Primary Soot Particles Research Articles

Aircraft soot from conventional fuels and biofuels during ground idle and climb-out conditions: Electron microscopy and X-ray micro-spectroscopy

Aircraft soot has a significant impact on global and local air pollution and is of particular concern for the population working at airports and living nearby. The morphology and chemistry of soot are related to its reactivity and depend mainly on engine operating conditions and fuel-type. We investigated the morphology (by transmission electron microscopy) and chemistry (by X-ray micro-spectroscopy) of soot from the exhaust of a CFM 56-7B26 turbofan engine, currently the most common engine in aviation fleet, operated in the test cell of SR Technics, Zurich airport. Standard kerosene (Jet A-1) and a biofuel blend (Jet A-1 with 32% HEFA) were used at ground idle and climb-out engine thrust, as these conditions highly influence air quality at airport areas. The results indicate that soot reactivity decreases from ground idle to climb-out conditions for both fuel types. Nearly one third of the primary soot particles generated by the blended fuel at climb-out engine thrust bear an outer amorphous shell implying higher reactivity. This characteristic referring to soot reactivity needs to be taken into account when evaluating the advantage of HEFA blending at high engine thrust. The soot type that is most prone to react with its surrounding is generated by Jet A-1 fuel at ground idle. Biofuel blending slightly lowers soot reactivity at ground idle but does the opposite at climb-out conditions. As far as soot reactivity is concerned, biofuels can prove beneficial for airports where ground idle is a common situation; the benefit of biofuels for climb-out conditions is uncertain.

Characterizing particulate matter emissions in an aviation kerosene-fueled model combustor at elevated pressures and temperatures

The present study investigated the characteristics of particulate matter (PM) emissions in a single-dome, Rich-Quench-Lean, model combustor under different operating conditions typical for aero-engines, with special emphasis on identifying the individual effects of pressure, temperature, and equivalence ratio on PM emissions in aero-combustors. Results showed that the number-based particle size distribution was shifted from the nucleation mode (<50 nm) to the accumulation mode (50–1000 nm) as the dome equivalence ratio increased from 0.944 to 1.267. When the dome equivalence ratio further increased from 1.342 to 1.814, the number distribution was shifted to smaller particle sizes that are dominated by the nucleation mode. With the increase of operating pressure, the PM emissions have a notably higher number concentration with smaller size particles. While increasing pressure can significantly enhance the nucleation rate yielding higher primary soot particle concentration, the hydrogen radicals that play a crucial role in the hydrogen-abstraction/carbon-addition soot surface growth mechanism are suppressed, thereby leading to the reduced surface growth rate and smaller particle size. On the other hand, the overall accumulation mode seems not being affected with increasing pressure, resulting in a similar size distribution beyond Dp > 60 nm for the different pressure cases investigated. Regarding the influence of combustion temperature, a non-monotonic response of particle geometric mean diameter with increasing primary zone average temperature was found in that particle size increased and decreased in the temperature range below and above 2270 K respectively. This non-monotonic response with temperature variation is likely caused by the competition between soot growth rate and oxidation rate. It is postulated that the increasing trend of particle size is a result of more carbonaceous particles formed under elevated temperatures providing surface areas available for the condensation of organic compounds, while the soot oxidation rate eventually overtakes at higher temperatures leading to the observed decreasing trend.

The evolution of soot morphology and nanostructure along axial direction in diesel spray jet flames

This study aims at providing new insights into the soot formation and oxidation processes in diesel spray jet flames under the diesel-like conditions. Soot particles are sampled within a reacting diesel jet spray flame in a constant-volume combustion chamber under diesel-like high-temperature and pressure ambient conditions. The experiments are conducted with an injector hole of 0.28 mm and injection mass of 30 mg per shot. Soot particles at different locations along the axis of the diesel jet flame are sampled by a thermophoretic probe, and the variation of soot morphology and nanostructure of the sampled soot particles along the axial location in the flame is analyzed by using the transmission electron microscopy (TEM). At upstream of the jet flame, large “liquid-like” particles are observed in the TEM images. As these particles are delivered to the downstream, (1) there are many immature primary particles formed on the surface of large “liquid-like” particles, and (2) they are carbonized into the larger soot aggregates with the matured structure, and then at last, (3) the larger soot aggregates gradually break up into the smaller particles with relatively compact structure. Both the diameter of primary particles and radius of gyration of the soot aggregates decrease as the distance between the injector nozzle tip and the center of TEM grid is increased, while the opposite trend is found in the behavior of the fractal dimensions of soot aggregates. The high resolution TEM images show that the soot primary particles exhibit the turbostratic state at the upstream of the jet flame, while more matured structures with the graphitic crystallite in outer shell and several fine particles in the inner core are observed as the sampling distance from orifice is increased. The average fringe length increases while the average tortuosity and separation decrease with the distance from the nozzle. These results suggest that the mechanism of mature soot aggregates generated directly from the large “liquid-like” particles should be one of the ways of soot particle formation along the axis of diesel jet flames.

Single-camera, single-shot, time-resolved laser-induced incandescence decay imaging.

Knowledge of soot particle sizes is important for understanding soot formation and heat transfer in combustion environments. Soot primary particle sizes can be estimated by measuring the decay of time-resolved laser-induced incandescence (TiRe-LII) signals. Existing methods for making planar TiRe-LII measurements require either multiple cameras or time-gate sweeping with multiple laser pulses, making these techniques difficult to apply in turbulent or unsteady combustion environments. Here, we report a technique for planar soot particle sizing using a single high-sensitivity, ultra-high-speed 10MHz camera with a 50ns gate and no intensifier. With this method, we demonstrate measurements of background flame luminosity, prompt LII, and TiRe-LII decay signals for particle sizing in a single laser shot. The particle sizing technique is first validated in a laminar non-premixed ethylene flame. Then, the method is applied to measurements in a turbulent ethylene jet flame.

Effects of carbon dioxide addition to fuel on soot evolution in ethylene and propane diffusion flames

The influence of carbon dioxide addition to the fuel on soot evolution in ethylene and propane diffusion flames was studied by optical diagnostics. The mole fraction of CO2 addition ranged from 0 to 0.5, while the flow rate of the fuel gas was kept constant for these two sets of flames. Spatial distributions of polycyclic aromatic hydrocarbons (PAHs), temperature, as well as volume fraction, primary particle size and number density of soot were observed by the methods of laser-induced fluorescence (LIF), ratio pyrometry and laser-induced incandescence (LII), respectively. It was found that the flame height decreased for ethylene flames while it was nearly constant for propane flames with increasing addition of CO2. The measurements showed a temperature reduction in the lower part but an increase in the upper part in the ethylene-based flames. By contrast, a slight temperature decrease was observed in overall propane-based flames with the addition of CO2. Similar suppression effects were observed in the total soot/PAHs loading, percentage of carbon conversion to soot, and the total number of primary soot particles regardless of the fuel type. Comparison between the total loading of soot and PAHs indicated that addition of CO2 inhibited the conversion of PAHs to soot. The results also showed that the addition of CO2 in the fuel had a small effect on the specific growth rate of soot regardless of the fuel type. Relative changes of particle surface area could reasonably well explain the shift in the peak volume fraction from the wings to the centerline with the addition of CO2 to the ethylene flames.

Experimental investigation of soot morphology and primary particle size along axial and radial direction of an ethylene diffusion flame via electron microscopy

The effect of oxygen concentrations on the formation and evolution of soot particles was investigated by analyzing soot morphology using SiC fiber deposition technique and thermophoretic sampling method in a co-flow diffusion ethylene flame. Soot particles were examined via transmission electron microscopy at different heights along the flame centerline. Results show that the flame temperature exhibits a bimodal distribution. As the flame height increases, the flame distribution gradually changes from bimodal to unimodal, and the increase in oxygen concentration not only causes the flame height to decrease, but also causes the bimodal distribution of the flame more pronounced. The morphological evolution of soot deposits are strongly dependent on the oxygen concentration, radial and axial position, and flame temperature. Soot deposits along the flame centerline begin to be oxidized into dense flocculent and fibrous mesh structures with the flame temperature increases. In front of the flame, the oxidation is enhanced with temperature rise at the same height, resulting in more dense morphology of the soot deposits and the decrease in primary particle size. The results of thermophoretic sampling show that soot growth undergoes various stages of nucleation, growth, coagulation, agglomeration and oxidation, and the average particle size distributions of soot increase first and then decrease. The increase in oxygen concentration leads to advances in all stages of soot formation, including surface growth, agglomeration and oxidation. Additionally, the flame temperatures increase sharply as the increase of flame heights, leading to the soot aggregates to be oxidized to loose chain-like agglomerates.

Soot evolution and flame response to acoustic forcing of laminar non-premixed jet flames at varying amplitudes

New details regarding the soot evolution and its controlling parameters in steady and forced flames have been studied using high spatial resolution laser diagnostic techniques. Steady laminar non-premixed ethylene/nitrogen flames with three different diameters burners were acoustically forced using a loudspeaker. 10-Hz-sinusoidal signals of different amplitudes were transmitted to the loudspeaker to drive the flames. The results reveal that the spatial correlation between the soot field and the temperature profile is influenced by the burner diameter and forcing conditions. The soot field in steady laminar flames is confined to a relatively narrow temperature range, 1500–2000 K. In contradiction, the soot field in forced flames spread across a wider range of temperature, 1400–2100 K. Furthermore, the spatial correlation between the normalised soot concentration and primary particle size can be described with an exponential function. While it is observed that the exponential coefficients vary with burner diameter and forcing conditions, further study is necessary for a better understanding. In general, laminar flames forced at a lower amplitude (α=25%) tend to produce less soot than moderately forced (α=50%) flames. Further increasing the forcing amplitude to α=75% does not increase the soot production in laminar flames; conversely, lower peak and volume-integrated soot volume fraction are observed in the strongly forced flame (α=75%) as relative to the moderately forced counterpart. These findings shed new light on the seemingly contradictory results published in the literature regarding the effect of the forcing intensity on the soot production.

Brownian dynamics simulation of soot primary particle aggregation in laminar ethylene diffusion flames

The aggregation of soot primary particles along the centerline of laminar ethylene diffusion flames is simulated using the Brownian dynamics method incorporating the cluster–cluster aggregation model. Unlike previous studies, this simulation approach takes more key parameters into account, such as the temperature, dynamic viscosity and velocity of the flame gas, the primary particle diameter distribution and the soot volume fraction. Given that the change in the O2 concentration would greatly affect these simulation parameters, three aggregation simulations for flames at different O2 concentrations are implemented to check the applicability of this simulation approach for different sets of simulation parameters. These simulations dynamically display the soot morphology evolution from isolated mature primary particles to fractal-like aggregates along the flame centerline, and simulation results indicate that the fractal properties of simulated soot aggregates are almost identical, yielding a fractal dimension of 1.70-1.74 and a fractal prefactor of 2.13-2.23, and the mean number of primary particles per simulated soot particle is in the range of 1-83. Moreover, as the O2 concentration increases from 21% to 40%, the simulated height of the soot aggregation region along the flame centerline decreases from 25 mm to 10 mm, and the simulated time of soot aggregation reduces from 18.5 ms to 8.8 ms. On the whole, these simulation results agree well with the experimental data.

Understanding in-cylinder soot reduction in the use of high pressure fuel injection in a small-bore diesel engine

This study shows how soot particles inside the cylinder of the engine are reduced due to high pressure fuel injection used in a light-duty single-cylinder optical diesel engine fuelled with methyl decanoate, a selected surrogate fuel for the diagnostics. For various injection pressures, planar laser induced incandescence (PLII) imaging and planar laser-induced fluorescence of hydroxyl (OH-PLIF) imaging were performed to understand the temporal and spatial development of soot and high-temperature flames. In addition, a thermophoresis-based particle sampling technique was used to obtain transmission electron microscope (TEM) images of soot aggregates and primary particles for detailed morphology analysis. The OH-PLIF images suggest that an increase in the injection pressure leads to wider distribution of high-temperature flames likely due to better mixing. The enhanced high-temperature reaction can promote soot formation evidenced by both a faster increase of LII signals and larger soot aggregates on the TEM images. However, the increased OH radicals at higher injection pressure accelerates the soot oxidation as shown in a higher decreasing rate of LII signals as well as dramatic reduction of the sampled soot aggregates at later crank angles. The analysis of nanoscale carbon layer fringe structures also shows a consistent trend that, at higher injection pressure, the soot particles are more oxidized to form more graphitic carbon layer structures. Therefore, it is concluded that the in-cylinder soot reduction at higher injection pressure conditions is due to enhanced soot oxidation despite increased soot formation.

Experimental investigation of soot evolution in a turbulent non-premixed prevaporized toluene flame

The formation, growth, and oxidation of soot in turbulent prevaporized toluene diffusion flames stabilized on a jet-in-hot-coflow (JHC) burner are investigated in this study. Flame structure, local gas temperature as well as local soot volume fraction and primary soot particle diameter, are simultaneously detected by means of OH planar laser-induced fluorescence (PLIF), non-linear excitation regime two-line atomic fluorescence (nTLAF) of indium, and time-resolved (TiRe) laser-induced incandescence (LII), respectively. The collected data sets were used to generate joint statistics of soot properties and flame characteristics and provided new insights into the interaction of the OH layer and soot in turbulent flames. The interaction of OH and soot as a driving mechanism for soot oxidation is of particular interest as it has been proven to be challenging to model. Statistics of soot volume fraction and primary particle size in the OH layer are employed to gain deeper insights into the soot oxidation process. Mean soot volume fraction and primary soot particle size conditioned on temperature and OH signal intensity indicate that, due to differential diffusion of soot with respect to the chemical species, high soot volume fraction and primary soot particle diameter of up to 50 nm are present at low temperatures and low OH concentration. In the soot oxidation region, statistical analysis of the soot parameters disclose that clusters of high soot volume fraction mostly consist of large primary particles. Observations from instantaneous images and the presence of large primary particles inside the OH layer suggest that the oxidation is not sufficiently fast to burn the soot completely.

Determining fractal properties of soot aggregates and primary particle size distribution in counterflow flames up to 10 atm

Abstract Experimental investigations of soot morphology are performed in counterflow flames of N2-diluted ethylene and air, up to 10 atm. A thermophoretic sampling device is attached to a pressure vessel containing a counterflow burner where flames with an ethylene mole fraction of 0.3 are stabilized at 3, 5, and 6 atm. To allow measurements at higher pressures, the fuel mole fraction is lowered to 0.2 to reduce the soot loading and flames are studied at 5, 7, and 10 atm. Thermophoretic sampling of the soot zone is performed using TEM grids. The sampling process causes minimal flame disturbances. Soot collected on TEM grids is analyzed under transmission electron microscope (TEM). Primary particle size distributions are inferred at each pressure by manually analyzing the primary particles from TEM images. Fractal properties of soot at each pressure are also obtained by analyzing the TEM images at comparatively low magnifications. Mean primary particle diameter increases from 17.5 to 47.1 nm as the pressure is increased from 3 to 10 atm, whereas the fractal dimension and prefactor do not change with pressure up to 10 atm. For the flames studied here, fractal dimension lies between 1.61 and 1.67 whereas fractal prefactor varies between 1.68 and 1.86 without following any apparent trend with pressure.

The role of reactive PAH dimerization in reducing soot nucleation reversibility

Abstract Nucleation of incipient soot or carbon black nanoparticles has significant scientific and industrial importance because it influences the particle size distribution, morphology and composition, hence its health and environmental impact as well as functional properties. R eversible P olycyclic Aromatic Hydrocarbon (PAH) C lustering (RPC) with van der Waals forces (vdW) was proposed recently as opposed to I rreversible P AH C lustering (IPC) for soot nucleation (Eaves et al., Proc. Combust. Inst, 35 (2015) 1787) to relax the assumption of stable dimer formation with physical bonds at flame temperatures. Here, the necessity of considering chemical bond formation between PAHs in a dimer for reducing soot nucleation reversibility in ethylene coflow diffusion flames with a wide range of nitrogen dilution ratios is demonstrated. An RPC model with C hemical B ond F ormation (RPC CBF) is developed and its performance is compared to those of RPC and IPC models. Only the RPC CBF model with low reversibility for PAH addition on the surface of soot primary particles can predict soot volume fraction, average primary particle diameter, primary particle number density and the bimodal size distribution of soot agglomerates on the flame centerline within experimental uncertainty. While the IPC model overpredicts soot concentrations by a factor of five with increased dilution, the RPC model underpredicts it by more than two orders of magnitude for the flame with 68% nitrogen dilution. The RPC model fails to predict the observed bimodality of agglomerate particle size distribution in flames with low dilution because its predicted nucleation rate is much weaker compared to that of growth. Accounting for chemical bond formation for dimers is essential to form enough nuclei with increasing dilution and temperature. Also, low reversibility for PAH addition is required to predict the balance between nucleation and surface growth, hence the average size of soot primary particles.

In-flame soot particle structure on the up- and down-swirl side of a wall-interacting jet in a small-bore diesel engine

This study shows how the structure of soot particles within the flame changes due to the relative direction of the swirl flow in a small-bore diesel engine in which significant flame–wall interactions cause about half of the flame travelling against the swirl flow while the other half penetrating in the same direction. The thermophoresis-based particle sampling method was used to collect soot from three different in-flame locations including the flame–wall impingement point near the jet axis and the two 60° off-axis locations on the up-swirl and down-swirl side of the wall-interacting jet. The sampled soot particle images were obtained using transmission electron microscopes and the image post-processing was conducted for statistical analysis of size distribution of soot primary particles and aggregates, fractal dimension, and sub-nanoscale parameters such as the carbon layer fringe length, tortuosity, and spacing. The results show that the jet-wall impingement region is dominated by many small immature particles with amorphous internal structure, which is very different to large, fractal-like soot aggregates sampled from 60° downstream location on the down-swirl side. This structure variation suggests that the small immature particles underwent surface growth, coagulation and aggregation as they travelled along the piston-bowl wall. During this soot growth, the particle internal structure exhibits the transformation from amorphous carbon segments to a typical core–shell structure. Compared to those on the down-swirl side, the soot particles sampled on the up-swirl side show much lower number counts and more compact aggregates composed of highly concentrated primary particles. This soot aggregate structure, together with much narrower carbon layer gap, indicates higher level of soot oxidation on the up-swirl side of the jet.

Experimental and numerical study of the evolution of soot primary particles in a diffusion flame

The evolution of primary soot particles is studied experimentally and numerically along the centreline of a co-flow laminar diffusion flame. Soot samples from a flame fueled with C2H4 are taken thermophoretically at different heights above the burner (HAB), their size and nano-structure are analysed through TEM. The experimental results suggest that after inception, the nascent soot particles coagulate and coalesce to form larger primary particles ( ∼ 5 to 15 nm). As these primary particles travel along the centreline, they grow mainly due coagulation and condensation and a layer of amorphous hydrocarbons (revealed by HRTEM) forms on their surface. This amorphous layer appears to promote the aggregation of primary particles to form fractal structures. Fast carbonisation of the amorphous layer leads to a graphitic-like shell around the particles. Further graphitization compacts the primary particles, resulting in a decrease of their size. Towards the flame tip the primary particles decrease in size due to rapid oxidation. A detailed population balance model is used to investigate the mechanisms that are important for prediction of primary particle size distributions. Suggestions are made regarding future model development efforts. Simulation results indicate that the primary particle size distributions are very sensitive to the parameterization of the coalescence and particle rounding processes. In contrast, the average primary particle size is less sensitive to these parameters. This demonstrates that achieving good predictions for the average primary particle size does not necessarily mean that the distribution has been accurately predicted.

Nanostructure and reactivity of soot produced from a turbodiesel engine using post injection

Post injection is commonly used for triggering regeneration of exhaust aftertreatment devices such as diesel particulate filters and NOx adsorber catalysts. Post injection is also a method to reduce engine-out particulate matter (PM) emissions while maintaining efficiency, to reduce or potentially eliminate exhaust aftertreatment. This study explores post injection in a light duty multi-cylinder turbodiesel engine to reduce PM. The impact of post injection dwell and duration on soot reduction and reactivity are investigated in this study. Soot samples were collected from the engine exhaust and subjected to analyses of oxidative reactivity and nanostructure of the primary soot particles. Close-coupled post injections conditions reduce soot emissions by 11–21%, while long-dwell post injection conditions reduce soot emissions by 28–33%. The reaction rate constants of soot collected from close-coupled post injection conditions decrease by 10–13% compared to baseline condition, while the reaction rate constants of soot collected from long dwell post injection conditions increase by 37–39% compared to the baseline condition. The difference in soot reactivity indicates fuel from the post injection interacts with the main injection flame. Moreover, with the increase of injection dwell and post injection size, soot surface oxygen content and amorphous carbon content increase. This explains the change in reactivity of soot from different injection dwells. Primary soot particle and particle aggregate sizes do not vary much with post injection. Soot from post injection conditions show shrinking-core-type oxidation without graphene layer rearrangement.

Soot aggregate morphology in coflow laminar ethylene diffusion flames at elevated pressures

The effect of combustion pressure on soot aggregate morphology and on primary soot particle diameter was investigated by thermophoretic sampling in nitrogen-diluted ethylene flames. Soot aggregate samples were collected using a high-pressure thermophoretic sampling system installed inside the high-pressure combustion chamber. Collected samples were imaged by transmission electron microscopy followed by an automated image analysis process to yield aggregate morphology information. The experiments covered pressures from 3 to 6 bar, and samples were collected at heights of 2 and 5 mm above the burner exit in nitrogen-diluted ethylene (ethylene to nitrogen mole ratio 2:1) flames with nominal visible flame heights of about 20 mm. It was observed that average primary soot particle diameter increased with increasing pressure within the pressure range considered, at both measurement locations within the flame. Fractal parameters of the soot aggregates were inferred directly from the experimental data (as opposed to the usual practice of assuming a priori values for fractal dimension and pre-factor). Near to the soot nucleation and growth regions of the co-flow laminar diffusion flames, at 2 mm above the burner exit, the fractal dimension of the aggregates was not as high as the accepted universal value of about 1.9, in spite of the fact that number of primary particles per aggregate is quite high and pressure is well above atmospheric. At 5 mm above the burner exit, however, the fractal dimension of the soot aggregates approached the accepted universal value of 1.9, at high pressures; at relatively lower pressures, the fractal dimension was about 1.6 at the same location within the flame. In view of the inferred fractal results, caution should be exercised when applying fractal analysis to low heights in flames at lower pressures where soot inception and growth are relatively dominant.

Soot and flow field in turbulent swirl-stabilized spray flames of Jet A-1 in a model combustor

Soot production processes and their interaction with the swirling flow field were studied experimentally in turbulent spray flames in a model gas turbine combustor. Two Jet A-1/air spray flames with two different air flow rates both with constant fuel flow corresponding to a thermal power of 10 kW were stabilized in the combustor with a swirl number of 0.55. Soot measurements were collected using auto-compensating laser-induced incandescence and velocity measurements were obtained using stereoscopic particle image velocimetry techniques. The particle image velocimetry displayed a distinct change in spray pattern from a V-shaped hollow cone to a V-shaped solid cone when the air flow rate was increased. The first soot event was not detected until 30 mm above the combustor inlet was reached, which was expected as it was necessary for liquid fuel to disperse and evaporate. The flow field featured strong inner and outer recirculation zones as well as the inner and outer shear layers. Soot concentration distributions were found to be confined to the outside of the inner recirculation zone; this is in contrast to the gaseous fuel swirl-stabilized flames in which soot particles are mainly detected within the inner recirculation zone. Time-averaged maximum soot volume fraction level decreased by about 60% with a 20% increase in air mass flow rate instigated by enhanced oxidation rate and turbulent mixing. The primary soot particle size was found to fall in the range of 30 and 60 nm for both cases. The results obtained emphasize the role played by the intermittent structures and air flow rate on soot processes in swirling spray combustion. Observed soot distributions in liquid spray flames in this work differ drastically from those reported previously for gaseous fuel combustion in swirl-stabilized flames.

Informing TiRe-LII assumptions for soot nanostructure and optical properties for estimation of soot primary particle diameter

Time-resolved laser-induced incandescence for primary particle size determination is tested using three model carbon blacks. Optical properties change as does the nanostructure upon laser annealing, whereas aggregate morphology and primary particle size remain equivalent to the original material, as shown by transmission electron microscopy (TEM). Primary particle diameters found from fitting experimentally measured time-resolved laser-induced incandescence (LII) signals with existing models do not match the particle diameters as directly visualized by TEM. The accommodation coefficient is shown to be a crucial parameter which can result in substantial variations in simulated conductive cooling profiles for particle sizing. Aggregate structure in the form of intra-aggregate connectivity and shielding is an additional underlying cause for erroneous particle sizing, not presently captured by LII models.

Aggregated particles caused by instrument artifact

Abstract. Previous studies have indicated that superaggregates, clusters of aggregates of soot primary particles, can be formed in large-scale turbulent fires. Due to lower effective densities, higher porosity, and lower aerodynamic diameters, superaggregates may pass through inlets designed to remove particles &lt; 2.5 µm in aerodynamic diameter (PM2.5). Ambient particulate matter samples were collected at Peavine Peak, NV, USA (2515 m) northwest of Reno, NV, USA from June to November 2014. The Teledyne Advanced Pollution Instrumentation (TAPI) 602 BetaPlus particulate monitor was used to collect PM2.5 on two filter types. During this time, aggregated particles &gt; 2.5 µm in aerodynamic diameter were collected on 36 out of 158 sample days. On preliminary analysis, it was thought that these aggregated particles were superaggregates, depositing past PM10 (particles &lt; 10 µm in aerodynamic diameter) pre-impactors and PM2.5 cyclones. However, further analysis revealed that these aggregated particles were dissimilar to superaggregates observed in previous studies, both in morphology and in elemental composition. To determine if the aggregated particles were superaggregates or an instrument artifact, samples were investigated for the presence of certain elements, the occurrence of fires, high relative humidity and wind speeds, as well as the use of generators on site. Samples with aggregated particles, referred to as aggregates, were analyzed using a scanning electron microscope for size and shape and energy dispersive X-ray spectroscopy was used for elemental analysis. It was determined, based on the high amounts of aluminum present in the aggregate samples, that a sampling artifact associated with the sample inlet and prolonged, high wind events was the probable reason for the observed aggregates.

Morphology and nano-structure analysis of soot particles sampled from high pressure diesel jet flames under diesel-like conditions

Soot particles emitted from diesel engines have a significant impact on the atmospheric environment. Detailed understanding of soot formation and oxidation processes is helpful for reducing the pollution of soot particles, which requires information such as the size and nano-structure parameters of the soot primary particles sampled in a high-temperature and high-pressure diesel jet flame. Based on the thermophoretic principle, a novel sampling probe minimally disturbing the diesel jet flame in a constant volume combustion vessel is developed for analysing soot particles. The injected quantity of diesel fuel is less than 10 mg, and the soot particles sampled by carriers with a transmission electron microscope (TEM) grid and lacey TEM grid can be used to analyse the morphologies of soot aggregates and the nano-structure of the soot primary particles, respectively. When the quantity of diesel fuel is more than 10 mg, in order to avoid burning-off of the carriers in higher temperature and pressure conditions, single-crystal silicon chips are employed. Ultrasonic oscillations and alcohol extraction are then implemented to obtain high quality soot samples for observation using a high-resolution transmission electron microscope. An in-house Matlab-based code is developed to extract the nano-structure parameters of the soot particles. A complete sampling and analysis procedure of the soot particles is provided to study the formation and oxidation mechanism of soot.