Indoor Radon Progeny Aerosol Size Measurements in Urban,Suburban, and Rural Regions

By using direct and indirect methods, we conducted size distribution measurements of radon progeny particles in a variety of indoor environments in urban, suburban, and rural areas. The radon progeny particle size distribution owing to indoor activities has two definable source categories: (1) gas combustion from stoves and kerosene heaters–particles were found to be smaller than 0.1 μm in diameter, mostly in the range 0.02–0.08 μm; and (2) cigarette smoking and food frying—particles were found to be larger, in the size range 0.1–0.2 μm. The radon progeny particle size distribution, without significant indoor activities, such as cooking, was found to be larger in rural areas than in urban or suburban areas. The modal diameters of the size spectra in the rural areas were two to three times larger than those in urban or suburban areas, around 0.3–0.4 vs. 0.1–0.2 μm. Results obtained by applying the attachment theory to the measured number-weighted size spectra from an electrical aerosol size analyzer support this rinding. These results, if confirmed by more extensive studies, will be useful for the assessment of the risk from the inhalation of radon progeny in various indoor environments.     

Determination of the Coarse Mode of the Atmospheric Aerosol Using Data from a Forward-Scattering Spectrometer Probe

The coarse mode of the atmospheric aerosol, containing mostly particles larger than 1 μm in diameter, can conveniently be measured by means of an optical-forward-scattering spectrometer probe mounted on an airplane. Although the instrument is able to count single particles, at least 10,000 particles have to pass the sensitive volume in order to reduce errors due to statistical fluctuations of the counts, especially in the bins of the larger particles. The fitting of a lognormal curve to the measured particle counts is possible by means of a least-squares technique, described in this paper. The quality of the fit can be examined by determination of the best fit for the particle number distribution and the particle volume distribution, and an intercomparison of the two. Data for atmospheric samples show geometric mean diameters for the number size distribution between 1 and 2.5 μm and standard deviations between 1.7 and 2.5. Aerosols dominated by one source have a small standard deviation. Standard deviations larger than 2.5 are an indication of an aerosol coming from several sources and in many cases a good fit can be obtained by using two lognormal distributions.     

Analytic and numerical calculations of the formation of a sulphuric acid aerosol in the upper troposphere

Analytic and numerical calculations are performed on the production of sulphuric acid aerosol in conditions of a very large nucleation event observed in the upper troposphere. The numerical results feature a growing peak in the size distribution whose magnitude is reproduced well analytically, and are consistent with the observed particle number concentration at sizes greater than 25 nm (measured dry diameter), but suggest that most of the aerosol was at unobserved smaller sizes. Because of growth and coagulation, number concentrations of the aerosol rapidly become independent of the number initially nucleated, so that conclusions as to the nucleation process, either homogeneous or ion-induced nucleation, cannot easily be drawn from existing atmospheric observations. The final concentration is very insensitive to the magnitude of the SO₂ source, but, if condensation on, and coagulation with, a remnant background aerosol occurs, such nucleation events will be cut off for source magnitudes less than a specific value. Anthropogenic emissions of SO₂ which exceed this value can produce higher aerosol number concentrations in the atmosphere with consequences for the indirect effect of aerosols on the climate.     

Size Characterization of Carbonaceous Particles Using a Lovelace Multijet Cascade Impactor / Parallel-Flow Diffusion Battery Serial Sampling Train

A serial sampling train consisting of a Lovelace multijet cascade impactor (LMJI) and a seven cell parallel-flow diffusion battery (PFDB) has been used to provide a comprehensive method for sizing aerosols with a wide size distribution ranging from less than 0.01 μm to over 10 μm. The fraction of the aerosol greater than 0.7 μm is collected by the impactor. The remaining fraction of the aerosol is sampled by the PFDB. Design of the PFDB is based on the theory of a screen-type diffusion battery. The concept of parallel flow is employed to provide a method for sampling aerosols that fluctuate too rapidly in concentration and size distribution to be measured by conventional methods. The LMJI/PFDB sampling system is useful for characterizing multimodel size distributions such as those that occur in ambient aerosols. It can also be used to determine the chemical composition of collected samples as a function of particle size. This sampling system has been used to size classify diesel and diesel-oil shale exposure atmospheres, and benzo(a)-pyrene-coated carbon black aerosols. The diffusion equivalent diameter (D _de) of the diesel exhaust was 0.07–0.08 μm, and the oil-shale dust had a mass median aerodynamic diameter (MMAD) of 2.6–2.9 μm. The size distribution of the carbon black aerosol was bimodal, with the fine fraction having a D _de of 0.2 μm, and the coarse fraction having a MMAD of 2.0 μm.     

Particle Emissions from Chrome Plating

Aerosol particles formed from the bursting of small gas bubbles emitted from the anode and cathode electrodes during electrolytic chrome plating were measured with a laboratory apparatus. The measured aerosol particle emission factors were about 5.15 mg particles/amp-hour or 3.81 mg Cr^{+ 6}/amp hour. The cathode gases generated more particles on a gas volume basis with 20 mg particles/liter hydrogen gas evolved from the cathode compared to 1 mg particles/liter oxygen gas evolved from the anode. With about 35% more cathode hydrogen gas evolved than anode oxygen gas evolved and with the much greater particle emissions caused by the cathode hydrogen on a gas volumetric basis, the aerosol particle emissions from the cathode bubble bursting were about 97% of the total particle mass emissions. The particle size distributions measured from the cathode had a mass median aerodynamic diameter of about 38 μ m whereas the anode mass median particle diameter was about 8 μ m. The anode particle mass size distribution was bimodal with peaks at about 0.6 and 24 μ m diameter. The cathode particle mass size distribution was trimodal with peaks at 2.5, 9, and about 60 μ m diameter.     

A Device for Generating Singly Charged Particles in the 0.1–1.0-μm Diameter Range

In this paper an aerosol charger that largely avoids the production of multiply charged particles in the 0.1–1.0 μm diameter range is described. The input aerosol is first passed through an electrostatic condenser to remove all charged particles and ions. The remaining neutral aerosol then flows into a 23-cm-long, 2.1-cm inner diameter cylindrical tube; the inner surface of this tube is uniformly coated with 0.09 μCi⁶³ Ni, a 0.067 MeV β-emitter with a half-life of 92 years. At typical airflow rates of 0.2–1.0 lpm, this low-activity source of ionizing radiation produces bipolar ion concentrations ranging from 1 × 10⁴ to 9 × 10⁴ ion/cm³, which is much lower than levels required to bring the aerosol to Boltzmann charge equilibrium. At a flow rate of 1.0 lpm, particles smaller than about 1.0 μm typically interact with no more than one ion en route through the charger. Therefore, particles at the charger exit are mostly either neutral or singly charged. Charge distributions of initially-neutral mono-disperse polystyrene latex particles were measured at the exit from the charger for particle diameters ranging in size from 0.09 to 1.09 μm. It was found that, at an airflow rate of 1.0 lpm and particle size 1.09 μm, the ratios of singly, doubly, and triply charged to total positively charged concentrations were 0.75, 0.19, and 0.06 respectively; particles with more than three charges were not detected. In contrast, the analogous charge ratio at Boltzmann equilibrium is 0.28 (+ 1), 0.24 (+ 2), 0.19 (+ 3), 0.13 (+ 4), 0.08 (+ 5), 0.05 (+ 6), and 0.7 (+ 02).     

Large Particle Size Distribution in Five U.S. Cities and the Effect on a New Ambient Participate Matter Standard (PM10)

A mobile aerosol-sampling system was used to determine the large particle ambient aerosol size distribution (up to approximately 100 μm particle diameter) in five cities across the United States: Birmingham, Alabama; Research Triangle Park, North Carolina; Philadelphia, Pennsylvania; Phoenix, Arizona; and Riverside, California. A mobile wide range aerosol classifier (WRAC) developed at the University of Florida was used. The study shows that any measurement of ambient particulate matter with a size-fractionating inlet sampler will be influenced by the ambient particle size distribution.

Mass distribution measurements determined by the WRAC were compared with mass measurements obtained simultaneously using TSP Hi-Vol and 15 μm cut-size inhalable particulate network samplers. Aerosol size-classification results showed the presence of a large particle mass mode at all sites sampled. The position and magnitude of the large particle mode varied and was not a simple function of concentration. The percentage of the total aerosol mass collected by the present EPA reference method high-volume air sampler varied from about 85 to 95%. The percentage of total aerosol mass less than 10 μm varied from about 50 to 90%, depending on the sampling location and sampling condition.     

Experimental Study of Particle Deposition on Semiconductor Wafers

A sensitive method for detecting particle deposition on semiconductor wafers has been developed. The method consisted of generating a monodisperse fluorescent aerosol, depositing the known-size monodisperse aerosol on a wafer in a laminar flow chamber, and analyzing the deposited particles using a fluorometric technique. For aerosol particles in the size range of 0.1–1.0 μm, the mobility classification-inertial impaction technique developed by Romay-Novas and Pui (1988) was used to generate the monodisperse test aerosols. Above a particle diameter of 1.0 μm, monodisperse uranine-tagged oleic acid aerosols were generated by a vibrating-orifice generator. The test wafer was a 3.8-cm diameter silicon wafer placed horizontally in a vertical laminar flow chamber which was maintained at a free stream velocity of 20 cm/s. A condensation nucleus counter and an optical particle counter were used to obtain the particle concentration profile in the test cross section and to monitor the stability of aerosol concentration during the experiment. The results show that the measured particle deposition velocities on the wafers agree well with the theory of Liu and Ahn (1987) in the particle size range between 0.15 and 8.0 μm. The deposition velocity shows a minimum around 0.25 μm in particle diameter and increases with both smaller and larger particle size owing to diffusional deposition and gravitational settling, respectively.     

Particle size analysis of dioctyl phthalate aerosols using the stöber aerosol spectrometer

Particle size distributions of nearly monodisperse dioctyl phthalate aerosols (dia. between 0–5 and 1–4 μm) have been determined using the Stöber aerosol spectrometer. The particle size distributions can be approximated very well by bimodal distribution functions. From a statistical analysis it turned out that the accuracy of the approximation is limited in case of small particles (dia. ～ 0·5 μm). This is due to evaporation of the particles during the analysis.The mean of the particle size distribution determined with the Stöber aerosol spectrometer was in fair agreement with the particle diameter determined with the higher order Tyndall spectrometer.     

Hygroscopic Properties of Pasadena,California Aerosol

The hygroscopic behavior of Pasadena, CA aerosol was continuously measured from August 15 to September 15, 1999 using a tandem differential mobility analyzer. Two dry particle sizes were sampled, 50 nm and 150 nm in diameter; humidification of the dry aerosol was carried out at 89% relative humidity. Complex growth patterns were observed for both size modes, with aerosol distributions splitting from a single mode at times to more than 6 modes. Diurnal profiles for the observed multiple peaks were noted, with the greatest number of measurable growth modes being found during the late night and predawn hours for 50 nm particles. For 150 nm particles, more modes were present during the afternoon hours, with the humidified aerosol becoming bimodal during the late night/early morning hours. Growth factors, defined as the ratio of humidified particle diameter (at 89%) to dry diameter, were determined for modes with significant number concentrations. Average growth factors over the sampling period for the 2 particle sizes ranged from 1.0 to 1.6. Hygroscopic growth increased in the latter half of the sampling period when forest fires were present. In short, treating this complex urban aerosol as a combination of "less" and "more" hygroscopic fractions is an oversimplification.     

Aerosol Characterization of a Smoldering Source

The aerosol emitted by a moderately large smoldering combustion source (16 cm in diameter) has been characterized in detail. The fuel is a permeable bed of cellulosic insulation (wood fibers) receiving its primary air supply by flow up from the bottom of the bed while the smolder wave propagates downward. The mass mean particle size of the aerosol is 2–3 μm; this shows no clear trend with smolder wave depth in the bed or with air flow velocity. The large average particle size is shown to imply that, compared to punk smoke, the present aerosol requires a sevenfold greater concentration to trigger an ionization detector. Coagulation of the aerosol in the plume above the source is shown to be minimal, but substantial coagulation can occur within the source. The apparent fractional conversion of gasified mass (60–75% of the fuel) to aerosol mass decreases with smolder wave depth in the bed and with decreasing air flow rate. The mass and number flow rate of the aerosol show these same trends. The decreasing aerosol emissions with wave depth or air flow rate are plausibly explained by filtration effects in the smolder bed.     

Monodisperse fine aerosol generation using fluidized bed

Monodisperse, fine aerosols are needed in many applications: filter testing, experiments for testing models, and aerosol instrument calibration, among others. Usually, monodisperse fine aerosols are generated in very low concentrations, or mass flow rates, in the laboratory scale. In this work, we needed to generate aerosols with higher mass flow rate than typically available by the laboratory-scale methods, such as atomizers, nebulizers, ultrasonic generators, vibrating orifice generators, and condensation generators. Therefore, we constructed a fluidized bed aerosol generator to achieve particle mass flow rates in the range of 15-100 g/h. Monodisperse, spherical SiO₂ particles of two sizes with geometrical diameters of 1.0 and 2.6 µm were used in the aerosol generator. The aerosol generator was used at both atmospheric pressure, and at high pressures up to 5 bar (abs).The particle size, mass concentration and the net average particle charge were measured after mixing the aerosol with nitrogen. The particle size distributions with both particle sizes were monodisperse, and no particle agglomerates were entrained from the fluidized bed. The behavior of the fluidized bed generator was found to be markedly different with the two particle sizes in regard to particle concentration, presumably due to different particle charging inside the generator. After determining the net average charge of the particles, an ion source Kr-85 was used to reduce the charge of the particles. This was found to be effective in neutralizing the particles.     

Size-resolved eddy covariance measurements of fine particle vertical fluxes

A method is described for field measurements of size-resolved aerosol vertical fluxes over rural areas. The method is based on a direct eddy covariance method and using an Electrical Low-Pressure Impactor (Outdoor ELPI). The main advantage of this method is to measure simultaneously the aerosol vertical fluxes at several submicron sizes. Due to the low response time of the experimental setup (about 1 s) compared to the turbulent time scales of the particle concentrations, a flux correction was applied and it is based on spectral analysis. As an example of results, the particles fluxes normalized by the concentration were measured over a maize field for atmospheric aerosols that ranged in size from 0.007 to 1.6 μm.     

Measurements of the total and regional deposition of inhaled particles in the human respiratory tract

The total and regional deposition of monodisperse aerosols in the human respiratory tract has been measured in 12 healthy subjects breathing through the mouth. Radioactively labelled polystyrene particles in the aerodynamic diameter range 3.5–10.0 μm were employed. The total deposition results are similar to those reported by Stahlhofen et al. (1980), showing only a slight progressive increase with particle size, from a mean fraction of 0.79 of the inhaled aerosol at 3.5 μm, to 0.88 for 10 μm particles. The extrathoracic airways show a very marked deposition at all sizes, predominantly in the throat. The throat values rise rapidly from a mean of 0.09 at 3.5 μm to 0.36 at 10 μm particle diameter. Two intrathoracic fractions were also obtained by the widely accepted method of measuring the relative amounts of activity cleared from the thorax as a function of time. Alveolar deposition was apparently still some 0.06 of the inhaled aerosol at 10 μm particle diameter. Tracheo-bronchial deposition showed little change at any particle size except at 3.5 μm, when it was 0.24 of the inhaled aerosol.     

Determining Size-Specific Emission Factors for Environmental Tobacco Smoke Particles

Because size is a major controlling factor for indoor airborne particle behavior, human particle exposure assessments will benefit from improved knowledge of size-specific particle emissions. We report a method of inferring size-specific mass emission factors for indoor sources that makes use of an indoor aerosol dynamics model, measured particle concentration time series data, and an optimization routine. This approach provides--in addition to estimates of the emissions size distribution and integrated emission factors--estimates of deposition rate, an enhanced understanding of particle dynamics, and information about model performance. We applied the method to size-specific environmental tobacco smoke (ETS) particle concentrations measured every minute with an 8-channel optical particle counter (PMS-LASAIR; 0.1 m 2+ w m diameters) and every 10 or 30 min with a 34-channel differential mobility particle sizer (TSI-DMPS; 0.01 m 1+ w m diameters) after a single cigarette or cigar was machine-smoked inside a low air-exchange-rate 20 m 3 chamber. The aerosol dynamics model provided good fits to observed concentrations when using optimized values of mass emission rate and deposition rate for each particle size range as input. Small discrepancies observed in the first 1-2 h after smoking are likely due to the effect of particle evaporation, a process neglected by the model. Size-specific ETS particle emission factors were fit with log-normal distributions, yielding an average mass median diameter of 0.2 w m and an average geometric standard deviation of 2.3 with no systematic differences between cigars and cigarettes. The equivalent total particle emission rate, obtained by integrating each size distribution, was 0.2-0.7 mg/min for cigars and 0.7-0.9 mg/min for cigarettes.     

Particle Suspension in a Rotating Drum Chamber When the Influence of Gravity and Rotation are Both Significant

In recent years, rotating chambers have been found to be an effective method of retaining particles suspended in the air for an extended period of time. Rotating drum chambers have the potential of providing a stable atmosphere of well-characterized inhalable particles for periods lasting from hours to days for use in inhalation toxicology studies. To aid in planning for the use of rotating drum chambers in inhalation studies, we created a model that describes (a) the concentration of particles in the chamber under various conditions and (b) the particle sizes for which gravity and rotation influence particle dynamics. Previous publications describe the suspension / deposition of particles when the rotational effect is dominant, but do not describe particle suspension / deposition when gravitational settling is significant as occurs when such drum chambers are operated at optimal conditions for retaining the highest fraction of particles over time. By using the limiting trajectory of particles, the fraction of particles that remain suspended in a 1-m diameter rotating drum chamber was derived for forces of gravity only, rotation only, and gravity plus rotation. For particles between 0.5 and 1 μm in diameter and for suspension times of < 96 h, there was no loss of the suspended particles for drum rotation rates from 0.1 to 10 rpm. For 2- and 5-μm diameter particles, > 98% and 91%, respectively, remain suspended after 96 h under optimal rotation of the drum chamber. Optimal rotation rates were independent of particle size for particles < 10 μm in diameter (agreeing with Gruel et al. [1987] even though we predicted suspended fractions higher by > 30% for 10-μm particles after 96 h). For 20-μm diameter particles and suspension times < 96 h, the maximum suspended fraction occurred for drum rotation rates between 0.3 and 0.5 rpm. The particles > 2 μm can be selectively removed from an airborne particle size distribution in time periods of < 15 h when the rotational rate is > 5 rpm.     

The effect of particle size distribution on the microstructure and properties of Al2O3 ceramics formed by stereolithography

Stereolithography (SL) shows advantages for preparing alumina-based ceramics with complex structures. The effects of the particle size distribution, which strongly influence the sintering properties in ceramic SL, have not been systematically explored until now. Herein, the influence of the particle size distribution on SL-manufactured alumina ceramics was investigated, including bending strength at room temperature, post-sintering shrinkage, porosity, and microstructural morphology. Seven particle size distributions of alumina ceramics were studied (in μm/μm: 30/5, 20/3, 10/2, 5/2, 5/0.8, 3/0.5, and 2/0.3); a coarse:fine particle ratio of 6:4 was maintained. At the same sintering temperature, the degree of sintering was greater for finer particle sizes. The particle size distribution had a larger influence on flexural strength, porosity and shrinkage than sintering temperature when the particle size distribution difference reached 10-fold but was weaker for 10 μm/2 μm, 5 μm/2 μm and 5 μm/0.8 μm. The sintering shrinkage characteristics of cuboid samples with different particle sizes were studied. The use of coarse particles influenced the accuracy of small-scale samples. When the particle size was comparable to the sample width, such as 30 μm/5 μm and 5 mm, the width shrinkage was consistent with the height shrinkage. When the particle size was much smaller than the sample width, such as 2 μm/0.3 μm and 5 mm, the width shrinkage was consistent with the length shrinkage. The results of this study provide meaningful guidance for future research on applications of SL and precise control of alumina ceramics through particle gradation.     

Effects of particle size and rubber content on fracture toughness in rubber-modified epoxies

The effects of particle size of core-shell rubber on the fracture toughness of rubber-modified epoxies were investigated. Various sizes of core-shell rubber particles, from 0.16 to 1.2 μm in diameter, were synthesized by seeded emulsion polymerization. Particle size effects were clearly seen for lower crosslinked diglycidyl ether of bisphenol A (DGEBA)/piperidine resin. Fracture toughness increased as the particle size of core-shell rubber decreased from 1.2 to 0.4 μm. On the other hand, fracture toughness was constant in this range of particle sizes for higher crosslinked DGEBA/diaminodiphenylmethane (DDM) resin. Cavitation in the rubbery core and shear deformation in the matrix are the toughening mechanisms for DGEBA/piperidine resin, whereas cavitation is the only mechanism for DGEBA/DDM resin. Toughening effectiveness decreased with <0.2 μm core-shell rubber particles since they are difficult to cavitate. The effects of core-shell rubber content on fracture toughness of rubber-modified epoxies were also examined. The optimum rubber content for maximum toughness of rubber-modified epoxies decreased with decreased particle size of core-shell rubber in shear deformable DGEBA/piperidine resin. But the fracture toughness of rubber-modified DGEBA/DDM resins increased as the rubber content increased.     

Effects of chicken eggshell filler size on the processing,mechanical and thermal properties of PVC matrix composite

The effects of filler particle size of poly(vinyl chloride)/chicken eggshell powder (PVC/ESP) composites on the processing, tensile properties, morphology and thermal degradation were investigated. The mixing of composites was done using Rheomix internal mixer. The processing torque of PVC/ESP composite at a particle of 0.2 μm exhibits lower processing torque compared to that at a particle size of 7 μm due to the dispersive resistance from larger ESP filler particles. Good interfacial adhesion exists between the filler and matrix in composites prepared via a filler particle size of 0.2 μm, which has improved the tensile strength and modulus of PVC/ESP composite compared to a filler particle size of 7 μm as justified from FESEM images on the tensile fracture surface of the composites. Thermogravimetric analysis results show that the filler particle size of 0.2 μm composite exhibits higher thermal stability compared to the filler particle size of 7 μm composite.     

Multiple charging of ultrafine particles in a corona charger

The charge distribution on ultrafine aerosol particles in the size range below 35 nm has been measured using a corona-based unipolar charger, in which ion generation and aerosol charging take place simultaneously in the region around a sharp-point discharge electrode. The mean number of charges per particle predicted by Fuchs’ diffusion charging theory is in relatively good agreement with the experimental results, and this implies that diffusion charging is the predominant mechanism in spite that the electric field in the charging region is very high. Since a steady state is unattainable in unipolar charging, the charge distribution depends on the geometry and operating conditions of each particular charger. When the present device operates under the conditions (nt-product) which yield the maximum charging efficiency, double charge appears on particles with diameter as small as 15 nm. At larger values of nt, 32 nm particles can acquire up to five charges. The critical particle diameter above which multiple charging occurs is about four times smaller than for bipolar radioactive chargers. In order to use corona charging in aerosol particle size measurement by electrical methods, the required mobility data inversion is thus straightforward for particle diameter below about 15 nm, but becomes quite complex for larger sizes.