Response Characteristics of PMP Compliant Condensation Particle Counters Toward Various Calibration Aerosols

Within the European legislation, a new limit for particle emissions of light and heavy duty engines based on the particle number (PN) was established in 2011. For PN determination, solid exhaust particles are quantified by means of a condensation particle counter (CPC). In literature, deviations in PN of up to 30% are reported for a comparison of different measurement set-ups. Among others variations in the counting efficiency (CE) of different CPCs have to be considered as possible error sources that contribute to the overall deviation in PN. Thereby the uncertainties in CE may result from variations in the calibration procedure of different manufacturers (e.g., calibration aerosol). To investigate this circumstance, devices from three different manufacturers were directly compared according to their CE for model aerosols. The subject CPCs exhibited differences of up to 17% (23 nm particles) in the counting efficiency when measuring simultaneously the same test aerosol. Depending on the PN size distribution in real exhaust, this might result in an error (～9%) in the finally determined PN. Additionally, the CPC response for selected volatile exhaust components was investigated. In this way, we found out that the fraction of detected nucleation mode particles increases approximately by factor 3 in case particles consist of or contain volatile material (e.g., sulfuric acid).

Copyright 2015 American Association for Aerosol Research     

Investigation of solid particle number measurement: Existence and nature of sub-23 nm particles under PMP methodology

A Particle Measurement Program (PMP) compliant system, an AVL advanced particle counter (APC) and an alternative volatile particle removal system, a catalytic stripper (CS) were evaluated and compared for measuring solid particle number (PN) emissions. The evaluations and comparisons were conducted using sulfuric acid and hydrocarbon particles as model volatile particles in laboratory tests, and diluted exhaust from a diesel particle filter (DPF)-equipped heavy-duty diesel vehicle operated on a heavy-duty chassis dynamometer under steady speed conditions at two different engine loads. For the laboratory test, both the APC and CS removed more than 99% of the volatile particles in terms of PN when using aerosols composed of pure sulfuric acid or hydrocarbons. When using laboratory test aerosols consisting of mixtures of sulfuric acid and hydrocarbons more than 99% of the particles were removed by the APC but the surviving particles were no longer entirely volatile, 12–14% were solid. For the chassis dynamometer test, PN emissions between 3 and 10 nm downstream the APC were ∼2 and 7 times higher than the PN emissions of particles above 10 nm at the 74% and 26% engine load, respectively. At the 26% engine load, PN level of the 3–10 nm particles downstream the APC were significantly higher than that in the dilution tunnel, demonstrating that the APC was making 3–10 nm particles. The PN emission of 3–10 nm particles downstream the APC was related to the heating temperature of the APC evaporation tube, suggesting these particles are artifacts formed by renucleation of semivolatiles. Considerably fewer particles between 3 to 10 nm were seen downstream of the CS for both engine loads due mainly to removal of semivolatile material by the catalytic substrates, although some of this difference could be attributed to diffusion and thermophoretic losses. The findings of this study imply that improvement of the current PMP protocol would be necessary if the PMP were to be used in other applications where the PN emissions of particles below 23 nm are important.     

Differences between tailpipe and dilution tunnel sub-23 nm nonvolatile (solid) particle number measurements

European Union vehicle and engine regulations require measurement of nonvolatile (solid) particles with diameter >23?nm at the dilution tunnel. In 2019 it was decided to include particles >10?nm in the post Euro 6/VI regulations. Recent studies showed that sub-23?nm measurements are not only susceptible to volatile artifacts (i.e., re-nucleation downstream of the evaporation tube of the particle number system) but also to nonvolatile artifacts (i.e., nonvolatile particles formed in the tubing between the vehicle and the particle number system or in the particle number system itself). In order to investigate the origin of the nonvolatile particle formation, steady-state tests with a moped, a compressed natural gas (CNG), and a diesel vehicle while regenerating were conducted. Systems at the tailpipe and the dilution tunnel with evaporation tubes or catalytic strippers and condensation particle counters (CPCs) with 50% detection efficiencies at 2.5?nm, 4?nm, 10?nm, and 23?nm were used. The results showed higher concentrations of sub-23?nm particles at the dilution tunnel than at the tailpipe when facility preconditioning was not appropriate, the exhaust gas temperature exceeded 300?°C, and high concentrations of semi-volatile material were emitted (e.g., regenerations, lubricant oil).     

Evaluation of the European PMP Methodologies during On-Road and Chassis Dynamometer Testing for DPF Equipped Heavy-Duty Diesel Vehicles

This study evaluated the UN-ECE Particle Measurement Programme (PMP) protocol for the measurement of solid particle number emissions under laboratory and on-road conditions for two passive diesel particle filters (DPF)–equipped medium and heavy-heavy duty diesel vehicles. The PMP number emissions were lower than the European light-duty certification value (9.6 × 10¹¹ #/mi) for all standardized cycles, but exceeded this value during some higher load on-road driving conditions. Particle number measurements were generally less variable than those of the PM mass for the on-road testing, but had comparable or greater variability than PM mass for the laboratory measurements due to outliers. These outliers appear to be real events that are not apparent with integrated filter methods. The particle number measurements for the low cut point CPCs (3–7 nm) below the PMP system were approximately an order of magnitude higher than those for the PMP-compliant CPC (23 nm), indicating the presence of a large fraction of solid sub-23 nm particles. Although such particles are defined as solid by the PMP method, their actual state is unknown. Nucleation particles with a large sulfate contribution formed under a variety of conditions when the exhaust temperature near the DPF exceeded a “critical” temperature, typically >300°C.     

Interpretation of Secondary Organic Aerosol Formation from Diesel Exhaust Photooxidation in an Environmental Chamber

Secondary organic aerosol (SOA) formation from diesel exhaust was investigated using an environmental chamber. Particle volume measurement based solely on mobility diameter underestimated the SOA formation from diesel exhaust due to the external void space of agglomerate particles. Therefore, particle mass concentration and fractal-like dimension was determined from the particle effective density as a function of particle mass using an aerosol particle mass analyzer and scanning mobility particle sizer (APM–SMPS). Continuous aging of aerosol measured by an increase of atomic ratio (O/C) underscored the importance of multigenerational oxidation of low-volatile organic vapors emitted from diesel engine as a possible significant source of ambient oxygenated SOA. Higher particle effective densities were observed when raw exhaust was injected into a full bag as opposed to filling a bag with diluted exhaust using an ejector diluter. This suggests that the dilution method, in addition to dilution ratio, may impact the evaporation of semivolatile species. This study demonstrates the critical need to evaluate particle mass when evaluating SOA formation onto fractal particles such as diesel exhaust.     

A Fast Scanning Mobility Particle Spectrometer for Monitoring Transient Particle Size Distributions

The Scanning Mobility Particle Spectrometer (SMPS) is a key tool for measuring particle size distribution. The application of the instrument to obtain size distributions throughout a wide range of particle sizes for transient systems, such as motor vehicle emissions, has been limited by the time resolution of the SMPS. In this paper, we present a fast-SMPS (f-SMPS) that utilizes a Radial Differential Mobility Analyzer (rDMA) and a Wixing Condensation Particle Counter (mCPC). The combination of these two components allows for the acquisition of particle size distributions on the time scale of several seconds. The Instrument has an operating range of 5–98 nm and can obtain particle size distributions at rates of up to 0.4 Hz. This paper presents the initial construction and calibration of the instrument followed by its application to several sampling scenarios. Samples from the on-road testing of a heavy-duty diesel (HDD) vehicle demonstrate the utility of this instrument for momtor vehicle emissions measurements as size distributions can now be associated with discrete events taking piace during vehicle onroad operation. For instance, these data indicate the presence of a number peak at 15 nm during transient vehicle operation. Previous work indicates that these particles are associated with the loss of engine lubricating oil.     

Measurement of number and size distribution of particles emitted from a mid-sized transportation multipoint port fuel injection gasoline engine

This study was carried out to characterize the engine-exhaust particulate emissions from a typical multipoint port fuel injection gasoline engine used in transportation sector. Though gasoline engine showed no visible tail pipe emissions yet its particle concentrations were comparable to mineral diesel, particularly at high engine loads. Average sizes of particles emitted in gasoline exhaust are found to be way smaller than particles emitted in diesel exhaust under similar operating conditions. The peak particle concentrations for mineral diesel never go below 40 nm size however for gasoline engine, it was as low as 20 nm for most engine operating conditions. Within a very limited operating range, gasoline engine performance was superior to its diesel counterparts in terms of particulate size and number distribution however it deteriorates very quickly as soon as the fuel-air mixture becomes closer to stoichiometric ratio, typically under high engine load and speed conditions.     

Measurement of Automotive Nonvolatile Particle Number Emissions within the European Legislative Framework: A Review

In 2011, the European Commission introduced a limit for nonvolatile particle number (PN) emissions >23 nm from light-duty (LD) vehicles and the stated intent is to implement similar legislation for on-road heavy-duty (HD) engines at the next legislative stage. This paper reviews the recent literature regarding the operation-dependent emission of PN from LD vehicles and HD engines, and the measurement procedure used for regulatory purposes. The repeatability of the PN method is of the order of 5% and higher scatter of the results can easily be explained by the effect of the vehicles or the aftertreatment devices on the PN emissions (e.g., the fill state of the diesel particulate filters). Reproducibility remains an issue since it may exceed 30%. These high-variability levels are mainly associated with calibration uncertainties of the PN instruments. Correlation measurements between the full-flow dilution tunnels (constant-volume samplers, CVS) and the proportional partial-flow dilution systems (PFDS) showed agreement within 15% for the PN method down to 1 × 10¹¹ p/kWh. At lower concentrations, the PN background of the CVS and/or the PFDS can result in larger inconsistencies. The filter-based particulate matter (PM) mass and the PN emissions correlate well down to 1–2 mg/km for LD vehicles and to 2–3 mg/kWh for HD applications. The correlation improves when only elemental carbon mass is considered: it is relatively good down to 0.1–0.3 mg/km or mg/kWh.

Copyright 2012 American Association for Aerosol Research     

Comparison of Vehicle Exhaust Particle Size Distributions Measured by SMPS and EEPS During Steady-State Conditions

Fast-sizing spectrometers, such as the TSI Engine Exhaust Particle Sizer (EEPS), have been widely used to measure transient particle size distributions of vehicle exhaust. Recently, size distributions measured during different test cycles have begun to be used for calculating suspended particulate mass; however, several recent evaluations have shown some deficiencies in this approach and discrepancies relative to the gravimetric reference method. The EEPS converts electrical charge carried by particles into size distributions based on mobility classification and a specific calibration, and TSI recently released a matrix optimized for vehicle emissions as described by Wang et al. (Submitteda). This study evaluates the performance of the new matrix (soot matrix) relative to the original matrix (default matrix) and reference size distributions measured by a scanning mobility particle sizer (SMPS). Steady-state particle size distributions were generated from the following five sources to evaluate exhaust particulates with various morphologies estimated by mass-mobility scaling exponent: (1) A diesel generator operating on ultralow sulfur diesel, (2) a diesel generator operating on biodiesel, (3) a gasoline direct-injection vehicle operating at two speeds, (4) a conventional port-fuel injection gasoline vehicle, and (4) a light-duty diesel (LDD) vehicle equipped with a diesel particulate filter. Generally, the new soot matrix achieved much better agreement with the SMPS reference for particles smaller than 30 nm and larger than 100 nm, and also broadened the accumulation mode distribution that was previously too narrow using the default matrix. However, EEPS distributions still did not agree with SMPS reference measurements when challenged by a strong nucleation mode during high-load operation of the LDD vehicle. This work quantifies the range of accuracy that can be expected when measuring particle size distribution, number concentration, and integrated particle mass of vehicle emissions when using the new static calibration derived based on the properties of classical diesel soot.

Copyright 2015 American Association for Aerosol Research     

Design Optimization of a Portable Thermophoretic Precipitator Nanoparticle Sampler

Researchers at the National Institute for Occupational Safety and Health (NIOSH) are developing methods for characterizing diesel particulate matter in mines. Introduction of novel engine and exhaust aftertreatment technologies in underground mines is changing the nature of diesel emissions, and metrics alternative to the traditional mass-based measurements are being investigated with respect to their ability to capture changes in the properties of diesel aerosols. The emphasis is given to metrics based on measurement of number and surface area concentrations, but analysis of collected particles using electron microscopy (EM) is also employed for detailed particle characterization. To collect samples for EM analysis at remote workplaces, including mining and manufacturing facilities, NIOSH is developing portable particle samplers capable of collecting airborne nano-scale particles. This paper describes the design, construction, and testing of a prototype thermophoretic precipitator (TP) particle sampler optimized for collection of particles in the size range of 1–300 nm. The device comprises heated and cooled metal plates separated by a 0.8 mm channel through which aerosol is drawn by a pump. It weighs about 2 kg, has a total footprint of 27 × 22 cm, and the collection plate size is approximately 4 × 8 cm. Low power consumption and enhanced portability were achieved by using moderate flow rates (50–150 cm³/min) and temperature gradients (10–50 K/mm with ΔT between 8 K and 40 K). The collection efficiency of the prototype, measured with a condensation particle counter using laboratory-generated polydisperse submicrometer NaCl aerosols, ranged from 14–99%, depending on temperature gradient and flow rate. Analysis of transmission electron microscopy images of samples collected with the TP confirmed that the size distributions of collected particles determined using EM are in good agreement with those determined using a Fast Mobility Particle Sizer.

Copyright 2012 American Association for Aerosol Research     

Investigation of vehicle exhaust sub-23 nm particle emissions

A solid particle number limit was applied to the European legislation for diesel vehicles in 2011. Extension to gasoline direct injection vehicles raised concerns because many studies found particles below the lower size limit of the method (23 nm). Here we investigated experimentally the feasibility of lowering this size. A nano condensation nucleus counter system (nCNC) (d_50% = 1.3 nm) was used in parallel with condensation particle counters (CPCs) (d_50% = 3 nm, 10 nm and 23 nm) at various sampling systems based on ejector or rotating disk diluters and having thermal pre-treatment systems consisting of evaporation tubes or catalytic strippers. An engine exhaust particle sizer (EEPS) measured the particle size distributions. Depending on the losses and thermal pre-treatment of the sampling system, differences of up to 150% could be seen on the final detected particle concentrations when including the particles smaller than 23 nm in diameter. A volatile artefact as particles with diameters below 10 nm was at times observed during the cold start measurements of a 2-stroke moped. The diesel vehicles equipped with the Diesel Particulate Filter (DPF) had a low solid sub-23 nm particles fraction (<20%), the gasoline with direct injection vehicles had higher (35–50%), the gasoline vehicles with port fuel injection and the two mopeds (two and four-stroke) had the majority of particles below 23 nm. The size distributions peaked at 60–80 nm for the DPF equipped vehicles, at 40–90 nm for the gasoline vehicles with a separate nucleation mode peak at approximately 10 nm sometimes. Mopeds peaked at sizes below 50 nm when their aerosol was thermally pre-treated.

© 2017 American Association for Aerosol Research     

Volatility characterization of nanoparticles from single and dual-fuel low temperature combustion in compression ignition engines

This work explores the volatility of particles produced from two diesel low temperature combustion (LTC) modes proposed for high-efficiency compression ignition engines. It also explores mechanisms of particulate formation and growth upon dilution in the near-tailpipe environment. The number distribution of exhaust particles from low- and mid-load dual-fuel reactivity controlled compression ignition (RCCI) and single-fuel premixed charge compression ignition (PPCI) modes were experimentally studied over a gradient of dilution temperature. Particle volatility of select particle diameters was investigated using volatility tandem differential mobility analysis (V-TDMA). Evaporation rates for exhaust particles were compared with V-TDMA results for candidate pure n-alkanes to identify species with similar volatility characteristics. The results show that LTC particles are mostly comprised of material with volatility similar to engine oil alkanes. V-TDMA results were used as inputs to an aerosol condensation and evaporation model to support the finding that smaller particles in the distribution are comprised of lower volatility material than large particles under primary dilution conditions. Although our results show that saturation levels are high enough to drive condensation of alkanes onto existing particles under the dilution conditions investigated, they are not high enough to allow homogeneous nucleation of these same compounds in the primary exhaust plume. Therefore, we conclude that observed particles from LTC operation must grow from low concentrations of highly nonvolatile compounds present in the exhaust.

Copyright © 2016 American Association for Aerosol Research     

A Unique Online Method to Infer Water-Insoluble Particle Contributions

Particle number, size, and composition information is important for constraining aerosol effects on air quality, climate, and health. The composition of particles, especially from vehicular sources, may contain insoluble black carbon (BC) materials that modify particle nucleating properties. In this study, we develop a method to provide quantitative and real-time information on the water-insoluble components found in near-road aerosol sources. A water-based condensation particle counter (W-CPC) and a butanol-based CPC (B-CPC) were used to measure the particle number concentration. Both instruments were coupled with a scanning mobility particle sizer (SMPS) to record the particle number and size data. Real time water-insoluble particle mass was estimated from the difference in particle number concentration between the two CPCs; theoretical water-insoluble mass was calculated from the ideal hygro- scopicity single parameter κ-values. This online method was calibrated with test compounds and then applied to data collected from a field study. Ambient aerosol was sampled from a monitoring station located 15 m from the I-710 freeway in Long Beach, California. The results show that near-roadway emissions contain water-insoluble (BC and non-BC) components. Particle number and BC concentrations increase after changes in wind direction near the freeway on both weekday and weekend measurements. Particles were less hygroscopic (κ?～?0.2) before changes in wind direction from downwind to upwind of the freeway (κ?>?0.6). Rapid changes in water-solubility can be captured with this technique. By assuming a two-component mixture, the water-insoluble mass fractions were inferred. BC shows a positive correlation with the water-insoluble mass however its presence may not account for the entire water-insoluble mass from the near-roadway source.

Copyright 2014 American Association for Aerosol Research     

In-service performance assessment of electrostatic precipitators serving a rubber vulcanization process

We examined two emission abatement systems of some vulcanization ovens, serving a unit producing small rubber-based parts for automotive application. Each emission control unit treats the gases exhausted by three to five ovens. A heat exchanger cools down the fumes to a temperature suitable for the correct operation of a couple of two-stage electrostatic precipitators in series. We performed quantitative analysis of concentrations and size distributions in these rubber fumes using aerosol technology instrumentation, namely optical particle spectrometers and electrical mobility particle sizers. The size of sampled particles was mainly between 100?nm and 1000?nm. We evaluated the performance of the exhaust fume abatement units, with focus on the electrostatic precipitator. Concerning batch ovens, the quantitative trend of the emissions follows the thermal cycle of the post-curing process. Time interval since the last maintenance operation causes a gradual reduction in the removal efficiency. The measured data demonstrate the reliability and the adequacy of aerosol instrumentation for the characterization of the emissions from rubber vulcanization ovens. The pair of electrostatic precipitators was shown to be effective in removing most of the particles detected in the fumes stream. The measurement protocol developed in this study allows assessing the influence of the maintenance schedule on the performance of the emission control units. New technologies for treating organic vapors can be evaluated in a reliable and effective way.

Copyright © 2019 American Association for Aerosol Research     

A Potential Soot Mass Determination Method from Resistivity Measurement of Thermophoretically Deposited Soot

Miniaturized detection systems for nanometer-sized airborne particles are in demand, for example in applications for onboard diagnostics downstream particulate filters in modern diesel engines. A soot sensor based on resistivity measurements was developed and characterized. This involved generation of soot particles using a quenched co-flow diffusion flame; depositing the particles onto a sensor substrate using thermophoresis and particle detection using a finger electrode structure, patterned on thermally oxidized silicon substrate. The generated soot particles were characterized using techniques including Scanning Mobility Particle Sizer for mobility size distributions, Differential Mobility Analyzer—Aerosol Particle Mass analyzer for the mass–mobility relationship, and Transmission Electron Microscopy for morphology. The generated particles were similar to particles from diesel engines in concentration, mobility size distribution, and mass fractal dimension. The primary particle size, effective density and organic mass fraction were slightly lower than values reported for diesel engines. The response measured with the sensors was largely dependent on particle mass concentration, but increased with increasing soot aggregate mobility size. Detection down to cumulative mass as small as 20–30 μg has been demonstrated. The detection limit can be improved by using a more sensitive resistance meter, modified deposition cell, larger flow rates of soot aerosol and modifying the sensor surface.     

Characterization of Soot Particles Produced in a Transparent Research CR DI Diesel Engine Operating with Conventional and Advanced Combustion Strategies

The effect of the combustion mode on particle emission was analyzed both in the cylinder and at the exhaust of a direct injection (DI) Common Rail (CR) transparent research diesel engine by means of spectroscopic and conventional methods. The engine was equipped with a flexible electronic control unit (ECU) capable of operating up to 5 injections per cycle with different start of injection and dwell time allowing performing different combustion modes. The conventional diesel combustion, the homogeneous charge compression ignition (HCCI), and the low temperature combustion (LTC) modes were analyzed. In-cylinder broadband UV–visible scattering and extinction measurements were carried out to follow the particle formation and oxidation processes as well as to have information about their chemical nature and size distribution. The characterization of the particulate emission at the exhaust was performed by means of an electrical low pressure impactor (ELPI), for the counting and the sizing of the particles, and an opacimeter, for measuring the smoke opacity. The in-cylinder measurements highlighted that particles ranged from 3 to 100 nm whatever was the combustion mode. Nevertheless, particles produced by a conventional diesel combustion process principally consist of soot. Whereas particles formed during HCCI and LTC modes are composed mainly of organic compounds. The exhaust particle emissions depend on the combustion mode both in terms of size and number. A larger amount of particles smaller than 100 nm was emitted during HCCI and LTC modes with respect to the conventional one. Moreover, HCCI mode showed a strong accumulation mode.

Copyright 2012 American Association for Aerosol Research     

Emissions and radiative impacts of sub-10 nm particles from biofuel and fossil fuel cookstoves

Abstract

Combustion sources have been shown to directly emit particles smaller than 10?nm. The emission of 1-3?nm particles from biofuel or fossil fuel cookstoves has not been studied previously, nor have the radiative impacts of these emissions been investigated. In this work, emissions (number of particles) were measured during a water boiling test performed on five different cookstoves (three-stone fire, rocket elbow, gasifier, charcoal, and liquified petroleum gas [LPG]) for particle diameters between ～1 and ～1000?nm. We found significant emissions of particles smaller than 10?nm for all cookstoves (>5?×?10¹⁵ # kg-fuel^?1). Furthermore, cleaner (e.g., LPG) cookstoves emitted a larger fraction of sub-10?nm particles (relative to the total particle counts) than traditional cookstoves (e.g., three-stone fire). Simulations performed with the global chemical transport model GEOS-Chem-TOMAS that were informed by emissions data from this work suggested that sub-10?nm particles were unlikely to significantly influence number concentrations of particles with diameters larger than 80?nm that can serve as cloud condensation nuclei (CCN) (<0.3%, globally averaged) or alter the cloud-albedo indirect effect (absolute value <0.005?W m^?2, globally averaged). The largest, but still relatively minor, localized changes in CCN-relevant concentrations (<10%) and the cloud-albedo indirect effect (absolute value <0.5?W m^?2) were found in large biofuel combustion source regions (e.g., Brazil, Tanzania, Southeast Asia) and in the Southern Ocean. Enhanced coagulation-related losses of these sub-10?nm particles at sub-grid scales will tend to further reduce their impact on particle number concentrations and the aerosol indirect effect, although they might still be of relevance for human health.

Copyright © 2020 American Association for Aerosol Research     

A PM 2.5 Inlet Impactor Designed for a High Flow Application

Two potential strategies for reducing diesel emissions are exhaust aftertreatment and the use of reformulated or alternative fuels. Little is yet known about the impact on ultrafine particle emissions of combining exhaust aftertreatment with such increasingly common fuels. This paper reports ultrafine particle size distribution measurements for a study in which the impact of such fuels on emissions from a heavy duty diesel engine employing different aftertreatment configurations was evaluated. Eight different fuels were tested: Canadian No. 1 and No. 2 diesel; low sulfur diesel fuel; two different ultra low sulfur diesel fuels (< 30 ppm S); Fischer-Tropsch diesel fuel; 20% biodiesel blended with ultra low sulfur diesel fuel; and PuriNO_x?. The fuels were tested in combination with four exhaust configurations: engine out, diesel oxidation catalyst (DOC), continuously regenerating diesel particle filter (CRDPF), and engine gas recirculation with CRDPF (EGR-DPF). In general, aftertreatment configuration was found to have a greater impact on ultrafine particle size distributions than fuel composition, and the effects of aftertreatment tended to be uniform across the entire particle size distribution. Steady state tests revealed complex behavior based on fuel type, particularly for PuriNOx. This behavior included bimodal particle size distributions with modes as low as 8–10 nm for some fuels. Unlike previous results for gravimetric PM from this study, no significant correlation for ultrafine emissions was found for fuel properties such as sulfur level.     

Chemical characterization of particulate emissions from diesel engines: A review

This review examines the chemical properties of particulate matter (PM) in diesel vehicle exhaust at a time when emission regulations, diesel technology development, and particle characterization techniques are all undergoing rapid change. The aim is to explore how changes in each of these areas impact the others. Particle composition is of central interest to the practical issues of health effects, climate change, source apportionment, and aerosol modeling. Thus, the emphasis here is to identify the emerging questions and examine how they can be addressed. As regulations drive down the allowed tailpipe emission levels, advances in engine and aftertreatment technology have made it possible to substantially reduce PM emissions. Besides the reduction in level, new technologies such as diesel particulate filters (DPFs) and selective catalytic reduction (SCR) can also affect the physical and chemical properties of PM. This in turn introduces new analytical demands that must address not only the issue of sensitivity, but also of specificity. New methods of aerosol chemical analysis are described that address these needs, improve our understanding of particle composition, and provide critical insight into the current issues surrounding motor vehicle PM emissions and their environmental impact.     

Measurement of Electrical Charge on Diesel Particles

Nanoparticles (D _p < 50 nm), which are formed as diesel engine exhaust cools and dilutes, constitute minority of total particle mass but majority of total particle number. There are several different theories to explain the nucleation of nanoparticles from diesel exhaust. The two main theories are homogeneous binary nucleation of sulfuric acid and water, and ion-induced nucleation. This study examined the ion-induced nucleation theory. In order to test the ionic nucleation theory, the charged fraction of the diesel particles were measured as a function of particle size using regular diesel fuel in this study. A very small amount of charge was found for the diesel nanoparticles in the nuclei mode, whereas there was a large charged fraction for the diesel particles in the accumulation mode. If ion-induced nucleation were the dominant mechanism for the nucleation of nanoparticles from diesel exhaust, one would expect a significant charge on the nuclei mode particles. The results from this study suggest that ion-induced binary nucleation is at least not a dominant mechanism for the nucleation of diesel exhaust when using regular diesel fuel.

This study also examined the influence of metal additives on nucleation and particle charging. The metal additives examined are of the type used to enhance particle oxidation in diesel particulate filters. When used, the additives led to a large increase in the concentration of solid particles in the nuclei mode, and significantly raised the level of particle charge for particles of all sizes. When additives were used, some of the solid particles in the nuclei mode carried a charge. We believe that these metal related particles form early enough in the combustion process to be charged by ions present during and shortly after combustion.