Extrapolation of single particle soot photometer incandescent signal data

The Single Particle Soot Photometer (SP2) is an instrument for quantifying the refractory black carbon (rBC) mass of individual aerosol particles. It heats the particle’s rBC component to vaporization and quantifies the resulting visible thermal radiation to infer rBC mass. For purely technical reasons, SP2s are unable to quantify rBC mass beyond an easily adjustable limit due to eventual saturation of the electronics that record the visible light signals. Here, we evaluate an extrapolation algorithm to estimate rBC masses exceeding this upper limit in an SP2. The algorithm is based on identifying the crossing points of linear fits to unsaturated data, and using the duration of the saturated data to constrain potential errors. We find that extrapolation performance is quite insensitive to instrument parameters including laser intensity, rate of data acquisition, and particle speed through the laser. However, this approach increases uncertainty on the detection limit of the instrument, and is hence only useful in unknown aerosols for very limited extrapolation to approximately a factor of 1.5 increase in the upper mass range, corresponding to a 15% increase in the upper diameter limit. This increased range small enough that early identification of meaningful saturation during measurement campaigns remains the only tenable approach to robustly characterizing rBC mass size distributions and, in some cases, rBC mass concentrations.     

Investigations of SP-AMS Carbon Ion Distributions as a Function of Refractory Black Carbon Particle Type

The soot particle aerosol mass spectrometer (SP-AMS) instrument combines continuous wave laser vaporization with electron ionization aerosol mass spectrometry to characterize airborne, refractory black carbon (rBC) particles. The laser selectively vaporizes absorbing rBC-containing particles, allowing the SP-AMS to provide direct chemical information on the refractory and non-refractory chemical components, providing the potential to fingerprint various rBC particle types. In this study, SP-AMS mass spectra were measured for 12 types of rBC particles produced by industrial and combustion processes to explore differences in the carbon cluster (C_n⁺) mass spectra. The C_n⁺ mass spectra were classified into three categories based on their ion distributions, which varied with rBC particle type. The carbon ion distributions were investigated as a function of laser power, electron ionization (on/off), and ion charge (positive or negative). Results indicate that the dominant positive ion-formation mechanism is likely the vaporization of small, neutral carbon clusters followed by electron ionization (C₁⁺ to C₅⁺). Significant ion signal from larger carbon cluster ions (and their fragment ions in the small carbon cluster range), including mid carbon (C₆⁺ to C₂₉⁺) and fullerene (greater than C₃₀⁺) ions, were observed in soot produced under incomplete combustion conditions, including biomass burning, as well as in fullerene-enriched materials. Fullerene ions were also observed at high laser power with electron ionization turned off, formed via an additional ionization mechanism. We expect this SP-AMS technique to find application in the identification of the source and atmospheric history of airborne ambient rBC particles.

Copyright 2015 American Association for Aerosol Research     

A New Method to Determine the Number Concentrations of Refractory Black Carbon Ice Nucleating Particles

Ice nucleating particles (INP) initiate heterogeneous ice nucleation in mixed-phase clouds, influencing cloud phase and onset temperatures for ice formation. Determination of particle types contributing to atmospheric INP populations requires isolation of the relatively rare INP from a total particle sample, typically followed by time-consuming single-particle characterization. We propose a method to estimate the contributions of light-absorbing, primarily refractory black carbon (rBC), particles to INP populations by selectively removing them prior to determination of INP concentrations. Absorbing particles are heated to their vaporization temperature using laser induced incandescence in a single particle soot photometer (SP2) and the change in INP number concentrations, compared to unheated samples, is assessed downstream in the CSU Continuous Flow Diffusion Chamber (CFDC). We tested this approach in the laboratory using strongly-absorbing and nonabsorbing aerosol types to confirm effective removal of rBC INP and to explore the impact of the processing on non-light-absorbing INP. An INP-active rBC particle type was efficiently removed, while nonabsorbing kaolinite and a soil-based INP were not affected by laser exposure. Results for the products of wiregrass combustion indicated that absorbing particles, primarily rBC, accounted for about 40% of all INP, consistent with electron microscopy of INP emitted during prescribed burns of this fuel type. However, kaolinite internally mixed with rBC exhibited reduced activity after passing through the SP2, suggesting that the validity of the method for realistic internal mixtures needs additional research. The sensitivity of the CFDC presently limits applicability of the method to relatively high INP number concentration samples.

Copyright 2014 American Association for Aerosol Research     

A novel inversion method to determine the mass distribution of non-refractory coatings on refractory black carbon using a centrifugal particle mass analyzer and single particle soot photometer

A novel inversion method is presented, which derives the two-variable number distribution for black carbon aerosol, using a coupled centrifugal particle mass analyzer (CPMA) and single particle soot photometer (SP2). The CPMA classifies all particles by their mass-to-charge ratio, and the SP2 detects the mass of refractive black carbon (rBC) in each individual particle. The results of the inversion are the simultaneous number distributions of both rBC mass and total particle mass. Using the distribution, the coating distribution on a population of rBC particles can be identified visually. Furthermore, the distribution can be integrated to find one-variable mass and number concentration distributions as a function of total or rBC particle mass. These capabilities were demonstrated via smog chamber experiments, where an organic (non-rBC) coating was grown onto uncoated rBC aerosol over several hours via photo-oxidation of p-xylene. The particle distributions were constructed using the inversion over a range of 1–60 fg of total particle mass. As the non-rBC coating thickness increased over time, a shift in the number distribution toward higher total mass was observed. At the end of the experiment, uncoated rBC was injected into the chamber, and the distribution was clearly resolved using the inversion. The CPMA-SP2 method offers several advantages over “SP-2 only” methods, namely, (i) coating mass information can be obtained over a wider range of total particle mass, (ii) total particle mass is measured directly, and (iii) it does not make core–shell morphology assumptions.

Copyright © 2018 American Association for Aerosol Research     

Evaluation of a Heated-Inlet for Calibration of the SP2

Black carbon (BC) calibration standards, such as fullerene soot, are routinely used to calibrate single-particle soot photometer (SP2) instruments. Impurities in BC standards create uncertainties in these calibrations, and thus it is desirable to remove non-BC compounds from the aerosol, though removal processes must not significantly alter BC microphysical properties. We present a series of experiments using mobility- and mass-selected fullerene soot particles to assess the performance of a high-temperature denuder system for treating BC prior to SP2 analysis. Particle mass, incandescence, and scattering properties were measured by tandem aerosol particle mass analyzers and an SP2, after thermal treatment at a range of temperatures and residence times (RT). For a longer RT (e.g., ～6 s at 300°C), monodisperse fullerene soot particles of initial mass 1.4 fg decreased in mass with increasing temperature, by 3% at 300°C to 15% at 600°C. Mass losses were similar for fullerene soot particles of initial mass 10.7 fg. The peak height of the particle laser-induced incandescence (LII) and scattering intensities of the 10.7 fg fullerene soot increased by 7% and 3%, respectively, at 300°C, and by over 15% and 10% at 400°C, possibly due to microphysical changes after heating. When sampling through a 300°C denuder with a particle RT of 2.5 s, the LII intensity of ambient BC particles of initial mass 1.1 fg increased by 8%. In light of these results, denuder temperatures of ～300°C with 0.4 s ≤ RT ≤ 2.5 s are recommended for SP2 calibration.

Copyright 2013 American Association for Aerosol Research     

Dependence of Laser-Induced Incandescence on Physical Properties of Black Carbon Aerosols: Measurements and Theoretical Interpretation

We used a single-particle soot photometer (SP2) to measure the mass of individual black carbon (BC) particles down to ～ 0.5 fg by means of laser-induced incandescence with an intra-cavity, continuous-wave laser. The incandescence of nine different types of BC samples was investigated to provide a physical basis for choosing appropriate BC materials for SP2 calibration. We estimated the vaporization temperatures of these BC samples from the spectral dependence of incandescence at the limit of the small size parameter x, for which spectral dependence of emissivity is known a priori. The vaporization temperatures differed by less than 2.2% among the samples. For the x < 1 regime of particle size, the peak amplitude of the incandescence signal measured by the SP2 was linearly proportional to the particle mass. The slopes of such linear proportionality were positively correlated with | (m ² -1)/(m ² +2)|, where the m is the complex refractive index of the BC particle. For particles in which x > 1, the rate of increase in the peak amplitude of the incandescence signal with increasing particle mass was negatively correlated with the compactness of particle shape, consistent with the theoretical prediction of emissivity, which accounts for particle shape. The incandescence–BC mass relationships were similar between fullerene soot and ambient soot sampled in Tokyo, thus suggesting that fullerene soot is a suitable calibration standard for SP2 measurements of ambient soot.     

Soot Particle Aerosol Mass Spectrometer: Development,Validation, and Initial Application

The Soot Particle Aerosol Mass Spectrometer (SP-AMS) was developed to measure the chemical and physical properties of particles containing refractory black carbon (rBC). The SP-AMS is an Aerodyne Aerosol Mass Spectrometer (AMS) equipped with an intracavity laser vaporizer (1064 nm) based on the Single Particle Soot Photometer (SP2) design, in addition to the resistively heated, tungsten vaporizer used in a standard AMS. The SP-AMS can be operated with the laser vaporizer alone, with both the laser and tungsten vaporizers, or with the tungsten vaporizer alone. When operating with only the laser vaporizer, the SP-AMS is selectively sensitive to laser-light absorbing particles, such as ambient rBC-containing particles as well as metal nanoparticles, and measures both the refractory and nonrefractory components. When operated with both vaporizers and modulating the laser on and off, the instrument measures the refractory components of absorbing particles and the nonrefractory particulate matter of all sampled particles. The SP-AMS design, mass spectral interpretation, calibration, and sensitivity are described. Instrument calibrations yield a sensitivity of greater than 140 carbon ions detected per picogram of rBC mass sampled, a 3σ detection limit of less than 0.1 μg·m^?3 for 60 s averaging, and a mass-specific ionization efficiency relative to particulate nitrate of 0.2 ± 0.1. Sensitivities were found to vary depending upon laser-particle beam overlap. The utility of the instrument to characterize ambient rBC aerosol is demonstrated.

Copyright 2012 American Association for Aerosol Research     

Practical limitations of aerosol separation by a tandem differential mobility analyzer–aerosol particle mass analyzer

A cavity ring-down spectrometer and condensation particle counter were used to investigate the limitations in the separation of singly and multiply charged aerosol particles by a tandem differential mobility analyzer (DMA) and aerosol particle mass analyzer (APM). The impact of particle polydispersity and morphology was investigated using three materials: nearly monodisperse polystyrene latex nanospheres (PSL); polydisperse, nearly spherical ammonium sulfate (AS), and polydisperse lacey fractal soot agglomerates. PSL and AS particles were easily resolved as a function of charge. For soot, the presence of multiply charged particles severely affects the isolation of the singly charged particles. In cases where the DMA–APM was unable to fully resolve the singly charged particles of interest, the peak mass deviated by up to 13% leading to errors in the mass specific extinction cross section of over 100%. For measurements of nonspherical particles, nonsymmetrical distributions of concentration as a function of mass were a sign of the presence of multiply charged particles. Under these conditions, the effects of multiply charged particles can be reduced by using a second charge neutralizer after the DMA and prior to the APM. Dilution of the aerosol stream serves to decrease the total number concentration of particles and does not remove the contributions of multiply charged particles.     

A High Resolution Study of the Effect of Morphology On the Mass Spectra of Single PSL Particles with Na-containing Layers and Nodules

The interpretation and quantification of measurements of particle composition by laser ablation based single particle mass spectrometry is complex. Among the most difficult systems to quantify are internally mixed particles containing alkali metals and organics. The alkali atoms in such particles tend to suppress the formation of other ions sometimes to below the detection limit. Here we present a study of the behavior of single particle mass spectral peak intensities as a function of the amount of the sodium containing compounds deposited on the surface of 240 nm polystyrene latex (PSL) spheres. We generate three morphologically distinct and well defined coating types: uniform layers, cubic nodules and rounded nodules, and measure the individual particle mass spectra as a function of the vacuum aerodynamic diameter with nanometer resolution. The data show that the probability of detecting the PSL spheres depends on the amount of the alkali metal on the PSL sphere surface, its morphological distribution and the ablation laser power. The data suggest that PSL spheres with localized Na-containing nodules are easier to detect than those which are completely encapsulated. We show, for example, that at low laser power, PSL particles that are completely encapsulated with Na-containing compounds, whose weight fraction is close to 50%, cannot be detected, while 35% of PSL spheres with same amount of coating can be detected if coating is localized in nodules on a fraction of the particle surface.     

Optimized detection of particulates from liquid samples in the aerosol phase: Focus on black carbon

Operational parameters for a single particle soot photometer (SP2) and a CETAC Marin-5 nebulizer were optimized for detection of particulates aerosolized from liquid samples. The sensitivity of nebulization efficiency on nebulizer input gas pressure, liquid sample flow rate, and alcohol doping of the sample were explored. The nebulization efficiency of the Marin-5 was found to be roughly independent of applied gas pressure once above a minimum pressure. The nebulization efficiency changed by ～50% for an order of magnitude change in liquid sample flow. Doping the sample with isopropyl alcohol at a 1:1 ratio results in a ～50% relative increase in nebulization efficiency over a broad range of liquid flows. These results should apply to all particulate materials in the size range studied. SP2 operational parameters including sheath and sample flow were explored to optimize detection of refractory black carbon (rBC) specifically via coupling to the nebulizer. The SP2 tested samples up to 5 cc s?1 with 100% detection of rBC in its size range of detection, with increased sample jet spread and corresponding lack of detected rBC in the air at higher flows, leading to a total undetected rBC mass fraction of ～15% at 16 cc s?1. Varying sheath flow does not improve this result, which is significant because under reasonable Marin-5 operating conditions, the SP2 only samples a fraction of the total air flow out of the nebulizer. Recommended operational parameters for cases of sample with low rBC loadings are presented: first, when very little liquid sample is available; second, when considerable sample is available.

© 2017 American Association for Aerosol Research     

Distinguishing fuel and lubricating oil combustion products in diesel engine exhaust particles

The main sources of particulate emissions from engines are fuel and lubricating oil. In this study, particles emitted by a medium speed diesel engine for locomotive use were characterized chemically by using a soot particle aerosol mass spectrometer (SP-AMS). Additionally, positive matrix factorization (PMF) was applied to the SP-AMS data for the separation of fuel from lubricating oil and/or oil additives in diesel engine emissions. The mass spectra of refractory species, i.e., metals and rBC, were included in the PMF input matrix in addition to organics in order to utilize the benefit of the SP-AMS to measure non-refractory and refractory species. In general, particulate matter emitted by the diesel engine was dominated by organics (51%) followed by refractory black carbon (rBC; 48%), trace metals and inorganic species (1%). Regarding the sources of particles, PMF indicated four factors for particle mass of which two were related to lubricating oil-like aerosol (LOA1, 29% and LOA2, 24%) and two others to diesel-like fuel aerosol (DFA1, 35% and DFA2, 12%). The main difference between LOA1 and LOA2 was the presence of soot in LOA1 and metals in LOA2 factors. DFA factors represented burned (DFA1) and unburned fuel (DFA2). The results from the PMF analysis were completed with particle size distributions, volatility measurements and particle morphology analyses.

Copyright © 2019 American Association for Aerosol Research     

Quantification of online removal of refractory black carbon using laser-induced incandescence in the single particle soot photometer

Refractory black carbon (rBC) is an aerosol that has important impacts on climate and human health. rBC is often mixed with other species, making it difficult to isolate and quantify its important effects on physical and optical properties of ambient aerosol. To solve this measurement challenge, a new method to remove rBC was developed using laser-induced incandescence (LII) by Levin et al. in 2014. Application of the method with the Single Particle Soot Photometer (SP2) is used to determine the effects of rBC on ice nucleating particles (INP). Here, we quantify the efficacy of the method in the laboratory using the rBC surrogate Aquadag. Polydisperse and mobility-selected samples (100–500 nm diameter, 0.44–36.05 fg), are quantified by a second SP2. Removal rates are reported by mass and number. For the mobility-selected samples, the average percentages removed by mass and number of the original size are 88.9 ± 18.6% and 87.3 ± 21.9%, respectively. Removal of Aquadag is efficient for particles >100 nm mass-equivalent diameter (d_me), enabling application for microphysical studies. However, the removal of particles ≤100 nm d_me is less efficient. Absorption and scattering measurements are reported to assess its use to isolate brown carbon (BrC) absorption. Scattering removal rates for the mobility-selected samples are >90% on average, yet absorption rates are 53% on average across all wavelengths. Therefore, application to isolate effects of microphysical properties determined by larger sizes is promising, but will be challenging for optical properties. The results reported also have implications for other instruments employing internal LII, e.g., the Soot Particle Aerosol Mass Spectrometer (SP-AMS).

© 2016 American Association for Aerosol Research     

Method to measure refractive indices of small nonspherical particles: Application to black carbon particles

We demonstrate a new method to measure refractive indices m of small nonspherical particles by simultaneous measurement of particle volume and light-scattering cross section. In contrast to traditional methods, this method can suppress uncertainties due to unknown particle shape. As demonstration of the method, laboratory experiments using an aerosol particle mass analyzer (APM) and single-particle soot photometer (SP2) have been conducted to determine the refractive indices of seven types of commercial black carbon particles and ambient soot at 1064 nm wavelength. Difference in measured |(m²?1)/(m²+2)| values among the seven commercial black carbon samples is consistent with the difference in their crystalline structures identified by a transmission electron microscope. The complex refractive index m=(n, k) of ambient soot particles in Tokyo urban area was determined to be m=(2.26±0.13, 1.26±0.13).     

Aerosol measurement by laser doppler spectroscopy—I. Theory and experimental results for aerosols homogeneous

The basic theory, experimental techniques and results are presented describing a technique for sizing aerosol particles in situ using laser Doppler spectroscopy. Unlike conventional light scattering procedures which use average intensity information, this technique utilizes the Doppler shifted frequency of the scattered light produced by the Brownian motion of the aerosol particles to determine particle diffusion coefficients and size. Experiments were carried out using monodisperse dibutylpthalate aerosols and monodisperse polystyrene latex spheres, in concentrations ranging from 10³ to 10⁶ particles per cubic centimeter. Measured particle sizes were within 10 per cent of the size predicted by conventional light scattering methods for the DBP particles and the reported sizes of the PSL particles. Based on these results it is concluded that laser Doppler spectroscopy can be utilized to accurately measure aerosol particle size in situ.     

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.     

Using ATOFMS to Determine OC/EC Mass Fractions in Particles

Historically, obtaining quantitative chemical information using laser desorption ionization mass spectrometry for analyzing individual aerosol particles has been quite challenging. This is due in large part to fluctuations in the absolute ion signals resulting from inhomogeneities in the laser beam profile, as well as chemical matrix effects. Progress has been made in quantifying atomic species using high laser powers, but very few studies have been performed quantifying molecular species. In this study, promising results are obtained using a new approach to measure the fraction of organic carbon (OC) associated with elemental carbon (EC) in aerosol particles using single particle laser desorption ionization. A tandem differential mobility analyzer (TDMA) is used to generate OC/EC particles by size selecting EC particles of a given mobility diameter and then coating them with known thicknesses of OC measured using a second DMA. The mass spectra of the OC/EC particles exiting the second DMA are measured using an ultrafine aerosol time-of-flight mass spectrometer (UF-ATOFMS). A calibration curve is produced with a linear correlation (R² = 0.98) over the range of OC/EC ion intensity ratios observed in source and ambient studies. Importantly, the OC/EC values measured in ambient field tests with the UF-ATOFMS show a linear correlation (R² = 0.69) with OC/EC mass ratios obtained using semi-continuous filter based thermo-optical measurements. The calibration procedure established herein represents a significant step toward quantification of OC and EC in sub-micron ambient particles using laser desorption ionization mass spectrometry.     

Design and characterization of a novel single-particle polar nephelometer

A new polar nephelometer (PN) has been developed to measure simultaneously the scattering angular distributions from 11.7° to 168.3° for individual particles in planes parallel and perpendicular to the polarization of the incident laser beam. Each detection plane had 21 silicon photodiode detectors to detect scattered light at a rate of 100 Hz. Laboratory experiments to validate the performance of the instrument were conducted using nearly mono-disperse spherical particles (polystyrene latex [PSL] and nigrosine) and nonspherical particles (sodium chloride [NaCl] and soot). The observed scattering angular distributions for individual PSL particles were in good agreement with the results of simulations based on Mie theory. Complex refractive index values for nigrosine particles were determined by comparing the observed scattering angular distributions with the results of simulations. Clear differences between the measured scattering angular distributions and the results of simulations based on Mie theory assuming spherical particles were observed for NaCl particles (mobility diameters of 500 and 700 nm) and propane soot particles (mobility diameters of 300, 500, and 700 nm). These results are reasonably explained by theoretical predictions. We also conducted initial observations of ambient particles in Nagoya city, Japan. Scattering angular distributions for particles with a mobility diameter of 500 nm and an average effective density of 1.4 or 0.3 g/cm³, which were selected with a combination of differential mobility analyzer and aerosol mass particle analyzer, were measured using the PN. As results, scattering angular distributions for nearly spherical inorganic and organic particles with an average effective density of around 1.4 g/cm³ were found to be distinguishable from nonspherical particles with an average effective density of around 0.3 g/cm³. This study has demonstrated that our PN has the potential to distinguish between spherical and nonspherical particles.

Copyright © 2016 American Association for Aerosol Research     

A novel optical aerosol detector utilizing an optic fiber with conductive polymer coating

The detection of atmospheric aerosol particles is becoming an important issue in many fields such as environmental science, occupational medicine, semiconductor industry and material science. In the present paper, we utilized the conductive polymer, polypyrrole (PPy), as a sensitive membrane for detecting aerosol particles optically. A polymer optical fiber reflectance probe is constructed by depositing the PPy nanofilm at the end face of the fiber. The sensor principle relies on the change in the refractive index of the PPy nanofilm upon its interaction with aerosol nanoparticles and on the electrostatic induction between aerosol particles and the PPy nanofilm, which leads to a change in the reflected intensity. For preliminary evaluation of optical aerosol detector, three types of aerosol particles, NaCl, black carbon (BC) and polystyrene latex (PSL), are selected. The fabricated fiber optic reflectance probe using the PPy nanofilm shows distinct variations in the reflected light intensity depending on the type of aerosol particle and its properties. The proposed sensing approach may promote the use of conductive polymers in optical techniques for the detection of atmospheric aerosols.     

Examining the Relationship Between Black Carbon and Soot in Flames and Engine Exhaust

This article investigates the black carbon (BC) content of soot formed in premixed and diffusion flames and emitted by light duty gasoline and diesel vehicles. BC is measured photoacoustically and compared with particulate mass collected by filter and calculated from particle size distributions. The BC fraction of soot from rich premixed ethylene flames increases with height above the burner, but can remain well below unity in modestly sooting flames. The BC fraction produced by a propane diffusion flame soot generator (combustion aerosol standard, CAST) falls as the fuel is diluted with nitrogen, the principal means used to adjust the desired particle size. Thermally treating the soot to remove possible condensed semivolatile species does little to change these trends. Transmission electron microscopy (TEM) images show that despite low BC content, these particles display the characteristic fractal-like agglomerate morphology of soot. Particle mass spectra reveal polycyclic aromatic hydrocarbon (PAH) and fullerene fragments associated with low BC soot, which disappear as the BC fraction approaches unity. The results suggest that low BC content reflects immature solid soot that has not carbonized. Particulate matter (PM) measurements from current technology diesel and gasoline vehicles exhibit a high, >80% BC fraction. This is attributed to effective soot carbonization during the expansion and exhaust strokes of the engine, and to the substantial reductions of condensable hydrocarbons by catalytic aftertreatment. These results are discussed with respect to using light absorption-based instruments to monitor engine exhaust PM and using flame-generated soot for PM instrument calibration.     

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 (NH₄)₂SO₄ aerosol since the spectra of diesel soot and (NH₄)₂SO₄ 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.