Parker C. Reist

Neutralizing charged aerosols with radioactive sources

Abstract Electrically charged aerosol particles reach an equilibrium charge distribution when mixed with positive and negative ions in a time which is inversely proportional to ionic concentration. One way to neutralize the charge on aerosols is to use radioactive sources in a neutralizing chamber. The differential equation governing ionic concentration is analyzed, and approximate solutions are given for various combinations of radioactive source geometry and neutralizing chamber geometry. It is shown that when the range of the radioactive particles emitted by the source is greater than or equal to the chamber dimensions, the ratio of the particle residence time to the characteristic time necessary for neutralization is proportional to the square root of the radioactive source strength, inversely proportional to flow rate, and proportional chamber dimensions to the second power. This ratio is computed for four successful neutralizers and found to be ≳ 10. This analysis should facilitate the design of radioactive neutralizers.

Drag on non-spherical, orthotropic aerosol particles

Abstract Inhaled particles have been implicated in many forms of respiratory disease. Mathematical lung deposition models have been developed for spheres and fibers, but not for non-spherical, noncylindrical particles because theory and experimental data have been unavailable. To satisfy this need, non-spherical, orthotropic particles settling in air were modeled under dynamically similar conditions using aluminum prisms settling in viscous oil. Twenty-five rectangular prisms, each with a different length: width: thickness ratio, were settled in each of their three primary orientations and the resulting drag forces determined. Linear regression using predictors related to prism geometry and orientation resulted in an empirical drag force equation. Equation drag predictions are in excellent agreement with published data for rectangular prisms and other orthotropic objects including cylinders, ellipsoids, and double-conicals. The equation was validated by settling a quasi-monodisperse aerosol of tungstic acid platelets in a Stober spiral centrifuge, and comparing predicted with observed aerodynamic behavior. Using these results, lung deposition models for flakes and other non-spherical particles are now possible.

Characterization of morphological changes in agglomerates subject to condensation and evaporation using multiple fractal dimensions

Multiple fractal dimensions are used to characterize morphological changes that occur when an aerosol composed of irregularly shaped agglomerates is subject to condensation followed by evaporation. The agglomerates change from a branched, chainlike structure to a more regular, near-spherical or clumplike structure reflected in a decrease in the structural fractal dimension. The textural fractal dimension remains constant because the primary particles, of which the agglomerates are composed, do not change in shape. The degree of supersaturation and the number of condensation-evaporation cycles that the aerosol undergoes are major factors that influence morphological change. Even at low supersaturations, increasing the number of condensation-evaporation cycles makes the agglomerates more regular and thus decreases the structural fractal dimension. The transition point in the Richardson plot is a good indicator of the size of the primary particles in the agglomerate.

Exposure to respirable crystalline silica in eastern North Carolina farm workers

Occupational exposure to crystalline silica has been linked to silicosis, some forms of cancer, and certain autoimmune diseases. Little information exists on exposure levels of respirable silica in the agricultural industry. This study assessed respirable silica exposure of farm workers in eastern North Carolina. Sandy soils in this region have been shown to contain high levels of respirable silica. Personal breathing zone samples (n = 37) were collected from 27 workers at seven farms during various agricultural activities. The highest respirable silica concentrations were measured during sweet potato transplanting (3.91 +/- 2.07 mg/m3). Respirable silica exposure was observed to be associated with agricultural activity, soil moisture, relative humidity, and wind speed. Most of the variation in exposure (79%) was explained by agricultural activity and soil moisture. The observed percentage of silica levels (mean 34.7%) were almost twice as high as was reported in studies of California agriculture. This may be due to the loamy sand and sandy loam soil types in the regions in this study. In agriculture, respirable silica exposure is highly variable, but the potential for exposures above the threshold limit value of 0.05 mg/m3 exists during particular agricultural activities.

Development and operation of an improved turntable dust feeder

Abstract An improved version of a dry dispersion turntable aerosol feeder has been developed which provides greater variation, control, and reliability in aerosol output. The heart of the system is a rotating dust cylinder that permits steady delivery of dust to a turntable groove where it is picked-up by an adjustable aspirator. This design feature eliminates the need for vibrators and ensures a smooth dust flow to the dust pick-up point. Depending on the cylinder diameter, unattended operating time can be greatly increased. The aspirator incorporates an adjustable venturi and pick-up apparatus. By varying venturi adjustment and air pressure, dust pick-up under differing operating conditions can be assured. A large range in aerosol output can be achieved using different groove widths, locations, and turntable speeds. Operational performance tests show steady aerosol output over the delivery range of 100 mg/min to 50 g/min, depending on the size of the feeder. To date, two sizes of the turntable aerosol feeder have been utilized effectively resulting in dust concentrations ranging from 100 μg/m 3 to 560 mg/m 3 .

Aspects of the performance of the electrical aerosol analyzer under nonideal conditions

Abstract We report experiments in which the Electrical Aerosol Analyzer makes measurements on constant polydisperse aerosols under conditions representative of those encountered in practice. By varying only the sheath air, we observe ambiguities in the data of up to an order of magnitude, contrary to assumed operational characteristics. Possible causes of this observation are discussed and consequences for the utility of the instrument are pointed out.

Fractal Characterization of the Structure of Aerosol Agglomerates Grown at Reduced Pressure

Experimentally grown metallic aerosol particles were found to form highly complex shapes. When the ambient pressure at which they were grown was decreased, distinctive changes in their appearance were observed. Fractal analysis was used to characterize these particles and their differences. The analysis was able to distinguish several different growth regimes and explain some of the growth-pressure effects.

The influence of molecular complexes on the photooxidation of sulfur dioxide

Abstract It is proposed that molecular interaction products and specifically donor-acceptor molecular complexes significantly affect SO2 photochemical reaction mechanisms. Product quantum yield and triplet SO2 quenching rates are related to the extent of complex formation as indicated by the ionization potential and dielectric constant of the donor species. Intra-complex rearrangement may lead to some of the reaction products observed in some studies. Molecular complexes appear to participate in aerosol formation mechanisms involving SO2. NO and various hydrocarbons at ambient levels.

Design and Operating Parameters for a Large Ambient Aerosol Chamber

Recent investigations of ambient aerosol behavior over urban areas have pointed out the need for controlled experimental data to link together field investigation results and computer simulation studies. This paper describes the design considerations, construction details and operating parameters of a large (8000 ft3) outside reaction chamber constructed in rural North Carolina. The chamber is triangular in cross-section, 20 ft wide, 20 ft high and 40 ft long, and is covered with clear 5 ml Teflon film. The outdoor location of the chamber permits the reaction volume to be exposed to the natural conditions of temperature and solar radiation. A recirculating air system allows the air in the chamber to be passed through an “absolute” fiberglas filter for adjustment of condensation nuclei concentration and also through driers for humidity adjustment. Internal fans are provided for mixing of the chamber contents without use of the recirculating system so that various degrees of turbulence can be approximated. ...

Characterization of an Aerosol Chamber for Human Exposures to Endotoxin

The objective of this study was to develop and characterize an exposure chamber in which human subjects could be exposed to low dust concentrations carrying an endotoxin coating. An exposure chamber, dust dispersion method, and endotoxin characterization technique were developed for inhalation exposures. A 6.27 m3 exposure chamber was designed and constructed from cinder block, glass windows, and Plexiglas. Using an acetone adhesion process, Enterobacter agglomerans were adsorbed onto respirable cellulose particles to create the endotoxin aerosol. The size distribution of the endotoxin-treated particles was verified using light microscopy and cascade impactors. A dry powder dust generator was refined to consistently disperse small quantities of the aerosol into the chamber to maintain dust concentrations at approximately 250 micrograms/m3. Dust levels during the chamber exposures were monitored using a portable continuous aerosol monitor (PCAM). During initial exposure runs, PCAM monitoring stations were positioned at different locations within a 0.5-meter matrix to document mixing patterns. Total dust and cascade impactor samples were collected throughout each exposure period to characterize the chamber operating system and insure the mean airborne dust concentration fulfilled target levels. A one-factor analysis of variance at the 95 percent confidence interval illustrated that there was not a statistically significant difference in the mean dust concentration throughout the exposure runs compared to the individual runs. Together the consistency of the total dust filters, endotoxin concentrations, and aerosol-monitoring instrument were adequate to allow use of the chamber for experimental studies involving human volunteers.