Browsing by Subject "particle size distribution"
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Item Analysis of dense colloidal dispersions with multiwavelength frequency domain photon migration measurements(2009-06-02) Dali, Sarabjyot SinghFrequency domain photon migration (FDPM) measurements are used to study the properties of dense colloidal dispersions with hard sphere and electrostatic interactions, which are otherwise difficult to analyze due to multiple scattering effects. Hard sphere interactions were studied using a theoretical model based upon a polydisperse mixture of particles using the hard sphere Percus Yevick theory. The particle size distribution and volume fraction were recovered by solving a non linear inverse problem using genetic algorithms. The mean sizes of the particles of 144 and 223 nm diameter were recovered within an error range of 0-15.53% of the mean diameters determined from dynamic light scattering measurements. The volume fraction was recovered within an error range of 0-24% of the experimentally determined volume fractions. At ionic strengths varying between 0.5 and 4 mM, multiple wavelength (660, 685, 785 and 828 nm) FDPM measurements of isotropic scattering coefficients were made of 144 and 223 nm diameter, monodisperse dispersions varying between 15% - 22% volume fraction, as well as of bidisperse mixtures of 144 and 223 nm diameter latex particles in 1:3, 1:1 and 3:1 mixtures varying between volume fractions of 15% - 24%. Structure factor models with Yukawa potential were computed by Monte Carlo (MC) simulations and numerical solution of the coupled Ornstein Zernike equations. In monodisperse dispersions of particle diameter 144 nm the isotropic scattering coefficient versus ionic strength show an increase with increasing ionic strength consistent with model predictions, whereas there was a reversal of trends and fluctuations for the particle diameter of 223 nm. In bidisperse mixtures for the case of maximum number of smaller particles, the isotropic scattering coefficient increased with increasing ionic strength and the trends were in conformity with MC simulations of binary Yukawa potential models. As the number of larger diameter particles increased in the dispersions, the isotropic scattering coefficients depicted fluctuations, and no match was found between the models and measurements for a number ratio of 1:3. The research lays the foundation for the determination of particle size distribution, volume fractions and an estimate of effective charge for high density of particles.Item Chemical and Physical Properties of Atmospheric Aerosols (a) A Case Study in the Unique Properties of Agricultural Aerosols (b) The Role of Chemical Composition in Ice Nucleation during the Arctic Spring(2011-08-08) Moon, Seong-GiThis study focuses on the analysis of atmospheric particles sampled from two different field campaigns: the field study at a cattle feeding facility in the summer from 2005 to 2008 and the Indirect and Semi-Direct Aerosol Campaign (ISDAC) in 2008. A ground site field study at a representative large cattle feeding facility in the Texas Panhandle was conducted to characterize the particle size distributions, hygroscopicity, and chemical composition of agricultural aerosols. Here, a first comprehensive dataset is reported for these physical and chemical properties of agricultural aerosols appropriate for use in a site-specific emission inventory. The emission rate and transport of the aerosols are also discussed. In addition, mixing ratios of total and gaseous ammonia were measured at the same field in 2007 and 2008. Measurements such as these provide a means to determine whether the fugitive dust emitted from a typical large feedlot represents a health concern for employees of the feeding operation and the nearby community. Detailed chemical composition of aircraft-sampled particles collected during ISDAC was studied. Filter samples were collected under a variety of conditions in and out of mixed phase and ice clouds in the Arctic. Specifically, particles were sampled from a mixed-phase cloud during a period of observed high concentrations of ice nuclei (IN), a biomass plume, and under relatively clean ambient conditions. Composition of particles was studied on a particle-by-particle basis using several microspectroscopy techniques. Based on the elemental composition analysis, more magnesium was found in Arctic cloud residues relative to ambient air. Likewise, based on the carbon speciation analysis, high IN samples contained coated inorganics, carbonate, and black or brown carbon particles. In the samples collected during a flight through a biomass burning plume, water-soluble organic carbon was the dominant overall composition. Due to their hygroscopic nature, these organics may preferably act as cloud condensation nuclei (CCN) rather than IN. Other ambient samples contained relatively higher fractions of organic and inorganic mixtures and less purely water-soluble organics than found in the biomass particles. The most likely source of inorganics would be sea salt. When present, sea salt may further enhance ice nucleation.Item Development of methodology to correct sampling error associated with FRM PM10 samplers(2009-05-15) Chen, JingCurrently, a lack of accurate emission data exits for particulate matter (PM) in agricultural air quality studies (USDA-AAQTF, 2000). PM samplers, however, tend to over estimate the concentration of most agricultural dusts because of the interaction of the particle size distribution (PSD) and performance characteristics of the sampler (Buser, 2004). This research attempts to find a practical method to characterize and correct this error for the Federal Reference Method (FRM) PM10 sampler. First, a new dust wind tunnel testing facility that satisfies the USEPA?s requirement of testing PM10 samplers was designed, built, and evaluated. Second, the wind tunnel testing protocol using poly-dispersed aerosol as the test dust was proved to be able to provide results consistent with mono-dispersed dusts. Third, this study quantified the variation of over sampling ratios for the various cut point and slopes of FRM PM10 samplers and proposed an averaged over sampling ratio as a correction factor for various ranges of PSD. Finally, a method of using total suspended particle (TSP) samplers as a field reference for determining PM10 concentrations and aerosol PSD was explored computationally. Overall, this dissertation developed successfully the methodology to correct the sampling error associated with the FRM PM10 sampler: (1) wind tunnel testing facilities and protocol for experimental evaluation of samplers; (2) the variation of the oversampling ratios of FRM PM10 samplers for computational evaluation of samplers; (3) the evaluation of TSP sampler effectiveness as a potential field reference for field evaluation of samplers.