Soot Aging Research Articles

Dark NO2 Reduction on a Graphene Surface with Implications for Soot Aging and HONO Formation.

Soot, primarily composed of elemental carbon (EC) and organic carbon (OC), is ubiquitous in PM2.5. In the atmosphere, the heterogeneous interaction between NO2 and soot is not only an important pathway driving soot aging but also of central importance to nitrous acid (HONO) formation. It is commonly believed that the surface redox reaction between reductive OC and NO2 dominates the night aging of soot and the conversion of NO2 to HONO. However, completely differing from the currently popular explanation, we find here that the redox reaction between EC and NO2 can also drive the conversion of NO2 to HONO during soot aging. By combining in situ experiments with density functional theory (DFT) calculations, we proposed that the surface carbon vacancy defects on graphite/graphene-like EC should be a type of potential primary adsorption and reactive sites inducing the heterogeneous reduction of NO2. We suggested a new mechanism that NO2 is reduced to form HONO on surface vacancy defects through the splitting of H2O molecules, and the carbon atoms adjacent to surface vacancy are simultaneously oxidized to form hydroxyl-functionalized EC. This novel finding provides insights into the chemical mechanism driving the NO2-to-HONO conversion and rapid soot aging, which expands our knowledge of the heterogeneous chemistry of soot in the atmosphere.

Distinct Photochemistry of Odd-Carbon PAHs from the Even-Carbon Ones During the Photoaging and Analysis of Soot.

Polycyclic aromatic hydrocarbons (PAHs) are the primary organic carbons in soot. In addition to PAHs with even carbon numbers (PAHeven), substantial odd-carbon PAHs (PAHodd) have been widely observed in soot and ambient particles. Analyzing and understanding the photoaging of these compounds are essential for assessing their environmental effects. Here, using laser desorption ionization mass spectrometry (LDI-MS), we reveal the substantially different photoreactivity of PAHodd from PAHeven in the aging process and their MS detection through their distinct behaviors in the presence and absence of elemental carbon (EC) in soot. During direct photooxidation of organic carbon (OC) alone, the PAHeven are oxidized more rapidly than the PAHodd. However, the degradation of PAHodd becomes preponderant over PAHeven in the presence of EC during photoaging of the whole soot. All of these observations are proposed to originate from the more rapid hydrogen abstraction reaction from PAHodd in the EC-photosensitized reaction, owing to its unique structure of a single sp3-hybridized carbon site. Our findings reveal the photoreactivity and reaction mechanism of PAHodd for the first time, providing a comprehensive understanding of the oxidation of PAHs at a molecular level during soot aging and highlight the enhanced effect of EC on PAHodd ionization in LDI-MS analysis.

Formation mechanism of environmentally persistent free radicals during soot aging

Environmentally persistent free radicals (EPFRs) are a newly emerging class of pollutants that are known to be formed on metal oxides (MOs). Recent experimental investigations have suggested that EPFRs may also be formed on soot. However, little is known about the structural and compositional characteristics of soot surfaces conducive to EPFRs formation. Concurrently, the changes in the surface structure and composition of soot indicate its aging process, yet the detailed mechanism underlying EPFRs formation in this process remain obscure. Additionally, it is uncertain whether the formation mechanism of EPFRs on soot differs from that on MOs. In this study, the density functional theory (DFT) and ab initio molecular dynamics (AIMD) were used to investigate EPFRs formation on soot at different stages of aging. The results indicate that EPFRs could be formed on both the complete soot surface with hydroxyl groups and the defective surface. Correspondingly, during the aging process of soot, the formation trend of EPFRs increases in the early stage of the aging process and then decreases over time. Additionally, the formation mechanism on soot is different from that on CuO. On soot, the EPFRs formation is initiated by free hydroxyl radical formed through the desorption process, following the Eley-Rideal mechanism, while on CuO surfaces, its formation involves the surface-bound hydroxyl groups, and thus follows the Langmuir-Hinshelwood mechanism. Furthermore, EPFRs triggered by soot are less stable than those by CuO, due to the easier desorption process of phenoxyl radicals on soot. These findings provide valuable insights into the detailed formation mechanism of EPFRs on soot at different stages of aging, thereby contributing to control EPFRs emissions from a mechanism perspective.

Quantifying evolution of soot mixing state from transboundary transport of biomass burning emissions

Incomplete combustion of fossil fuels and biomass burning emit large amounts of soot particles into the troposphere. The condensation process is considered to influence the size (Dp) and mixing state of soot particles, which affects their solar absorption efficiency and lifetimes. However, quantifying aging evolution of soot remains hampered in the real world because of complicated sources and observation technologies. In the Himalayas, we isolated soot sourced from transboundary transport of biomass burning and revealed soot aging mechanisms through microscopic observations. Most of coated soot particles stabilized one soot core under Dp<400nm, but 34.8% of them contained multi-soot cores (nsoot ≥ 2) and nsoot increased 3-9 times with increasing Dp. We established the soot mixing models to quantify transformation from condensation- to coagulation-dominant regime at Dp≈ 400nm. Studies provide essential references for adopting mixing rules and quantifying the optical absorption of soot in atmospheric models.

Morphology and Fractal Dimension of Size‐Resolved Soot Particles Emitted From Combustion Sources

AbstractDatabases of the morphological parameters of fresh size‐resolved soot particles and their systematic comparisons among various combustion sources are important to trace the soot aging process and evaluate their optical properties. Here, the mixing state, effective density (ρeff), monomer number and diameter (N and dp), and fractal dimension (Df) of size‐resolved soot particles from vehicle emissions (VE), biomass burning (BB), coal combustion (CC), tunnel air (TA) and urban air (UA) were characterized based on electron microscopy observations and analysis. We determined that freshly‐emitted soot particles from combustion sources contain not only bare‐like soot but also some coated‐soot particles. ρeff (157–689 kg/m3) decreased while N (46–1,500) and dp (24–42 nm) increased with the increased diameter of soot particles. The Df of BB and CC were independent of the diameter changes and fluctuated between 1.65 and 1.80, while the Df of VE and TA (1.62–1.71) increased and UA Df (1.87–1.80) decreased with increasing diameter. Based on Df obtained in this study, we found that the ensemble Df of VE, BB, CC, TA, and UA could not represent the finer (&lt;150 nm) soot particles and coarser soot particles (&gt;600 nm) in VE, TA, and UA. This study highlights that the morphological parameters of freshly‐emitted soot particles vary among different combustion sources and have a size effect, particularly for finer and coarser soot particles. The database of size‐resolved soot particles will be helpful to improve soot models based on numerical simulation and better evaluate atmospheric optical properties.

A new thermodeactivation model for soot surface reactivity toward O2 in turbulent hydrocarbon diffusion flames

ABSTRACT Accurate soot modeling in flames is one important tool contributing to an understanding of soot behavior in combustion. Soot models reflect the four principal processes that govern soot dynamics: nucleation, coagulation, surface growth, and oxidation. Existing oxidation models reflect both the temperature-dependent surface oxidation chemical kinetics and also the fact that the surfaces become less reactive as they age. This surface deactivation process has been modeled primarily by using in-flame soot measurements as a basis for developing functional forms and parameters to describe the decay of α, the ratio of active oxidation sites divided by total surface sites. These existing models, in general, overpredict oxidation rates in turbulent diffusion flames. This study presents an alternative soot aging factor model that is based on the analogous process of coal char oxidation deactivation, a well-studied problem. The proposed framework provides a basis for application to a wide range of turbulent diffusion flames. This new model is configured to provide the same parameter α that appears in existing approaches. As such it can be easily applied to the existing published models. The approach is implemented in the fixed-pivot sectional soot model, with the performance compared against literature data for ethylene/air turbulent diffusion flames. With only very minor adjustment from the baseline parameters developed for coal char, the model shows good agreement with the literature flame data.

Highly oxygenated organic molecules with high unsaturation formed upon photochemical aging of soot

<h2>Summary</h2> Highly oxygenated organic molecules are a key component of atmospheric secondary organic aerosol. The origin and formation mechanism of highly oxygenated organic molecules with high unsaturation (HU-HOMs) commonly observed in the atmosphere, however, remain unknown. Through molecular-level investigations of the photochemical aging of soot by O₂ in the air, we find that the multigenerational photo-oxidation of large polycyclic aromatic hydrocarbons (PAHs) on soot by singlet oxygen (¹O₂) and superoxide anion radicals (O₂^⋅−) can be an important source of the unexplained HU-HOMs. The PAH-derived HU-HOMs exhibit lactone and anhydride functional groups and can substantially increase the hydrophilicity of soot. As PAHs are a characteristic and ubiquitous component of combustion aerosols, their photochemical oxidation can generate substantial amounts of HU-HOMs, influencing the atmospheric fate and effects of combustion aerosols.

Quantifying the Fractal Dimension and Morphology of Individual Atmospheric Soot Aggregates

AbstractThe complex morphology of soot aggregates is a major source of uncertainty in evaluating their warming effects in the atmosphere. Fractal dimension (Df) is a key parameter in quantifying the morphology of soot particles. Previous studies are mostly based on manual identification of soot monomers in electron microscopic images and are hard to provide comparable results in determination of Df. Here we develop a novel image recognition technique to automatically determine the Df of individual soot aggregates from electron microscopy images. The novel method has been shown to be able to trace the small change of the soot Df from an urban tunnel (1.61 ± 0.19) to its exit (1.70 ± 0.15). By applying this new method, we show a substantial difference in average Df of soot particles emitted from vehicles (1.66 ± 0.17) than from biomass burning (1.75 ± 0.18) and coal burning (1.76 ± 0.18). Average Df of soot from an urban atmosphere (1.77 ± 0.18) is close to that from biomass and coal combustion but much lower than that from a rural atmosphere (1.85 ± 0.13). In summary, the new technique provides an automatic, accurate and reliable quantification of soot morphology Df, enabling an improved understanding of soot aging processes and a more accurate modeling of soot impact on their climate.

Effect of Diesel Soot on the Heterogeneous Reaction of NO2 on the Surface of γ-Al2O3

Soot and aged soot are often found to be mixed with atmospheric particles, which inevitably affect various atmospheric heterogeneous reactions and secondary aerosol formation. Previous studies have investigated the heterogeneous reaction of NO2 with different types of soot, but there are few studies on the heterogeneous reaction of NO2 with mixtures containing diesel soot (DS) or aged DS and mineral dust particles. In this study, the effects of DS and aged DS on the heterogeneous reaction of NO2 on the surface of γ-Al2O3 were investigated via in-situ diffuse reflectance infrared Fourier transform spectrometry (DRIFTS). The results showed that the DS or DS n-hexane extract significantly inhibited the formation of nitrate on γ-Al2O3 particles and promoted the formation of nitrite. At 58% RH, with the increase of DS or DS n-hexane extract loading amount, the effect of DS or DS n-hexane extract on the formation of nitrate changed from promotion to inhibition, but DS or DS n-hexane extract always promoted the formation of nitrite. The results also showed that light was conducive to the formation of nitrate on the DS-γ-Al2O3 or DS-n-hexane extract-γ-Al2O3 particles. Furthermore, the influence of soot aging on the heterogeneous reaction of NO2 was investigated under light and no light. In the dark, O3-aged DS-γ-Al2O3 or O3-aged DS-n-hexane extract-γ-Al2O3 firstly inhibited the formation of nitrate on the mixed particles and then promoted it, while the effect of aged DS on nitrite formation was complex. Under light, the O3-aged DS-γ-Al2O3 firstly promoted the formation of nitrate on the mixed particles and then inhibited it, while the O3-aged DS-n-hexane extract-γ-Al2O3 promoted the formation of nitrate on the mixed particles. Our results further showed that the production of nitrate on the O3-aged particles under light or no light was greater than that of the UV-nitrate-aged particles. This study is helpful to deeply understand the atmospheric chemical behavior of soot and the heterogeneous conversion of atmospheric NO2.

Wetting properties of fresh urban soot particles: Evaluation based on critical supersaturation and observation of surface trace materials

Soot particles strongly absorb solar radiation and contribute to global warming. Also, wetting properties of soot at emission can affect its lifetime. We investigated surface conditions related to wetting and hydrophobic properties of fresh soot using data from measurements taken in Tokyo. A cloud condensation nuclei (CCN) counter was used to clarify surface conditions of particles composed mainly of water-insoluble (WI) materials: total and active particles as CCN around critical supersaturation (Sc) of 203-nm-diameter WI particles. Averaged number fractions of inactivated particles as CCN at 1.05% supersaturation (SS), which is Sc of hydrophilic WI particles, were estimated as 1.4%. Number fractions of inactive particles changed less at 1.78%SS during rush hour and increased at 0.89%SS, implying that most of the WI particles included small amounts of water-soluble (WS) materials rather than being completely hydrophobic. Based on transmission electron microscope (TEM) analysis of samples collected during rush hour, 69% of the mostly bare soot particles had Na or K small domains that are regarded as originating in fossil fuels. Based on water dialysis analysis results, some Na and K on soot were WS. Combination results with CCN measurements suggest that these WS materials decrease the Sc of soot. Moreover, the morphological structure of sulfate covering Na and K domains on the soot surface implicates pre-existing sodium and potassium compounds on soot as a trigger of soot aging. However, inactive particles at Sc at poor-hydrophilic particles and soot particles composed solely of WI materials on TEM samples were also found, although they were minor. Such particles, which are unfavorable for obtaining a wettable surface, might retain non-hygroscopicity for a longer period in the atmosphere. Evaluation of long-range soot transport can benefit from consideration of slight and inhomogeneous differences of chemical compounds on soot that occur along with their emission.

Future warming exacerbated by aged-soot effect on cloud formation

Clouds play a critical role in modulating the Earth’s radiation balance and climate. Anthropogenic aerosol particles that undergo aging processes, such as soot, aid cloud droplet and ice crystal formation and thus influence the microphysical structure of clouds. However, the associated changes in cloud radiative properties and climate effects remain uncertain and are largely omitted in climate models. Here we present global climate simulations of past and future effects of both ozone-aged soot particles acting as cloud condensation nuclei and sulfuric acid-aged soot particles acting as ice-nucleating particles on the structure and radiative effects of clouds. Under pre-industrial conditions, soot aging led to an increase in thick, low-level clouds that reduced negative shortwave effective radiative forcing by 0.2 to 0.3 W m−2. In the simulations of a future, warmer climate under double pre-industrial atmospheric carbon dioxide concentrations, soot aging and compensating cloud adjustments led to a reduction in low-level clouds and enhanced high-altitude cirrus cloud thickness, which influenced the longwave radiative balance and exacerbated the global mean surface warming by 0.4 to 0.5 K. Our findings suggest that reducing emissions of soot particles is beneficial for future climate, in addition to air quality and human health. Aged soot particles in the atmosphere enhance future warming through their influence on cloud formation and thus Earth’s radiation balance, according to global climate simulations.

Hydrophilic properties of soot particles exposed to OH radicals: A possible new mechanism involved in the contrail formation

This work investigates the role of soot particles in the early steps of formation of condensation trails (contrails). Contrails are thin linear clouds that form behind cruising aircrafts and can evolve into persistent clouds, and therefore can contribute to the radiative forcing of the atmosphere. Mitigating their contribution is considered an efficient way for decreasing the radiative forcing. Condensation freezing is proposed as a major pathway leading to the heterogeneous nucleation of ice particles in the aircraft exhausts. Soot particles, freshly produced in the flames and not significantly oxidized in the post-flame, are hydrophobic. Therefore, many theoretical and experimental approaches that investigate contrails formation introduce prior activation by surface adsorption of sulfur compounds issued from kerosene combustion. Even though the role of sulfur content of the fuel is unclear, and may only weakly impacts contrails formation, many simulations still rely on this assumption. In this work, to elucidate the role of sulfur compounds on the activation of soot chemically aged with OH radicals, a comparative study is performed with soot sampled from a turbulent jet flame burning a sulfur-containing fuel (kerosene) and a sulfur-free fuel (diesel). Soot aging experiments are performed in the atmospheric simulation chamber CESAM, with controlled generation of OH radicals. Soot activation (ratio between the number of nucleated droplets and seeding particles) is measured in water supersaturation conditions. OH exposure significantly enhances soot activation regardless of the sulfur presence in the fuel. The possibility for soot oxidation by OH radicals formed in aeronautical engines to be sufficient to promote soot activation is explored. Large eddy simulation in the high pressure turbine is used to model the fluid particle and OH trajectories and their chemical evolution. Residence time in the turbine is found sufficient to activate soot particles, opening a new possible route to explain ice particles formation.

Effects of pressure on soot formation in laminar coflow methane/air diffusion flames doped with n-heptane and toluene between 2 and 8 atm

There is currently a lack of adequate understanding of soot formation in flames fueled with liquid hydrocarbons at elevated pressures. In this study, laminar coflow n-heptane and toluene doped CH4/air diffusion flames were numerically investigated under a constant carbon mass flow rate at pressures between 2 and 8 atm to understand how pressure affects the sooting propensity of these two main components of surrogate fuels mimicking gasoline. Numerical simulations were performed using a detailed reaction mechanism containing 175 species and 1175 reactions and a sectional soot model. Soot inception is modeled by collisions among pyrene, BAPYR and BGHIF. Soot surface growth and oxidation are modeled using the hydrogen abstraction acetylene addition (HACA) mechanism as well as PAH surface condensation. The predicted sensitivity of soot production to pressure is in better agreement with the measurements of Daca and Gülder (2017) when the soot aging effect is considered in HACA surface growth. The predicted soot concentrations are in overall good agreement with measurements. Propargyl recombination and propargyl reaction with propyne are important pathways for the formation of benzene. In methane+toluene flames, attack of toluene by H radical is an effective benzene formation pathway low in the flame, but the relative importance of benzene formation from toluene is reduced with increasing pressure. Although all the soot formation processes are enhanced with increasing pressure, PAH condensation is enhanced the most, followed by HACA and inception. At 6 and 8 atm, PAH condensation becomes comparable to HACA. The pressure dependence of the sooting propensity follows the order of methane+toluene < methane+n-heptane < methane, consistent with measurements. This result can be explained by the pressure dependence of benzene formation pathways, the kinetic effect of pressure, and the scrubbing effect of soot production on the gas-phase species involved in soot formation.

Absorption and scattering of light by soot aggregates with uniform and pendular ring coatings

The magnitude of radiative forcing by soot is strongly dependent on its morphology and mixing state, which continuously evolve during atmospheric aging. One of the major soot aging mechanisms is via interaction with condensable trace-gas chemicals, and our previous study has shown the formation of two distinct coating distributions on fractal soot aggregates, primarily depending on the radius of monomer spheres and supersaturation of the condensing vapor. The two coating distributions, spherical shells around monomers and pendular rings in junctions between monomers, differ profoundly in their ability to restructure soot aggregates. Here, we investigate the impact of these two distributions on the optical properties of soot, using Discrete Dipole Approximation (DDA) calculations, which are also compared against the Rayleigh Debye Gans (RDG) approximation and the core-shell model. For most optical parameters, the difference between the two coating distributions was minor, except for the asymmetry parameter, where it was significant (5–13%). We found that the placement of condensate in junctions significantly depressed the asymmetry parameter for most coating volumes, thus promoting isotropic scattering by soot. Compared to a uniform condensate distribution, the presence of coating between monomers always resulted in slightly larger absorption, scattering and single scattering albedo (SSA), with the difference reaching ~10% for scattering at high coating volumes. We found that the RDG approximation underestimated absorption, but was able to predict scattering to within ~3%. On the other hand, the core-shell model significantly overestimated absorption, scattering, and SSA, especially for thinly coated soot.

Photochemical Aging of Soot in the Aqueous Phase: Release of Dissolved Black Carbon and the Formation of 1O2.

The photochemical aging of soot in the aqueous phase could have an important influence on water environments such as fog water and wet aerosols in the atmosphere, as well as lakes and oceans. In this study, we systematically investigated the photochemistry of soot in the aqueous phase. Soot releases dissolved black carbon into the aqueous phase during photoreactions, which is attributed to the phototransformation of the nonpolar unsaturated C-H species in soot to polar carbonyl-containing species. More importantly, we found that soot suspensions, particularly those of the dissolved part of soot, were effective photosensitizers for the generation of singlet oxygen (1O2). The 1O2 apparent quantum yield of the dissolved part reached 33 ± 2% under 377 nm irradiation, which is an order of magnitude higher than those of most types of well-studied dissolved organic matter in water. As a result, soot could impact the environmental fate of coexisting organic contaminants, such as the photodegradation of bisphenol A. This study will not only give insight into the photochemistry of soot in the liquid phase but also reveal the significant implications of soot photoaging in the aqueous phase by the release and degradation of organic matter.

Mixing State and Fractal Dimension of Soot Particles at a Remote Site in the Southeastern Tibetan Plateau.

The mixing state and fractal dimension (Df) of soot particles are two major factors affecting their absorption capacity and their climate effects. Here we investigated these factors of soot particles found in a typical valley of the southeastern Tibetan Plateau where wood burning in local villages was one major source of soot particles. Our motivation revealed Df and the aging property of soot particles in remote air and discussed their regional climatic implications. We found that 64% of total analyzed particles by number were soot-bearing particles and most of them aged with sulfate or organic coating. The Df sequence is bare-like soot (1.75 ± 0.08) < partly coated soot (1.82 ± 0.05) < embedded soot (1.88 ± 0.05). The aging process enlarged the overall size of the soot-bearing particles and increased the compactness of soot. Soot aging critically depended on high relative humidity (RH) during nighttime. Besides emission sources and coating processes, the coating aerosol phase under different RHs is another important factor affecting the soot Df.

Laser-induced structural modifications of differently aged soot investigated by HRTEM

Laser heating was applied to soot probed in the first inception region (nascent soot) and downstream (mature soot) of premixed methane and ethylene flames, burning in similar temperature and equivalence ratio conditions. Different properties of nascent and mature soot were investigated by TEM/HRTEM and EELS along with elemental analysis, UV–visible and Raman spectroscopy. Structural modifications were followed by HRTEM imaging applied to the soot before and after laser heating. Laser heating produced varied modifications of the lamellae organization dependent upon the morphology of pristine soot, in turn dependent on soot aging and fuel type (ethylene and methane). Specifically, structures in form of void shells versus multifaceted rosette structures were observed in the case of young nascent and mature soot, respectively, showing a relationship with their different nanostructures, namely amorphous versus disordered turbostratic structures. Additionally, the differentiation between void shells or multifaceted rosette structures can be used as signature for the occurrence of laser heating effects, which have to be avoided when the laser is instead employed for diagnostic purposes.

Understanding the formation and growth of polycyclic aromatic hydrocarbons (PAHs) and young soot from n-dodecane in a sooting laminar coflow diffusion flame

The mechanistic pathways of PAH formation and growth remain uncertain. In addition, our current understanding of the transformation of PAHs into young soot is limited. The PAHs participating in this transformation remain ambiguous. Simultaneous measurements of PAHs and particles are necessary to better understand how particulates are formed in flames. A comprehensive analysis has been performed on a n-dodecane doped methane coflow diffusion flame. PAHs have been analyzed with GC/MS while the particulates have been collected from the flame centreline for studying under a Transmission Electron Microscope (TEM). The soot measurements from a previous study have been used to complete the comprehensive analysis. The novelty of the current study lies in the fact that it provides quantitative information on the growth of PAHs ranging from naphthalene (A2) to pyrene and fluoranthene (together they are referred to as A4 in this study) and how these species participate in the formation, growth and aging of the young soot. The experimental results show that before the commencement of the young soot, the gaseous phase is dominated by species equal to and smaller than A4. PAHs larger than A4 were not detected. With the growth of the young soot, the mole fraction of A4 decreases. The results suggest that small and medium-sized PAHs (A4 and smaller) are responsible for the formation and growth of the young soot. As the young soot transform into mature solid soot, A4 increases abruptly. The study shows that PAH growth and soot maturity are not mutually exclusive. The target flame has also been simulated numerically to identify the problems associated with the state-of-the-art model. The numerical model with irreversible nucleation can not capture the formation of young soot even though it considers nucleation from A4. A comprehensive database of PAHs and soot has been created for future numerical model validations.

Role of elemental carbon in the photochemical aging of soot.

Soot, which consists of organic carbon (OC) and elemental carbon (EC), is a significant component of the total aerosol mass in the atmosphere. Photochemical oxidation is an important aging pathway for soot. It is commonly believed that OC is photoactive but EC, albeit its strong light absorption, is photochemically inert. Here, by taking advantage of the different light absorption properties of OC and EC, we provide direct experimental evidence that EC also plays an important role in the photochemical aging of soot by initiating the oxidation of OC, even under red light irradiation. We show that nascent soot, in addition to undergoing photochemical oxidation under blue light with a wavelength of 440 nm, undergoes similar oxidation under red light irradiation of λ = 648 nm (L648). However, separated OC (extracted from soot by n-hexane) and EC exhibit little reactivity under L648 These observations indicate that EC plays a pivotal role in photoaging of soot by adsorbing light to initiate the oxidation of OC. Comparison of in situ IR spectra and photoelectrochemical behaviors suggests that EC-initiated photooxidation of OC proceeds through an electron transfer pathway, which is distinct from the photoaging induced by light absorption of OC. Since the absorption spectra of EC have a much larger overlap with the solar spectra than those of OC, our results provide insight into the chemical mechanism leading to rapid soot aging by organic species observed from atmospheric field measurements.

Effects of Flame Configuration and Soot Aging on Soot Nanostructure and Reactivity in n-Butanol-Doped Ethylene Diffusion Flames

Soot has received considerable attention since it is a major pollutant in exhaust gas from fossil fuel combustion and it causes adverse climate and health effects. This work focused on soot morphology, nanostructure, and reactivity variations regarding the soot collected in different flame configurations of n-butanol-doped ethylene inverse diffusion flame (IDF) and normal diffusion flame (NDF) at different heights above burner surface (HAB = 20, 30, and 40 mm). The effects of flame configuration and soot aging on structural and reactivity characteristics were analyzed with emphasis on the differences of young and mature soot generated in IDF and NDF without and with n-butanol addition at specific positions, respectively. The effects of fuel-side n-butanol addition on IDF and NDF soot structures and reactivity were also evaluated. The soot obtained using thermophoretic sampling technique was analyzed by transmission electron microscopy (TEM) to investigate soot morphology evolutions along the centerline an...