Calibration of an Aerosol Composition Mass Spectrometer with Sulfuric Acid Water Aerosol

The first quantitative chemical analysis of polar stratospheric cloud particles has recently been performed using a balloon-borne aerosol composition mass spectrometer (ACMS). A similar spectrometer is presently used in a large cryo-chamber experiment to study low temperature aerosols. All experiments require prior to their employment an accurate calibration to convert mass spectrometer signals into molecular species contained in the aerosols. For the calibration, pure H 2 SO 4 /H 2 O droplets are generated having known composition and diameters between 0.4 w m and 1 w m. The size distribution and the number concentration can be controlled. A flow reactor with a rotating inner glass cylinder placed in a H 2 SO 4 /H 2 O bath solution of known concentration is used to condition the droplets. The residence time of the particles in the flow reactor is long enough that the droplets adopt the composition of the bath solution before entering the ACMS. The result is a linear relationship between the mole ratio of the H 2 SO 4 /H 2 O droplets and the mass spectrometer count rate ratio of water to sulfuric acid. The evaluation takes the dissociation of H 2 SO 4 inside the ACMS into account. The calibration error varies between 3 and 4 wt. % H 2 SO 4 for stratospheric particles with a composition of 30-70 wt. % H 2 SO 4 . Besides the calibration of the instrument, the analysis of the aerosols is a valuable diagnostic tool to investigate impurities in the particles.     

Investigation of Volatility Method for Measuring Aqueous Sulfuric Acid on Mixed Aerosols

The thermal discrimination or volatility technique is a widely used method exploiting differences in aerosol volatility to discriminate between particles of different chemical composition. In recent years numerous investigators applied this technique to determine the existence and the amount of sulfuric acid in the aerosol phase of aircraft contrails forming in the upper troposphere and lower stratosphere (UT/LS). Although the potential for systematic errors due to incomplete evaporation and recondensation of volatile material as well as internal wall losses was recognized by other investigators, we are not aware of any study on polydisperse aerosol (broad size distribution) incorporating these effects into the volatility technique. Here, a tandem differential mobility analyzer (TDMA) is employed to investigate the performance of a thermal discriminator designed at the University of Missouri-Rolla (UMR). Since sulfuric acid is of particular interest for atmospheric aerosol, this study focused on aqueous sulfuric acid (H 2 SO 4 /H 2 O) aerosol. For an operating temperature of 300°C and an aerosol residence time of more than 0.25 s, we found that complete evaporation of H 2 SO 4 /H 2 O aerosol occurred up to diameters of at least 1.9 w m, which is consistent with theoretical calculations. No evidence for recondensation was found for H 2 SO 4 /H 2 O particle surface area and mass concentrations typical for UT/LS background and aircraft plume conditions. Wall losses were measured and incorporated into a size-resolved version of the volatility method, allowing more accurate measurements of the volatile (H 2 SO 4 /H 2 O) volume fraction of polydisperse aerosol. The increased accuracy was demonstrated using well-characterized, mixed (partially volatile) H 2 SO 4 /H 2 O/NaCl aerosol.     

Size Distribution Change of Titania Nano-Particle Agglomerates Generated by Gas Phase Reaction,Agglomeration, and Sintering

In the manufacturing of nanometer-sized material particlulates by aerosol gas-to-particle conversion processes, it is important to analyze how the gas-phase chemical reaction, nucleation, agglomeration, and sintering rates control the size distribution and morphology of particles. In this study, titania particles were produced experimentally by the thermal decomposition of titanium tetraisopropoxide (TTIP) and oxidation of titanium tetrachloride (TiCl 4 ) using a laminar flow aerosol reactor. The effect of reaction temperature on the size and morphology of the generated particles was investigated under various conditions. The size distributions of agglomerates were measured using a DMA/CNC system. The size distributions of primary particles were measured using TEM pictures of the agglomerates sampled by a thermophoretic aerosol sampler. In order to model the growth of both agglomerates and primary particles simultaneously, a two-dimensional discrete-sectional representation of the size distribution was employed, solving the aerosol general dynamic equation for chemical reaction, agglomeration, and sintering. Qualitative agreement between the experimentally observed results and the simulation are satisfactory for the large variations in reactor temperature explored.     

A High Volume Apparatus for the Condensational Growth of Ultrafine Particles for Inhalation Toxicological Studies

A high volume (2500 LPM) system for the condensational growth of ultrafine particles was developed and evaluated using indoor air as a test aerosol. The main features of this system are the following: (a) ultrafine particles grow condensationally to supermicron sizes using high purity deionized water as a condensing medium; (b) the supersaturation ratio is adjustable and can be precisely controlled; (c) the system can operate for a wide range of ambient air temperature and relative humidity conditions; and (d) a thermal dryer is used to return the condensationally grown particles back to their original size. Restoring the original ambient size distribution and preserving the composition of the ambient ultrafine particles is very important for inhalation studies. The system is fully automated and has computerized feedback controls. In addition, saturation of the aerosol with water vapor occurs at close to ambient temperatures to minimize particle losses of volatile components. Saturation of sample air is obtained using a direct steam-injecting, fully modulating electric humidifier. The sample air after saturation is drawn through the supersaturator, which is a refrigerant-to-air heat exchanger and is cooled down to obtain the desirable supersaturation ratio. Supersaturation ratios can be precisely adjusted, with the optimum operational level found to be in the range of 2 to 3. The performance of the system was evaluated as a function of critical operation parameters, including the supersaturation ratio as well as the saturation and supersaturation temperatures. A series of virtual and conventional impactors was used to characterize the condensational growth of ultrafine particles. This new high volume apparatus was shown to grow ambient ultrafine particles to supermicron sizes with a particle size growth of approximately 1.8 w m. Particle losses in the system were found to be minimal (about 10%). The thermal dryer was used successfully to restore the grown particles back to their original size distribution. Particle concentration, aerosol temperature, and residence time (aerosol flow) are key parameters shown to affect the performance of the thermal dryer was used successfully to restore the grown particles back to their original size distribution. Particle concentration, aerosol temperature, and residence time (aerosol flow) are key parameters shown to affect the performance of the thermal dryer.     

Preparation of Polymer Particles in Aerosol-Phase Reaction

ABSTRACT

Polymer (polysiloxane, polystyrene) particles were prepared by polymerizing droplets of monomer (methyltrichlorosilane, styrene) with vapor (water, trifluoromethanesulfonic acid) in aerosol reactor. The droplets were produced by either evaporation-condensation or nebulization. The applicability of the droplet production methods depends on the nature of the polymerization systems. In the step polymerization of polysiloxane, the evaporation-condensation route gave better results while, in the chain polymerization of polystyrene, the nebulization method was preferred to the other. The evaporation-condensation method was limited in its ability to produce appreciable amounts of polymer product due to its dependence on monomer vapor pressure. In the nebulization technique for the preparation of polystyrene particles, the addition of various volatile additives to the styrene monomer to be nebulized made it possible to change the average size, size distribution and surface characteristics of the particles. Also, the combination of the aerosol techniques made it possible to prepare composite particles of polysiloxane-coated polystyrene with a core-shell structure.     

Real-Time Chemical Analysis of Organic Aerosols Using a Thermal Desorption Particle Beam Mass Spectrometer

An instrument has been developed for real-time, quantitative chemical analys is of organic particles in laboratory environments. In this apparatus, which we call a Thermal Desorption Particle Beam Mass Spectrometer (TDPBMS), particles are sampled into a differentially-pumped vacuum chamber, focused into a narrow, low-divergence particle beam using aerodynamic lenses, and then transported into a high-vacuum region where they impact on a heated surface, evaporate, and the vapor is mass analyzed in a quadrupole mass spectrometer. The average composition of a continuous stream of particles is thus measured in real time, and size-dependent composition can be obtained by passing the incoming aerosol through a differential mobility analyzer. The TDPBMS can analyze multi component organic particles in the 0.02-0.5mu m size range for compound concentrations 0.1-1mu g m3 without particle matrix effects. By using careful calibration techniques that account for particle shape and transport efficiency, the particulate organic components can be quantified with an estimated uncertainty of 20%. The utility of TDPBMS for laboratory studies of aerosol chemistry is demonstrated by monitoring the tridecanoic acid concentration in secondary organic aerosol formed during a smog chamber reaction of 1-tetradecene and ozone.     

Generation of Submicron Arizona Test Dust Aerosol: Chemical and Hygroscopic Properties

This article describes a submicron dust aerosol generation system based on a commercially available dust disperser intended for use in laboratory studies of heterogeneous gas–aerosol interactions. Mineral dust particles are resuspended from Arizona Test Dust (ATD) powder as a case study. The system output in terms of number and surface area is adjustable and stable enough for aerosol flow reactor studies. Particles produced are in the 30–1000 nm size range with a lognormal shape of the number size distribution. The particles are characterized with respect to morphology, electrical properties, hygroscopic properties, and chemical composition. Submicron particle elemental composition is found to be similar for the particle surface and bulk as revealed by X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma optical emission spectroscopy (ICP-OES), respectively. A significant difference in chemical composition is found between the submicron aerosol and the ATD bulk powder from which it was generated. The anionic composition of the water-soluble fraction of this dust sample is dominated by sulfate. Resuspended dust particles show, as expected, nonhygroscopic behavior in a humid environment. Small hygroscopic growth of about 1% (relative change in mobility diameter) was observed for 100 nm particles when the relative humidity (RH) was changed from 12 to 94%. Particles larger than 100–200 nm shrank about 1% once exposed to RH > 90%. This was interpreted as a restructuring of the larger agglomerates of dust to particles of smaller mobility diameter, under the influence of water vapor.     

Ammonium Chloride Aerosol Nucleation and Growth in a Cross-Flow Impinging Jet Reactor

The nucleation and growth of an ammonium chloride aerosol starting from gaseous ammonia and hydrogen chloride was investigated experimentally and with the use of a mathematical model. The reactor was composed of 2 opposed jets perpendicular to a main stream and was operated under laminar/transition flow conditions. The reactants were segregated when they entered the reactor. The parameter observed was the particle size distribution of the aerosol described by its moments. Considerable scatter in the experimental results complicated their analysis, but some important trends could be identified. The model results were strongly affected by the fluid mechanics model, which influenced the predicted mixing inside the reactor. This work shows the importance of fluid mixing in controlling the aerosol size distribution.     

Development of an Aerosol Mass Spectrometer for Size and Composition Analysis of Submicron Particles

The importance of atmospheric aerosols in regulating the Earth's climate and their potential detrimental impact on air quality and human health has stimulated the need for instrumentation which can provide real-time analysis of size resolved aerosol, mass, and chemical composition. We describe here an aerosol mass spectrometer (AMS) which has been developed in response to these aerosol sampling needs and present results which demonstrate quantitative mea surement capability for a laboratory-generated pure component NH₄ NO₃ aerosol. The instrument combines standard vacuum and mass spectrometric technologies with recently developed aerosol sampling techniques. A unique aerodynamic aerosol inlet (developed at the University of Minnesota) focuses particles into a narrow beam and efficiently transports them into vacuum where aerodynamic particle size is determined via a particle time-of-flight (TOF) measurement. Time-resolved particle mass detection is performed mass spectrometrically following particle flash vaporization on a resistively heated surface. Calibration data are presented for aerodynamic particle velocity and particle collection efficiency measurements. The capability to measure aerosol size and mass distributions is compared to simultaneous measurements using a differential mobility analyzer (DMA) and condensation particle counter (CPC). Quantitative size classification is demonstrated for pure component NH₄ NO₃ aerosols having mass concentrations 0.25_mu g m -3. Results of fluid dynamics calculations illustrating the performance of the aerodynamic lens are also presented and compared to the measured performance. The utility of this AMS as both a laboratory and field portable instrument is discussed.     

Production and Characterization of Pd/SiO2 Catalyst Nanoparticles from a Continuous MOCVS/MOCVD Aerosol Process at Atmospheric Pressure

A continuous 2-step aerosol process is described for the generation of SiO₂ supported palladium (Pd/SiO ₂ ) catalyst particles from metal-organic (MO) precursors. In a first flow reactor, submicron SiO ₂ support particles are generated by chemical vapor synthesis (CVS) from TEOS [tetraethyl(ortho)silicate]. These silica particles are then coated with palladium in a second flow reactor by chemical vapor deposition (CVD) from ( η ³ -allyl)( η ⁵ -cyclopentadienyl)palladium [Pd(allyl)Cp].

The sublimation and decomposition behavior of both metal-organic precursors was measured by thermo-gravimetric analysis (TGA) and FTIR; the vapor concentration of Pd(allyl)Cp was determined for the range of process conditions used.

Each process step was characterized both with regard to aerosol properties as well as morphology and composition of individual particles. This was done with a variety of on-line and off-line techniques including electrical mobility analysis, TEM, energy-dispersive X-ray analysis, and physisorption methods like BET. The coating thickness was also measured on line by a high-resolution single-stage impactor (SS-LPI) technique.

It is shown that the continuous CVS process can be set to generate constant concentrations and sizes of silica support particles with a specific surface area of 350 m ² g ^?1 , which are carbon free and non-porous. The silica particles can be restructured to spheres if desired. The continuous palladium CVD process was able to generate variable and defined coatings of narrowly distributed Pd nanoparticles with mean sizes between 0.75 and about 3.5 nm. The on-line measurements by SS-LPI showed equivalent coating thicknesses from 0.3 nm up to 3 nm, which were stable over several hours.     

Modeling and Control of a Titania Aerosol Reactor

We focus on modeling and control of an aerosol flow reactor used to produce titania powder. We initially present a detailed population balance model for the process which accounts for simultaneous nucleation, Brownian and shearinduced coagulation, and convective transport and describe the spatio-temporal evolution of the aerosol volume distribution. Then, under the assumption of lognormal aerosol volume distribution, the method of moments is employed for the derivation of a model that describes the evolution of the three leading moments of the volume distribution. The moment model, together with the fundamental model that describes the temperature in the reactor and concentrations of the gas-phase species, are subsequently used to synthesize a nonlinear output feedback controller which manipulates the temperature of the reactor wall to achieve an aerosol size distribution in the outlet of the reactor with desired geometric average particle diameter. The nonlinear controller is successfully implemented on the process model and is shown to deal effectively with external disturbances.     

Analysis of the Rapid Carbothermal Reduction Synthesis of Ultra-Fine Silicon Carbide Powders

A nondimensionalized and scaled nonisothermal model is developed for the "rapid carbothermal reduction" synthesis of sub-micron silicon carbide particles in an aerosol flow reactor to determine the minimum parametric representation of the system. Seven dimensionless groups are needed to completely describe the system, and these dimensionless groups are varied to determine the effects of the furnace wall temperature, inlet carbon particle size, carrier gas flow rate, and solids feed rate on final product quality. Analysis shows that radiation dominates the heating process, sintering dominates the primary particle growth, and conversion is controlled with precursor carbon particle size, wall temperature, and carrier gas flow rate.     

Monte Carlo Simulation of Multicomponent Aerosols Undergoing Simultaneous Coagulation and Condensation

A Monte Carlo method was developed to simulate multicomponent aerosol dynamics, specifically with simultaneous coagulation and fast condensation where the sectional method suffers from numerical diffusion. This method captures both composition and size distributions of the aerosols. In other words, the composition distribution can be obtained as a function of particle size. In this method, particles are grouped into bins according to their size, and coagulation is simulated by statistical sampling. Condensation is incorporated into the Monte Carlo method in a deterministic way. If bins with fixed boundaries are used to simulate the condensation process numerical dispersion occurs, and thus a moving bins approach was developed to eliminate numerical dispersion. The method was validated against analytical solutions, showing excellent agreement. An example of the usefulness of this model in understanding aerosol evolution is presented. The effects of the number of particles and number of bins on the accuracy of the numerical results are also discussed. It was found that with 20 bins per decade and 105particles in the control volume results with less than 5%error can be obtained. The results are further improved to within 2%error by filtering the statistical noise with a cubic spline algorithm.     

Design and Characterization of a Fluidized Bed Aerosol Generator: A Source for Dry,Submicrometer Aerosol

A fluidized bed aerosol generator has been designed and built for the purpose of generating a constant output of dry, submicrometer particles with a large number density. The output of the fluidized bed for generating aerosol particles from dry soot powder has been characterized using a differential mobility analyzer and a condensation particle counter. The particle size distribution is bimodal, with a mode in the submicrometer diameter size range and a mode in the supermicrometer diameter size range. The larger diameter mode is fully separated from the smaller mode and can thus be easily removed from the aerosol flow using impaction techniques. The distribution in the submicrometer size range is nearly log-normal, with a count median diameter falling between 0.1 and 0.3 micrometers. A number density of greater than 10⁵ particles cm^-3 of soot particles in the submicrometer range can be produced, constant to within 25% (1 standard deviation) over a 4 h time period. The number density of particles produced in the submicrometer range was found to vary with the ratio of soot to bronze beads in the bed mixture, whether or not a feed system was used, and nitrogen flow rate through the fluidized bed and feed system.     

Ultrafine Particle Sampling with the UNC Passive Aerosol Sampler

A model is presented to describe the collection of ultrafine particles by the UNC passive aerosol sampler. In this model, particle deposition velocity is calculated as a function of particle size, shape and other properties, as well as a function of sampler geometry. To validate the model, deposition velocities were measured for ultrafine particles between 15 and 90 nm in diameter. Passive aerosol samplers were placed in a 1 m ³ test chamber and exposed to an ultrafine aerosol of ammonium fluorescein. SEM images of particles collected by the samplers were taken at 125 kX magnification. Experimental values of deposition velocity were then determined using data from these images and from concurrent measurements of particle concentration and size distribution taken with an SMPS. Deposition velocities from the model and from the experiments were compared and found to agree well. These results suggest that the deposition velocity model presented here can be used to extend the use of the UNC passive aerosol sampler into the ultrafine particle size region.     

Low Pressure Aerosol Flow Reactor

In this work we report the development of a novel low pressure aerosol flow reactor for the determination of the kinetic parameters of fast heterogeneous processes. The experimental apparatus consists of a spray atomizer to introduce aerosols into a low pressure zone; a fast flow reactor for kinetic measurements and an IR spectrometer and mass spectrometer for concentration measurements. The surface area distribution and number density of the aerosol particles are determined from their infrared spectra and the decay kinetics are determined by monitoring the disappearance rates of the gas phase species (with a mass spectrometer) as a function of the aerosol properties. We report the application of this apparatus to the investigation of the uptake of acetone by liquid water aerosols (0.1–20 μ m diameter) at room temperature and a pressure of 35 Torr. These measurements yielded a value of the mass accommodation coefficient, α, of 3.6 _{? 2} ^{+ 3.1} × 10 ^{? 3} .     

Detection and Analysis of Aerosol Particles by Laser-Induced Breakdown Spectroscopy

Laser-induced breakdown spectroscopy (LIBS) was evaluated as a means for quantitative analysis of the size, mass, and composition of individual micron-to submicron-sized aerosol particles over a range of well-characterized experimental conditions. Conditional data analysis was used to identify LIBS spectra that correspond to discrete aerosol particles under low aerosol particle loadings. The size distributions of monodisperse particle source flows were measured using the LIBS technique for calcium- and magnesium-based aerosols. The resulting size distributions were in good agreement with independently measured size distribution data. A lower size detection limit of 175 nm was determined for the calcium- and magnesium-based particles, which corresponds to a detectable mass of approximately 3 femtograms. In addition, the accuracy of the LIBS technique for the interference-free analysis of different particle types was verified using a binary aerosol system of calcium-based and chromium particles.     

Simulation study of production of fine ceramic powders in a cyclone reactor

A numerical simulation study of production of fine ceramic powders in an innovative vapor-phase aerosol reactor is described. Arrangement is typical of reverse-flow cyclone equipment; no similar device is present in current scientific literature and industrial technology. The cyclone reactor has a potential technological application as it realizes process intensification by two simultaneous operating advantages: (i) curly flow reduces recirculation of as-synthesized particles towards flame region, and (ii) cyclone arrangement segregates large particles. As a result, ceramic powders with narrower particle size distribution can be produced with regard to traditional equipment. The study is based on the re-modeling of an existing industrial reactor for production of fine TiO₂ according to a cyclone configuration; particle size distributions from simulation and plant are compared.     

The Behavior of Constant Rate Aerosol Reactors

An aerosol reactor is a gaseous system in which fine particles are formed by chemical reaction in either a batch or flow process. The particle sizes of interest range from less than 10 Å (molecular clusters) to 10μm. Such reactors may be operated to study the aerosol formation process, as in a smog reactor, or to generate a product such as a pigment or a catalytic aerosol. Aerosol reactors can be characterized by three temporal or spatial zones or regions of operation for batch and flow reactors, respectively. In zone I, chemical reaction results in the formation of condensable molecular products which nucleate and form very high concentrations of small particles. The number density depends on the concentration of preexisting aerosol. Zone II is a transition region in which the aerosol number concentration levels off as a result of hetergeneous condensation by the stable aerosol. In zone III coagulation becomes sufficiently rapid to reduce the particle number concentration. There may be a zone IV in which agglomerates form. Chemical reaction may continue to generate condensable material throughout the various zones. This paper deals with reactors in which aerosol material is generated at a constant rate. Design parameters of interest are the particle size distribution, number density, surface area, and mass loadings. For ideal systems composed of spherical coalescing particles, these can be predicted theoretically for certain limiting cases. However, the irregular agglomerates which may form in zone IV are more difficult to characterize theoretically.     

Development of an Electrostatic Precipitator for Off-Line Particle Analysis

Due to the lake of in-situ aerosol particle analysis systems, aerosol samples are taken and analyzed off-line. For detailed analysis of particle properties such as shape, morphology, and composition, off-line operating analytical tools like light microscopes, scanning electron microscopes (SEM), total reflection x-ray fluorescence (TXRF), and so on are used. The analysis must be performed on a representative sample of particles homogeneously deposited on a flat sample plate. This avoids sample preparation steps which may change the sample. In this paper we describe the design, construction, and evaluation of a continuous sampling device that deposits gasborne particles on an analytically suitable sample plate. The collection efficiency and the deposition pattern were optimized using a numerical model and experiments. It turned out that representative samples appropriate for further analysis can be taken in the particle size range from 0.03 mu m < Dp < 10 mu m. Additionally, the sampling efficiency was investigated for particles smaller than 0.03 mu m using electrical and non-electrical deposition mechanisms like diffusion and thermophoresis. The investigations performed demonstrate that the designed electrostatic precipitator (ESP) is a very useful tool for homogeneous particle deposition on analytically suitable flat sample plates and can be used as a back-up filter. Further, the ESP especially can be used in combination with a differential mobility analyzer (DMA) if detailed investigations of a narrow particle size range of a polydisperse aerosol are required.