Aerodynamic Lens Research Articles

Application of Aerodynamic Lenses to In Situ Particle Monitors (ISPM) for Higher Reliability in Semiconductor Fabrication Process

In-Situ Particle Monitors (ISPMs) are widely used today to monitor particles within and downstream of semiconductor processing equipment, for they can detect particles under vacuum conditions on real-time basis. However, ISPMs have some limitations. Firstly, since only a small fraction of the flow passes through the laser beam, the counting efficiency of ISPMs is quite low. Also, particles are not uniformly distributed in low pressure flows and this results in the poor correlation between ISPM counts and the actual particle concentration. Secondly, the intensity of the laser light in the viewing volume is not uniform, but instead has a profile that is typically Gaussian. This leads to a broad distribution of scattered intensities for particles of a given size. Therefore, ISPMs cannot offer accurate sizing resolution. In this experimental study, we improved ISPM performance by using aerodynamic lenses to focus particles through the center of the illuminated optical viewing volume. Aerodynamic lenses consist of a series of circular orifices in a cylindrical tube. The flow converges and then diverges as it flows through these orifices. Because of their inertia, particles do not perfectly follow the flow stream lines. Particles smaller than a critical size concentrate along the centerline of the flow, while particles larger than this critical size are lost by inertial deposition. Therefore, it is possible to design a lens system that focuses all the particles through the viewing volume. We found that by using aerodynamic lenses we were able to increase the counting efficiency of an HYT Model 70 ISPM from 0-10% to near 100% for particles of 0.494 μm in diameter at pressures ranging from 0.1 to 10 torr. Also, uniform illumination of particles in the center region of the viewing volume led to a more uniform pulse height distribution of scattered light, which implies the possibility that ISPM can be used as a particle sizer.

Technical Note: Use of a beam width probe in an Aerosol Mass Spectrometer to monitor particle collection efficiency in the field

Abstract. Two Aerodyne Aerosol Mass Spectrometers (Q-AMS) were deployed in Mexico City, during the Mexico City Metropolitan Area field study (MCMA-2003) from 29 March–4 May 2003 to investigate particle concentrations, sources, and processes. We report the use of a particle beam width probe (BWP) in the field to quantify potential losses of particles due to beam broadening inside the AMS caused by particle shape (nonsphericity) and particle size. Data from this probe show that no significant mass of particles was lost due to excessive beam broadening; i.e. the shape- and size-related collection efficiency (Es) of the AMS during this campaign was approximately one. Comparison of the BWP data from MCMA-2003 with other campaigns shows that the same conclusion holds for several other urban, rural and remotes sites. This means that the aerodynamic lens in the AMS is capable of efficiently focusing ambient particles into a well defined beam and onto the AMS vaporizer for particles sampled in a wide variety of environments. All the species measured by the AMS during MCMA-2003 have similar attenuation profiles which suggests that the particles that dominate the mass concentration were internally mixed most of the time. Only for the smaller particles (especially below 300 nm), organic and inorganic species show different attenuation versus particle size which is likely due to partial external mixing of these components. Changes observed in the focusing of the particle beam in time can be attributed, in part, to changes in particle shape (i.e. due to relative humidity) and size of the particles sampled. However, the relationships between composition, atmospheric conditions, and particle shape and size appear to be very complex and are not yet completely understood.

Simultaneous Measurement of the Effective Density and Chemical Composition of Ambient Aerosol Particles

Simultaneous measurements of the effective density and chemical composition of individual ambient particles were made in Riverside, California by coupling a differential mobility analyzer (DMA) with an ultrafine aerosol time-of-flight mass spectrometer (UF-ATOFMS). In the summer, chemically diverse particle types (i.e., aged-OC, vanadium-OC-sulfate-nitrate, biomass) all had similar effective densities when measured during the same time period. This result suggests that during the summer study the majority of particle mass for the different particle types was dominated by secondary species (OC, sulfates, nitrates) of the same density, while only a small fraction of the total particle mass is accounted for by the primary particle cores. Also shown herein, the effective density is a dynamic characteristic of the Riverside, CA ambient aerosol, changing by as much as 40% within 16 h. During the summer measurement period, changes in the ambient atmospheric water content correlated with changes in the measured effective densities which ranged from approximately 1.0 to 1.5 g x cm(-3). This correlation is potentially due to evaporation of water from particles in the aerodynamic lens. In contrast, in the fall during a Santa Ana meteorological event, ambient particles with a mobility diameter of 450 nm showed three distinct effective densities, each related to a chemically unique particle class. Particles with effective densities of approximately 0.27 g x cm(-3), 0.87 g x cm(-3), and 0.93 g x cm(-3) were composed mostly of elemental carbon, lubricating oil, and aged organic carbon, respectively. It is interesting to contrast the seasonal differences where in the summer, particle density and mass were determined by high amounts of secondary species, whereas in the fall, relatively clean and dry Santa Ana conditions resulted in freshly emitted particles which retained their distinct source chemistries and densities.

Chemical and microphysical characterization of ambient aerosols with the aerodyne aerosol mass spectrometer

The application of mass spectrometric techniques to the real-time measurement and characterization of aerosols represents a significant advance in the field of atmospheric science. This review focuses on the aerosol mass spectrometer (AMS), an instrument designed and developed at Aerodyne Research, Inc. (ARI) that is the most widely used thermal vaporization AMS. The AMS uses aerodynamic lens inlet technology together with thermal vaporization and electron-impact mass spectrometry to measure the real-time non-refractory (NR) chemical speciation and mass loading as a function of particle size of fine aerosol particles with aerodynamic diameters between approximately 50 and 1,000 nm. The original AMS utilizes a quadrupole mass spectrometer (Q) with electron impact (EI) ionization and produces ensemble average data of particle properties. Later versions employ time-of-flight (ToF) mass spectrometers and can produce full mass spectral data for single particles. This manuscript presents a detailed discussion of the strengths and limitations of the AMS measurement approach and reviews how the measurements are used to characterize particle properties. Results from selected laboratory experiments and field measurement campaigns are also presented to highlight the different applications of this instrument. Recent instrumental developments, such as the incorporation of softer ionization techniques (vacuum ultraviolet (VUV) photo-ionization, Li+ ion, and electron attachment) and high-resolution ToF mass spectrometers, that yield more detailed information about the organic aerosol component are also described.

Review of Measurement Methods and Compositions for Ultrafine Particles

Impactor, virtual impactor/aerosol concentrator, and aerodynamic lenses are used to separate the ultrafine particle (UP) fraction from other particle sizes for chemical analysis. Cascade impactors, such as the Micro-Orifice Uniform Deposit Impactor (MOUDI), are most commonly used in field studies, with sampling onto substrates amenable to different chemical analyses. Impactors need sufficient sampling flow rates and homogeneous deposits on the impaction surfaces for multiple chemical analyses. Mass, elements, ions, and carbon fractions can be measured on these substrates by several analytical methods. Specific organic compounds measured by solvent extraction require substantial mass loadings that can only be obtained by compositing samples from several measurement periods unless aerosol concentrators or high-volume sampling devices are used. Thermal desorption-gas chromatographic/mass spectrometry has potential to obtain organic speciation with small sample sizes. Studies of UP composition began in the late 1990s, with 25 ambient studies surveyed here. These are mostly from urban areas. Organic material, including polycyclic aromatic hydrocarbons (PAHs), usually constituted the most abundant portion of UP, with high elemental concentrations found near industrial sites. Much of the UP < 50 nm appears to be semi-volatile, consistent with it being composed by organic materials such as hopanes from engine oils or condensed secondary organic aerosol such as organic acids.

Collimation of metal nanoparticle beams using aerodynamic lenses

With the objective of reducing the divergence, aerodynamic lenses were applied to collimate a beam of metal nanoparticles in a size range up to 20nm. Influences of the aerodynamic devices on the particle aggregation process and the beam properties have been evaluated by time-of-flight measurements on mass filtered clusters. Additional transmission electron microscopy and atomic force microscopy studies on deposited particles complement these investigations. Perspectives for the application of aerodynamic lenses in the collimation of ultrafine particles are discussed.

Evaporation of Water from Particles in the Aerodynamic Lens Inlet: An Experimental Study

The extremely high particle transmission efficiency of aerodynamic lens inlets resulted in their wide use in aerosol mass spectrometers. One of the consequences of transporting particles from high ambient pressure into the vacuum is that it is accompanied by a rapid drop in relative humidity (RH). Since many atmospheric particles exist in the form of hygroscopic water droplets, a drop in RH may result in a significant loss of water and even a change in phase. How much water is lost in these inlets is presently unknown. Since water loss can affect particle size, transmission efficiency, ionization probability, and mass spectrum, it is imperative to provide definitive experimental data that can serve to guide the field to a reasonable and uniform sampling approach. In this study, we present the results of a number of highly resolved measurements, conducted under well-defined conditions, of water evaporation from a range of particles, during their transport through an aerodynamic lens inlet. We conclude that the only sure way to avoid ambiguities during measurements of aerodynamic diameter in instruments that utilize low-pressure aerodynamic lens inlets is to dry the particles prior to sampling.

An experimental study of nanoparticle focusing with aerodynamic lenses

High sampling efficiencies of analyte ions, molecules or particles are needed to maximize the sensitivity of mass spectrometers. “Ion funnels”, which utilize electrodynamic focusing, have been shown to effectively focus ions with mass-to-charge ratio ( m/ z) ranging from ∼100 to 5000. Focusing efficiencies of ion funnels drop for higher m/ z values because very high voltages are needed to overcome the particle inertia. Conventional “aerodynamic lenses” utilize inertia to focus down to 25 nm in diameter (∼5 MDa); to date, Brownian diffusion has prevented the effective focusing of particles smaller than this. We recently reported a design procedure that should, in principle, extend focusing with aerodynamic lenses to particles as small as 3 nm (∼10 kDa), thereby bridging the gap between the ion funnel and the conventional aerodynamic lenses. In this paper, we report for the first time experimental results for the performance of these new “nanolenses”. Measurements were done using spherical oil droplets, proteins, and sodium chloride particles ranging in size from 3 to 30 nm diameter. We found that particle transport efficiencies from atmospheric pressure to vacuum through the aerodynamic lens system were greater than 80% for 10–30 nm particles, and greater than 50% for a ∼3.8 nm protein (Lysozyme from chicken egg white, molecular weight 14.3 kDa). Particle beam diameters were about a factor of two greater than predicted by our numerical simulations, but provide clear evidence that the nanolenses effectively focus all three particle types.

Instruction Manual for the Aerodynamic Lens Calculator

This is an instruction manual for the aerodynamic lens design tool: the “Aerodynamic Lens Calculator”. We explain how to install and use this software. Examples are provided to use this tool to design or test a lens system. The structures of the source code programming are also provided in this manual. The design tool and its manual mentioned in this article are available in the publisher's online edition of Aerosol Science and Technology. To access this file, click on the link for this issue, then select this article. In order to access the full article on-line, you must either have an institutional subscription or a member subscription accessed through http://www.aaar.org.

A Design Tool for Aerodynamic Lens Systems

We report in this article the development of a tool to design and evaluate aerodynamic lens systems: the Aerodynamic Lens Calculator. This Calculator enables quick and convenient design of aerodynamic lens systems based on the parameterized knowledge gained from our detailed numerical simulations of flow and particle transport through aerodynamic lens systems. It designs the key dimensions of a lens system: pressure limiting orifice, relaxation chamber, focusing lenses, spacers and the accelerating nozzle. It also provides estimates of particle terminal axial velocities, particle beam width, and particle transmission efficiencies. This article describes in detail the information that is used in the design tool, including equations for pressure drop through orifices, contraction factors as a function of Reynolds, Mach and Stokes numbers, spacer length as a function of Reynolds number and estimations for the relaxation chamber dimensions. The article also evaluates the performance of the design tool by comp...

Coupling a versatile aerosol apparatus to a synchrotron: Vacuum ultraviolet light scattering, photoelectron imaging, and fragment free mass spectrometry

An aerosol apparatus has been coupled to the Chemical Dynamics Beamline of the Advanced Light Source at Lawrence Berkeley National Laboratory. This apparatus has multiple capabilities for aerosol studies, including vacuum ultraviolet (VUV) light scattering, photoelectron imaging, and mass spectroscopy of aerosols. By utilizing an inlet system consisting of a 200μm orifice nozzle and aerodynamic lenses, aerosol particles of ∼50nm–∼1μm in diameter can be sampled directly from atmospheric pressure. The machine is versatile and can probe carbonaceous aerosols generated by a laboratory flame, nebulized solutions of biological molecules, hydrocarbon aerosol reaction products, and synthesized inorganic nanoparticles. The sensitivity of this apparatus is demonstrated by the detection of nanoparticles with VUV light scattering, photoelectron imaging, and charged particle detection. In addition to the detection of nanoparticles, the thermal vaporization of aerosols on a heater tip leads to the generation of intact gas phase molecules. This phenomenon coupled to threshold single photon ionization, accessible with tunable VUV light, allows for fragment-free mass spectrometry of complex molecules. The initial experiments with light scattering, photoelectron imaging, and aerosol mass spectrometry reported here serve as a demonstration of the design philosophy and multiple capabilities of the apparatus.

Elastic light scattering from nanoparticles by monochromatic vacuum-ultraviolet radiation

Elastic light scattering is reported using monochromatic vacuum-ultraviolet radiation to study free, spherical silica nanoparticles prepared by approaches from colloidal chemistry, with diameters between 100 and 240 nm. The colloidal nanoparticles of defined size are transferred from an aqueous solution into the gas phase using a particle beam experiment. After focusing of the particle beam by an aerodynamic lens, the scattered light from monochromatic synchrotron radiation is measured. Angle-resolved elastically scattered light is detected, showing a strong forward-scattering component. Additional evidence for the detection of elastically scattered light comes from plotting the scattered light intensity as a function of the dimensionless parameter qR, where q is the magnitude of the scattering wave vector and R is the particle radius. This yields different power-law regimes that are assigned to scattering from the surface and the bulk of the nanoparticles. Furthermore, there is evidence for modulations in the scattered light intensity as a function of scattering angle, which is clearly distinguished from the forward-scattering component. The experimental results are compared to Mie scattering simulations for isolated particles, yielding general agreement with the experimental results. Deviations from Mie simulations are observed for samples consisting of significant amounts of aggregates. The present results indicate that the optical properties of free nanoparticles are sensitively probed by vacuum-ultraviolet radiation.

Design, Modeling, Optimization, and Experimental Tests of a Particle Beam Width Probe for the Aerodyne Aerosol Mass Spectrometer

Aerodynamic lens inlets have revolutionized aerosol mass spectrometry by allowing the introduction of a very narrow particle beam into a vacuum chamber for subsequent analysis. The real-time measurement of particle beam width after an aerodynamic lens is of interest for two reasons: (1) it allows a correction to be made to the measured particle concentration if the beam is so broad, due to poor focusing by non-spherical particles, that some particles miss the detection system; and (2) under constant lens pressure it can provide a surrogate particle non-sphericity measurement. For these reasons, a beam width probe (BWP) has been designed and implemented for the Aerodyne Aerosol Mass Spectrometer (AMS), although this approach is also applicable to other instruments that use aerodynamic lens inlets. The probe implemented here consists of a thin vertical wire that can be precisely positioned to partially block the particle beam at fixed horizontal locations in order to map out the width of the particle beam. A computer model was developed to optimize the BWP and interpret its experimental data. Model assumptions were found to be reasonably accurate for all laboratory-generated particle types to which the model was compared. Comparisons of particle beam width data from a number of publications are also shown here. Particle losses due to beam broadening are found to be minor for the AMS for both laboratory and ambient particles. The model was then used to optimize the choice of the BWP dimensions, and to guide its use during continuous operation. A wire diameter approximately 1.55 times larger than the beam width to be measured provides near optimal sensitivity toward both collection efficiency and surrogate non-sphericity information. Wire diameters of 0.62 mm and 0.44 mm (for the AMS “long” and “short” chambers, respectively) provide reasonable sensitivity over the expected range of particle beam widths, for both spherical and non-spherical particles. Three other alternative BWP geometries were also modeled and discussed.

Effect of process parameters on the structure of Si–Ti–N nanostructured coatings deposited by hypersonic plasma particle deposition

Si–Ti–N nanostructured coatings were synthesized by inertial impaction of nanoparticles using a process called hypersonic plasma particle deposition (HPPD). A study detailing the effect of plasma gases showed that in the case of an Ar + H 2 plasma gas mixture, crystalline phases in the coatings consisted of TiN, TiSi 2, and Ti 5Si 3. When the system was switched to a plasma gas mixture of Ar + N 2, the only crystalline phase was TiN. Transmission electron microscopy confirmed the presence of TiN crystallites in an amorphous matrix. Warren–Averbach analysis indicated the average size of the TiN crystallites to be 14.9 nm. In separate experiments with the same conditions, aerodynamic lenses were used to deposit particles directly onto TEM grids. We observed agglomerated structures with an Ar + N 2 plasma, while with an Ar + H 2 plasma we found discrete TiSi x and TiN x particles. In-situ particle diagnostics indicated only small changes in the particle size distributions when the plasma gases were changed.

Synchrotron Radiation Based Aerosol Time-of-Flight Mass Spectrometry for Organic Constituents

A synchrotron radiation based aerosol time-of-flight mass spectrometer using tunable vacuum-ultraviolet (VUV) light is described for real-time analysis of organic compounds in ultrafine and large aerosol particles. Particles are sampled from atmospheric pressure and are focused through an aerodynamic lens assembly into the mass spectrometer. As the particles enter the source region, they impinge on a cartridge heater and are vaporized. The particle vapor expands back into the source region and is softly ionized with tunable, quasicontinuous VUV light generated with synchrotron radiation. The radiation can be tuned to an energy close to the ionization energy of the sample molecules, thus minimizing the complications resulting from ion fragmentation. Photoionization efficiency scans (photon scans) can be readily collected, which permit measurement of the molecule's ionization energy and fragmentation onsets. Four high molecular weight, low vapor pressure organic compounds of importance in atmospheric aerosols are analyzed and their ionization energies measured with uncertainties of +/-60 meV. These are oleic acid (8.68 eV), linoleic acid (8.52 eV), linolenic acid (8.49 eV), and cholesterol (8.69 eV).

Aerodynamic Focusing of Nanoparticles: I. Guidelines for Designing Aerodynamic Lenses for Nanoparticles

This article describes the challenges in focusing nanoparticles (<30 nm) into tightly collimated beams, and provide guidelines for designing aerodynamic lens systems for nanoparticles. The major difficulties of focusing nanoparticles arise from their low inertia and high diffusivity. Because of their low inertia, nanoparticles tend to closely follow gas streamlines; their high diffusivities lead to beam broadening and diffusional deposition. We have identified the minimum particle size that can be focused to the axis with a single lens when diffusion is neglected, assuming that the flow is continuum and subsonic. We show that lighter carrier gases are preferred for focusing small particles, and that multiple lenses operating at suboptimal Stokes numbers can be designed to focus particles smaller than was recognized previously. There exists a maximum pressure under which particles can be optimally focused, while particle diffusion and pumping requirements are minimized. Finally, we describe the procedure for designing aerodynamic lens systems for focusing nanoparticles, and present a case study of designing a single aerodynamic lens to focus 5 nm particles.

Aerodynamic Focusing of Nanoparticles: II. Numerical Simulation of Particle Motion Through Aerodynamic Lenses

We have developed a numerical simulation methodology that is able to accurately characterize the focusing performance of aerodynamic lens systems. The commercial computational fluid dynamics (CFD) software FLUENT was used to simulate the gas flow field. Particle trajectories were tracked using the Lagrangian approach. Brownian motion of nanoparticles was successfully incorporated in our numerical simulations. This simulation tool was then used to evaluate the performance of an aerodynamic lens assembly that was designed to focus 3 nm spherical unit density particles following the guidelines described in Paper I (Wang et al. 2005). Our simulations show that the performance of this lens assembly is close to what is predicted by the design guidelines. The simulations also demonstrate the ability of aerodynamic lenses to focus sub-30 nm spherical unit density particles.

A New Time-of-Flight Aerosol Mass Spectrometer (TOF-AMS)—Instrument Description and First Field Deployment

We report the development and first field deployment of a new version of the Aerosol Mass Spectrometer (AMS), which is capable of measuring non-refractory aerosol mass concentrations, chemically speciated mass distributions and single particle information. The instrument was constructed by interfacing the well-characterized Aerodyne AMS vacuum system, particle focusing, sizing, and evaporation/ionization components, with a compact TOFWERK orthogonal acceleration reflectron time-of-flight mass spectrometer. In this time-of-flight aerosol mass spectrometer (TOF-AMS) aerosol particles are focused by an aerodynamic lens assembly as a narrow beam into the vacuum chamber. Non-refractory particle components flash-vaporize after impaction onto the vaporizer and are ionized by electron impact. The ions are continuously guided into the source region of the time-of-flight mass spectrometer, where ions are extracted into the TOF section at a repetition rate of 83.3 kHz. Each extraction generates a complete mass spect...

Transportable real-time single-particle ion trap mass spectrometer

A transportable ion trap mass spectrometer for real-time detection and characterization of individual airborne particles was constructed by minimal modification of a commercial ion trap mass spectrometer. A blank flange was replaced with a flange containing an aerodynamic lens based inlet, light scattering detection optics and ablation/ionization laser optics. Four holes were drilled into the ring electrode. Timing electronics boards running off of in-place power systems were added and integrated with the existing software. The modified mass spectrometer and laser system was packaged in a rugged wheeled frame for easy transport. Particles entered the instrument through a 100μm orifice and were passed through an aerodynamic lens system that produced a well-collimated particle beam over a wide range of sizes. The particle beam passed through a skimmer into the main chamber where individual particles were optically detected and sized with two focused 532nm diode lasers on their way to the ion trap. When the particles reached the center of the trap, they were ablated and ionized with a focused 266nm laser. The nascent ions were then mass analyzed using standard ion trap techniques, including tandem mass spectrometry. Each detected particle was characterized with a mass spectrum and an aerodynamically determined particle size. Careful design minimized the weight and size of the instrument to 104kg and 69×71×76cm, with power consumption less than 1.5kW. Tandem mass spectrometry was demonstrated for identification of ions through collision-induced dissociation (CID) up to mass spectrometry (MS).4 Unit mass resolution was observed in both the parent and CID mass spectra.

Determination of Single Particle Mass Spectral Signatures from Light-Duty Vehicle Emissions

In this study, 28 light-duty gasoline vehicles (LDV) were operated on a chassis dynamometer at the California Air Resources Board Haagen-Smit Facility in El Monte, CA. The mass spectra of individual particles emitted from these vehicles were measured using aerosol time-of-flight mass spectrometry (ATOFMS). A primary goal of this study involves determining representative size-resolved single particle mass spectral signatures that can be used in future ambient particulate matter source apportionment studies. Different cycles were used to simulate urban driving conditions including the federal testing procedure (FTP), unified cycle (UC), and the correction cycle (CC). The vehicles were selected to span a range of catalytic converter (three-way, oxidation, and no catalysts) and engine technologies (vehicles models from 1953 to 2003). Exhaust particles were sampled directly from a dilution and residence chamber system using particle sizing instruments and an ATOFMS equipped with an aerodynamic lens (UF-ATOFMS) analyzing particles between 50 and 300 nm. On the basis of chemical composition, 10 unique chemical types describe the majority of the particles with distinct size and temporal characteristics. In the ultrafine size range (between 50 and 100 nm), three elemental carbon (EC) particle types dominated, all showing distinct EC signatures combined with Ca, phosphate, sulfate, and a lower abundance of organic carbon (OC). The relative fraction of EC particle types decreased as particle size increased with OC particles becoming more prevalent above 100 nm. Depending on the vehicle and cycle, several distinct OC particle types produced distinct ion patterns, including substituted aromatic compounds and polycyclic aromatic hydrocarbons (PAH), coupled with other chemical species including ammonium, EC, nitrate, sulfate, phosphate, V, and Ca. The most likely source of the Ca and phosphate in the particles is attributed to the lubricating oil. Significant variability was observed in the chemical composition of particles emitted within the different car categories as well as for the same car operating under different driving conditions. Two-minute temporal resolution measurements provide information on the chemical classes as they evolved during the FTP cycle. The first two minutes of the cold start produced more than 5 times the number of particles than any other portion of the cycle, with one class of ultrafine particles (EC coupled with Ca, OC, and phosphate) preferentially produced. By number, the three EC with Ca classes (which also contained OC, phosphate, and sulfate) were the most abundant classes produced by the nonsmoking vehicles. The smoker category produced the highest number of particles, with the dominant classes being OC comprised of substituted monoaromatic compounds and PAHs, coupled with Ca and phosphate, thus suggesting used lubricating oil was associated with many of these particles. These studies show, by number, EC particles dominate gasoline emissions in the ultrafine size range particularlyforthe lowest emitting newer vehicles, suggesting the EC signature alone cannot be used as a unique tracer for diesels. This represents the first report of high time- and size-resolved chemical composition data showing the mixing state of nonrefractory elements in particles such as EC for vehicle emissions during dynamometer source testing.