Diesel Soot Particles Research Articles

Characterization of Diesel Soot Aggregates by Scattering and Extinction Methods

Characteristics of diesel soot particles sampled from diesel exhaust of a common-rail turbo-charged diesel engine are quantified by scattering and extinction diagnostics using newly build two laser-based instruments. The radius of gyration representing the aggregates size is measured by the angular distribution of scattering intensity, while the soot mass concentration is measured by a two-wavelength extinction method. An approach to estimate the refractive index of diesel soot by an analysis of the extinction and scattering data using an aggregates scattering theory is proposed.

Effect of Oxidation Catalysts on Diesel Soot Particles

The effect of a conventional oxidation catalyst and a novel particle oxidation catalyst (POC) on diesel particles is studied using identical methodology. Regulated particulate matter emission measurement is followed by analyzing soluble organic fraction. In addition, size distributions are measured using a partial flow sampling system with a thermodenuder as an option. A parallel ELPI-SMPS method is used to study the particle effective density and, further, the mass. Tests are conducted using a heavy duty diesel engine with a very low sulfur fuel. A decrease in particle mass was observed when using a catalyst. When using a conventional catalyst the decrease was attributed to the decrease of soluble organic fraction, while using POC the nonsoluble fraction was also found to decrease, by 8-38%. This observation is confirmed by particle number measurement, and POC was found to decrease the dry particle number concentration measured downstream of a thermodenuder by 13-28%. Further particle structure analysis indicated lower density values when using conventional catalyst or POC. The physical size of the particles was not changed noticeably over either catalyst--implying the soluble organic fraction was condensed onto the soot, filling the voids in the porous structure of soot agglomerates, when no catalyst is used.

Update in Environmental and Occupational Medicine 2005

Although air pollution consists of a heterogeneous mixture of gases and particles, most recent research has been devoted to the adverse effects of particulate matter (PM). PM consists of primary particles, such as diesel soot or other combustion-derived particles,whichare emitteddirectly into the atmosphere, and secondary particles, which are created in the atmosphere through complex physicochemical transformation of gases (1, 2). Airborne PM is generally defined on the basis of the size distribution of the particles. Thus, PM10 andPM2.5 stand for PMwithmedian aerodynamic diameters less than 10 m and 2.5 m, respectively. Ultrafine particles (UFPs) have a diameter less than 100 nm (0.1 m). These particles demonstrate very high deposition in human alveoli (3).

On-line determination of the optical properties of particles produced by test fires

The performance of smoke detectors depends largely on the characteristics of the smoke produced in a fire situation. A better knowledge of the properties of the aerosol particles originating in fires will therefore help to improve the performance of existing detectors, reduce the amount of false alarms, and serve as guideline in the development of new measuring techniques. Here, we present a study of the optical properties of the standard EN54 test fires. The measurements were done using the diesel particle scatterometer [Hunt A, Quinby-Hunt M, Sheperd I. Polarized light scattering for diesel exhaust particulate characterization. In: Proceedings of the DOE diesel engine emissions reduction workshop. Castine, MA, 1998; Hull P, Shepherd I, Hunt A. Modeling light scattering from diesel soot particles. Appl Opt 2004;43(17):3433–41.] This instrument measures the angular distribution of the Mueller scattering matrix elements S 11 , S 12 , and S 34 [Bohren C, Huffman D. Absorption and scattering of light by small particles. New York, USA: Wiley Science Paperback Series; 1983.] with a time resolution faster than 1 Hz . The size distribution and the complex refractive index of the particles can then be determined by fitting the data with Mie-scattering calculations. The optical information was complemented with the aerosol size distribution measured using a SMPS system. Our results show that the simultaneous measurement of several scattering-matrix elements can serve as a good discrimination criterion for the different types of fire (flaming and smoldering). In contrast, the type of fire cannot be determined from the size distribution data alone.

Absorption amplification of black carbon internally mixed with secondary organic aerosol

The scattering and absorption properties of black carbon (BC) particles internally mixed with secondary organic aerosol (SOA) were investigated experimentally at the large aerosol chamber facility AIDA. Diesel soot particles were coated with secondary organic compounds produced by the in situ ozonolysis of α‐pinene. It was found that the organic coating strongly affects the optical and microphysical properties of the soot aggregates. Amplification factors of the internally mixed BC of 1.8 to 2.1 compared to the specific absorption cross section of externally mixed BC were measured. These amplification factors are well reproduced by a Mie model for concentrically coated spheres over a wide range of organic coating/BC mixing ratios. Other optical properties in particular of thinly coated soot particles, namely, the single scattering albedo, the Ångstrøm exponent, and the hemispheric backscattering ratio, are less well reproduced by the model, most likely because of the restructuring and the incomplete enclosure of the porous soot aggregates.

Nanometric La1−xKx MnO3 Perovskite-type oxides – highly active catalysts for the combustion of diesel soot particle under loose contact conditions

The La1−xKxMnO3 perovskite-type oxides whose sizes were in nanometric range were prepared by the citric acid-ligated method. The structures of these perovskite-type oxides were examined by XRD and FT-IR. The catalytic activity for the combustion of soot particulate was evaluated by a technique of the temperature-programmed reaction. In the LaMnO3 catalyst, the partial substitution of K for La at A-site enhanced the catalytic activity for the combustion of soot particle. In the La1−xKxMnO3 catalysts, the combustion temperature of soot particle decreases with increasing x values. The La1−xKxMnO3 oxides with the substitution quantity between x=0.20 and x=0.25 are good candidate catalysts for the soot particle removal reaction, and the combustion temperature of soot particle is between 285 and 430 °C when the contact of catalysts and soot is loose, and their catalytic activities for the combustion of soot particle are as good as supported Pt catalysts, which is the best catalyst system so far reported for soot combustion under loose contact conditions.

Combustion characteristics of water-insoluble elemental and organic carbon in size selected ambient aerosol particles

Abstract. Combustion of elemental carbon (EC) and organic carbon (OC) contained in ambient aerosol matter was explored using scanning electron microscopy (SEM) in combination with energy dispersive X-ray analysis (EDX). To ease identification of the particles of interest and to avoid or at least reduce interaction with simultaneously sampled inorganic oxides and salts, the approach used in this work differed in two ways from commonly applied procedures. First, rather than using a mixture of particles of vastly different sizes, as in PM10 or PM2.5, aerosol matter was collected in a 5-stage impactor. Second, the water soluble fraction of the collected matter was removed prior to analysis. Diesel soot particles, which appeared in the well-known form of chain-type aggregates, constituted the major fraction of EC. In contrast, OC containing particles were observed in a variety of shapes, including a sizable amount of bioaerosol matter appearing mostly in the size range above about 1 µm. During heating in ambient air for 1h, diesel soot particles were found to be stable up to 470°C, but complete combustion occurred in a narrow temperature interval between about 480 and 510°C. After diesel soot combustion, minute quantities of "ash" were observed in the form of aggregated tiny particles with sizes less than 10 nm. These particles could be due to elemental or oxidic contaminants of diesel soot. Combustion of OC was observed over a wide range of temperatures, from well below 200°C to at least 500°C. Incompletely burnt bioaerosol matter was still found after heating to 600°C. The results imply that the EC fraction in aerosol matter can be overestimated significantly if the contribution of OC to a thermogram is not well separated.

Appraisal of three-dimensional numerical simulation for sub-micron particle deposition in a micro-porous ceramic filter

A computational, three-dimensional approach to investigate the behavior of diesel soot particles in the micro-channels of a wall-flow, porous-ceramic particulate filter is presented. Particle size examined is in the PM2.5 range. The flow field is simulated with a finite-volume Navier–Stokes solver and the Ergun equation is used to model the porous material. The permeability coefficients were obtained by fitting experimental data. Particle flow, dispersion, deposition and wall–particle interactions are investigated tracking large swarms of 2 and 0.2 μ m diameter particles in a Lagrangian frame of reference. Particle dynamics included rarefied gas hypotheses (the Knudsen number being larger than unity) and bounce/capture models based on impact kinetic energy loss. The influence of gas molecules–particle interaction on overall particle behavior is also examined by including Brownian motion and partial slip in particle equation of motion. Simulations help to highlight three-dimensional non-uniform particle deposition, mainly due to flow distribution in the micro-channel. All particles deposit onto the porous filter wall following the distribution of the through-wall velocity. The larger, 2 μ m , particles show a larger tendency to deposit at the end of the filter. Due to the flow contraction at the inlet, virtually no particle deposit in the inlet section of the filter. Reasons for the scarce influence on particle deposition due to particle-flow slip and Brownian motion are given.

The Reno Aerosol Optics Study: An Evaluation of Aerosol Absorption Measurement Methods

The Reno Aerosol Optics Study (RAOS) was designed and conducted to compare the performance of many existing and new instruments for the in situ measurement of aerosol optical properties with a focus on the determination of aerosol light absorption. For this study, simple test aerosols of black and white particles were generated and combined in external mixtures under low relative humidity conditions and delivered to each measurement system. The aerosol mixing and delivery system was constantly monitored using particle counters and nephelometers to ensure that the same aerosol number concentration and amount reached the different instruments. The aerosol light-scattering measurements of four different nephelometers were compared, while the measurements of seven light-absorption instruments (5 filter based, 2 photoacoustic) were evaluated. Four methods for determining the aerosol light-extinction coefficient (3 cavity ring-down instruments and 1 folded-path optical extinction cell) were also included in the comparisons. An emphasis was placed on determining the representativeness of the filter-based light absorption methods, since these are used widely and because major corrections to the raw attenuation measurements are known to be required. The extinction measurement from the optical extinction cell was compared with the scattering measurement from a high-sensitivity integrating nephelometer on fine, nonabsorbing ammonium sulfate aerosols, and the two were found to agree closely (within 1% for blue and green wavelengths and 2% for red). The wavelength dependence of light absorption for small kerosene and diesel soot particles was found to be very near λ− 1, the theoretical small-particle limit. Larger, irregularly shaped graphite particles showed widely variable wavelength dependencies over several graphite runs. The light-absorption efficiency at a wavelength of 530 nm for pure kerosene soot with a number size distribution peak near 0.3 μ m diameter was found to be 7.5 ± 1.2 m2 g− 1. The two most fundamental independent absorption methods used in this study were photoacoustic absorption and the difference between suspended-state light extinction and scattering, and these showed excellent agreement (typically within a few percent) on mixed black/white aerosols, with the photoacoustic measurement generally slightly lower. Excellent agreement was also observed between some filter-based light-absorption measurements and the RAOS reference absorption method. For atmospherically relevant levels of the aerosol light-absorption coefficient (< 25 Mm− 1), the particle soot absorption photometer (PSAP) absorption measurement at mid-visible wavelengths agreed with the reference absorption measurement to within ∼ 11% for experiment tests on externally mixed kerosene soot and ammonium sulfate. At higher absorption levels (characterized by lower single-scattering albedo aerosol tests), this agreement worsened considerably, most likely due to an inadequate filter loading correction used for the PSAP. The PSAP manufacturer's filter loading correction appears to do an adequate job of correcting the PSAP absorption measurement at aerosol single-scattering albedos above 0.80–0.85, which represents most atmospheric aerosols, but it does a progressively worse job at lower single-scattering albedos. A new filter-based light-absorption photometer was also evaluated in RAOS, the multiangle absorption photometer (MAAP), which uses a two-stream radiative transfer model to determine the filter and aerosol scattering effects for a better calculation of the absorption coefficient. The MAAP absorption measurements agreed with the reference absorption measurements closely (linear regression slope of ∼ 0.99) for all experimental tests on externally mixed kerosene soot and ammonium sulfate.

Depth‐resolved analysis of traffic soot sampled in the vicinity of a German motorway

AbstractParticulate air pollution is caused to a high extent by soot particles arising from traffic. The formation of soot particles and their interaction with the atmosphere usually results in an inhomogeneous depth distribution of its chemical inventory. Because the potential reactivity of the compounds strongly depends on their distance from the surface, the chemical composition at or near the surface is important. Therefore, airborne particles were sampled for depth‐resolved analysis near a busy German motorway. The particles' sizes were classified with a multi‐stage impactor and deposited on indium foil for analysis with plasma‐based secondary neutral mass spectrometry (Plasma‐SNMS), dynamic secondary ion mass spectrometry (SIMS) as well as XPS. At a distance of 2 m from the motorway, the fine particles smaller than 100 nm mainly consisted of carbon and organic hydrogen similar to virgin diesel soot particles produced by a test diesel engine. With increasing particle size, nitrogen, sulfur and oxygen occurred additionally in substantial amounts with a sulfur–oxygen ratio of 1:4. These elements were found in the surface region not deeper than 30 nm predominantly as ammonium sulfate. A minor fraction of the nitrogen was found to be part of nitro‐organic compounds in the surface. Further away from the motorway no virgin soot particles were found. All soot particles (except for minor soil dust contributions) were found covered by the same surface layer as the motorway‐near particles. Copyright © 2004 John Wiley &amp; Sons, Ltd.

SOA formation from the photooxidation of α-pinene in the presence of freshly emitted diesel soot exhaust

Abstract Outdoor smog chamber experiments were performed to investigate the effects of the preexisting diesel soot particles on secondary aerosol formation. Diesel exhaust (50–240 μg m −3 of diesel soot particles) was added to the outdoor chambers without or with α -pinene (0.14 ppmV). The increase of medium polarity and organic composition change of diesel soot particles were tracked by analyzing deuterated alkanes, which were dosed to the system. Changes due to photooxidation of diesel soot exhaust were accelerated when α -pinene was added. Major α -pinene oxidation products were also analyzed from gas and particle samples and they were mostly carbonyl compounds such as cis -pinonaldehyde, cis -pinonic acid, pinalic-4-acid and 10-hydroxy pinonaldehyde. The partitioning coefficients of the major carbonyl products were much higher in diesel soot particles than predicted by theory. The high particle concentration of carbonyl products on diesel soot particles could be explained by heterogeneous acid-catalyzed reactions.

Effect of engine load on diesel soot particles.

This study concentrates on characterization of nonvolatile fraction of diesel particles. These particles have an impact on earth's radiation balance as well as on health effects of vehicle emissions. In addition to composition and size distribution of particles, an important factor affecting their health effects and properties and lifetimes in the atmosphere is their morphology. The effect of engine parameters on soot particle size distributions and also on particle morphology has been studied. It was found that the shape of the size distribution and also the structure of diesel particles depend on engine load. The number distributions were found to obey log-normal assumption. The width of the distribution increased with increasing engine load. The geometric standard deviations of measured distributions varied from 1.7 to 2.1. Simultaneously, the fractal dimension of particles decreased with increasing engine load. The values for mass fractal dimensions based on sealing of particle mass and mobility size were between 2.6 and 2.8. Both electron microscopy and measurements of aerodynamic size versus mobility size suggest that the morphology of particles in different size regimes vary, with the large particles being less compact than the small ones.

Measuring diesel soot with a scanning mobility particle sizer and an electrical low-pressure impactor: performance assessment with a model for fractal-like agglomerates

The scanning mobility particle sizer (SMPS) and the electrical low-pressure impactor (ELPI) are frequently used to measure particle size distributions of combustion aerosols. The instruments are especially popular for diesel exhaust measurements since the emission of “particulate matter” is restricted by legislation. A problem is that the interpretation of the results that these instruments give is not straightforward: fractal-like diesel soot agglomerates in the exhaust have a complex interaction with their carrier gas. This paper focuses on this complex interaction in order to assess its effect on the instruments’ output. A theoretical model for the aerodynamic behavior of fractal-like agglomerates is used as a tool to assess the performance of the ELPI and the SMPS for the measurement of diesel soot particles. The model couples the aerodynamic diameter and the mobility diameter for fractal-like soot agglomerates. Visual analysis with scanning electron microscopy (SEM) aids model development and instrument performance assessment. It is concluded that the performance of both instruments is affected by the fractal-like structure of diesel soot. The ELPI, if it is set to measure the aerodynamic diameter, gives an underestimation of the apparent size of particles due to their fractal-like structure. As a result, the number of particles is overestimated. The model presented in this paper helps interpretation of the ELPIs response. The SMPS is affected by multiple charging of soot agglomerates, larger than 1000 nm , which are abundantly present in diesel exhaust gas and are not removed by the impaction stage at the input due to their small effective density. The diesel engine in this paper emits many of such particles. Multiple charging leads to an underestimation of the size of the agglomerates and hampers size classification in the instrument.

Dynamic Measurements of Diesel Soot Particles by Extinction and Scattering Methods(Measurement, PM in Exhaust I)

Temporal measurements of mass concentration and mean volume equivalent diameter of soot particles in diesel exhaust were made simultaneously by a two-wavelength extinction method and a scattering method. It is shown that the soot mass concentration changes dynamically depending on the variation in engine load, while the mean particle size varies within a narrow range centering 150 nm. A theoretical approach to quantify the scattering to absorption ratio of diesel soot particles is proposed whereby the transmittance values measured at two wavelengths and electron micrographs of sampled soot particles are incorporated into the soot aggregate scattering theory.

UV-VIS-NIR spectral optical properties of soot and soot-containing aerosols

The UV-VIS-NIR spectral optical properties of soot and soot containing aerosols were investigated in detail during the AIDA Soot Aerosol Campaign 1999. One aim of the campaign was a comprehensive comparison of the microphysical properties of Diesel and spark generator soot. The mass specific extinction cross section at λ=450 nm of Diesel soot is 10.6±0.5 m 2 g −1 which is almost a factor of two larger than the corresponding value of 5.7±0.3 m 2 g −1 measured for spark generator soot. Coagulation-induced particle growth does not affect the soot extinction cross section and has a weak influence on the scattering properties of the soot aggregates. Atmospheric processing of freshly emitted soot was simulated in mixing experiments. The formation of mixed Diesel soot and dry ammonium sulfate particles by coagulation has only a minor effect on the soot absorption cross section. The coating of spark generated soot with organic material results in a strong increase of the single scattering albedo. A significant increase of the absorption coefficient at λ=473 nm during the coating process can be attributed to an enhancement of the specific soot absorption cross section by more than 30%.

Single particle MS, SNMS, SIMS, XPS, and FTIR spectroscopic analysis of soot particles during the AIDA campaign

Within the framework of the AIDA soot aerosol campaign diesel soot particles, spark generated soot particles, and aerosol mixtures were characterized with respect to their chemical state using different surface sensitive analysis methods. A comparison between diesel soot and graphite spark generated soot revealed a significant difference in the chemical composition of the particle surfaces. No distinct change from external to internal mixing could be detected by single particle mass spectrometry for mixtures of diesel soot and (NH 4) 2SO 4 aerosol since the spectra of diesel soot and (NH 4) 2SO 4 aerosol were surprisingly similar due to sulfate on the surface of diesel soot particles and traces of carbon impurities on ammonium sulfate particles. In addition to the expected formation of new particles a considerable change of the soot particle surface was detected while exposing diesel soot or spark generated soot to α-pinene and ozone, indicating a surface layer formed by oxidation products of α-pinene. However, the oxygen-containing hydrocarbon fragments detected by single particle mass spectrometry were distinctly different for the two soot types, which can be explained by either the different product adsorption or ionization behavior. Depositions of α-pinene reaction products on the surface could be confirmed by QMS-SIMS and XPS for particles of both types of soot. Due to the high mass resolution of TOF-SIMS acidic derivatives were identified as reaction products of α-pinene and ozone. The analytical methods applied in this work elucidated the different properties of spark generated soot compared to diesel soot. Therefore, spark generated soot should only be used with care as a general diesel soot surrogate.

Significance of semivolatile diesel exhaust organics for secondary HONO formation.

The atmospheric origin of nitrous acid (HONO) is largely unknown despite its estimated importance as an OH source during daytime due to its rapid photolysis. Recently, primary HONO contained in automobile exhaust as well as secondary HONO formation on soot particles have been invoked as possible HONO sources, but none of them is able to account for the observed HONO to NOx ratios of up to 0.04 in the atmosphere. In this paper, we show that semivolatile and/or water-soluble species contained in diesel exhaust are significantly involved in secondary HONO formation. These species are not associated with soot when the exhaust exits the tailpipe. To quantify these species and to assess the reaction kinetics leading to HONO, experiments were performed in which filtered but hot diesel exhaust gas interacted with a glass surface as well as a water film mimicking dry and wet surfaces to which exhaust might be exposed. A fraction of 0.023 of the NOx emitted was heterogeneously converted to HONO, which is at least three times more than the primary HONO emissions by diesel engines and a fraction of 50 larger than HONO formed on diesel soot particles that do not contain the semivolatile organics.

SAMPLE PREPARATION FOR SIZE ANALYSIS OF DIESEL SOOT PARTICLES USING FIELD-FLOW FRACTIONATION

Sample treatment procedure, and the experimental parameters that need to be considered in size analysis of diesel soot particles using field-flow fractionation (FFF), are discussed. A probe-type sonication of 10 min in water containing 0.05% of Triton X-100 provides well-dispersed suspension of the soot particles. Sample treatment is found to be important for reduction in particle-particle interaction and, thus, for preparation of well-dispersed soot suspension. FFF elution profile of untreated suspension has a long tailing that does not return to the baseline until the external field is removed. The scanning electron micrographs show that the untreated suspension contains aggregated particles. Extraction of chemicals is also found to be important for preparation of welldispersed suspension. The suspensions prepared without the extraction step, have broader and larger size distributions than those prepared with the extraction step, due to the presence of soluble organic fractions (SOF) that cause particle-aggregation. FFF provides an excellent repeatability in size analysis of soot particles. The standard deviation of the mean diameter measured by FlFFF was 3.6%. Both Flow FFF (FlFFF) and sedimentation FFF (SdFFF) are tested for size analysis of soot particles. SdFFF provides higher size-selectivity than FlFFF and is less problematic in terms of the particle-channel interaction. Unlike in SdFFF, FlFFF analysis does not require the density information and, thus, the fractogram can be directly transformed to the size distribution. But FlFFF provides lower size-selectivity and has narrower dynamic range than SdFFF.

Heterogeneous reaction of NO2 on diesel soot particles.

Soot particles were collected from a diesel engine using a procedure that realistically mimics exhaust gas conditions in tailpipes and during dilution at room temperature. After being sampled, the particles were exposed to NO2 concentrations and relative humidity in ranges relevant for the troposphere using 13N as tracer. Gas-phase nitrous acid(HONO) and irreversibly bound (i.e., chemisorbed) species were the main reaction products with initial yields of 80-90% and about 10%, respectively. Neither NO nor HNO3 were detectable. The HONO formation increased with increasing engine load (i.e., with a decreasing air to fuel ratio, lambda). The reaction rates of HONO and chemisorbed NO2 increased with increasing NO2 concentration and did not depend on relative humidity. At the beginning of reaction, the uptake coefficient averaged over 3 min ranged from 5 x 10(-6) to 10(-5) for NO2 concentrations between 2 and 40 ppb. The HONO formation rates decreased with time, indicating consumption of reactive surface species, while the chemisorption rates remained almost constant. The total HONO formation potential of the particles was estimated to about 1.3 x 10(17) molecules/mg of diesel soot or to about 4.7 mg/kg of diesel fuel, indicating that the reaction between NO2 and diesel soot particles does not provide a significant secondary HONO source in the atmosphere. A Langmuir-Hinshelwood type reaction mechanism was proposed that adequately describes the observed results and also allows discussing important general features of reactions on soot.

Selection and development of a reactor for diesel particulate filtration

The diesel engine is an efficient power generator but its exhaust gas needs to be cleaned. A diesel particulate filter, in fact a multi-phase multi-purpose chemical reactor for environmental protection, can lower the emission of diesel soot particles. The aim is to develop a diesel particulate filter that is robust, dependable, energy efficient and resistant to plugging. Moreover, ultrafine particles should be trapped and no poisoning gases should be emitted. A strategic approach is used for the design of a diesel particulate filter, inspired by the method of Krishna and Sie (1994) Chemical Engineering Science 49, 4029–4065, for multiphase reactor selection. The method involves three strategic levels for reactor selection: catalyst design, heat and mass injection and dispersion, and hydrodynamic flow regime; in this paper, the emphasis is on hydrodynamics. This approach has led to the design of a novel filter type for diesel soot filtration: the turbulent precipitator with foam collector plates. In this filter the gas flow is divided over two zones with different hydrodynamic characteristics: fast gas flow in an open channel and slow gas flow in stagnant zones. The open channel enables low pressure drop and prevents plugging, the stagnant zones enable deposition of diesel soot particles and, if desired, the placement of the catalytic material. The results for two different geometries of the turbulent precipitator are presented, they indicate that 90 ppi ceramic foam collector plates perform the best and that it is possible to tune different turbulent precipitators for different diesel engines. Computational fluid dynamics can be used to optimize the turbulent precipitator because it identifies the two hydrodynamic zones in the filter.