2009. "SPLAT II: An Aircraft Compatible, Ultra-Sensitive, High Precision Instrument for In-Situ Characterization of the Size and Composition of Fine and Ultrafine Particles." Aerosol Science and Technology 43(5):411-424. Abstract The properties of aerosols depend on the size and internal compositions of the individual particles. The vast majority of atmospheric aerosols are smaller than 200 nm, yet the single particle mass spectrometers, the only instruments that can characterize the size and internal compositions of individual particles, typically detect these small particles with extremely low efficiencies. In this paper we describe a new instrument called SPLAT II that provides unparalleled sensitivity to small particles, detecting 100% of particles that are larger than 125 nm and 40% of 100 nm particles. This instrument also brings an increase by a factor of 10 in temporal resolution, sizing up to 500 particles per second and characterizing the composition of up to 100 of them. SPLAT II uses a two-laser, two-step process to evaporate the particles and generate ions, producing high quality, reproducible mass spectra of the refractive and non-refractive aerosol fractions to yield the complete compositions of individual particles. The instrument control board provides for size dependent delays for lasers’ triggers to eliminate a size dependent hit rate. The mass spectra are recorded with 14-bit vertical resolution and analyzed using custom software packages. The instrument’s high sizing resolution and sensitivity makes it possible to combine it with the differential mobility analyzer(s) and measure particle size, composition, density, dynamic shape factor, hygroscopicity, and fractal dimension.
2009. "Comparison Between Mass Spectra of Individual Organic Particles Generated by UV Laser Ablation and in the IR/UV Two-Step Mode." International Journal of Mass Spectrometry 282(1-2):6-12. Abstract One of the most fundamental aspects of single particle mass spectrometry is that the individual particle mass spectra are first classified and only then averaged. When ions are generated by ablation with a UV laser the mass spectra of particles with identical compositions exhibit large particle to particle fluctuations in the mass spectral intensity pattern. This is particularly true when it comes to particles containing organic molecules. At laser fluence that is sufficient to ionize sulfates many of the organic molecules exhibit high degree of fragmentation, often to the point where they cannot be distinguished from elemental carbon. In contrast, when ion generation is separated into two steps, in which the first step uses infra red to evaporate the semi-volatile components and the second, time delayed UV pulse to ionize the evaporating plume, the quality of the individual particle mass spectra are significantly improved. We present an experimental investigation of the properties and behavior of individual particle mass spectra of organic particles that are generated by ablation and in two steps. We investigate the effect of UV laser fluence and the delay between the two lasers. The study shows that the two step approach yields highly reproducible mass spectra that contain sufficient detail to allow clear molecular assignments. The two step approach also produces 10 times as many ions, and the mass spectral intensity can be related to the amount of organics in the particle. In contrast, ablation generated mass spectra were found to exhibit high degree of fragmentation and particle-to-particle fluctuations.
2009. "Beyond single particle mass spectrometry: multidimensional characterisation of individual aerosol particles." International Reviews in Physical Chemistry 28(2):309-358. Abstract The behavior of small aerosol particles depends on a number of their physical and chemical properties, many of which are strongly coupled. The size, internal composition, density, shape, morphology, hygroscopicity, index of refraction, activity as cloud condensation nuclei and ice nuclei, and other attributes of individual particles - all play a role in determining particle properties and their impacts. The traditional particle characterization approaches rely on separate parallel measurements that average over an ensemble of particles of different sizes and/or compositions and later attempt to draw correlations between them. As a result such studies overlook critical differences between particles and bulk and miss the fact that individual particles often exhibit major differences. Here we review the recently developed methods to simultaneously measure in-situ and in real time several of the attributes for individual particles using single particle mass spectrometer, SPLAT or its second generation SPLAT II. We also discuss novel approaches developed for classification, visualization and mining of large datasets produced by the multidimensional single particle characterization.
2009. "Achieving Size Independent Hit-Rate in Single Particle Mass Spectrometry." Aerosol Science and Technology 43(4):305-310. Abstract Recent improvements in single particle mass spectrometers make it possible to optically detect, size, and characterize the compositions of individual particles with diameters larger than a micron and smaller than 100 nm. Based on particle detection in two stages of optical detection these instruments generate a precisely timed trigger pulse, which is used to fire the ion generation laser or lasers. Practical experience shows that the wide size range results in small, but significant differences in laser trigger timing between small and large particles. If not treated, the instrument hit-rate becomes size dependent and instrument operator is forced to optimize the instrument for the desired size range, while having to contend with a lower hit-rate for the other. The present paper presents an analysis of the problem, demonstrating that size dependence of laser trigger timing stems from the differences in the particle position within the detection laser beam at the instant of detection. It shows that it is possible to compensate for these differences by generation a laser trigger delay coefficient for individual particles as a function of particle time of flight, i.e. its size. The study also shows that a single function can be used to characterize particles with a wide range of densities.
2009. "Single Wall Diesel Particulate Filter (DPF) Filtration Efficiency Studies Using Laboratory Generated Particles." Chemical Engineering Science 64(8):1625-1634. Abstract Diesel offers higher fuel efficiency, but produces higher exhaust particulate matter. Diesel particulate filters are presently the most efficient means to reduce these emissions. These filters typically trap particles in two basic modes: at the beginning of the exposure cycle the particles are captured in the filter holes, and at longer times the particles form a "cake" on which particles are trapped. Eventually the "cake" removed by oxidation and the cycle is repeated. We have investigated the properties and behavior of two commonly used filters: silicon carbide (SiC) and cordierite (DuraTrap® RC) by exposing them to nearly-spherical ammonium sulfate particles. We show that the transition from deep bed filtration to "cake" filtration can easily be identified by recording the change in pressure across the filters as a function of exposure. We investigated performance of these filters as a function of flow rate and particle size. The filters trap small and large particles more efficiently than particles that are ~80 to 200 nm in aerodynamic diameter. A comparison between the experimental data and a simulation using incompressible lattice-Boltzmann model shows very good qualitative agreement, but the model overpredicts the filter’s trapping efficiency.
2008. "Simultaneous Measurements of Individual Ambient Particles Size, Composition, Effective Density, and Hygroscopicity." Analytical Chemistry 80(5):1401-1407. doi:10.1021/ac701723v Abstract The interaction between atmospheric particles and water vapor impacts directly and significantly the effect that these particles exert on the atmosphere. The hygroscopicity of individual particles, which is a quantitative measure of their response to changes in relative humidity, is related to their internal compositions. To properly include atmospheric aerosols in any model requires knowledge of the relationship between particle size, composition and hygroscopicity. Here we demonstrate the capability to conduct in real-time the simultaneous measurements of individual ambient particle hygroscopic growth factors, densities and compositions using a hydrated tandem differential mobility analyzer that is coupled to an ultra-sensitive single particle mass spectrometer. We use as an example the class of particles that are composed of sulfate mixed with oxygenated organics to illustrate how such multidimensional single particle characterization can be used to yield quantitative information about the composition of individual particles. We show that the data provide the relative concentrations of organics and sulfates, the density of the two fractions and particle hygroscopicity.
2008. "ClusterSculptor: Software for Expert-Steered Classification of Single Particle Mass Spectra." International Journal of Mass Spectrometry 275(1-3):1-10. doi:10.1016/j.ijms.2008.04.033 Abstract To take full advantage of the vast amount of highly detailed data acquired by single particle mass spectrometers requires that the data be organized according to some rules that have the potential to be insightful. Most commonly statistical tools are used to cluster the individual particle mass spectra on the basis of their similarity. Cluster analysis is a powerful strategy for the exploration of high-dimensional data in the absence of a-priori hypotheses or data classification models, and the results of cluster analysis can then be used to form such models. More often than not, when examining the data clustering results we find that many clusters contain particles of different types and that many particles of one type end up in a number of separate clusters. Our experience with cluster analysis shows that we have a vast amount of non-compiled knowledge and intuition that should be brought to bear in this effort. We will present new software we call ClusterSculptor that provides comprehensive and intuitive framework to aid scientists in data classification. ClusterSculptor uses k-means as the overall clustering engine, but allows tuning its parameters interactively, based on a non-distorted compact visual presentation of the inherent characteristics of the data in high-dimensional space. ClusterSculptor provides all the tools necessary for a high-dimensional activity we call cluster sculpting. ClusterSculptor is designed to be coupled to SpectraMiner, our data mining and visualization software package. The data are first visualized with SpectraMiner and identified problems are exported to ClusterSculptor, where the user steers the reclassification and recombination of clusters of tens of thousands particle mass spectra in real-time. The resulting sculpted clusters can be then imported back into SpectraMiner. Here we will greatly improved single particle chemical speciation in an example of application of this new tool to a number of particle types of atmospheric importance.
2008. "A New Real-Time Method for Determining Particles Sphericity and Density: Application to Secondary Organic Aerosol Formed by Ozonolysis of alpha-Pinene." Environmental Science & Technology 42(21):8033-8038. doi:10.1021/es8013562 Abstract Particle volumes are most often obtained by measuring particle mobility size distributions and assuming that the particles are spherical. These volumes are then converted to mass loads by using particle densities that are commonly either assumed or estimated from the measured mobility and vacuum aerodynamic diameters assuming again that the particles are spherical. Depending on the system, these assumptions can introduce significant errors. We present a new method that can be applied to any particle system to determine in real-time whether the particles are spherical or not. We use our 2nd generation single particle mass spectrometer (SPLAT II) to measure with extremely high precision the vacuum aerodynamic size distributions of particles classified by differential mobility analyzer (DMA) and demonstrate that the line shape of these distributions provide a way to unambiguously distinguish between spherical and aspherical particles. Moreover, the very same experimental system is used to obtain in addition to individual particle size, its density, composition and dynamic shape factor. We illustrate the application of this method to secondary organic aerosols formed as a result of ozonolysis of α-pinene in the presence and absence of an OH scavenger and find these particles to be spherical with densities of 1.198±0.004 gcm-3 and 1.213±0.003 gcm-3 respectively.
2008. ""Depth-Profiling" and Quantitative Characterization of the Size, Composition, Shape, Density, and Morphology of Fine Particles with SPLAT, a Single-Particle Mass Spectrometer." Journal of Physical Chemistry A 112(4):669-677. doi:10.1021/jp077308y Abstract A significant fraction of atmospheric particles are composed of inorganic substances that are mixed or coated with organics. The behavior of these particles depends on the particle internal composition and on the arrangement of the specific constituents in each particle. It is important to know which constituent is on the surface and whether it entirely covers the particle surface. We present a study that demonstrates that an instrumental system that includes an ultra-sensitive single particle mass spectrometer that is coupled with a differential mobility analyzer can be used to quantitatively measure in real-time individual particle composition, size, density, shape and determine which substance is on the surface and whether it entirely covers the particle. Here we use liquid dioctyl phthalate to coat NaCl seeds and generate spherical particles that are encapsulated with the organic coat and pyrene, a solid poly aromatic hydrocarbon, to produce aspherical particles with pyrene nodules and exposed NaCl cores. We show that the behavior of the mass spectral intensities as a function of laser fluence yields information that can be used to determine the morphological distribution of individual particles constituents.
2008. "Photooxidation of Alpha-Pinene at High Relative Humidity in the Presence of Increasing Concentrations of NOx." Atmospheric Environment 42(20):5044-5060. doi:10.1016/j.atmosenv.2008.02.026 Abstract The photooxidation of ~1 ppm alpha-pinene in the presence of increasing concentrations of NO2 was studied in a Teflon chamber at relative humidities from 70 - 88% and temperatures from 296 - 304 K. The loss of alpha-pinene and formation of gas phase products were followed using proton transfer reaction mass spectrometry (PTR-MS). Gas phase reaction products measured by PTR-MS and their yields include formaldehyde (5 + 1%), formic acid (2.5 + 1.4%), methanol (0.6 + 0.3%), acetaldehyde (3.9 + 1.7%), acetic acid (10 + 2%), acetone (11.5 + 3.1%), pinonaldehyde (22 + 6%), and pinene oxide (0.9 + 0.1%). There was evidence of organic nitrates in the gas phase and small peaks were tentatively assigned to norpinonaldehyde, 4-oxopinonaldehyde, propanedial, 2,3-dioxobutanal and 3,5,6-trioxoheptanal or 3-hydroxymethyl-2,2-dimethylcyclobutylethanone. The formation and growth of new particles were followed using a scanning mobility particle sizer (SMPS), and their chemical composition was probed using single particle mass spectrometry (SPLAT II). SPLAT II analysis also provided measurements of the vacuum aerodynamic diameters of the newly formed secondary organic aerosol (SOA) particles and, in combination with the electrical mobility diameter, a particle density of 1.21 + 0.02 g cm-3 was calculated, 20% larger than often assumed in calculating SOA yields. SPLAT II showed that the suspended SOA consisted of a complex mixture of organic nitrates and organics, possibly including pinonic acid, pinic acid and trans-sobrerol. Three-wavelength light scattering measurements made using an integrating nephelometer were consistent with particles having a refractive index characteristic of organic compounds, but the data could not be well matched at all three wavelengths with a single refractive index. The effect of addition of cyclohexane or NO on particle formation showed that ozonolysis was the major mechanism of SOA formation in this system. However, unlike simple ozonolysis, organic nitrates are formed in both the gas and particle phases. Identifying and measuring specific organic nitrates in both the gas and particle phases in air may help to elucidate why SOA formation has been reported in field studies to be associated with polluted urban areas, yet the carbon in these particles is largely contemporary, i.e., non-fossil fuel carbon.
2008. "Nitrate Ion Photochemistry at Interfaces: A New Mechanism for Oxidation of alpha-Pinene." Physical Chemistry Chemical Physics. PCCP 10(21):3063-3071. doi:10.1039/b719495a Abstract The photooxidation of 0.6 - 0.9 ppm α-pinene in the presence of a deliquesced thin film of NaNO3, and for comparison increasing concentrations of NO2, was studied in a 100 L Teflon® chamber at relative humidities from 70 − 88% and temperatures from 296 − 304 K. The loss of α-pinene and the formation of gaseous products were followed with time using proton transfer mass spectrometry. The yields of gas phase products were smaller in the NaNO3 experiments than in NO2 experiments. In addition, pinonic acid, pinic acid, trans-sobrerol and other unidentified products were detected in the extracts of the wall washings only for the NaNO3 photolysis. These data indicate enhanced loss of α-pinene at the NaNO3 thin film during photolysis. Supporting the experimental results are molecular dynamics simulations which predict that α-pinene has an affinity for the surface of the deliquesced nitrate thin film, enhancing the opportunity for oxidation of the impinging organic gas during the nitrate photolysis. This new mechanism of oxidation of organics may be partially responsible for the correlation between nitrate and the organic component of particles observed in many field studies, and may also contribute to the missing source of SOA needed to reconcile model predictions and field measurements. In addition, photolysis of nitrate on surfaces in the boundary layer may lead to oxidation of co-adsorbed organics.
2007. "On the Effect of Particle Alignment in the DMA." Aerosol Science and Technology 41(2):112-124. Abstract The Differential Mobility Analyzer (DMA) is designed to Measure particle mobility diameter, which for spherical particles is equal to particle volume equivalent diameter. In contrast, the mobility diameter of aspherical particles is a function of the particle shape and orientation. The magnitude of the DMA electric fields is such that it can cause aspherical particles to align preferentially in a specific orientation. The same electric field and the sheath flow rate (qsh) define the particle mobility diameter. But, the fact that particle orientation depends on the electric field makes the dynamic shape factor and hence the mobility diameter depend on qsh. Here, we describe an operating procedure that relies on a tandem DMA system, in which the second DMA is operated at a number of qsh, to obtain information about particle shape by measuring the effect of particle alignment on the particle mobility diameter. We show how the relationship between the mobility diameter and qsh can even be used to physically separate particles according to their shapes. In addition, we explore the use of simultaneous measurements of particle alignment and particle vacuum aerodynamic diameters to gain further information on particle shape and account for particle alignment in the calculations of dynamic shape factor. We first test this approach on doublets and compact triplets of PSL spheres, for which the orientation dependent dynamic shape factors are known. We then investigate applications on a number of polydisperse particle systems of various shapes.
2007. "Measurements and Interpretation of the Effect of a Soluble Organic Surfactant on the Density, Shape and Water Uptake of Hygroscopic Particles." Journal of Aerosol Science 38(9):903-923. Abstract A large fraction of atmospheric particles are composed of hygroscopic salts that are mixed with variety of organic molecules, of which surfactants represent an interesting and potentially important class. Because of the tendency of surfactant molecules to coat the particles' surface, a monolayer has the potential to completely alter the particles' interactions with the rest of the atmosphere. Given the important role that is played by the interaction of particles with the ambient relative humidity it is critical to develop an understanding of the impact surfactants may exert on particle hygroscopic properties. We present an experimental study of the relationship between the concentrations of a soluble surfactant that is internally mixed with two different hygroscopic salts and particle density, shape and water uptake. We show that the observed particle density provides evidence that the density of the surfactant fraction changes with concentration and that once this is properly taken into account the water uptake data can quantitatively be understood.
2007. "Effect of Hydrophobic Primary Organic Aerosols on Secondary Organic Aerosol Formation from Ozonolysis of α-Pinene." Geophysical Research Letters 34(20):Paper # L20803. doi:10.1029/2007GL030720 Abstract Semi-empirical secondary organic aerosol (SOA) models typically assume a well-mixed organic aerosol phase even in the presence of hydrophobic primary organic aerosols (POA). This assumption significantly enhances the modeled SOA yields as additional organic mass is made available to absorb greater amounts of oxidized secondary organic gases than otherwise. We investigate the applicability of this critical assumption by measuring SOA yields from ozonolysis of α-pinene (a major biogenic SOA precursor) in a smog chamber in the absence and in the presence of dioctyl phthalate (DOP) and lubricating oil seed aerosol. These particles serve as surrogates for urban hydrophobic POA. The results show that these POA did not enhance the SOA yields. If these results are found to apply to other biogenic SOA precursors, then the semi-empirical models used in many global models would predict significantly less biogenic SOA mass and display reduced sensitivity to anthropogenic POA emissions than previously thought.
2006. "SpectraMiner, an Interactive Data Mining and Visualization Software for Single Particle Mass Spectroscopy: A Laboratory Test Case." International Journal of Mass Spectrometry 258(1-3):58-73. Abstract Single Particle Mass Spectrometers are sophisticated instruments designed to measure the sizes and compositions of a wide range of individual particles in-situ, in real-time. They characterize hundreds of thousands or millions of particles, generating vast amounts of rich and complex data, the proper mining of which requires dedicated state of the art tools. The analysis of individual particle mass spectra is particularly difficult because of their high dimensionality — each data point, representing a single particle, includes the 450 mass spectral peak intensities, particle size, and time of detection. The first step is to organize the data; a process typically accomplished by grouping particles of similar attributes. Since the common assumption is that the data must be reduced to become manageable, they are typically classified into a small number of clusters (~10) and represented by their average/representative spectra. Our approach is quite different. We have developed a data mining and visualization software package we call SpectraMiner that makes it possible to handle hundreds of clusters without loss of information and thus overcome the limits set by traditional statistical data analysis approaches. Data, which often include over 1 million particle spectra, are organized using K-mean clustering algorithm. The clusters are merged into nodes by sequentially combining similar clusters. The final structure is displayed in a hierarchical dynamical tree or circular dendogram. This interactive dendogram is the visual interface that allows for real-time data mining and exploration. Clicking on any of the clusters/nodes in the dendogram reveals the mass spectral and other detailed information about the particles that reside at that position. At each step the scientist is in control of the level of detail and the visualization format, rapidly switching between them while running the program on an office PC.
2006. "From Agglomerates of Spheres to Irregularly Shaped Particles: Determination of Dynamic Shape Factors from Measurements of Mobility and Vacuum Aerodynamic Diameters." Aerosol Science and Technology 40(3):197-217. Abstract With the advert of aerosol instrumentation it has become possible to simultaneously measure individual particle mobility and vacuum aerodynamic diameters. For spherical particles these two diameters yield individual particle density. In contrast, assigning a physical meaning to the mobility or aerodynamic diameter of aspherical particles is not straightforward. This paper presents an experimental exploration of the effect of particle shape on the relationship between mobility and vacuum aerodynamic diameters. We make measurements on systems of three types: 1) Agglomerates of spheres, for which the density and the volume are known; 2) Ammonium sulfate, sodium chloride, succinic acid and lauric acid irregularly shaped particles of known density; and 3) Internally mixed particles, containing organics and ammonium sulfate, of unknown density and shape. For agglomerates of spheres we observed alignment effects in the DMA and report the first measurements of the dynamic shape factors (DSFs) in free molecular regime. We present here the first experimental determination of the DSF of ammonium sulfate particles. We find for ammonium sulfate particles a DSF that increases from 1.03 to 1.07 as particle mobility diameter increases from 160 nm to 500 nm. Three types of NaC1 particles were generated and characterized: nearly spherical particles with DSF of ~1.02; cubic with DSF that increases from 1.065 to 1.17 as particle mobility diameter increases from 200 nm to 900 nm; and compact agglomerates with DSF 1.3-1.4. Organic particles were found very nearly spherical. The data suggest that particles composed of binary mixtures of ammonium sulfate and succinic acid have lower dynamic shape factors than pure ammonium sulfate particles. However, for internally mixed ammonium sulfate and lauric acid particles we cannot distinguish between nearly spherical particles with low density and particles with DSF of 1.17.
2006. "Evaporation of Water from Particles in the Aerodynamic Lens Inlet: An Experimental Study." Analytical Chemistry 78(19):6942-6947. Abstract The extremely high particle transmission efficiency of aerodynamic lens inlets resulted in their wide use in aerosol mass spectrometers. One of the consequences of a transport of 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. To predict how much water will be evaporated is not feasible. Because water loss can effect in addition to particle size, its 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 carefully conducted measurements that provide the first experimentally determined benchmark 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.
2006. "A High Resolution Study of the Effect of Morphology on the Mass Spectra of Single PSL Particles with Na-Containing Layers and Nodules." Aerosol Science and Technology 40(12):1111-1122. doi:10.1080/02786820601001677 Abstract The interpretation and qualification of measurements of particle composition by laser ablation based single particle mass spectroscopy is complex. Among the most difficult system to quantify are internally mixed particles containing alkali metals. The Alkali atoms in such particles tend to suppress the formation of other ions sometimes to below detection limit. Here we present a study of the behavior of the single particle mass spectral peak intensities as a function of the amount of the sodium containing compounds deposited on the surface of 240 nm PSL spheres. We generate three morphologically distinct and well defined coating types: uniform layer, cubic nodules and rounded nodules, and measure the individual particle mass spectra as a function of the vacuum aerodynamic diameter with nanometer resolution. We find that the probability of detecting the PSL spheres depends on the amount of the alkali metal on the PSL sphere surface and the ablation laser power. We also find that the morphological distribution of the sodium containing coating plays a role in determining mass spectral intensities. The data suggest that PSL spheres with localized Na-containing nodules are easier to detect than those completely encapsulated. We show that 80% of PSL spheres with nodules, whose weight fraction is close to 50%, can be detected at high laser power compared with 60% detection of encapsulated PSL particles with the same amount of coating. At low laser powers these detection limits drop to 35% and ~0% respectively.
2005. "Single Particle Laser Ablation Time-of-Flight Mass Spectrometer: An Introduction to SPLAT." Aerosol Science and Technology 39(6):554-568. Abstract We present our single particle mass spectrometer we call SPLAT. SPLAT was designed to make possible the detection and characterization of particles down to 100nm, generate reproducible single particle mass spectra, and collect data at a high rate. The final instrument presented here is capable of characterizing individual particles down to 50nm at the rate of 20 particles per second. In SPLAT, mass spectra are generated by a two-step process, that uses a pulsed infrared laser to heat the particle and a time delayed pulsed UV laser to create ions. We describe a mode of operation that makes it possible to take advantage of the gas phase ionization of the semivolatile components of the particle, while also generating mass spectral signatures of the nonvolatile fraction, thereby providing complete particle mass spectra. We present some sample results from two field deployments of SPLAT.
2005. "High Precision Density Measurements of Single Particles: The Density of Metastable Phases." Aerosol Science and Technology 39(10):972-986. Abstract We describe a system designed to measure the size, composition and density of individual particles in real-time. It uses a DMA to select a monodisperse particle population and the single particle mass spectrometer to measure individual particle mass spectrometer to measure individual particle aerodynamic diameter and composition. Mobility and aerodynamic diameters are used to extract particle density. The addition of individual particle density to the mass spectrum is intended to improve the data classification process. In the present paper we demonstrate that the system has the requisite accuracy and resolution to make this approach practicable. We also present a high precision variant that uses an internal calibrant to remove any of the systematic errors and significantly improves the measurement quality. The high precision scheme is most suitable for laboratory studies making it possible to follow slight changes in particle density. An application of the system to measure the density of hygroscopic particles of atmospheric importance in metastable phases near zero relative humidity is presented. The density data are consistent with conclusions reached in a number of other studies that some particle systems of atmospheric significance once deliquesced persist as droplets down to near zero relative humidity.