PAH Growth Research Articles

Effect of Hydrogen Addition on Soot Formation and Emission in Acetylene Laminar Diffusion Flame.

Hydrogen (H2) has been regarded as a highly competitive alternative fuel that does not produce CO2 and soot during combustion. There are few studies of cofiring H2 with hydrocarbons. Meanwhile, the effect of hydrogen addition on soot formation and emission is questionable. In this study, the effect of H2 addition with varying ratios (between 0 and 50% by molar fraction while the remainder is nitrogen) on soot formation in acetylene (50% by molar fraction) laminar diffusion flames was experimentally and numerically investigated. Results show that with H2 addition, the flame height increases and the temperature decreases. The total soot emission and the primary particle size both increase with H2 addition. The addition of H2 promotes soot formation. In addition, the soot oxidation is weakened due to the lower flame temperature. Chemical kinetic analysis shows that the concentrations of A1, H, and H2O increase with H2 addition, which is consistent with the experimental results. According to the HACA reaction, the increase of the molar fraction of A1 and H radicals could promote PAH growth and soot formation.

Inhibitory mechanisms of ammonia addition on soot formation during n‑decane pyrolysis

Soot formation during the imperfect combustion or pyrolysis of fuels is harmful to engine efficiency·NH3 has been proved to reduce soot emission effectively in flames. In this work, the effect of NH3 addition on soot inhibition during n-decane pyrolysis was systematically investigated from a theoretical principle. The MD simulations indicate that the coking phenomenon is weakened with the NH3 addition. Besides, the NH3 additives make the concentration peak of the dominant gas products lag. Thus, the concentrations of crucial precursors (acetylene and propargyl) are reduced by NH3 addition. Moreover, the NH3 additives can also influence the PAH growth process through the PAHs with N atoms (N-PAHs). Then, the chemical kinetic analysis shows that proper addition of NH3 additives can meet the heat absorption demand of fuel cracking. Furthermore, the mass fractions of PAHs and soot particles decrease with NH3 addition, which is consistent with the results from MD simulations. The formation of the main carbonitride species originates from the reaction of CH3 and C2H5 with NH2 followed by the dehydrogenation or C-C bond breaking reaction. The carbon source directly involved in the formation of soot particle will be reduced and the growth of the soot particle will be inhibited by preventing the formation of the key reactants, such as the acetylene molecule.

Propylene Oxide Addition Effect on the Chemical Speciation of a Fuel-Rich Premixed n-Heptane/Toluene Flame.

1,2-Propylene oxide (PO, C3H6O) is considered as a promising agent for improving fuel. In this work, the effect of PO additives on the species pool in a premixed burner-stabilized fuel-rich (ϕ = 1.6) flame fueled by n-heptane/toluene mixture (7/3 by volume of liquids) at atmospheric pressure is studied by the flame-sampling molecular beam mass spectrometry and numerical modeling in order to get insight into the chemical aspects of the influence of oxygenates with an epoxy group on the formation of abundant intermediates (including PAH precursors) during combustion of fossil fuels. The flames with various loadings of PO in the fuel blend (from 0 to 16.3% in mole basis) are examined, and detailed kinetic mechanisms available in the literature are validated against the measurements of mole fraction profiles of reactants, major products, and many intermediate species. A higher reactivity of the fresh mixture and a reduction in the peak mole fractions of intermediates playing an important role in PAH formation (benzene, styrene, ethylbenzene, phenol, acetylene, diacetylene, etc.) are observed when PO is added. This was found to be due to simultaneously two factors: the partial replacement of "sooting" fuel (toluene, which is the main precursor of these species) with oxygenated additive, and the changes in the flame radical pool caused by PO addition. Propylene oxide additive was found to change the ratio between H, OH, O, and CH3 toward an increase in the proportion of O and CH3. The detailed kinetic mechanisms considered in the work are found to overpredict the peak mole fraction of acetylene, a key species playing a crucial role in PAH growth. Its chemistry is revisited in order to provide a better prediction of C2H2 and, as a result, PAHs.

Bay capping via acetylene addition to polycyclic aromatic hydrocarbons: Mechanism and kinetics

The bay-capping mechanism on PAH armchair edges and the kinetics of acetylene addition to 6–6–5 and 5–6–5 bays have been explored by ab initio/RRKM-ME calculations. The bays on the edges were modeled by C21H11 and C20H9 radicals produced by H abstractions from 7H-benzo[c]cyclopenta[e]pyrene and dicyclopenta[cf]pyrene. The C20H9 + C2H2 reaction is shown to have a low entrance barrier and to rapidly form the capped product, indaceno[2,1,8,7-cdefg]pyrene, along with ethynyl substituted dicyclopenta[cf]pyrene at temperatures above 1400 K. The reactivity of C21H11 is shown to be governed by the location of the unpaired electron; the π radical R1 formed by H abstraction from the CH2 group in 7H-benzo[c]cyclopenta[e]pyrene reacts with C2H2 very slowly owing to a high entrance barrier, with the bay-capping rate constant approaching 10−16 cm3 molecule−1 s−1 only at temperatures above 2000 K. This result reaffirms that the growth of π aryl radicals via acetylene addition is inefficient and reflects the generally low reactivity of such radicals where the spin density is highly delocalized over the entire polyaromatic system. Alternatively, the σ C21H11 radical R2 produced by H abstraction from the five-membered ring at the bay rapidly reacts with C2H2 forming the bay-capped product, with the rate constant on the order of 10−12 cm3 molecule−1 s−1 at T ≥ 1500 K. Rate constants for the capping reactions at the 6–6–5 and 5–6–5 bays are compared with those at the 6–0–6, 6–6–6, and 6–5–6 bays. The site-specific bay-capping rate constants have been utilized in kMC simulations of the PAH growth and the results showed measurable differences when the 6–6–5 and 5–6–5 bay-capping reactions are taken into account, including an increase of the growth rate and the formation of closed-shell PAH and a rise of the number of embedded five-membered rings accompanied with a slight decrease of their overall amount.

Soot and PAH formation in high pressure spray pyrolysis of gasoline and diesel fuels

Time-resolved soot and PAH formation from gasoline and diesel spray pyrolysis are visualized and quantified using diffuse back illumination (DBI) and laser induced fluorescence (LIF) at 355 nm, respectively, in a constant-volume vessel at 60 bar from 1400 to 1700 K for up to 30 ms. The delay, maximum formation rate, and yield of soot and PAHs are compared across fuels and temperatures and correlated with the yield sooting indices on either the mass or mole basis. The delays generally decrease with increasing temperature, and the formation rates of both PAHs and soot generally increase with temperature. The apparent PAH-LIF yield may decrease with temperature due to PAH growth and conversion into larger species, signal trapping, and thermal quneching. Soot yield generally increases with temperature. The mass-based YSI correlates reasonably well with soot delay, but YSI does not correlate well with soot yield. The mass-based YSI is a more appropriate predictor of sooting propensity than the mole-based YSI.

Phytochemical effects of Apium graveolens on the abundances of functional genes associated with PAH degradation in soil

The biodegradation of polycyclic aromatic hydrocarbons in soil is enhanced by chemical constituents of various plant species that support the growth and activity of PAH degrading bacteria in the rhizosphere. Here, we investigated the phytochemical effects of celeriac (Apium graveolens var. rapaceum) root tissues, a known stimulator of PAH degradation, on the copy numbers of nahAc, xylE, and 16S rRNA genes to determine the relative contributions of growth-linked degradation versus selective enrichment of PAH degraders. Real-time quantitative PCR analysis showed that the copy numbers of 16S rRNA genes, as a reporter of general bacterial population size, increased approximately 2- to 5-fold in soil amended with celeriac, but were not affected by exposure to naphthalene alone. In contrast, copy numbers of two functional genes (nahAc and xylE), commonly used as reporters for monitoring the population size of PAH degraders, were significantly increased following exposure to naphthalene but were differently affected by soil amendment with celeriac. As compared with the copy numbers of nahAc gene in unamended control soil, the nahAc copy numbers were increased by 460-fold in soil amended with naphthalene alone and by 610-fold in soil amended with both naphthalene and celeriac after 10 days of incubation. The copy numbers of xylE were increased by 20-fold in soil amended with naphthalene alone and increased by 200-fold in soil amended with both naphthalene and celeriac after 10 days of incubation. The result suggests that increased degradation rates of PAHs in the presence of celeriac involves both growth linked processes and selective enrichment of the degrader population.

Study of effects of ammonia addition on soot formation characteristics in n-heptane co-flow laminar diffusion flames

Ammonia is a compelling carbon-free fuel, and co-firing ammonia with conventional hydrocarbon fuels receives growing interest as a feasible solution to mitigate CO2 emission. This study experimentally and numerically investigated the soot formation characteristics of n-heptane co-flow laminar diffusion flames with different contents of ammonia (up to 40% by volume, balanced using argon). The soot volume fraction (SVF) in the flames was quantitatively measured via laser-induced incandescence technique; PAHs and OH radical were qualitatively determined via laser-induced fluorescence technique. Results showed that ammonia addition reduced the SVF, i.e., a peak SVF decrease of 35% at 40% ammonia. Meanwhile, PAH content also decreased with ammonia addition, demonstrating that ammonia would effectively inhibit the formation of PAHs. Thus, the inception and growth of soot decreased. Interestingly, OH radical was reduced with ammonia addition, revealing that ammonia also chemically inhibited soot oxidation. The results confirmed that ammonia reduced the soot loading in the flames due to inhibitory effect on the inception and growth of soot rather than the enhancement on oxidation. The established mechanism effectively captured the variation of different PAHs in the experiments. Numerical analysis showed that the formation of the first aromatic ring benzene was reduced by ammonia as a result of the reduction of C4H5 and R203:C4H5+C2H2 reaction. In addition, the reduced hydrogen abstraction due to decrease in H, coupled with the reduced small PAH, led to the decrease in PAH growth. Nevertheless, a more comprehensive mechanism including the reactions between aromatics and nitrogen species, such as NH, CN, and NOx, is still required in future studies.

Growth of Polycyclic Aromatic Hydrocarbons by C2H2 Mediated by Five-membered Rings: Acenaphthylene Conversion to Phenanthrene

ABSTRACT To investigate the role of five-membered rings in the growth of PAHs, the conversion of acenaphthylene to phenanthrene is studied using density functional theory and composite method. Two pathways are considered that initiate with H-abstraction from different sites on acenaphthylene and are followed by acetylene addition on the radical sites. The reactions in both pathways progress with the formation of two fused five-membered rings and H-migration reactions that eventually get converted to a six-membered ring to form either phenanthrene or phenanthryl radical. The reaction kinetics are calculated using RRKM theory, and the new reactions are added to a detailed hydrocarbon mechanism to test their effect on the profiles of phenanthrene and acenaphthylene. The flame simulations indicate that the phenanthrene concentration increases by 58% with the addition of the proposed reactions to the mechanism and helps in matching the experimental phenanthrene profile with the computed one. Thus, the pathways studied in this work can be considered to be important for phenanthrene formation.

Experimental and numerical study on sooting transition process in iso-octane counterflow diffusion flames: Diagnostics and combustion chemistry

The present work focused on the combustion chemistry in sooting transition process in iso-octane counterflow diffusion flames. A series of iso-octane flames ranging from sooting-free to heavy-sooting were investigated by stepwise increasing the oxygen concentration (XO) in the oxidizer stream. In particular an optical approach with two different algorithms based on the three wavelengths distribution of flame images captured by digital single lens reflex (DSLR) camera was proposed to identify the critical condition for the sooting transition process. The results showed that the sharp increase in signal ratio of red luminosity with simultaneously obvious decrease in signal rations of blue luminosity could be used as an effective indicator of sooting onset in flames. On-line gas chromatography (GC) measurements were used to investigate the flame chemistry and determine quantitative mole fraction profiles of stable and reactive species formed in the combustion process. Particular attention was given to the combustion chemistry changes caused by the occurrence of sooting transition. Numerical kinetic simulations were performed along with the comparisons with experimental results to obtain deep insights into the sooting transition process, with focus on benzene chemistry considering the critical role of benzene formation in the subsequent large aromatic species/soot formation process. In most cases, the trend observed in the experiments could be satisfactorily reproduced and explained by the models. For major species and C1–C4 species, it can be seen that the concentration increased with the increase of XO in sooting transition process. For the decomposition of fuel and the formation of benzene, the influence in sooting transition process was mainly reflected in the contributions of different pathways due to temperature effects. The impact of sooting transition on flame chemical structure was mainly reflected in the growth of PAHs. The increasing trend for the PAHs growth caused by the increase of oxygen concentration in the transition process was more and more obvious.

Chemical effects of anisole and toluene addition to n‑heptane on PAH characteristics in laminar premixed flames by LIF measurement and kinetic model

Anisole is a candidate renewable fuel that displays satisfying combustion characteristics, but its sooting characteristics are not well known. The goal of this study is to investigate the chemical effects of anisole and toluene on PAH formation in n-heptane laminar premixed flames, using LIF measurement and chemical kinetic simulation. To focus on chemical effects, the equivalence ratios, dilution ratios, and flame temperatures were kept nearly unchanged when anisole and toluene were blended separately into n-heptane flame. LIF experimental results indicated that PAH formation was promoted with the addition of anisole and toluene, and the effect of toluene was stronger than the promotional effect of anisole. The chemical kinetic model predicted the observed PAH tendencies well in the LIF experiments. Based on this model, reaction pathway and sensitivity analyses were performed to interpret the chemical effects. Results revealed that, due to their different molecular structures, the difference in chemical effects between anisole and toluene was notable in PAH growth processes. Anisole decomposed first via the O-CH3 bond dissociation reaction, and then proceeded to a CO elimination reaction to yield an important PAH precursor, C5H5, which contributed to PAH formation via styrene and indene reaction networks. In the toluene added flame, PAH formation was enhanced because of ring expansion reaction on the dehydrogenated branch of the toluene.

Lethal, behavioral, growth and developmental toxicities of alkyl-PAHs and non-alkyl PAHs to early-life stage of brine shrimp, Artemia parthenogenetica

Alkyl-PAHs are the predominant form of PAHs in crude oils which are supposed to demonstrate different toxicities compared to non-alkyl PAHs. Little information is available about the toxicity of alkyl-PAHs on marine Artemia. This study addressed and compared the lethal, behavioral, growth and developmental toxicities of three alkyl-PAHs, namely 3-methyl phenanthrene (3-mPhe), retene (Ret) and 2-methyl anthracene (2-mAnt), to their non-alkyl forms, phenanthrene (Phe) and anthracene (Ant) using Artemia parthenogenetica (nauplii, <24 h) as test organism following a 48 h and a 7 d of exposure, respectively. Benzo-a-pyrene (Bap) was selected as a reference toxicant for the comparison with the above alkyl-PAHs and non-alkyl PAHs. Results showed that for all tested endpoints, A. parthenogenetica nauplii had the highest sensitivity to Bap while Ant had no significant effect on nauplii survival or development within given concentrations. Considering the aqueous freely dissolved PAH concentrations, the 48 h-LC50 (survival), 48 h-EC50 (immobility) and 7 d-LC10 (survival) of Bap were calculated as 0.321, 0.285 and 0.027 μg/L, respectively, which were twofold to fivefold lower than those of Phe, 3-mPhe, Ret, Ant and 2-mAnt. A higher acute toxicity of alkyl-PAHs (3-mPhe and 2-mAnt) than their non-alkyl forms (Phe and Ant) was observed. Not limited to Phe, the common non-polar narcotic mode of action was also observed for Bap, 3-mPhe, Ret and 2-mAnt, which was evident by the inhibited mobility of nauplii. The decreased body lengths were found for all PAH treatments compared to the solvent control, whereas instar retardations were only found in nauplii exposed to Bap, Phe and Ret. Our findings emphasized the sensitivity differences of A. parthenogenetica nauplii to selected alkyl PAHs and non-alkyl PAHs and confirmed the application of lethal, behavioral and growth indicators in the toxicity evaluation of selected PAHs other than Ant. However, the distinct toxicities of these PAHs suggested other toxic modes of action may play more important roles apart from narcotic mode of action and need to be elucidated in future studies. In addition, a strong correlation between the body length and the instar of A. parthenogenetica nauplii was observed for each PAH exposure, suggesting that body length can be representative for both growth and developmental indicators during biological monitoring of PAH pollution in marine environment.

Experimental and kinetic modeling study of ammonia addition on PAH characteristics in premixed n-heptane flames

Due to concerns over pollutant emissions and ever-increasing energy crisis, ammonia is being considered as a promising alternative fuel to control soot particles and their precursors (polycyclic aromatic hydrocarbons, PAHs) emissions in combustion for diesel engines. This paper aims to investigate the chemical effects of NH3 addition on PAH formation in n-heptane laminar premixed flames. The laser-induced fluorescence (LIF) technique was used to measure the relative PAH fluorescence intensities qualitatively. A kinetic modeling study was performed to predict PAH concentrations by comparing with the experimental results, and then clarify the underlying effects of NH3 on PAH formation. Both experimental and numerical results revealed that PAH concentrations decreased monotonically as ammonia fraction increased, suggesting a PAHs-inhibiting role for NH3 in n-heptane baseline flames. Reaction pathway analysis revealed that the suppression of NH3 on PAH formation could be attributed to the competition for H, OH and O radicals in fuels' decomposition, and chemical interactions of amine radicals with the C1-C2 species. For this reason, the NH3 addition produced a relatively decreased C2H2, thereby inhibiting the formation of propargyl. As a result, benzene formation was inhibited through C3H3 self-recombination reaction, resulting sequentially in the suppression of PAH formation and growth. Besides, the decreased C2H2 yield in the post-flame region of NH3 doping flames also inhibited the formation of PAHs through the hydrogen-abstraction-acetylene-addition (HACA) mechanism.

Formation of Phenanthrene via Recombination of Indenyl and Cyclopentadienyl Radicals: A Theoretical Study.

This work presents quantum chemical G3(MP2,CC)//B2PLYPD3/6-311G(d,p) calculations of the potential energy surface for the indenyl (C9H7) + cyclopentadienyl (C5H5) reaction followed by unimolecular decomposition of the C14H11 radicals formed as the primary products, as well as the Rice-Ramsperger-Kassel-Marcus master equation (RRKM-ME) calculations to predict temperature- and pressure-dependent reaction rate constants and product branching ratios. The reaction begins with the barrierless recombination of indenyl and cyclopentadienyl forming a C14H12 molecule with a new C-C bond connecting two five-membered rings, which subsequently dissociates to C14H11 radicals by H losses. The primary products of the C9H7 + C5H5 → C14H11 + H reaction can directly decompose by another H loss to benzofulvalene, and this pathway is most favorable in terms of the entropy factor and hence is preferable at higher temperatures. Otherwise, the initial C14H11 isomers can undergo significant structural rearrangements before eliminating an H atom and producing phenanthrene, anthracene, or benzoazulenes, among which the formation of phenanthrene via the "spiran" pathway is clearly preferred. The calculated barriers along the computed favorable dissociation pathways are relatively low, in the ∼30-40 kcal/mol range, making the C14H11 radicals unstable at temperatures above 1000-1250 K at 1 atm. The results of RRKM-ME calculations show that, under typical combustion conditions, the decomposition of the C14H11 radicals predominantly leads to benzofulvalene. However, the latter can be rapidly converted to phenanthrene via H-assisted isomerization with the rate constant for the benzofulvalene + H → phenanthrene + H reaction being close to 10-11 cm3 molecule-1 s-1 at 1000-1500 K and 1 atm. The results provide further support for the hypothesis that recombination of two π radicals containing five-membered rings can lead to a growth of PAH with the formation of two fused six-membered rings, but the reaction mechanism may not be direct and is likely to involve two consecutive H atom losses leading to a fulvalene-like product, with subsequent H-assisted isomerization of the latter to a benzenoid PAH.

Identification of the molecular-weight growth reaction network in counterflow flames of the C3H4 isomers allene and propyne

The reaction networks responsible for aromatics formation in counterflow flames of the C3H4 isomers allene and propyne are identified through a combined experimental and modeling study. Mole fraction profiles of near-atmospheric pressure (933 mbar) diffusion flames fueled by the C3H4 isomers are analyzed by means of a newly assembled, chemically detailed kinetic mechanism. The experiment consists of a counterflow burner system that is coupled to a high-resolution time-of-flight molecular-beam mass spectrometer with single-photon ionization via synchrotron-generated vacuum-ultraviolet photons. Flame-sampled, mass-specific photoionization efficiency curves are used to identify the presence of aliphatically substituted aromatic species in addition to the commonly considered pericondensed ring structures. The new mechanism describes the formation and growth of aromatics through repetitive sequences of radical–radical and radical–molecule reactions that include C1C6 intermediates. Higher concentrations of aromatic species are observed in the allene flame and the new mechanism captures the observed experimental trends very accurately. The results indicate the importance of the aliphatically substituted aromatics and of ring-enlargement reactions for the growth reactions. According to the model simulations, radical+radical recombination and PAH-radical+molecule reactions play an important role in PAH growth.

Pyrene dimerization in controlled temperature environment: An experimental study

Nucleation step in soot formation can be defined as the transition from gas phase to condensed phase state at flame temperature. This aspect of particle formation in flame is the least understood despite of the great effort of the last decades. Most of the models and of the experimental setups are not able to isolate the single step of nucleation. The formation of the first particles or species in condensed phase is studied in combustion environments in which many other processes – namely PAH growth from other small hydrocarbons and oxidation – are taking place. A tubular flow reactor working in the temperature range of 800–1100 K with a stream containing pure PAH – pyrene in the current work - was used to isolate the pathways leading from gas to condensed phase. To avoid the formation of smaller compounds from the pyrene pyrolysis, temperature was kept lower than 1100 K allowing a residence time long enough to favour mass growth. DMA measurements at the exit of the reactor showed the presence of particles. Particle formation was found to be sensitive to the temperature. In particular, the formation rate was higher at the higher temperature, suggesting the intervention of some chemical-driven pathway for the formation of the first condensed phase species. The gas and the particles exiting the reactor were trapped in ethanol and analyzed by UV-Visible and fluorescence spectroscopy, and size exclusion chromatography (SEC). The formed particles have lost molecular spectral features and have a molecular weight (MW) >= 400: these characteristics allow to consider them the smallest species in condensed phase. Their MW evaluated by SEC and the analysis of their absorption spectra hinted to the predominance of oligomers formation, from dimers to tetramers, being trimers the most favored and stable species in the investigated conditions.

PAH 成長にセクショナル法を適用したガソリンサロゲート燃料用すす生成モデル（第2 報）:－ イソオクタン/トルエン/エタノール混合燃料のすす生成特性 －

PAH 成長にセクショナル法を適用したガソリンサロゲート燃料用すす生成モデル（第2 報）:－ イソオクタン/トルエン/エタノール混合燃料のすす生成特性 －

Soot formation of n-decane pyrolysis: A mechanistic view from ReaxFF molecular dynamics simulation

The chemical-kinetic mechanism of soot formation for the n-decane pyrolysis was comprehensively investigated from theoretical perspective. We found the formation of carbonaceous nanoparticle with a multi-layer closed cylindrical structure needs four processes including initial rings formation, PAH growth, particle nucleation via coalescence of PAH and particle growth by coagulation or surface reactions. The mechanisms of HACA, CAHM and CHRCR play crucial roles in the processes of the formation and growth of PAH molecules. In addition, the temperature effect was discussed and found the higher temperature can accelerate the formation of soot particle, which is in agreement with the experiment findings.

Chemical Effects of CO2 and H2O Addition on Aromatic Species in Ethanol/Air Diffusion Flame

ABSTRACT Exhaust gas recirculation (EGR) is an effective technique used widely to meet current emission regulations. In this work, the mechanism of H2O and CO2 addition (a main component of exhaust gas) on polycyclic aromatic hydrocarbons (PAHs, the precursor of soot) formation was investigated numerically in ethanol diffusion flames, motivated by current concerns on particulate matter (PM) emissions when ethanol is applied to modern gasoline engines, especially under high load and temperature. To understanding the underlying mechanisms of H2O and CO2 on ethanol flame, a comprehensive chemical kinetic analysis was performed by modeling gas-phase chemistry with PAHs formation up to pyrene. In order to distinguish the important chemical role of H2O and CO2 addition through partial replacement of N2 from their thermal effects, additional calculations were performed with two fictitious species. The numerical results show that the thermal effect of H2O and CO2 reduces the peak flame temperature, but their chemical effect influences the peak flame temperature differently. The results also showed that the addition of either H2O or CO2 led to a significant reduction of PAHs via thermal and chemical effects. The chemical suppressing effect of H2O and CO2 addition on PAHs formation was identified and attributed to reduced PAHs precursor species and slow of PAH growth process. However, reaction pathway analysis showed that the chemical effects of H2O and CO2 influence the PAHs chemistry differently.

Chemical kinetics of hydroxyl reactions with cyclopentadiene and indene

Oxidation reactions of highly unsaturated cyclic hydrocarbons are very important in combustion, competing with their participation in PAH and soot growth. One of the key oxidation pathways at combustion conditions is the reaction with hydroxyl radicals. Cyclopentadiene (C5H6) and indene (C9H8) are typical PAH precursors, which have highly unsaturated penta-ring structure. In this study, rate coefficients of the reactions of hydroxyl radical with cyclopentadiene and indene were measured behind reflected shock waves over the temperature range of 828–1390 K and pressure near 1 atm. Hydroxyl radicals were monitored by a narrow line-width laser absorption near 306.7 nm. The measured rate coefficients may be expressed as kC5H6+OH=3.68−0.23+0.27×1013e−(1742.5−77+75)/T and kC9H8+OH=1.44−0.14+0.10×1013e−(1497.8−72+130)/T cm3 mol−1 s−1. Our experimental results showed that the reaction of hydroxyl radicals with cyclopentadiene is about two times faster than that of indene, and the indene + OH reaction exhibited a relatively weaker temperature dependence. Chemical kinetic simulations, carried out with a detailed model, showed the sensitivity of model performance to these reactions and the potential of model improvement with our measured rate coefficients.

The Polycyclic Aromatic Hydrocarbon Mass Fraction on a 10 pc Scale in the Magellanic Clouds

Abstract We present maps of the dust properties in the Small and Large Magellanic Clouds (SMC, LMC) from fitting Spitzer and Herschel observations with the Draine &amp; Li dust model. We derive the abundance of the small carbonaceous grain (or polycyclic aromatic hydrocarbon; PAH) component. The global PAH fraction ( , the fraction of the dust mass in the form of PAHs) is smaller in the SMC ( %) than in the LMC ( %). We measure the PAH fraction in different gas phases (H ii regions, ionized gas outside of H ii regions, molecular gas, and diffuse neutral gas). H ii regions appear as distinctive holes in the spatial distribution of the PAH fraction. In both galaxies, the PAH fraction in the diffuse neutral medium is higher than in the ionized gas, but similar to the molecular gas. Even at equal radiation field intensity, the PAH fraction is lower in the ionized gas than in the diffuse neutral gas. We investigate the PAH life-cycle as a function of metallicity between the two galaxies. The PAH fraction in the diffuse neutral medium of the LMC is similar to that of the Milky Way (∼4.6%), while it is significantly lower in the SMC. Plausible explanations for the higher PAH fraction in the diffuse neutral medium of the LMC compared to the SMC include: more effective PAH production by fragmentation of large grains at higher metallicity, and/or the growth of PAHs in molecular gas.