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Atmospheric mineral dust has a large impact on the earth’s radiation balance and climate. The radiative effects of mineral dust depend on factors including, particle size, shape, and composition which can all be extremely complex. Mineral dust particles are typically irregular in shape and can include sharp edges, voids, and fine scale surface roughness. Particle shape can also depend on the type of mineral and can vary as a function of particle size. In addition, atmospheric mineral dust is a complex mixture of different minerals as well as other, possibly organic, components that have been mixed in while these particles are suspended in the atmosphere. Aerosol optical properties are investigated in this work, including studies of the effect of particle size, shape, and composition on the infrared (IR) extinction and visible scattering properties in order to achieve more accurate modeling methods. Studies of particle shape effects on dust optical properties for single component mineral samples of silicate clay and diatomaceous earth are carried out here first. Experimental measurements are modeled using T-matrix theory in a uniform spheroid approximation. Previous efforts to simulate the measured optical properties of silicate clay, using models that assumed particle shape was independent of particle size, have achieved only limited success. However, a model which accounts for a correlation between particle size and shape for the silicate clays offers a large improvement over earlier modeling approaches. Diatomaceous earth is also studied as an example of a single component mineral dust aerosol with extreme particle shapes. A particle shape distribution, determined by fitting the experimental IR extinction data, used as a basis for modeling the visible light scattering properties. While the visible simulations show only modestly good agreement with the scattering data, the fits are generally better than those obtained using more commonly invoked particle shape distributions. The next goal of this work is to investigate if modeling methods developed in the studies of single mineral components can be generalized to predict the optical properties of more authentic aerosol samples which are complex mixtures of different minerals. Samples of Saharan sand, Iowa loess, and Arizona road dust are used here as test cases. T-matrix based simulations of the authentic samples, using measured particle size distributions, empirical mineralogies, and a priori particle shape models for each mineral component are directly compared with the measured IR extinction spectra and visible scattering profiles. This modeling approach offers a significant improvement over more commonly applied models that ignore variations in particle shape with size or mineralogy and include only a moderate range of shape parameters. Mineral dust samples processed with organic acids and humic material are also studied in order to explore how the optical properties of dust can change after being aged in the atmosphere. Processed samples include quartz mixed with humic material, and calcite reacted with acetic and oxalic acid. Clear differences in the light scattering properties are observed for all three processed mineral dust samples when compared to the unprocessed mineral dust or organic salt products. These interactions result in both internal and external mixtures depending on the sample. In addition, the presence of these organic materials can alter the mineral dust particle shape. Overall, however, these results demonstrate the need to account for the effects of atmospheric aging of mineral dust on aerosol optical properties. Particle shape can also affect the aerodynamic properties of mineral dust aerosol. In order to account for these effects, the dynamic shape factor is used to give a measure of particle asphericity. Dynamic shape factors of quartz are measured by mass and mobility selecting particles and measuring their vacuum aerodynamic diameter. From this, dynamic shape factors in both the transition and vacuum regime can be derived. The measured dynamic shape factors of quartz agree quite well with the spheroidal shape distributions derived through studies of the optical properties.

Mineral dust aerosol plays an important role in determining the physical and chemical equilibrium of the atmosphere. The radiative balance of the Earth's atmosphere can be affected by mineral dust through both direct and indirect means. Mineral dust can directly scatter or absorb incoming visible solar radiation and outgoing terrestrial IR radiation. Dust particles can also serve as cloud condensation nuclei, thereby increasing albedo, or provide sites for heterogeneous reactions with trace gas species, which are indirect effects. Unfortunately, many of these processes are poorly understood due to incomplete knowledge of the physical and chemical characteristics of the particles including dust concentration and global distribution, as well as aerosol composition, mixing state, and size and shape distributions. Much of the information about mineral dust aerosol loading and spatial distribution is obtained from remote sensing measurements which often rely on measuring the scattering or absorption of light from these particles and are thus subject to errors arising from an incomplete understanding of the scattering processes. The light scattering properties of several key mineral components of atmospheric dust have been measured at three different wavelengths in the visible. In addition, measurements of the scattering were performed for several authentic mineral dust aerosols, including Saharan sand, diatomaceous earth, Iowa loess soil, and palagonite. These samples include particles that are highly irregular in shape. Using known optical constants along with measured size distributions, simulations of the light scattering process were performed using both Mie and T-Matrix theories. Particle shapes were approximated as a distribution of spheroids for the T-Matrix calculations. It was found that the theoretical model simulations differed markedly from experimental measurements of the light scattering, particularly near the mid-range and near backscattering angles. In many cases, in the near backward direction, theoretical models predicted scattering intensities for near spherical particles that were up to 3 times higher than the experimentally measured values. It was found that better agreement between simulations and experiments could be obtained for the visible scattering by using a much wider range of more eccentric particle shapes.

Metal oxides in the atmosphere emitted from various natural and anthropogenic processes alter the chemical balance of the Earth's atmosphere due to heterogeneous and photochemical processes with atmospheric trace gases. Therefore, understanding the heterogeneous chemistry and heterogeneous photochemistry of atmospheric trace gases on these oxide surfaces has become vital to precisely predict the effect of mineral dust loading on the Earth's atmosphere. Among the various components of mineral dust, light absorbing oxides play a significantly important role during the daytime. The work reported herein has focused mainly on TiO2 and Α-Fe2O3. These are light adsorbing components found in atmospheric mineral dust. Apart from being a component of mineral dust, TiO2 is heavily used in a number of industrial applications ranging from uses in self-cleaning, water purification to cosmetics. These applications have led to their presence in the atmosphere as anthropogenic dust particles and in contact with the atmosphere as a stationary phase. Iron-containing particles are transferred to the atmosphere mainly from wind and volcanic activities in the form of iron-containing mineral dust and volcanic ash aerosols. Α-Fe2O3 is the most stable iron containing compound found in the Earths' crust which constitutes in significant amounts in mineral dust. The presence of these oxide surfaces in the atmosphere can play a major role in heterogeneous chemistry and photochemistry. In this dissertation research, transmission FTIR spectroscopy and X-ray photoelectron spectroscopy are used to probe the details of heterogeneous chemistry and photochemistry of CO2, SO2, NO2, HCOOH, and HNO3 on titanium dioxide and hematite surfaces. Adsorption sites, surface speciation and surface species stability have been determined from analysis of FTIR and XPS spectra. Isotope labeling experiments were also carried out in order to obtain mechanistic information about the details of surface hydroxyl group reactivity on these oxide particle surfaces. Furthermore, heterogeneous photochemical reactions of adsorbates from atmospheric trace gas adsorption on TiO2 and Α-Fe2O3 were investigated under the conditions pertinent to troposphere. The role of adsorbed water on the stability of adsorbed species that form as a result of heterogeneous reactions and the effect of relative humidity on photochemistry on these oxide particles surfaces has also been investigated due to its important implications in the atmospheric chemistry of oxide surfaces. The research adds to our overall scientific understanding of the molecular level details of heterogeneous chemistry and photochemistry of light absorbing components in the atmosphere.

Particulate matter in the atmosphere emitted from various natural and anthropogenic sources is important due to their effects on the chemical balance of the atmosphere, the Earth's climate, human health and biogeochemical cycles. Although there have been many studies performed to understand the above effects, there still remains substantial uncertainty associated with processes involved and thus it is difficult for current atmospheric chemistry and climate models to reconcile model results with field measurements. Therefore, it is important to have better agreement between models and observations as the accuracy of future atmospheric chemistry and climate predictions depends on it. In this research, a greater understanding of the role of mineral dust chemistry was pursued through focused laboratory studies in order to better understand fundamental processes involved. In particular, studies to further understand the photochemistry of adsorbed nitrate, an important inorganic ion associated with particulate matter exposed to gas-phase nitrogen oxides, were conducted using Al2O3, TiO2 and NaY zeolite to represent non-photoactive components, photoactive components and aluminosilicate respectively, present in mineral dust. These studies reveal that photochemistry of nitrate adsorbed on mineral dust is governed by wavelength of light, physicochemical properties of dust particles and adsorption mode of nitrate. Gas phase NO2, NO and N2O are the photolysis products of nitrate on oxide particles under dry conditions. In contrast, nitrate adsorbed on zeolite is converted mainly to adsorbed nitrite upon irradiation. This nitrite yield is decreased with increasing relative humidity. Gas phase N2O is the main photolysis product of nitrate adsorbed in zeolite in the presence of co-adsorbed ammonia. Water adsorbed on semiconducting TiO2 can be photochemically converted to hydroxyl radicals. These hydroxyl radicals can be involved in surface mediated as well as gas phase oxidation reactions in the presence of cyclohexane. Another focus of this dissertation was to investigate the oxidation of sulfur dioxide oxidation in the presence of mineral aerosol, particularly, coal fly ash (FA), γ-Fe2O3 and Arizona test dust (AZTD), a model for mineral dust aerosol. Depending on the temporal evolution of Fe(II), we proposed that S(IV) oxidation in the presence of FA and γ-Fe2O3 initially occurs through a heterogeneous pathway and a homogeneous pathway is also possible over later time scales. S(IV) oxidation in the presence of AZTD appears to be mostly heterogeneous and does not lead to iron dissolution. Overall, these studies suggest that the rate, extent and products of atmospheric S(IV) oxidation can be highly variable and heavily dependent upon the nature of aerosol sources, thereby precluding simple generalizations about this reaction when modeling atmospheric processes involving diverse mineral dust aerosols. With the recent development in nanotechnology, nanoparticles are becoming a major fraction of atmospheric particulate matter. These particles can undergo aging under ambient conditions at any stage of their life cycle. This impacts the fundamental properties of these materials and therefore the behavior in the environment and interactions with biomolecules and biological systems. ZnO and CuO nanoparticles form adsorbed carbonate phases upon exposure to CO2 and water vapor. These carbonates become more solvated as the relative humidity is increased. Presence of carbonate phases on ZnO particles increases their water solubility. Thus, overall the work reported in this dissertation provides insights into heterogeneous and multiphase atmospheric chemical reactions in the presence of mineral aerosol and atmospheric aging of nanoparticles.

Particulate nitrates play important roles in the atmosphere. They consist mainly of NH4NO3 and NaNO3, products from the reactions of gaseous HNO3 with gaseous NH3 and sea salt, respectively. The gas-to-particle phase conversion of nitrate changes its deposition characteristics and ultimately changes the transport and deposition rates of the locally produced species. Studies were conducted to develop background information on the particle concentrations and size distributions and the chemistry and kinetics behind the interactions. The predominant nitrate species in the Tampa Bay area was identified as coarse mode NaNO3. NH4NO3 was not detected as it has high volatility at ambient temperatures. Spatial distribution sampling determined a gradient of NaCl and NaNO3 with increased distance from the coastal shore and an increase in the gas-to-particle conversion of nitric acid along a predominant air mass trajectory. The EQUISOLV II thermodynamic equilibrium model was evaluated. It was determined that the model can be used to predict gas and size-distributed particulate matter concentrations. The model was also used to examine the gas-to-particle partitioning of nitric acid to nitrate by NaCl and CaCO3. Both sodium and calcium partitioned nitrate to the particle phase. The magnitude of the partitioning was directly dependent on the equilibrium coefficients. The fine mode percentage of the total nitrate was determined using two methods. The results were used to expand the current data set to account for the coarse mode nitrate, and they indicated that particle nitrate accounted for 9% of the total nitrogen deposition flux to Tampa Bay. The formation of particle nitrate was examined using a nitrate accumulation model. Results indicated that the equilibrium time for particles less than 10 um in diameter was significantly less than their atmospheric residence time, with fastest conversion occurring under the highest relative humidity conditions. This information is vital in the development of atmospheric nitrogen dry deposition estimates, which are used to assess water quality and nutrient loading. These data can be used to determine air-monitoring strategies and to develop models that account for the coarse particle nitrogen species.

Atmosphärisches Aerosol kann den Strahlungstransport signifikant beeinflussen. Mineralstaub, der über der Sahara und anderen Wüsten in die Atmosphäre gelangt, ist das hinsichtlich der in letzterer dauerhaft verbleibenden Masse bedeutendste Aerosol. Darüber hinaus sind Saharamineralstaubpartikel nichtkugelförmig, und die Wirkungen dieser Partikeleigenschaft auf den Strahlungstransport in der Erdatmosphäre sind bislang nur ungenügend untersucht worden. Es werden die optischen Eigenschaften, Strahlungs- und Erwärmungseffekte von Saharamineralstaub unter Berücksichtigung der Nichtkugelförmigkeit seiner Partikel quantitativ untersucht, wobei der gesamte, im Hinblick auf den Strahlungshaushalt energetisch relevante Spektralbereich zugrunde gelegt wird. Zunächst werden auf Basis in-situ-gemessener Experimentaldaten die atmosphärischen Umgebungsbedingungen, Größenverteilungen, Brechungsindizes, Bodenalbedo und Partikelgestalt festgelegt, die in einem zweiten Schritt in ein Strahlungstransportmodell einfließen. Mit dessen Hilfe wird in umfangreichen numerischen Simulationen des Strahlungstransports in einer realistischen mineralstaubhaltigen Modellatmosphäre im Vergleich zu Messdaten beispielsweise geklärt, welche Partikelform und Größenäquivalenz angenommener sphäroidaler Modellpartikel am meisten realistisch sind. Des Weiteren werden im Zusammenhang mit der Partikelnichtkugelförmigkeit Sensitivitätsstudien zur Beantwortung der Fragen durchgeführt, inwieweit diese das Strahlungsfeld beeinflusst und zu veränderten Strahlungserwärmungswirkungen führt.

Modeling the single-scattering properties of nonspherical particles in the atmo¬sphere (in particular, ice crystals and dust aerosols) has important applications to climate and remote sensing studies. The first part of the dissertation (Chapters II¬V) reports a combination of exact numerical methods, including the finite-difference time-domain (FDTD), the discrete-dipole-approximation (DDA), and the T-matrix methods, and an approximate method-the physical-geometric optics hybrid (PGOH) method-in the computation of the optical properties of the non-spherical particles in a complete range of size parameters. The major advancements are made on the modeling capabilities of the PGOH method, and the knowledge of the electromag¬netic tunneling effect – a semi-classical scattering effect. This research is important to obtain reliable optical properties of nonspherical particles in a complete range of size parameters with satisfactory accuracy and computational efficiency. The second part (Chapters VI-VII) of the dissertation is to investigate the de¬pendence of the optical properties of ice crystals and mineral dust aerosols in the atmosphere on the spectrum, the particle size and the morphology based on compu¬tational models. Ice crystals in the atmosphere can be classified to be simple regular faceted particles (such as hexagon columns, plates, etc.) and imperfect ice crystals. Modeling of the scattering by regular ice crystals is straightforward, as their morphologies can be easily defined. For imperfect ice crystals, the morphology is quite diverse, which complicates the modeling process. We present an effective approach of using irregular faceted particle to characterize the imperfectness of ice crystals. As an example of application, less-than-unity backscattering color ratio of cirrus clouds is demonstrated and explained theoretically, which provides guidance in the calibra¬tion algorithm for 1.064-µm channel on the Calipso lidar. Dust aerosols have no particular morphology. To develop an approach to modeling the optical properties of realistic dust particles, the principle of using simple shapes (triaxial ellipsoids and nonsymmetric hexahedra) to represent irregular dust particles is explored. Simulated results have been compared with those measured in laboratory for several realistic aerosol samples. Agreement between simulated results and measurement suggests the potential applicability of the two aforementioned aerosol models. We also show the potential impact of the present study to passive and active atmospheric remote sensing and future research works.

Includes vita. / Title from PDF of title page. / Document formatted into pages; contains 236 pages. / Thesis (Ph.D.)--University of South Florida, 2003. / Includes bibliographical references. / Text (Electronic thesis) in PDF format. / ABSTRACT: Particulate nitrates play important roles in the atmosphere. They consist mainly of NH4NO3 and NaNO3, products from the reactions of gaseous HNO3 with gaseous NH3 and sea salt, respectively. The gas-to-particle phase conversion of nitrate changes its deposition characteristics and ultimately changes the transport and deposition rates of the locally produced species. Studies were conducted to develop background information on the particle concentrations and size distributions and the chemistry and kinetics behind the interactions. The predominant nitrate species in the Tampa Bay area was identified as coarse mode NaNO3. NH4NO3 was not detected as it has high volatility at ambient temperatures. Spatial distribution sampling determined a gradient of NaCl and NaNO3 with increased distance from the coastal shore and an increase in the gas-to-particle conversion of nitric acid along a predominant air mass trajectory. / ABSTRACT: The EQUISOLV II thermodynamic equilibrium model was evaluated. It was determined that the model can be used to predict gas and size-distributed particulate matter concentrations. The model was also used to examine the gas-to-particle partitioning of nitric acid to nitrate by NaCl and CaCO3. Both sodium and calcium partitioned nitrate to the particle phase. The magnitude of the partitioning was directly dependent on the equilibrium coefficients. The fine mode percentage of the total nitrate was determined using two methods. The results were used to expand the current data set to account for the coarse mode nitrate, and they indicated that particle nitrate accounted for 9% of the total nitrogen deposition flux to Tampa Bay. The formation of particle nitrate was examined using a nitrate accumulation model. / ABSTRACT: Results indicated that the equilibrium time for particles less than 10 um in diameter was significantly less than their atmospheric residence time, with fastest conversion occurring under the highest relative humidity conditions. This information is vital in the development of atmospheric nitrogen dry deposition estimates, which are used to assess water quality and nutrient loading. These data can be used to determine air-monitoring strategies and to develop models that account for the coarse particle nitrogen species. / System requirements: World Wide Web browser and PDF reader. / Mode of access: World Wide Web.

Mineral dust aerosols, the tiny soil particles in the atmosphere, play a key role in the atmospheric radiation budget through their radiative and cloud condensation nuclei effects. It is therefore important to evaluate the radiative forcing of mineral dust and subsequent changes in atmospheric dynamics. The Weather Research and Forecasting with Chemistry (WRF/Chem) regional model with the integrated dust modules and available observations have been used to investigate the three-dimensional distribution of mineral dust over Australia. Additionally, the WRF/Chem model was used to estimate the direct radiative forcing by mineral dust over Australia. Particular emphasize has been given to direct radiative feedback effect of mineral dust on boundary layer dynamics. Two dust emission schemes embedded within the WRF/Chem model have been utilized in this study: the dust transport (DUSTRAN) and the Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) schemes. The refractive index of mineral dust depends on the mineralogy, size and composition of dust over a given region. The refractive index of mineral dust for shortwave radiation was considered to be wavelength independent and set based on previous mineralogical studies over North Africa and Australia. However, the refractive index of mineral dust for longwave radiation was considered to be wavelength dependent and to vary for 16 longwave spectral bands. Model results were compared with observations to validate the performance of the model, including satellite datasets from the Moderate Resolution Imaging Spectroradiometer (MODIS), Multi-angle Imaging SpectroRadiometer (MISR) and Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP), as well as ground-based measurements obtained from air quality monitoring sites over Australia. The major results can be summarized as follows: (1) Lake Eyre Basin is the most important source of dust in Australia, with a peak activity identified to be during austral spring and summer, and dust emission within the basin is often associated with the passage of dry cold fronts; (2) Mineral dust from Lake Eyre Basin can be transported long distances to southeastern Australia in association with eastward propagating frontal systems, reaching as far as New Zealand and beyond, and to northern tropical Australia by postfrontal southerly winds, and subsequently towards northwestern Australia and the Indian Ocean by southeasterly trade winds; (3) Australian dust plumes are mainly transported in the lower atmosphere, although variation of boundary layer depth during the passage of cold frontal systems, as well as ascending motion at the leading edge of these systems and descending motion where postfrontal anticyclonic circulation is dominant contribute to the vertical extent of mineral dust aerosols; (4) the shortwave direct radiative effect of mineral dust results in cooling of the atmosphere from the surface to near the boundary layer top, but warming of the boundary layer top and lower free atmosphere; (5) changes in the vertical profile of temperature result in an overall decrease of wind speed in the lower boundary layer and an increase within the upper boundary layer and lower free atmosphere; (6) the longwave warming effect of mineral dust partly offsets its shortwave cooling effect at the surface. This compensation is significantly larger over and immediately downwind of dust source regions where coarse particles are more abundant, as they have stronger interaction with longwave radiation emitted from the Earth’s surface; (7) both shortwave and longwave radiative forcing by mineral dust was found to have a diurnal variation in response to changes in solar zenith angle and in the intensity of longwave radiation, respectively; (8) the absorptive nature of dust was shown to be associated with the shortwave heating of the atmosphere; (9) on the other hand, longwave cooling of the atmosphere was identified because absorption of longwave radiation by dust is less than its emission to the surface and top of the atmosphere (TOA).