Fluorescence Of Polycyclic Aromatic Hydrocarbons Research Articles

Quenching of Fluorescence of Polycyclic Aromatic Hydrocarbons by 4-OH-TEMPO

The quenching of fluorescence of four polycyclic aromatic hydrocarbons (anthracene, benzo(a)anthracene, pyrene, and fluoranthene) by 4-OH-TEMPO was investigated in 5% Triton X-100 in the temperature range from 15 to 55°C. No deviations from linearity in the Stern-Volmer plots were observed. The quenching mechanism was found to be entirely dynamic. The quenching rate constants were obtained for all temperatures studied. The findings demonstrate the possibility of developing an analytical method for the quantitative determination of nitrogen dioxide radical scavenger, 4-OH-TEMPO.

Laser diagnostics of soot precursors in a heavy-duty diesel engine at low-temperature combustion conditions

To better understand in-cylinder soot formation processes for modern, low-emissions, low-temperature combustion (LTC) operating conditions in diesel engines, soot and its precursors are imaged by laser diagnostics in a heavy duty optical engine. Virtually simultaneous images of planar laser-induced incandescence of soot (soot-PLII) using 1064 nm excitation and combined soot-PLII and planar laser-induced fluorescence of poly-cyclic aromatic hydrocarbons (PAH-PLIF) using 532 nm excitation reveal the temporal and spatial evolution of soot and its precursors during combustion. With increasing dilution of the intake air stream by various levels of nitrogen to simulate the use of exhaust-gas recirculation (EGR) to achieve LTC, the residence time of PAH increases as soot formation is delayed. At zero dilution (21% intake oxygen), soot appears nearly simultaneously with PAH, while at higher dilution (12.7% or 9.5% intake oxygen), soot formation is delayed by hundreds of microseconds to a millisecond or more. At all dilution levels, at its onset, PAH rapidly fills most of the downstream cross-section of the jet, with a relatively uniform fluorescence intensity distribution. The spatial evolution of soot, however, shifts from a relatively rapid filling of the downstream head of the jet at low dilution, to inception at the midstream periphery of the jet followed by a gradual progression downstream at high dilution. Effects of fuel-bound aromatics are also explored by comparing soot and PAH formation for a representative diesel fuel containing 27% aromatics by weight to that for neat n-heptane. The residence time of PAH before the onset of soot is slightly longer with the non-aromatic fuel, but otherwise the spatial and temporal evolution of PAH-PLIF and soot-PLII are indistinguishable from that of the diesel fuel at LTC conditions. Spectral analysis shows a characteristic soot emission spectra from 1064 nm excitation, while 532 nm excitation yields significant short-wavelength emission that cannot be attributed solely to soot incandescence, and is likely due to fluorescence of large PAH soot precursors. No narrow spectral features, such as from C2 or C3 emission, are apparent in the spectra, even at high laser fluence.

Laser-induced fluorescence of polycyclic aromatic hydrocarbons: an approach to gas standards

Rapid methods are needed to analyse air pollutants such as polycyclic aromatic hydrocarbons (PAHs). Reliable semi-quantitative gas standards were required for the development of a laser-induced fluorescence (LIF) method for polycyclic aromatic hydrocarbon analysis, based on sampling of air onto multi-channel polydimethylsiloxane rubber traps. Easily constructed diffusion tubes provided naphthalene vapour at ~2 ng s−1. A gas chromatographic fraction collection method for loading less volatile PAHs onto the traps from a flame ionization detector outlet was developed and optimized. The accuracy of the method, which can be further optimized, was sufficient for initial LIF screening tests to flag samples exceeding threshold PAH levels for subsequent quantitative GC–MS analysis.

Investigation of early soot formation process in a diesel spray flame via excitation—emission matrix using a multi-wavelength laser source

In order to investigate the early soot formation processes in diesel combustion, spectral analysis of polycyclic aromatic hydrocarbons (PAHs) formed in the early soot formation region in a diesel spray flame was conducted via the excitation—emission matrix (EEM) technique using a multi-wavelength laser source. The experiments were conducted using an optically accessible constant volume combustion vessel under diesel-like conditions (ambient temperature, Ta = 750–1130 K and ambient pressure, Pa = 2.0–3.0 MPa) at different ambient oxygen concentrations (10–21 per cent). The PAHs formed in a diesel spray flame in the combustion vessel were excited by a coherent multi-wavelength ‘rainbow’ laser light (mainly 266, 299, 342, and 398 nm, total 20 mJ) generated by converting the fourth harmonic (266 nm, 60 mJ) of a pulsed neodymium-doped yttrium aluminium garnet (Nd:YAG) laser using a Raman cell frequency converter filled with hydrogen (500 kPa). The spectra of laser-induced fluorescence from the PAHs excited by the different laser wavelengths in the flame were simultaneously captured with a spectrometer and an intensified charge-coupled device (ICCD) camera as EEM images. The EEM measured in diesel spray flames at different ambient temperatures and oxygen concentrations revealed that the timing and the region for PAH growth and soot particle formation are delayed and extended downstream at lower ambient temperatures and oxygen concentrations. At an ambient temperature of 940 K and an ambient oxygen concentration of 21 per cent, a variety of PAHs are detected around the ignition timing in the central region of the diesel spray flame and the PAHs grow into larger PAHs and soot particles as the combustion process progresses. At a lower ambient temperature of 750 K or lower ambient oxygen concentrations down to 10 per cent, the fluorescence from PAHs is detected in the central region of the diesel spray flame before the ignition timing and the intensity and the spectral characteristics of the PAH fluorescence do not change for a longer period until they are finally converted to soot particles in the downstream regions in the spray flame.

PAHs Distribution and Transition to Soot in Laminar Diffusion Flame of Diesel Fuel

In this study, formation mechanisms of PM (particulate matter) and its precursor (PAHs (polycyclic aromatic hydrocarbons)) were investigated as a fundamental PM study of diesel fuel combustion. A laminar diffusion flame of diesel fuel at a small pool-wick combustion equipment was used as the test flame. spatial distributions of PAHs and soot in the flame were measured by a laser induced fluorescence (LIF) and a laser induced incandescence (LIT) methods. Emission spectra of PAHs fluorescence were analyzed by a spectroscope. since the peak wavelength of the LIF spectrum of PAHs could be described as a function of the carbon number of PAHs, the molecular size of PAHs could be estimated. PAH species in the flame were measured using a gas chromatography (GC). LIF and GC data were compared for understanding the transition behavior from PAH to soot. These results suggested that the PAHs molecules rapidly grew up to the soot particles at nucleation area of PM. It was confirmed that relatively small PAHs species such as benzene and toluene existed in sooting region, however, the larger PAHs such as fluorine and pyrene could not be observed in the sooting region.

Laser Spectroscopy of Soot Formation Process in a Disel Spray Flame via Excitation-Emission Matrix Technique

In order to investigate the early soot formation processes in diesel combustion, spectral analysis of polycyclic aromatic hydrocarbons (PAHs) formed in the early soot formation region in a diesel spray flame was conducted via a newly developed Excitation-Emission Matrix (EEM) technique using a multi-wavelength laser source. The experiments were conducted using an optically accessible constant volume combustion vessel under diesel-like conditions (Ta =750 to 1 130 K and Pa=2.0 to 3.0 MPa). The PAHS formed in a diesel spray flame were excited by a coherent multiwavelength “rainbow” laser light (mainly 266, 299, 342 and 398 nm, total 20 mJ) generated by converting the 4th harmonic (266 nm, 60 mJ) of a pulsed Nd : YAG laser using a Raman cell frequency converter filled with hydrogen (500 kPa). The laser-induced fluorescence (LIF) from the PAHs excited by the different laser wavelengths in the flame was simultaneously captured using a spectrometer and an ICCD camera as EEM images. The EEM measured in diesel spray flames at different ambient temperatures and oxygen concentrations revealed that at ambient temperature of 940 K and oxygen concentration of 21%, a wide varieties of PAHs are detected around the ignition timing in the central fuel rich region of diesel spray flame and the PAHs are converted to larger PAHs and soot particles as the combustion process progresses. In contrast, at lower ambient temperature of 750 K or lower oxygen concentrations down to 10%, the fluorescence from PAHs are detected in the central fuel rich region before ignition and the intensity and the spectral characteristics of the PAH fluorescence remain unchanged until they are finally converted to soot particles in the downstream regions.

Fullerene C60 as a superefficient quencher of singlet exited states of polycyclic aromatic hydrocarbons

Quenching of fluorescence of polycyclic aromatic hydrocarbons (PAH), namely, naphthalene, anthracene, 9,10-diphenylanthracene, 9,10-dibromoanthracene by C60 fullerene in ethylbenzene at 293 K was found and investigated. The phenomenon is characterized by abnormally high values of bimolecular rate constants for quenching (k bim = (0.18–6.78)·1012 L mol−1 s−1) determined from the Stern—Volmer dependence of the PAH fluorescence intensity on the C60 concentration and occurs through the inductive-resonance (dominant channel) and exchange-resonance (minor channel) energy transfer from 1PAH* to C60. The overlap integrals of the PAH fluorescence spectra with the C60 absorption spectrum and the critical energy transfer distances were calculated.

Mid‐ to Far‐Infrared Spectral Energy Distribution of the Diffuse Galactic Radiation

Mid-infrared (MIR; 12-60 μm) diffuse emission in the Galactic plane and the Carina Nebula is investigated relative to the far-infrared (FIR; ≥100 μm) emission. Observations show that the ratio of the 12 μm emission to the total FIR intensity is more or less constant in the Galactic plane but exhibits a slight decrease followed by an increase as the FIR color becomes bluer in the Carina Nebula. The constancy is compatible with predictions from models of stochastic heating of very small grains or infrared fluorescence of polycyclic aromatic hydrocarbons. The decrease can be attributed to a weakening of the unidentified infrared band emission in strong radiation fields. Contrarily, the ratio of the 25 and 60 μm emission to FIR intensity increases linearly with field strength, which is incompatible with the model predictions. The Carina Nebula data show a much bluer FIR color than the Galactic plane, whereas the ratio of MIR to FIR emission is in a range similar to that of the plane. We interpret these characteristics in terms of a superposition of emission from dust grains in various radiation field strengths. The linear increase can be accounted for by an increasing contribution from emission by dust grains in strong radiation environments, whereas the shift in FIR color between the Galactic plane and the Carina Nebula can be attributed to different contributions from grains in weak radiation fields. Other possibilities, such as the effect of multiphoton processes, variations in the incident radiation spectrum, and possible contributions from iron grains, have also been examined, but none can account for the observations consistently. Applications of the present model to external galaxies are also discussed.

Epifluorescence microscopy and image analysis of high‐level polycyclic aromatic hydrocarbon contamination in soils

Interactions between polycyclic aromatic hydrocarbons (PAHs) and soil are an important determinant of their chemical availability and transport. Laboratory examination of microscale PAH-soil interaction is limited by the availability of methods for particle-scale observation. Inverted epifluorescence microscopy, combined with digital photography and computer image analysis, was evaluated for specificity and linearity using dissolved PAHs. A pyrene filter (excitation wavelength, 360-400 nm; emission wavelength, 450-520 nm) gave nonspecific PAH fluorescence, and bias for fluoranthene, benzo[b]fluoranthene, benzo[g,h,i]perylene, and benz[a]anthracene was quantified in comparison to that for pyrene. Concentrations ranging from 1 to 10 mM for anthracene, fluoranthene, and pyrene and from 1 to 50 mM for naphthalene produced a linear response with low interpixel variability. Liquid-phase analyses validated use of the technique for the descriptive analysis of PAH distribution in solid samples, but liquid-phase calibration was not quantitative for spiked or field-contaminated soils. The mean luminance for three field soils was proportional to the values predicted from their chemically measured concentrations and to values from spiked, aged, uncontaminated materials. Image analysis of laboratory- and field-contaminated samples determined the area distribution of fluorescent intensity and the size of fluorescent areas exceeding a threshold luminance. These qualitative descriptions of the microscale spatial distribution of PAH contamination are presented as potential endpoints for future research on biogeochemical interactions in heavily contaminated solids.

RELATIVE POLYCYCLIC AROMATIC HYDROCARBON CONCENTRATIONS IN UNSTEADY COUNTERFLOW DIFFUSION FLAMES

ABSTRACT Qualitative measurements of polycyclic aromatic hydrocarbon (PAH) concentrations made in counterflow diffusion flames subjected to both steady and oscillating strain rates are presented. Planar laser-induced fluorescences was used to make spatially and temporally resolved measurements of the relative concentrations of PAH. The measurements for each PAH size class were made as a function of strain rate, forcing frequency, and fuel type. The peak laser-induced fluorescence spectra from PAHs is known to be a function of the number of benzene rings that make up the particular PAH. It was found that, with increasing strain rate, the intensity of PAH fluorescence decreases dramatically, though unequally, among the three size classes measured. With an unsteady strain rate, the PAH intensity oscillates in phase for frequencies below 50 Hz. When nondimensionalized relative concentration is plotted, it is shown that the smaller PAHs continue to respond to the instantaneous strain rate at a higher frequency than larger PAHs.

Experimental Analysis of Soot Formation in Sooting Diffusion Flame by Using Laser-Induced Emissions

Two-dimensional images of OH fluorescence, polycyclic aromatic hydrocarbons (PAHs) fluorescence, and laser-induced incandescence (LII) from soot were measured in a sooting diffusion flame. To obtain an accurate OH fluorescence image, two images were taken with the laser wavelength tuned to (“on”) and away from (“off”) the OH absorption line. An accurate OH fluorescence image was obtained by subtracting the off-resonance image from the on-resonance image. For the PAH fluorescence and LII measurements, temporally resolved measurements were used to obtain the individual images; the LII image was obtained by detecting the LII signal after the PAH fluorescence radiation had stopped and the PAH fluorescence image was obtained by subtracting the LII image from the simultaneous image of PAH fluorescence and LII. Based on the obtained images, the relative location of OH, PAH, and soot in the flame was discussed in detail. To investigate the PAH size distribution in a sooting flame using LIF, an estimation strategy for PAH size is proposed. Emission spectra were measured at several heights in the flame using a spectrograph. Since the emission wavelength of PAH fluorescence shifts toward longer wavelengths with increasing PAH size, the main PAH components in the emission spectra could be estimated. The results suggest that PAH grows and the type of PAH changes as the soot inception region was approached. Near the soot inception region, we estimated that the PAHs, which have over 16 carbon atoms, mainly constituted the emission spectrum.

Experimental study of the effect of fuel mass flux on a 1-m-diameter methane fire and comparison with a hydrogen fire

The effect of fuel mass flux on the flow field of a 1-m-diameter methane fire is examined using particle image velocimetry (PIV) and data from visible flame emission and polycyclic aromatic hydrocarbon (PAH) fluorescence. The fuel mass flux is varied from 0.040 to 0.066 kg/m 2 s, which is representative of a significant fraction of the range of fuel mass fluxes that occur in liquid hydrocarbon pool fires. The results show that increasing the fuel mass flux increases the size of the region of unmixed fuel vapor but has little effect on the dynamics in the mixing region within the first diameter above the burner surface. In-plane time-averaged velocities ( U ¯ , V ¯ ) and time-averaged turbulent stresses ( u ′ 2 ¯ , v ′ 2 ¯ , and u ′ v ′ ¯ ) are given for a vertical plane passing through the burner centerline. Visible emission data are used to infer the probability of the maximum mixture fraction along a path normal to the PIV plane failing to exceed a stoichiometric value. PAH fluorescence data are used to infer the radial position of maximum time-averaged turbulent reaction rate as a function of elevation. Methane (0.054 kg/m 2 s) and hydrogen (0.022 kg/m 2 s) fires with the same inlet enthalpy flow rate (heat release rate) of 2.1 MW are examined. The comparison of results highlights the effect on the flow field of differences in fuel thermochemistry and density. The hydrogen fire reaches a maximum velocity at a lower elevation than that of the methane fire and has a correspondingly shorter unmixed fuel vapor region and greater lateral (radial) extent of the mixing region for a given elevation. The experiments were conducted with controlled and characterized boundary conditions for the purpose of providing validation data sets relevant to conditions found in fully turbulent pool fires.

Study of deposit morphology in a propane diffusion-flame under fuel-rich conditions

The morphologies of deposits on 15-μm diameter SiC fibres have been investigated with a scanning electron-microscope and compared with UV-excited laser-induced broadband fluorescences in a confined co-flowing, propane/air laminar diffusion-flame under a fuel-rich condition. A double structure of polycyclic aromatic hydrocarbon (PAH) fluorescence, having a hollow region between the central and outer fluorescence images, was observed in confined flames under extremely fuel-rich conditions. A comparison of the PAH fluorescence with the morphologies of the deposits indicates that the “dark” hollow zone is caused by a decreased number density of large grown-up droplet-like condensed-phase molecules, and the outer weak fluorescence zone is caused by the diffusion of gas-phase small ones. The evolution of soot precursors is stopped at the earlier stage of transition, to soot from condensed-phase large molecules in the non-sooting diffusion flame, under a fuel-rich condition.

Vortex-induced flame extinction in two-phase counterflow diffusion flames with CH planar laser-induced fluorescence and particle-image velocimetry.

Here the interaction between a laminar two-phase, non-pre-mixed counterflow flame and a vortex is examined. Special emphasis is given to the influence of different flame and vortex parameters on the extinction behavior of the flame. Simultaneous planar laser-induced fluorescence of the CH radical layer produced by the flame and particle-image velocimetry measurements of the flowfield are used to characterize the flame-vortex interaction. These simultaneous diagnostics are used for the first time in this configuration. The extinction processes occurring during the flame-vortex interaction can be analyzed by this method, especially the influence of strain at the flame surface. The influence of the droplets on the extinction behavior appears clearly compared with a fully gaseous flame. The spray flame is weaker and extinguishes earlier than does a gaseous flame. In the measurements an additional broadband signal in the vicinity of the CH layer is probably due to the induced fluorescence of polycyclic aromatic hydrocarbons, excited at the same wavelength.

High-temperature vapor detection of polycyclic aromatic hydrocarbon fluorescence

Polycyclic aromatic hydrocarbons (PAHs) are generated from the incomplete combustion of organic matters in a wide variety of natural and man-made materials, such as fuels. It is important to develop an analytical technique for rapidly monitoring PAH generation at high temperatures during pyrolysis processes. This work describes a real-time and in situ technique using laser-induced fluorescence to detect PAH vapors at elevated temperatures. We report fluorescence spectra of several typical PAH model compounds, such as pyrene, phenanthrene and chrysene. We have also examined the temperature effects on PAH fluorescence spectra such as spectral shift, intensity and band width.

Fluorescence spectroscopy of aromatic species produced in rich premixed ethylene flames

The fluorescence spectra of the condensed species (CS) collected in the soot inception region of a rich premixed laminar ethylene/oxygen flame have been measured by excitation in the UV at 266 and 355 nm excitation wavelength. The contribution of the most abundant polycyclic aromatic hydrocarbons (PAH) to the CS fluorescence has been evaluated in order to attribute the CS fluorescence at different emission wavelengths to specific aromatic structures. The fluorescence peaks detected in the UV region of the CS fluorescence spectrum was found to be mainly due to a typical PAH like fluorene, that is, the most fluorescent one among the PAH analyzed in the CS by chromatographic analysis. The CS exhibited the larger emission in the visible where the PAH contribution has been shown to be negligible and other fluorescing aromatic species, not identified by chromatographic analysis of the CS, have to be considered responsible for the visible fluorescence. Laser induced fluorescence (LIF) flame measurements excited at 266 nm and detected at two selected wavelengths (310 and 410 nm) have been performed along the flame axis and compared with the CS fluorescence intensity. The LIF and CS fluorescence signals show quite similar axial trends demonstrating that the LIF signals are related to CS fluorescence. In particular, the LIF fluorescence signals detected in the UV could be attributed to the PAH fluorescence whereas the unidentified species contained in the CS can be followed by LIF detection in the visible region.

Spectrochemical evaluation of pyridinium chloride as a possible selective fluorescence quenching agent of polycyclic aromatic hydrocarbons in water and neat acetonitrile

Pyridinium chloride (PC) is examined as a selective, fluorescence quenching agent for alternant as opposed to nonalternant polycyclic aromatic hydrocarbons (PAHs) in two polar solvents — water and acetonitrile. Nine alternant and 13 nonalternant PAHs were dissolved in water and acetonitrile and a total of 0.2 M of pyridinium chloride was added. The resulting change in fluorescence intensity was observed and reported as the Stern–Volmer quenching constant. Results show that PC is a selective quencher in both polar solvents. It selectively quenches the fluorescence emission intensity of alternant PAHs while leaving the nonalternant PAH fluorescence emission virtually unchanged. These results agree with the selective quenching behavior seen for PC surfactant analogs, cetylpyridinium chloride (CPC) and dodecylpyridinium chloride (DDPC). Furthermore, these results illustrate that the presence of a surfactant or micelle is not a requirement for selective quenching. The selective quencher PC is applicable to situations where a surfactant is not desirable or soluble.

Experimental study of nonfuel hydrocarbons and soot in coflowing partially premixed ethylene/air flames

Centerline profiles of gas temperature, C1 to C12 nonfuel hydrocarbon concentrations, polycyclic aromatic hydrocarbon (PAH) laser-induced fluorescence (LIF), and soot volume fraction are reported for coflowing ethylene nonpremixed and partially premixed flames with primary equivalence ratios ranging from 24 to 3. Concentrations of acetylene and C4 hydrocarbons were lower in nearly all of the partially premixed flames than in the nonpremixed flame, whereas concentrations of methane and C 3H 4 were larger in all of the partially premixed flames than in the nonpremixed flame. These results indicate that the primary effect of partial premixing is not to uniformly increase the concentrations of pyrolysis products, but to shift the pyrolysis mechanism towards odd-carbon species. The concentration of benzene was larger in several of the richer partially premixed flames than in the nonpremixed flame, probably because the shift in pyrolysis mechanism enhances self-reaction of C 3H 3 radicals. Increases in soot volume fraction and other aromatics were observed that matched the increases in benzene. Profiles of PAH fluorescence agreed closely with those for specific gas-phase PAH such as naphthalene, and the maximum PAH signals were a good predictor of the eventual maximum soot volume fractions. Concentrations of oxygenated hydrocarbons such as formaldehyde and ketene were dramatically increased in the partially premixed flames; for formaldehyde this trend was confirmed with in situ LIF measurements.

On-line and in-situ detection of polycyclic aromatic hydrocarbons (PAH) on aerosols via thermodesorption and laser-induced fluorescence spectroscopy.

A fiber optical sensor system for the determination of polycyclic aromatic hydrocarbons (PAH) on aerosols by laser-induced, time-resolved fluorescence is combined with a thermodesorption device. The sensor system is based on an aerosol flow cell, which is fibre-optically coupled to a pulsed nitrogen laser for excitation and the detection system. Time-resolved fluorescence emission spectra are detected by a monochromator equipped with a photomultiplier and a fast digital storage oscilloscope. The analytical figures of merit of the thermodenuder are reported for benzo[a]pyrene, benzo[b]fluoranthene, and benzo[ghi]-perylene on ultrafine soot and NaCl aerosols. By thermodesorption of the PAH, problems due to quenching of the PAH fluorescence by the bulk aerosol material or excimer formation on the aerosol surface were avoided. For the PAH under study, the sensitivity was improved considerably and detection limits between 110 and 850 ng m(-3) were attained, while a response time of 2-3 min was achieved with the thermodenuder. A calibration for PAH on ultrafine soot and NaCl aerosols was established independent of the aerosol substrate.

Aspects of soot dynamics as revealed by measurements of broadband fluorescence and flame luminosity in flickering diffusion flames

Numerous investigators have attributed laser-induced broadband fluorescence observed in both rich, premixed flames and in diffusion flames to small polycyclic aromatic hydrocarbons (PAH). However, the wide variety of experimental flame conditions and excitation/detection wavelengths have clouded the interpretation of such measurements, for example, in terms of indicating either the presence of soot precursors or unreactive by-products (or both). This paper presents PAH fluorescence measurements excited at 283.5 nm and detected at 400–447 nm in a series of steady and flickering methane, propane, and ethylene diffusion flames burning at atmospheric pressure in an axisymmetric, coflow configuration. In the flickering flame experiments, acoustic forcing of the fuel rate is used to phase lock the periodic flame flicker close to the natural flame flicker frequency caused by buoyancy-induced instabilities. When compared to our earlier measurements of soot concentrations in the same flames, soot inception in the annular region is found to occur at the interface between the fluorescing PAH and the region of high radical concentrations. Although the peak PAH fluorescence signals and maximum soot concentrations do not occur at the same spatial locations, indirect evidence is presented that the species responsible for PAH fluorescence participate in either soot inception or growth. In contrast to prior suggestions that PAH fluorescence intensities scale with soot concentrations, the relative peak PAH fluorescence signals are observed to be 1.0:9.8:5.4 for the steady methane, propane, and ethylene flames, respectively, whereas the maximum soot levels follow a different trend of 1.0:19:39. Similar results are observed in the flickering flames, all of which exhibit enhanced PAH fluorescence signals compared to the steady flames. PAH fluorescence excited at 560.3 nm in the steady flames is also strongest for propane. Measurements of flame radiation arising from soot particles have also been made, with detection at 395–547 nm and to a limited degree at 833–900 nm. Visible flame emission is particularly sensitive to the local soot temperature. Comparison of the luminosity images with those of OH· fluorescence and soot scattering shows that the luminosity is strongest where the hydroxyl radicals and soot layers overlap, i.e., in regions of active soot oxidation.