Soot Particle Size Distribution Research Articles

Cloud droplet activation of black carbon particles coated with organic compounds of varying solubility

Abstract. Atmospheric black carbon (BC) particles are a concern due to their impact on air quality and climate. Their net climate effect is, however, still uncertain. This uncertainty is partly related to the contribution of coated BC particles to the global cloud condensation nuclei (CCN) budgets. In this study, laboratory measurements were performed to investigate CCN activity of BC (REGAL 400R pigment black) particles, in pure state or coated through evaporating and subsequent condensation of glutaric acid, levoglucosan (both water-soluble organics) or oleic acid (an organic compound with low solubility). A combination of soot particle aerosol mass spectrometer (SP-AMS) measurements and size distribution measurements with a scanning mobility particle sizer (SMPS) showed that the studied BC particles were nearly spherical agglomerates with a fractal dimension of 2.79 and that they were coated evenly by the organic species. The CCN activity of BC particles increased after coating with all the studied compounds and was governed by the fraction of organic material. The CCN activation of the BC particles coated by glutaric acid and levoglucosan were in good agreement with the theoretical calculations using the shell-and-core model, which is based on a combination of the CCN activities of the pure compounds. The oleic acid coating enhanced the CCN activity of the BC particles, even though the pure oleic acid particles were CCN inactive. The surprising effect of oleic acid might be related to the arrangement of the oleic acid molecules on the surface of the BC cores or other surface phenomena facilitating water condensation onto the coated particles. Our results show that present theories have potential for accurately predicting the CCN activity of atmospheric BC coated with organic species, given that the identities and amounts of the coating species are known. Furthermore, our results suggest that even relatively thin soluble coatings (around 2 nm for the compounds studied here) are enough to make the insoluble BC particles CCN active at typical atmospheric supersaturations and thus be efficiently taken up by cloud droplets. This highlights the need for an accurate description of the composition of atmospheric particles containing BC to unravel their net impact on climate.

Measurements of sooting limits in laminar premixed burner-stabilized stagnation ethylene, propane, and ethylene/toluene flames

The systematic investigation of the effect of two major experimental parameters (equivalence ratio and temperature) as well as fuel structure on the sooting limits of ethylene, propane, and ethylene/toluene flames was carried out in premixed burner-stabilized stagnation flames through qualitatively visual observations of flame yellow luminosity and quantitatively direct measurements of incipient soot particle size distributions by a scan mobility particle sizer. A strong temperature dependence of incipient soot formation was observed in the three flames. Detailed chemical kinetic modeling of the flame structure and the soot precursor chemistry was performed to explain the experimental observations. The results show that the incipient soot formation is prohibited in either low or high temperature flames. While the former is due to slow reactions induced by both low temperatures and small concentrations of soot precursors, the latter is attributed to the thermodynamic reversibility of polycyclic aromatic hydrocarbons (PAHs) at high temperatures. Sooting limits have been found to be not a function of concentrations of PAHs alone at a fixed flame temperature; sooting propensity generally increases with increasing fuel carbon/hydrogen atom ratio, presenting the following order: propane < ethylene < ethylene/toluene.

Effects of molecular O2 and NO2 on particle size distribution, morphology and nanostructure of diffusion flame soot oxidized in a flow reactor

The oxidation process of soot particles in O2 and NO2 atmospheres is investigated in a flow reactor at various temperatures. The particles are generated using an ethylene diffusion flame and sampled at the tip of the flame using an ejector diluter sampling system. Soot is oxidized by three kinds of diluted gas (pure N2, 10000 ppm O2 with N2 and 10000 ppm NO2 with N2) in the flow reactor and measured with DMS500, transmission electron microscopy (TEM), high-resolution TEM, Raman microscopy and thermogravimetric analysis (TGA). The results show that the particle size, primary particle diameter, agglomeration degree and concentration of soot particles decrease as the oxidation temperature increases. The overall oxidation capacity of NO2 is stronger than that of O2 at low temperatures (200 and 400 °C), whereas at high temperatures (600 and 800 °C) O2 is stronger. Both internal oxidation and external oxidation exist during the oxidation process of soot with O2 and NO2. The average fringe length of the lattice increases and the tortuosity of the lattice and the Id/Ig ratios of the soot particles decrease as temperature increases, meaning that the nanostructure of the soot particles becomes more ordered. The results of the multi-diagnostics above indicate three main forms of oxidation during the overall oxidation process: internal oxidation, external oxidation and internal collapse. The oxidation of O2 is dominated by internal oxidation and internal collapse at high temperatures (600 and 800 °C), whereas the oxidation of NO2 is dominated by external oxidation throughout the oxidation temperature range.

Modeling soot formation from solid complex fuels

A detailed model is proposed for predicting soot formation from complex solid fuels. The proposed model resolves two particle size distributions, one for soot precursors and another for soot particles. The precursor size distribution is represented with a sectional approach while the soot particle-size distribution is represented with the method of moments and an interpolative closure method is used to resolve fractional methods. Based on established mechanisms, this model includes submodels for precursor coagulation, growth, and consumption, as well as soot nucleation, surface growth, agglomeration, and consumption. The model is validated with comparisons to experimental data for two systems: coal combustion over a laminar flat-flame burner and biomass gasification. Results are presented for soot yield for three coals at three temperatures each, and for soot yield from three types of biomass at two temperatures each. These results represent a wide range of fuels and varying combustion environments, demonstrating the broad applicability of the model.

Transported probability density function based modelling of soot particle size distributions in non-premixed turbulent jet flames

The need to establish actual particle size distributions (PSDs) of soot emissions from the nanoscale upwards, along with the current global indicators based on soot mass, stems from increasingly strict regulatory demands. In the current work, a mass and number density preserving sectional model is coupled with a transported probability density function (PDF) method to study the evolution of soot PSDs in two non-premixed turbulent jet flames at Reynolds numbers of 10,000 and 20,000. The transported PDF approach is closed at joint-scalar level and includes mass fractions of gas phase species, soot sections, as well as enthalpy, leading to a fully coupled 78-dimensional joint-scalar space, treating interactions between turbulence and gas phase/soot chemistry as well as radiation without further approximation. The gas phase chemistry features 144 reactions, 15 solved and 14 steady-state species and an acetylene-based soot inception model is calibrated using comprehensive detailed chemistry up to pyrene and applied to a well-stirred/plug flow reactor configuration. The derived nucleation rate is subsequently applied in the turbulent flame calculations. Soot surface growth is treated via a PAH analogy and oxidation via O, OH and O2 is accounted for. The sectional model features 62 sections covering particle sizes in the range 0.38 nm ≤ dp ≤ 4.4 µm and includes a model for the collision efficiency of small particles ( ≤ 10 nm) based on the Lennard–Jones potential. The computed results reproduce the evolution of the PSDs with encouraging accuracy. It is also shown that the distribution of soot in mixture fraction space is affected by local extinction events.

Joint-scalar transported PDF modelling of soot in a turbulent non-premixed natural gas flame

The focus of the present work is on the prediction of soot in the turbulent Delft III/Adelaide natural gas flame at a Reynolds number of 9700. A parabolic flow solver with the SSG (Speziale, Sarkar and Gatski) Reynolds stress transport model for turbulence is coupled to a joint-scalar transported PDF (probability density function) approach allowing exact treatment of the interactions of turbulence with the solid and gas phase chemistries. Scalar mixing is treated via the modified Curl's coalescence/dispersion model and two different closures for the scalar dissipation rate are explored. The gas phase chemistry is represented by a systematically reduced mechanism featuring 144 reactions, 15 solved and 14 steady-state species. The dynamics of soot particles, including coagulation and aggregation in the coalescent and fractal aggregate limits, is treated either via a simplified two-equation model or via the MOMIC (method of moments with interpolative closure). The inclusion of soot surface reactions based on a second ring PAH (polycyclic aromatic hydrocarbon) analogy is also investigated. Soot oxidation via O, OH and O is taken into account and the sensitivity to the applied rates is investigated. An updated acetylene-based soot nucleation rate is formulated based on consistency with detailed chemistry up to pyrene and combined with a sectional model to compute soot particle size distributions in the NIST well-stirred/plug flow reactor configuration. The derived rate is subsequently used in turbulent flame calculations and subjected to a sensitivity analysis. Radiative emission from soot and gas phase species is accounted for using the RADCAL method and by the inclusion of enthalpy as a solved scalar. Computed soot levels reproduce experimental data comparatively well and approximately match absolute values. The axial location of peak soot in the Delft III/Adelaide flame is consistent with previous large eddy simulations and possible causes for the discrepancy with experimental data are analysed.

Characteristics of evolution of in-cylinder soot particle size and number density in a diesel engine

Particulate matter (PM) emission from diesel engines has been a critical issue due to its environmental impact. The soot size and number density inside the cylinder can severely affect the size and mass of PM emission emitted from the diesel exhaust pipe. In the present study, the temporal and spatial variations of the mean and instantaneous in-cylinder soot size and number density in a single cylinder direct injection diesel engine have been simulated numerically at different operating conditions, injection system parameters, fuel types and piston bowl geometry using AVL-FIRE code. The results show that the engine load has great effects on the mean soot particle size and number density with crank angle while the engine speed and the fuel injection system parameters have no obvious effects on them. It is observed that there is an obvious shift to larger particles at higher load. The mean soot number densities firstly increase with the increase of loads, but decrease at later crank angle. The in-cylinder soot size distribution shows a unimodal shape at different crank angle under all operating conditions. The soot particle numbers are concentrated from the start of combustion to 40 °CA after top dead center (ATDC). The spatial soot particle size distribution in the combustion chamber is closely related to engine load, impinging angle and airflow motion. The piston bowl diameter has large effect on the mean soot size and number density during the latter phase of the expansion stroke. The results also indicate that both in-cylinder mean soot size and number from biodiesel-diesel blends are affected not only by the oxygen content, but also engine operating conditions.

Semi-empirical model for indirect measurement of soot size distributions in compression ignition engines

This work proposes a semi-empirical model, which provides soot particle size distribution functions emitted by compression ignition engines. The model is composed of a phenomenological model based on the collision dynamics of particle agglomerates and an empirical model, which provides key input parameters such as primary particle size and a mathematical relationship between the size of the agglomerate and number of primary particles. The phenomenological model considers the relevant fluid-dynamics phenomena influencing the collision frequency function. It is observed that Brownian motion is the predominant phenomenon and in a much lesser degree inertial turbulent motion. The experimental model requires air/fuel ratio, engine speed, soot density and mean instantaneous in-cylinder pressure. A Dirac delta is used as a seed for the agglomerate size function whose magnitude depends on the soot volume concentration and the mean primary particle size at each engine operation condition. In a further step, the obtained modelled agglomerate size functions are fitted to lognormal size distributions defined by the modelled mean size and standard deviation. Modelled lognormal agglomerate size distribution functions are validated with respect to experimental distributions obtained using a Scanning Mobility Particle Sizer (SMPS).

Detailed modeling of soot particle formation and comparison to optical diagnostics and size distribution measurements in premixed flames using a method of moments

Recently improved knowledge about key processes for the formation and growth of soot particles lead to very extensive soot models describing several kinetic pathways for particle nucleation and growth in detail. One of these models has been proposed by D’Anna and coworkers (D’Anna et al., 2010, Sirignano et al., 2010). In these original studies, the multivariate formulation was solved with a sectional approach, while a more recent study (Salenbauch et al., 2017) also showed the suitability of moment methods such as the Conditional Quadrature Method of Moments (CQMOM) (Yuan et al., 2011). However, being a moment method, CQMOM does not allow to directly access the soot particle size distribution (PSD). This prevents the consistent comparability of CQMOM results to soot measurements based on a scanning mobility particle sizer (SMPS). Furthermore, the comparison to laser-induced fluorescence (LIF) experiments is also limited, as the LIF signal only represents small particles and their concentration is only extractable from the simulation results if the PSD shape is known.The aim of this study is to extend the previously developed CQMOM soot model by a PSD reconstruction step applying the concept of entropy maximization. Maintaining the efficiency of the CQMOM moment inversion algorithm to close the moment equations, the model extension enables to evaluate the diameter-based PSD in a post-processing step without prescribing a specific shape as input. The updated algorithm is applied to simulate soot formation in two different burner-stabilized premixed C2H4/O2/N2 flames with a very lightly sooting (C/O = 0.67) and a heavily sooting (C/O = 0.77) character. Numerical results are compared to recently published LIF, laser-induced incandescence (LII) and SMPS measurement data. The analysis investigates the model’s capability to predict phenomena such as uni- or bimodal distribution shapes and the transition of small nanostructures to agglomerates in the two target flames both of which exhibit very different sooting behaviour.

Effect of mixing methane, ethane, propane and ethylene on the soot particle size distribution in a premixed propene flame

The fuel mixing effect on soot formation was investigated based on a premixed propene flame, in which the C/O mole ratio was 0.6 and the maximum flame temperature was 1829 K. Different proportions of methane, ethane, propane and ethylene were mixed into the propene flame, with the constant C/O mole ratio and similar maximum flame temperatures and temperature–time histories. The particle size distribution function (PSDF) was measured in the burner stabilized stagnation (BSS) flame configuration, using the micro-orifice probe sampling technique and the scanning mobility particle sizer (SMPS). It was observed that the PSDF of the propene flame was not sensitive to methane, ethane or propane addition, while the number density of small nucleated particles was strengthened with ethylene addition. The particle growth in flame with a small amount of ethylene addition was stronger than that of either the pure propene or ethylene flame, indicating the synergistic effect between propene and ethylene on soot formation.

Size Distribution of Soot Particles in Premixed n-Heptane and Methylcyclohexane Flames

The size distributions of soot particles from premixed n-heptane and methylcyclohexane flames were measured and compared at different particle residence times. The micro-orifice probe sampling technique and the scanning mobility particle sizer were used for the particle size measurement in the burner-stabilized stagnation flame configuration. For both flame series, the mixture C/O mole ratio was 0.6 and the maximum flame temperature was controlled around 1835 K to investigate the fuel structure effects on soot propensity. It was observed that, with the increased particle residence time, the particle size distribution function of both flames evolved from the unimodal (only the nucleation mode) to the bimodal (both the nucleation and coagulation modes) distribution. The particle nucleation strength and particle growth rate of the methylcyclohexane flame were quantitatively lower than those of the n-heptane flame. Considering the equivalence ratio difference between the two flames, the sooting propensity dis...

Coupling an LES approach and a soot sectional model for the study of sooting turbulent non-premixed flames

Due to their negative impacts on environment and human health, future regulations on soot emissions are expected to become stricter, in particular by controlling the size of the emitted particles. Therefore, the development of precise and sophisticated models describing the soot production, such as sectional methods, is an urgent scientific and industrial challenge. In this context, the first objective of this work is to use for the first time a sectional model to perform an LES of a sooting turbulent flames in order to demonstrate its capacities. For this, the whole LES formalism for this approach is developed. It includes state-of-art models for the description of the gaseous phase and an extension of a soot subgrid intermittency model to the sectional approach, originally proposed for the hybrid method of moments. Then, the LES is used to analyze a turbulent non-premixed ethylene–air jet diffusion flame and results are validated by available experimental data. The quality of results for the gaseous phase is satisfactory and results for the solid phase show a reasonable agreement with the experimental results in terms of localization, intermittency and soot volume fraction magnitude. Once the coupled LES-sectional approach validated, having access to the full information on the spatial and temporal evolution of the soot Particle Size Distribution (PSD), the second objective of this work is to provide a new fundamental insight on soot production in turbulent non-premixed flames. First, it is observed that a one-peak and a two-peak PSD shapes are observed at the bottom and downstream of the flame, respectively. Second, high fluctuations of the PSD distribution is observed all along the flame. In particular, a time bimodal behavior is observed with the presence of a zone with regular transitions between one- and two-peak PSD shapes. By analyzing soot particles Lagrangian paths, these high fluctuations are shown to be linked with the wide range of history paths of soot particles, which are mainly driven by turbulence.

The Effect of Mixing Ratio and Fuel Purity on Smoke and Burning Characteristics of N-heptane/Toluene Test Fire

Abstract Experiments were conducted in a fire test room to investigate soot particle and burning behavior of n-heptane/toluene mixtures blended by liquids in different mixing ratio and different purity. Analytical reagent n-heptane (AR, purity ≥ 99.5 %), chemically pure n-heptane (CP, purity ≥ 97 %) and toluene (purity ≥ 99.5 %) were selected to prepare four kinds of mixtures, mixing by volume and by mass were two modes available. Extinction coefficient of smoke was obtained by extinction meter on the room ceiling. Size distribution of soot particles was measured by fast particulate spectrometer DMS500, then count median diameter (CMD) and geometric standard deviation of diameter (GSD) were calculated. Mass loss curve and temperature profile were measured by electronic balance and thermocouples, respectively. Experimental results revealed that even small difference in the incorporation of toluene, such as mixed by volume and mixed by mass, had significant influences on smoke extinction and CMD. When fuels were mixed in a same mass ratio, slight gap in purities could also affect smoke extinction. Thus fuels mixed by mass could not be employed as test fire, while CP n-heptane could be selected to prepare fuels of test fire based on our preliminary results. As for burning behavior, temperature profiles were almost identical for all mixtures, mass loss curves also had good consistencies.

Size Distribution of Ultrafine Particles Generated from Residential Fixed-bed Coal Combustion in a Typical Brazier

Ultrafine particles (with a small mean diameter) released from domestic coal combustion are an important parameter to consider in air pollution, as they affect air quality and human health. It has been suggested that poor combustion conditions release particles of different sizes enriched with health-damaging chemicals, such as polycyclic aromatic hydrocarbons. Furthermore, both smouldering and highly efficient combustion conditions release particles, which are often carcinogenic. Information on the particle size distribution (PSD) of char or soot emitted from fixed-bed domestic coal combustion is limited, with many studies reporting on wood combustion. This study investigated the influence of coal combustion phases (ignition, flaming, and coking) on the particle number concentration and size distribution of ultrafine particles. D-grade bituminous coal was crushed to a particle diameter (O) of 40–60 mm and combusted in a laboratory designed coal brazier (Imbaula) during experimental investigations of the particle size distribution normalised to the particle number concentration against the particle diameter. Experiments were carried out using the reduced smoke top-lit updraft method, colloquially known as the Basa njengo Magogo (BnM) method. The tests were carried out in a laboratory-controlled environment. Particulate matter was monitored using a NanoScan Scanning Mobility Particle Sizer (SMPS). Particles from the top-lit updraft (TLUD) showed an ultrafine geometric mean diameter centred at approximately 109 ± 18.4 nm for the ignition phase, 54.9 ± 5.9 nm for the pyrolysis/flaming phase, and 31.1 ± 5.1 nm for the coking phase. The particle mode diameter rapidly increased during the ignition phase (145 nm) and gradually decreased during the flaming phase (35 nm) and the coking phase (31 nm). This study shows that during smouldering combustion conditions (ignition), the particle diameter increases, whereas it decreases as the temperature increases. This information is essential for estimating particle deposition in the lungs and the associated health risks.

Modelling of secondary particulate emissions during the regeneration of Diesel Particulate Filters

Significant nanoparticle emission during the regeneration of Diesel Particulate Filters (DPFs) has been observed in experiments. A numerical reactive-flow model is coupled with a sectional particle method and phenomenological filtration model to describe the behavior of the DPF, and in particular the evolution of soot particle size distribution. The ability of the model to predict the pressure drop and flow profile in the DPF is critically assessed against experimental and simulated results from the literature. The capability to describe the impact of oxidative fragmentation on the size distribution of trapped particles is demonstrated. The model is shown to be able to qualitatively describe the decrease in average soot particle size during regeneration which will allow better prediction of particle number emissions.

An LES-PBE-PDF approach for predicting the soot particle size distribution in turbulent flames

In this article, we combine the large eddy simulation (LES) concept with the population balance equation (PBE) for predicting, in a Eulerian fashion, the evolution of the soot particle size distribution in a turbulent non-premixed hydrocarbon flame. In order to resolve the interaction between turbulence and chemical reactions/soot formation, the transport equations for the gas phase scalars and the PBE are combined into a joint evolution equation for the filtered pdf associated with a single realization of the gas phase composition and the soot number density distribution. With view towards an efficient numerical solution procedure, we formulate Eulerian stochastic field equations that are statistically equivalent to the joint scalar-number density pdf. By discretizing the stochastic field equation for the particle number density using an explicit adaptive grid technique, we are able to accurately resolve sharp features of evolving particle size distributions, while keeping the number of grid points in particle size space small. Compared to existing models, the main advantage of our approach is that the LES-filtered particle size distribution is predicted at each location in the flow domain and every instant in time and that arbitrary chemical reaction mechanisms and soot formation kinetics can be accommodated without approximation. The combined LES-PBE-PDF model is applied to investigate soot formation in the turbulent non-premixed Delft III flame. Here, the soot kinetics encompass acetylene-based rate expressions for nucleation and growth that were previously employed in the context of laminar diffusion flames. In addition, both species consumption by soot formation and radiation based on the assumption of optical thinness are accounted for. While the agreement of our model predictions with experimental measurements is not perfect, we indicate the benefits of the LES-PBE-PDF model and demonstrate its computational viability.

Evaluation of a Lagrangian Soot Tracking Method for the prediction of primary soot particle size under engine-like conditions

This paper reports the implementation and evaluation of a Lagrangian soot tracking (LST) method for the modeling of soot in diesel engines. The LST model employed here has the tracking capability of a Lagrangian method and the ability to predict primary soot particle sizing. The Moss-Brookes soot model is used here as the Eulerian method to simulate soot formation and oxidation processes. The inception, surface growth and oxidation models are adopted and modified such that the associated reaction rates can be computed using the Lagrangian approach. The soot nuclei are treated as Lagrangian particles when the mass of incipient soot exceeds a designated threshold value. Their trajectories are then computed using the particle momentum equation. The change of primary soot particle size is dependent on the modified Lagrangian surface growth and soot oxidation models. Performance of the LST model in predicting temporal soot cloud development, mean soot diameter and primary soot size distribution is evaluated using measurements of n-heptane and n-dodecane spray combustion obtained under diesel engine-like conditions. In addition, sensitivity studies are carried out to investigate the influence of soot surface ageing and oxidation rates on the primary soot particle size distribution. With the use of surface ageing, the predicted maximum primary soot particle sizes are closer to the experimentally measured maximum primary soot sizes. Also, the associated particle size distribution shows a lognormal shape. A higher rate of soot oxidation due to OH causes the soot particles to be fully oxidized downstream of the flame. In general, the LST model performs better than the Eulerian method in terms of predicting soot sizing and accessing information of individual soot particles, both of which are shortcomings of the Eulerian method.

Study on the influence of ethanol and butanol addition on soot formation in iso-octane flames

The emphasis of the work was to study the influence of ethanol and n-butanol addition on soot formation in the combustion process of iso-octane. Soot particle size distribution functions were measured at different heights above the burner in selected fuel-rich atmospheric pressure laminar premixed iso-octane/ethanol and iso-octane/n-butanol flames. The results showed that the addition of both alkanols to the reference fuel iso-octane reduces the soot formation potential in the flames. The comparison of the particle size distribution functions indicated that both the process of soot particle nucleation in the lower heights of the flame and coagulation in the larger heights of the flame are influenced by the positive effect of the addition of the oxygenated fuels. When comparing the influence of ethanol and n-butanol, it is obvious that n-butanol addition leads to a higher reduction of soot formation in iso-octane flames.

Detailed modeling of CO2 addition effects on the evolution of soot particle size distribution functions in premixed laminar ethylene flames

This study investigates computationally the influence of CO2 addition on the sooting behavior in premixed laminar ethylene/ oxygen/ argon burner stabilized stagnation (BSS) flames at the atmospheric pressure. The discrete sectional aerosol dynamics method combined with a reversible nucleation model and a novel model of reversible polycyclic aromatic hydrocarbon (PAH) condensation were employed to predict the size evolution of the particle size distribution (PSD) function. The predicted temperature profiles and PSD functions are in reasonably good agreement with the experimental data for nascent soot measured in the BSS configuration. The evolution of the PSD functions shows that CO2 addition reduces the soot nucleation and mass growth rates, consequently lowering the soot yield. The addition of CO2 reduces the concentrations of H, C2H2, C6H6, and large PAHs (e.g., pyrene) which all suppress the soot formation process through a chemical effect; the thermal effect of CO2 is not as strong as the chemical effect.

Size evolution of soot particles from gasoline and n-heptane/toluene blend in a burner stabilized stagnation flame

The evolution of soot particle size distribution function (PSDF) in premixed flames of gasoline (34% aromatics by volume) and a n-heptane/toluene blend (66% n-heptane/34% toluene by volume) was investigated in the burner stabilized stagnation (BSS) flame configuration, using the micro-orifice probe sampling and scanning mobility particle sizer (SMPS). The aim of this study is to illustrate the similarities and differences of sooting propensity between the real fuel and the simple hydrocarbon blend in premixed flame conditions. The mole ratio of carbon to oxygen (C/O) in the unburned gas was kept constant at 0.6 and similar maximum flame temperatures and temperature-time histories were kept between the two cases, so that we could focus on the fuel composition effects on sooting propensity. In addition, the size distribution, the total number density, and the volume fraction of soot were also compared to those previously measured for ethylene and propene flames under comparable conditions. It was observed that the particle size distributions of both gasoline and heptane/toluene flames evolve from the unimodal distribution (nucleation mode only) to the bimodal (both nucleation and coagulation mode) distribution. Compared to the heptane/toluene blend, the soot formation in gasoline flame features more persistent nucleation and much faster growth rate.