in a roadside environment. It was found that EC accounts for 33–37% of total carbon (TC) in FPs, whereas EC accounts for 12–20% of TC in UFPs. Both char-EC and soot-EC account for similar proportions of the total EC concentration in UFPs, while soot-EC accounts for only a small amount of the total EC in FPs. Positive and negative correlations between OC and soot-EC were observed for UFPs and FPs, respectively. The observed positive correlation in the case of UFPs possibly reflects the compactness (high density) of UFPs coated with condensed material, such as unburned fuel or lubricating oil emitted by motor vehicles, whereas the negative correlation in the case of FPs possibly indicates that whether or not the spaces between primary soot particles in FPs can be filled depends on the engine load of diesel vehicles operated near the sampling site. The positive and negative correlations were stronger for UFPs (r2 = 0.69, n = 29, p < 0.001) and FPs (r2 = –0.62, n = 29, p < 0.001) when the data collected at wind speeds greater than 2.5 m/s were excluded. The different morphological characteristics of the particles observed by transmission electron microscopy also support the observed correlations between OC and soot-EC. The possible emission of char or char-like particles from motor vehicles was shown and discussed in this study.

Water Adsorption on Oxidized Single Atomic Vacancies Present at the Surface of Small Carbonaceous Nanoparticles Modeling Soot

Quantum calculations are used to study the interaction of water molecules with carbonaceous clusters containing one single carbon atom vacancy. This is a simple but realistic way to model the active surfaces found in soot emitted by aircrafts. Prior to water adsorption, the atomic vacancy is oxidised by an approaching oxygen molecule, which is also likely to occur behind planes. The results of the calculations show that this oxidation process results in the formation of one ketone-like site and one epoxide-like site around the atomic vacancy. These sites may act as nucleation centers for water molecules, which are, however, physisorbed on the oxidized surface, leading to very weak charge transfer with the surface. Although less attractive for water than, for instance, a carboxyl-like site, the ketone-like site can also participate in the hydrophilic behavior of soot primary particles. In contrast, the epoxide-like site formed around the vacancy shows a very low affinity for water molecules.

Molecular Dynamics Simulations of Laser-Induced Incandescence of Soot Using an Extended ReaxFF Reactive Force Field

Laser-induced incandescence (LII) of soot has developed into a popular method for making in situ measurements of soot volume fraction and primary particle sizes. However, there is still a lack of understanding regarding the generation and interpretation of the cooling signals. To model heat transfer from the heated soot particles to the surrounding gas, knowledge of the collision-based cooling as well as reactive events, including oxidation (exothermic) and evaporation (endothermic) is essential. We have simulated LII of soot using the ReaxFF reactive force field for hydrocarbon combustion. Soot was modeled as a stack of four graphene sheets linked together using sp(3) hybridized carbon atoms. To calculate the thermal accommodation coefficient of various gases with soot, graphene sheets of diameter 40 Å were used to create a soot particle containing 2691 atoms, and these simulations were carried out using the ReaxFF version incorporated into the Amsterdam Density Functional program. The reactive force field enables us to simulate the effects of conduction, evaporation, and oxidation of the soot particle on the cooling signal. Simulations were carried out for both reactive and nonreactive gas species at various pressures, and the subsequent cooling signals of soot were compared and analyzed. To correctly model N(2)-soot interactions, optimization of N-N and N-C-H force field parameters against DFT and experimental values was performed and is described in this paper. Subsequently, simulations were performed in order to find the thermal accommodation coefficients of soot with various monatomic and polyatomic gas molecules like He, Ne, Ar, N(2), CO(2), and CH(4). For all these species we find good agreement between our ReaxFF results and previously published accommodation coefficients. We thus believe that Molecular Dynamics using the ReaxFF reactive force field is a promising approach to simulate the physical and chemical aspects of soot LII.

Optical soot characterization using two-color laser-induced incandescence (2C-LII) in the soot growth region of a premixed flat flame

The soot growth region of a premixed one-dimensional ethylene/air flame has been investigated using two-color laser-induced incandescence (2C-LII) with focus on optical soot properties. From the 2C-LII technique, primary soot particle sizes were deduced together with the temperature from pyrometry of the laser-heated particles, while the gas temperatures were obtained from pure rotational CARS nitrogen thermometry. Soot particle sizes were also measured from thermophoretically sampled soot particles analyzed by transmission electron microscopy (TEM), and the results showed growing isolated primary soot particles up to a height of 10mm, after which strong soot aggregation occurred and the increase in soot primary particle size ceased. The measured data was used in the evaluation of the soot absorption function, E(m), and a strong increase by a factor of two was observed from the lowest heights to the highest for assumed constant values of soot density and specific heat at all heights. By comparing the 2C-LII particle sizes with the sizes obtained from TEM, differences were observed. Part of the explanation is that the 2C-LII evaluation did not take aggregation into account, but it is additionally speculated that the thermal accommodation coefficient decreases with height above burner. These results are interesting in view of morphological and composition changes of the soot during the growth process and have implications for the use of 2C-LII as a soot diagnostic technique.

Solution of the inverse problem for the parameters of primary carbon nanoparticles from electron-microscopy data

From the data on the size of primary carbon particles formed in ethylene pyrolysis in a shock tube in the temperature range 2100-2800 K, calculation of their growth time and temperature is performed. It is shown that the growth time of primary nanosize soot particles does not exceed 7-10 μsec; they are heated up to temperatures hundreds of degrees higher than the gaseous-phase temperature, which is confirmed by the results of measuring the soot temperature.

Interaction of water molecules with defective carbonaceous clusters: An ab initio study

First-principle calculations are used to study the interaction of water molecules with carbonaceous clusters containing single carbon atom vacancy, similar to those which may be found in soot nanoparticles. It is shown that the dissociative adsorption of one water molecule at the vacancy site may lead to the formation of a “ketone-like” structure which can then act as a nucleation center for additional water molecules. Such a mechanism can thus participate in the hydrophilic behavior of soot primary particles although it appears less favorable than water nucleation around more hydrophilic sites such as carboxyl or hydroxyl groups.

Investigations on Soot Formation in Heptane Jet Diffusion Flames by Optical Techniques

Two-dimensional optical measurements were performed for the investigation of soot formation of n-heptane laminar gas-jet diffusion flames under buoyant and non-buoyant conditions utilizing the Bremen Drop Tower. Techniques employed were laser-induced incandescence for the determination of soot concentration and primary particle sizes and two-color emission pyrometry with a three-point Abel inversion for the measurement of temperature fields. In line with former experiments for other hydrocarbon fuels the investigations revealed drastic differences in the sooting behavior between flames under normal and microgravity. With the lack of buoyancy maximum soot temperatures were reduced by roughly 300 K and maximum primary particle sizes were more than doubled.

A theoretical characterization of the interaction of water with oxidized carbonaceous clusters

Quantum mechanical calculations are used to study the interaction of water molecules with oxidized carbonaceous clusters, modeling soot primary particles emitted by aircraft engines. First, the interaction of atomic oxygen with the face or the edges of small graphite crystallites of nanometer size is modeled at a quantum level, leading to the formation of graphene-oxide-like clusters. Then, a similar quantum approach is used to characterize the adsorption of one to six water molecules on the resulting clusters, in order to better understand, at a molecular level, the role of such oxidized sites on the hydrophilic properties of soot. It is shown that epoxide-like sites can participate in the hydrophilic behavior of soot primary particles although they are less attractive for water than hydroxyl or carboxyl sites.

Modelling aerosol number distributions from a vehicle exhaust with an aerosol CFD model

Vehicular traffic contributes significantly to the aerosol number concentrations at the local scale by emitting primary soot particles and forming secondary nucleated nanoparticles. Because of their potential health effects, more attention is paid to the traffic induced aerosol number distributions. The aim of this work is to explain the phenomenology leading to the formation and the evolution of the aerosol number distributions in the vicinity of a vehicle exhaust using numerical modelling. The emissions are representative of those of a light-duty diesel truck without a diesel particle filter. The atmospheric flow is modelled with a computational fluid dynamics (CFD) code to describe the dispersion of pollutants at the local scale. The CFD code, coupled to a modal aerosol model (MAM) describing the aerosol dynamics, is used to model the tailpipe plume of a vehicle with emissions corresponding to urban driving conditions. On the basis of available measurements in Schauer et al. (1999), three surrogate species are chosen to treat the semi-volatile organic compounds in the emissions. The model simulates the formation of the aerosol distribution in the exhaust plume of a vehicle as follows. After emission to the atmosphere, particles are formed by nucleation of sulphuric acid and water vapour depending strongly on the thermodynamic state of the atmosphere and on the dilution conditions. The semi-volatile organic compounds are critical for the rapid growth of nanoparticles through condensation. The semi-volatile organic compounds are also important for the evolution of primary soot particles and can contribute substantially to their chemical composition. The most influential parameters for particle formation are the sulphur fuel content, the semi-volatile organic emissions and also the mass and initial diameter of the soot particles emitted. The model is able to take into account the complex competition between nucleation, condensation and dilution, as well as the interactions among the different aerosol modes. This type of model is a useful tool to better understand the dynamics leading to the formation of traffic induced aerosol distributions. However, some key issues such as the turbulence in the exhaust plume and in the wake of the car, the magnitude and chemical composition of semi-volatile organic emissions and the possible nucleation of organic species need to be investigated further to improve our understanding of ultrafine particle formation.

Nanostructure of Soot Collected from Ethanol Droplet Flames in Microgravity

The nanostructure of soot particles collected from spherically symmetric ethanol droplet flames were analyzed using a high resolution transmission electron microscopy (HRTEM). Nanostructure properties, including fringe length and curvature of carbon lamellae, were measured for soot particles collected in various inert environments. The sampling experiments were performed in the reduced gravity environments produced in the NASA 2.2 sec Droptower at the Glenn Research Center in Cleveland, Ohio. Microgravity droplet combustion experiments provide unique opportunities to vary the residence times over a large range and to independently vary the temperature. In this study, the time-temperature histories experienced by the soot particles were adjusted by means of inert substitutions (argon vs. helium) and variations in the initial droplet diameters (ranging from 1.6 to 2.2 mm). The variations in the initial droplet diameter were found to affect only the residence time necessary for soot inception and growth, whereas inert substitutions modified both residence time and temperature. The measurements of soot nanostructure properties indicated that the higher temperatures produced in the argon inert environment produced more graphitic nanostructures, while the lower temperatures produced in the helium inert environment produced more amorphous nanostructures at the inner core of the soot primary particle. The variations in the initial droplet diameter produced distinct soot nanostructures on the periphery of the soot particle. The higher residence times experienced for the largest initial droplet diameter experiments produced longer carbon lamellae with negligible curvature, while the lower residence times for the smallest initial droplet diameter experiments produced shorter carbon lamellae with higher degrees of curvature. These experimental results provide important foundational understanding of the influence of residence time and temperature on the soot nanostructure that has not been studied previously.

A numerical study of soot aggregate formation in a laminar coflow diffusion flame

Soot aggregate formation in a two-dimensional laminar coflow ethylene/air diffusion flame is studied with a pyrene-based soot model, a detailed sectional aerosol dynamics model, and a detailed radiation model. The chemical kinetic mechanism describes polycyclic aromatic hydrocarbon formation up to pyrene, the dimerization of which is assumed to lead to soot nucleation. The growth and oxidation of soot particles are characterized by the HACA surface mechanism and pyrene-soot surface condensation. The mass range of the solid soot phase is divided into thirty-five discrete sections and two equations are solved in each section to model the formation of the fractal-like soot aggregates. The coagulation model is improved by implementing the aggregate coagulation efficiency. Several physical processes that may cause sub-unitary aggregate coagulation efficiency are discussed. Their effects on aggregate structure are numerically investigated. The average number of primary soot particles per soot aggregate n p is found to be a strong function of the aggregate coagulation efficiency. Compared to the available experimental data, n p is well reproduced with a constant 20% aggregate coagulation efficiency. The predicted axial velocity, OH mole fraction, and C 2H 2 mole fraction are validated against experimental data in the literature. Reasonable agreements are obtained. Finally, a sensitivity study of the effects of particle coalescence on soot volume fraction and soot aggregate nanostructure is conducted using a coalescence cutoff diameter method.

Modeling of soot aggregate formation and size distribution in a laminar ethylene/air coflow diffusion flame with detailed PAH chemistry and an advanced sectional aerosol dynamics model

Soot aggregate formation and size distribution in a laminar ethylene/air coflow diffusion flame is modeled with a PAH-based soot model and an advanced sectional aerosol dynamics model. The mass range of solid soot phase is divided into 35 discrete sections and two variables are solved for in each section. The coagulation kernel of soot aggregates is calculated for the entire Knudsen number regime. Radiation from gaseous species and soot are calculated by a discrete-ordinate method with a statistical narrow-band correlated-k based band model. The discretized sectional soot equations are solved simultaneously to ensure convergence. Parallel computation with the domain decomposition method is used to save computational time. The flame temperature, soot volume fraction, primary particle size and number density are well reproduced. The number of primary particles per aggregate is overpredicted. This discrepancy is presumably associated with the unitary coagulation efficiency assumption in the current sectional model. Along the maximum soot volume fraction pathline, the number-based and mass-based aggregate size distribution functions are found to evolve from unimodal to bimodal and finally to unimodal again. The different shapes of these two aggregate size distribution functions indicate that the total number and mass of aggregates are dominated by aggregates of different sizes. The PAH-soot condensation efficiency γ is found to have a small effect on soot formation when γ is larger than 0.5. However, the soot level and primary particle number density are significantly overpredicted if the PAH-soot condensation process is neglected. Generally, larger γ predicts lower soot level and primary particle number density. Further study on soot aggregate coagulation efficiency should be pursued and more experimental data on soot aggregate structure and size distribution are needed for improving the current sectional soot model and for better understanding the complex soot aggregation phenomenon.