Soot Refractive Index Research Articles

Estimation of soot refractive index from its nanostructural parameters with the dispersion model

Particles derived from combustion processes, mainly composed of soot agglomerates, are acknowledged to be among the main contributors to climate change. Their effects depend mostly on their size, shape, and internal structure. Specifically, the latter has a significant effect on their optical properties, mainly through the refractive index. This index has been widely evaluated, but scarcely correlated with the soot internal characteristics. In this work, relationships between the nanostructural parameters (such as the degree of graphitization, among others) obtained with conventional analytical techniques and the input parameters of the dispersion model (a representation of the electromagnetic radiation through the Lorentz-Drude approach) are proposed with the aim to determine the refractive index. From experiments in a chassis dynamometer, it has been observed that as the vehicle speed increases, the soot samples have, in general, higher degree of graphitization, due to increased combustion temperature. The method proposed allows quantifying how both the real and imaginary parts of the complex refractive index increase as the degree of graphitization increases. Much lower dependence on the average crystal length has been observed. Different combinations of techniques can be used to determine the nanostructural parameters, depending on the analytical technique used. As far as the resulting parameters are reliable, the effect of the technique selected is minor, thus providing flexibility to the application of the method.

Sooting propensities of novel cage hydrocarbon propellants

The absolute sooting propensities of three bishomocubane-based cage hydrocarbon fuels– bis(nitratomethyl)-1–3-bishomocubane (DNMBHC), 1,3-Bishomocubane dimer (BHCD), and diazido-dimethyl-bishomocubane (DADMBHC) having potential for propulsive applications were estimated using the full field light extinction technique (LE). The results were compared against relative soot volume fraction measurements obtained using color-ratio pyrometry (CRP). The Rayleigh assumption used in this study was validated by TEM (transmission electron microscopy) measurements. The experiments were conducted using the tethered droplet combustion technique in ambient air under normal gravity conditions. The sooting propensities of 10% w/w blends of the novel BHC fuels with an RP-1 surrogate fuel (RP-1-s) were also studied along with other fuels, includingn-dodecane, methylcyclohexane, isocetane, and dicyclopentadiene (DCPD), which was the starting fuel for the synthesis of the novel BHC compounds. Results indicate an excellent match in the sooting propensities estimated using the two methods despite considerable differences in the estimation methodologies. The novel BHC fuels were extremely sooty compared to RP-1-s and its constituent components on account of their caged molecular structure that favoured the formation of polyaromatic hydrocarbons (PAHs). The assumption of a constant soot dispersion exponent in CRP was judged as having consequences similar to the assumption of a constant soot refractive index in the LE method. The LE technique was assessed as more robust in comparison to CRP as it could capture soot emissions outside the luminous flame zone and had significantly lower uncertainties than CRP for sooty fuels..

The impact of organic carbon on soot light absorption

The impact of organic carbon (OC) on the light absorption of soot is determined by discrete element modeling coupled with the discrete dipole approximation for computing the scattering of radiation by soot particles. The mass absorption cross-section (MAC) of soot is used widely to determine its light absorption. Typically MAC is obtained from the mass average refractive index of OC and elemental carbon (EC) with large C/H that make up mature soot. As such, MAC can be overestimated by a factor of 3 in fuel-rich flames where newly-formed young soot contains EC with small C/H and OC that predominantly scatters light reducing its absorption by soot. Here a relation for the soot refractive index is derived accounting for soot morphology, maturity and OC content through its band gap at wavelength, λ = 266–1064 nm. Using this relation, the MAC of soot containing OC (up to 50 wt%) is in excellent agreement with carbon black, graphene and soot data. This confirms that soot morphology, maturity and OC content greatly influence light absorption during characterization of in-flame and freshly-emitted soot by laser induced incandescence and light extinction, especially in fuel-rich flames, and need to be properly accounted for in the soot refractive index.

The change of soot refractive index function along the height of premixed ethylene/air flame and its correlation with soot structure

It is crucial to understand the behavior of the optical properties of soot in the visible and near-IR spectral range to calculate the magnitude of their emission and absorption. Such information also helps in the development of diagnostic methods for soot. We measured the absolute value of the refractive index functions of soot nanoparticles E(m,1064) at a wavelength of 1064 nm, as well as the ratio of the refractive index functions at laser wavelengths of 532 nm and 1064 nm in a premixed ethylene/air flame using laser-induced incandescence. E(m,1064) increased with the flame height from 0.29 to 0.43. Depending on the height above the burner, the ratio of E(m,1064)/E(m,532) increased from 0.88 to 1. These changes in the optical properties are attributed to differences in the soot structure, such as the different sizes of graphene planes, which increased from 0.96 to 1.24 nm as the height above the burner increased from 10 to 20 mm. The presented data provide comprehensive information about the optical properties of soot in a premixed ethylene/air flame according to the height above a burner and make it possible to compare measurements of the soot optical properties in various target flames.

Radiative properties of soot fractal superaggregates including backscattering and depolarization

The modeling of radiative properties of soot particles can be addressed by a number of methods. Among them, the canonical Rayleigh-Debye-Gans (RDG) theory and the Superposition T-Matrix Method (STMM) are particularly well adapted to the task. Here, both RDG and STMM are used to model the radiative properties of two types of fractal aggregates indicative of soot relevant to lidar sensing. One type, a canonical aggregate with a chain-like morphology, corresponds to low-sooting flames and is well characterized by a single fractal dimension. The other type, a superaggregate, exhibits multiple fractal dimensions and is seen in heavily sooting flames. Radiative properties such as the differential scattering cross-section, total scattering cross-sections, backscattering cross-sections, and linear depolarization-ratio (LDR) are calculated for a range of wavelengths from 300−1100 nm while including the wavelength dependence of the soot refractive index. Of particular interest is that depolarization by a superaggregate is found to be approximately ten times larger than the depolarization by a canonical chain-like aggregate and both exhibit a power-law with wavelength. Results for specific wavelengths of interest to lidar (355, 532, and 1064 nm) are tabulated for lidar applications. Additionally, RDG theory yields fairly accurate results for modeling the radiative properties, including lidar relevant parameters such as backscattering cross-sections, only for canonical soot fractal aggregates.

Impact of the primary particle polydispersity on the radiative properties of soot aggregates

Abstract Combustion generated soot appears as fractal aggregates formed by polydisperse nearly spherical primary particles. Knowledge of their radiative properties is a prerequisite for laser based diagnostics of soot. In this parametric study, the effect of primary particle polydispersity on soot aggregate absorption and scattering properties is investigated numerically. Two series of fractal aggregates formed by normal and lognormal distributed primary particles of different levels of standard deviation were numerically generated for typical flame soot with a fractal dimension and prefactor fixed to D f = 1.73 and kf ≈ 1.5, respectively. Three aggregate sizes consisting of N p = 15 , 50 and 150 monomers per aggregate were investigated. Due to the uncertainty in soot refractive index, radiative properties were calculated by considering two different refractive indices at λ ≈ 532 nm recommended in the literature using the Discrete Dipoles Approximation and the Generalized Multiparticle Mie method. The results are interpreted in terms of correction factors to the Rayleigh–Debye–Gans theory for fractal aggregates (RDG-FA) for the forward scattering cross section A and for the absorption cross section h. It is shown that differential cross section for vertically polarized incident light, total scattering and absorption cross sections are well predicted by the RDG-FA theory for all considered aggregates formed by normally ( σ / d p ¯ ≤ 30%) and lognormally (σgeo ≤ 1.6) distributed primary particles. The refractive index is found to be of greater impact than primary particle polydispersity on the importance of multiple scattering. The radiative force per unit laser power experienced by the soot aggregates was found primarily determined by the aggregate volume, regardless of the level of primary particle polydispersity.

Light scattering and absorption by fractal aggregates including soot

This paper addresses how well and under what conditions the Rayleigh-Debye-Gans (RDG) approximation describes scattering and absorption of light by fractal aggregates (FA) including soot. The RDGFA theory, which is the prevailing, first order description of this problem, has two assumptions: the monomers, or primary particles, of the aggregate scatter and absorb in the Rayleigh regime, and the aggregate scatters in the diffraction limit weighted by this Rayleigh scattering and absorbs as a system of independent monomer particles. The aggregates studied here are formed via Diffusion Limited Cluster Aggregation (DLCA) and have a fractal dimension D = 1.78 ± 0.04 and prefactor of k0 = 1.35 ± 0.10. The aggregates are a collection of monodisperse spherical monomers with point contacts. Optical calculations were performed with the multiple sphere T-matrix (MSTM) and DDSCAT codes for incident light polarized perpendicular to the scattering plane. The scattering considered is the forward scattering intensity and the angular scattering as parameterized by the scattering wave vector. The total absorption cross section for aggregates is also calculated. This work stresses the systematic study of the effects of the variables of monomers per aggregate, which ranged from one to 502, two monomer size parameters of 0.157 and 0.314, and a wide range of refractive index real and imaginary parts. It also considers soot refractive indices with three representative dispersions. A summary of results for both scattering and absorption includes deviations from RDGFA theory ranging as large as 35% with positive deviations increasing with the real part of the refractive index and negative deviations growing with the imaginary part. These deviation from the RDG limit are shown to be similar to deviations for spheres.

Soot optical properties determined by analyzing extinction spectra in the visible near-UV: Toward an optical speciation according to constituents and structure

To improve the optical diagnostics of soot particles and gain an increased comprehension of the soot formation and aging mechanisms requires a better understanding of the soot refractive index. The present study evaluated the refractive index of soot generated by a miniCAST, an ethylene diffusion flame, and a PALAS GFG by interpreting specific extinction measurements in the near-UV spectrum with the help of an improved RDG-FA theory. Soot’s morphology, size distribution, and bulk densities were determined and taken into account for the inversion process. Findings indicate that absorption and scattering capacities of the considered soot were strongly dependent on the amount of organic or volatile carbons and the degree of graphitization of the carbon atoms in the primary sphere (amorphous to graphitic arrangement). The proposed speciation of particles according to these properties is based on the spectral dependence of the determined refractive index for three reference aerosols representative of organic, amorphous, and ordered soot particles. Results show that by combining the refractive indices of these three aerosols it is possible to explain the high variability of most of the optical properties of soot, paving the way for a simple optical technique that permits the speciation of soot particles.

Development of an improved data analysis approach for combined laser extinction and two-angle elastic light scattering diagnostics of soot aggregates.

An improved data analysis approach has been developed for the combined laser extinction and two-angle elastic light scattering diagnostics to relate the various measured optical cross sections to soot aggregate properties. The performance of the proposed approach is assessed using the comprehensive dataset of Santoro ethylene-air co-flow diffusion flame. Compared to previously reported studies, the proposed approach can be applied to a wider range of soot sources by removing the assumption made to the scattering regime or moment ratio of aggregate size distribution. The proposed approach also considers the contribution of scattering to extinction in determining the soot volume fraction, and this contribution is shown to increase as soot aggregate size becomes larger. The sensitivity of the calculation to the assumed parameters of the approach is examined and discussed. The mean radius of gyration of soot aggregates and the ratio of scattering intensities at the two measurement angles are shown to be independent of soot refractive index and are therefore recommended for soot model validation purposes.

Effect of morphology on the optical properties of soot aggregated with spheroidal monomers

The monomers of fractal aggregated soot particles are usually considered to be standard spheres in simulations, but a number of less regular shapes may be found in some burning conditions. In this paper, we simulated and investigated the optical properties of fresh dry soot particles as the aggregations of spheroidal monomers with different aspect ratios. Their optical properties were calculated using the numerically exact discrete dipole approximation (DDA) method. The simulated results indicated that the optical properties of soot aggregates composed of spheroidal monomers with highly nonspherical morphologies were considerably different from those composed of spherical monomers. The soot aggregates composed of the oblate spheroids with larger aspect ratios or the prolate spheroids with smaller aspect ratios may have led to larger cross sections of extinction, absorption and scattering. In extreme cases with Ra/Rb=3 and Ra/Rb=1/3 for the soot spheroidal monomers, the relative deviations compared to spherical monomers models reached up to 15% for the absorption cross sections, 10% for the single scattering albedo (SSA) and −25% for the asymmetry parameter (ASY). Moreover, by assuming a soot refractive index of 1.95+0.79i, a mass density of 1.8g/cm3 and a mean volume-equivalent spherical monomer radius of 0.02µm, the estimated mass absorption cross sections (MAC) of soot aggregates composed of the oblate spheroidal monomers with large aspect ratios (Ra/Rb=3) reached up to 7.5m2/g, which was closer to the measurements of 7.5±1.2m2/g than the ~6.5m2/g determined by the spherical monomers models. For future research with this type of small aggregated aerosol particles, it would be valuable to consider the monomer morphologies used in this paper for their optical simulations.

A Study of Optical Properties of Soot Aggregates Composed of Poly-Disperse Monomers Using the Superposition T-Matrix Method

The diameters of soot monomers may not be constant in the single fractal aggregated soot particle. The optical properties of light absorbing soot particles aggregated with poly-disperse monomers were studied using the superposition T-matrix method. Soot aggregates were generated with different log-normal probability distribution functions (PDF) of soot monomer diameter, according to the same soot volumes and monomer numbers. The single scattering properties of soot particles were calculated at a wavelength of 550 nm, assuming a soot refractive index of 1.95 + 0.79i and a mass density of 1.8 g/cm3. The random-orientation averaging results indicated that the optical properties of soot aggregates were fairly varied for the different distributions of the monomer diameters. In these simulations, the extinction and absorption of soot aggregates were slightly (<10%) affected by the monomer poly-dispersity. The simulated mass absorption cross-sections (MAC) of fresh dry soot particles aggregated with poly-disperse monomers reached up to 6.62 ± 0.07 m2/g, which was closer to the measurement (7.5 ± 1.2 m2/g) than the assumption of volume-equivalent mono-disperse monomer (6.36 ± 0.06 m2/g). Moreover, the optical properties of soot coated with an organic shell were calculated, and the optical results showed that the absorption cross-sections of the internally mixed soot particles were also slightly (<8%) influenced by the monomer poly-dispersity. We found that the effect of the monomer poly-dispersity on the light scattering and the single scattering albedo may be considerably large (up to ˜50% in extreme cases) for fresh dry soot aggregates. This effect on light scattering should be taken into account for those aggregates composed of monomers with widely distributed diameters.Copyright 2015 American Association for Aerosol Research

Measurement of soot concentration and bulk fluid temperature and velocity using modulated laser-induced incandescence

The use of a modulated LII two-colour technique to measure soot temperature in a laminar diffusion flame is described, and the results compared to CARS experiments. A sinusoidal modulated diode laser is used to excite the soot, and both the modulated LII intensity and its relative phase to the excitation source are measured with a lock-in amplifier and recorded. Modulation frequencies from 25 to 200,000 Hz were employed. The temperature is derived from the ratio of the modulated LII radiation signal at 445 and 750 nm, and the results compared to values obtained by CARS spectroscopy. The modulated LII temperatures were largely independent of modulation frequency and agreed well with the CARS temperatures. A theory is developed to explain the dependence of the phase delay of the modulated LII signal (with reference to that of the laser excitation source) on gas replacement time in the sample volume, soot cooling rate and soot volume fraction. The theory is shown to give a reasonable fit to the experimental results at all frequencies. At lower frequencies, the phase delay is dominated by the gas replacement time in the sample volume and at higher frequencies by the cooling rate of the heated soot. Time constants for both processes and the soot volume fraction are derived from the data and shown to be largely in agreement with the expected values. Using modulated LII-determined soot volume fraction and inverted and scatter corrected line-of-sight attenuation-determined absorption coefficients, the soot refractive index absorption function E(m) was measured to be between 0.45 and 0.42 over the wavelength range of 436–825 nm.

On the radiative properties of soot aggregates part 1: Necking and overlapping

There is a strong interest in accurately modelling the radiative properties of soot aggregates (also known as black carbon particles) emitted from combustion systems and fires to gain improved understanding of the role of black carbon to global warming. This study conducted a systematic investigation of the effects of overlapping and necking between neighbouring primary particles on the radiative properties of soot aggregates using the discrete dipole approximation. The degrees of overlapping and necking are quantified by the overlapping and necking parameters. Realistic soot aggregates were generated numerically by constructing overlapping and necking to fractal aggregates formed by point-touch primary particles simulated using a diffusion-limited cluster aggregation algorithm. Radiative properties (differential scattering, absorption, total scattering, specific extinction, asymmetry factor and single scattering albedo) were calculated using the experimentally measured soot refractive index over the spectral range of 266–1064nm for 9 combinations of the overlapping and necking parameters. Overlapping and necking affect significantly the absorption and scattering properties of soot aggregates, especially in the near UV spectrum due to the enhanced multiple scattering effects within an aggregate. By using correctly modified aggregate properties (fractal dimension, prefactor, primary particle radius, and the number of primary particle) and by accounting for the effects of multiple scattering, the simple Rayleigh–Debye–Gans theory for fractal aggregates can reproduce reasonably accurate radiative properties of realistic soot aggregates.

Investigation of Absorption and Scattering Properties of Soot Aggregates of Different Fractal Dimension at 532 nm Using RDG and GMM

Radiative properties of numerically generated fractal soot aggregates of different fractal dimensions were studied using the numerically accurate generalized Mie-solution method (GMM) and the Rayleigh-Debye-Gans (RDG) approximate theory. Fractal aggregates of identical prefactor but different fractal dimensions, namely, 1.4, 1.78, and 2.1, were generated numerically using a tunable algorithm of cluster–cluster aggregation for aggregates containing up to 800 primary particles. Radiative properties of these aggregates were calculated at a wavelength of 532 nm assuming a soot refractive index of 1.6 + 0.6i. Four commonly used structure factors in the RDG approximation were used to investigate the effect of structure factor on the differential and total scattering cross-sections and the asymmetry factor. The differential and total scattering properties calculated using the RDG approximation become increasingly sensitive to the structure factor with increasing the fractal dimension. Primary particle interactions are the fundamental mechanism for the aggregate absorption enhancement for small aggregates and the shielding effect for larger aggregates. The extent of these two competing factors is dependent on the fractal dimension and aggregate size. RDG reasonably predicts the effect of fractal dimension on the scattering properties, but fails to account for the effect of aggregation or fractal morphology on the absorption property of fractal soot aggregates, though the error is in general less than 15%.Copyright 2013 American Association for Aerosol Research

Instantaneous in-flame measurement of soot volume fraction, primary particle diameter, and aggregate radius of gyration via auto-compensating laser-induced incandescence and two-angle elastic light scattering

A new combination of soot diagnostics employing two-angle elastic light scattering and laser-induced incandescence is described that is capable of producing non-intrusive, instantaneous, and simultaneous, in situ measurements of soot volume fraction, primary particle size, and aggregate radius of gyration within flames. Controlled tests of the new apparatus on a well-characterized laminar flame show good agreement with existing measurements in the literature. From a detailed and comprehensive Monte Carlo uncertainty analysis of the results, it was found that the uncertainty in all three measured parameters is dominated by knowledge of soot properties and aggregation behavior. The soot volume fraction uncertainty is dominated by uncertainty in the soot refractive index light absorption function; the primary particle diameter uncertainty is dominated by uncertainty in the fractal prefactor; while the uncertainty in the aggregate radius of gyration is dominated by the uncertainty in the width of the distribution of aggregate sizes.

Examination of wavelength dependent soot optical properties of diesel and diesel/rapeseed methyl ester mixture by extinction spectra analysis and LII measurements

The refractive index of soot is an essential parameter for its optical diagnostics. It is necessary for quantitative interpretation of LII (Laser Induced Incandescence) signals, light scattering or extinction measurements as well as for emissivity calculations. The most cited values have been determined by intrusive methods or without taking into account the soot size distribution and its specific morphology. In the present study, soot generated by the combustion of diesel and diesel/rapeseed methyl ester (RME) mixture (70% diesel and 30% RME) are extensively characterized by taking into account the morphology, the aggregate size distribution, the mass fraction and the spectral dispersion of light. The refractive index m for wavelengths λ between 300 and 1000 nm is determined for diesel and diester fuels by both in-situ and ex-situ methods. The ex-situ method is based on the interpretation of extinction spectra by taking into account soot sizes and fractal morphology with the RDG-FA (Rayleigh–Debye–Gans for Fractal Aggregate) theory. The in-situ approach is based on the comparison of the LII signals obtained with two different excitation wavelengths. The absorption function E(m) and the scattering function F(m) are examined. This study reveals similar optical properties of soot particles generated by both studied fuels even at ambient and flame temperatures. The function E(m) is shown to reach a maximum for λ=250 nm and to tend toward a plateau-like behavior close to E(m)=0.3 for higher wavelength (600<λ (nm)<1000). The function F(m) is found to be quite constant for 400<λ (nm)<1000 and equal to 0.31.

Investigation on the influence of soot size on prompt LII signals in flames

Theoretical papers predict that prompt LII signals are weakly dependent on the soot size due to the fact that larger particles reach higher temperatures during the heating process by nanosecond laser pulses. This question is of crucial importance for establishing LII as a practical technique for soot volume fraction measurements. In this work two-color prompt LII measurements have been performed in several locations of diffusion and rich premixed ethylene-air flames. The experimental apparatus was carefully designed with a probe volume of uniform light distribution and sharp edges, a 4 ns integration time around the signal pulse peak and narrow spectral bandwidth. Measurements did not confirm the theoretical predictions concerning an increase of temperature for larger particles. On the contrary, larger particles in richer premixed flames exhibit a lower 400/700 signal ratio. This can probably be attributed to small differences in the refractive index of soot.

MEASUREMENT OF SOOT MORPHOLOGY, CHEMISTRY, AND OPTICAL PROPERTIES IN THE VISIBLE AND NEAR-INFRARED SPECTRUM IN THE FLAME ZONE AND OVERFIRE REGION OF LARGE JP-8 POOL FIRES

The dimensionless extinction coefficient, K e , was measured for soot produced in 2 m JP-8 pool fires. Light extinction and gravimetric sampling measurements were performed simultaneously at 635 and 1310 nm wavelengths at three heights in the flame zone and in the overfire region. Measured average K e values of 8.4 ± 1.2 at 635 nm and 8.7 ± 1.1 at 1310 nm in the overfire region agree well with values from 8–10 recently reported for different fuels and flame conditions. The overfire K e values are also relatively independent of wavelength, in agreement with recent findings for JP-8 soot in smaller flames. K e was nearly constant at 635 nm for all sampling locations in the large fires. However, at 1310 nm, the overfire K e was higher than in the flame zone. Chemical analysis of physically sampled soot shows variations in carbon-to-hydrogen (C/H) ratio and polycyclic aromatic hydrocarbon (PAH) concentration that may account for the smaller K e values measured in the flame zone. Rayleigh–Debye–Gans theory of scattering for polydisperse fractal aggregate (RDG-PFA) was applied to measured aggregate fractal dimensions and found to under-predict the extinction coefficient by 17–30% at 635 nm using commonly accepted refractive indices of soot, and agreed well with the experiments using the more recently published refractive index of 1.99–0.89i. This study represents the first measurements of soot chemistry, morphology, and optical properties in the flame zone of large, fully-turbulent pool fires, and emphasizes the importance of accurate measurements of optical properties both in the flame zone and overfire regions for models of radiative transport and interpretation of laser-based diagnostics of soot volume fraction and temperature.

Determination of the ratio of soot refractive index function E(m) at the two wavelengths 532 and 1064 nm by laser induced incandescence

A new method is proposed to measure the ratio of the refractive index function of soot particles E(m) at the two fixed wavelengths: 532 and 1064 nm. Using a non-intrusive, in-situ laser based technique, the ratio E(m,1064 nm)/E(m,532 nm) can be determined by comparing laser induced incandescence (LII) intensities at 532 and 1064 nm excitation wavelengths. The method consists of selecting laser energies that insure the equality of the LII signals in the low fluence regime under given conditions. Such equality is consistent with the fact that the soot particle will have reached the same temperature independently of the laser wavelength, i.e. the soot particle has absorbed the same energy. As the absorbed energy is proportional to the laser irradiance times E(m), the measurement of the laser energies required to insure perfect concordance of the LII intensities (spatially and temporally) serves to deduce the ratio E(m,1064 nm)/E(m,532 nm). The method is demonstrated in an acetylene/air flame, validated against extinction measurements performed by cavity ring-down spectroscopy (CRDS) by using laser radiations at 532 nm and 1064 nm and finally applied to different flame conditions.

Measurement of the dimensionless extinction coefficient of soot within laminar diffusion flames

The dimensionless extinction coefficient ( K e) of soot must be known to quantify laser extinction measurements of soot concentration and to predict optical attenuation through smoke clouds. Previous investigations have measured K e for post-flame soot emitted from laminar and turbulent diffusion flames and smoking laminar premixed flames. This paper presents the first measurements of soot K e from within laminar diffusion flames, using a small extractive probe to withdraw the soot from the flame. To measure K e, two laser sources (635 nm and 1310 nm) were coupled to a transmission cell, followed by gravimetric sampling. Coannular diffusion flames of methane, ethylene and nitrogen-diluted kerosene burning in air were studied, together with slot flames of methane and ethylene. K e was measured at the radial location of maximum soot volume fraction at several heights for each flame. Results for K e at both 635 nm and 1310 nm for ethylene and kerosene coannular flames were in the range of 9–10, consistent with the results from previous studies of post-flame soot. The ethylene slot flame and the methane flames have lower K e values, in some cases as low as 2.0. These lower values of K e are found to result from the contributions of (a) the condensation of PAH species during the sampling of soot, (b) the wavelength-dependent absorptivity of soot precursor particles, and, in the case of methane, (c) the negligible contribution of soot scattering to the extinction coefficient. RDG calculations of soot scattering, in combination with the measured K e values, imply that the soot refractive index is in the vicinity of 1.75–1.03 i at 635 nm.