Date of Degree
PhD (Doctor of Philosophy)
Paul D. Kleiber
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 that is blown into the atmosphere has an impact on the earth’s climate by absorbing and scattering the incoming sun light and outgoing infrared radiation from the earth. Different kinds of mineral dust can absorb and scatter radiation differently depending on the particle’s size, shape, and composition, all of which can be very complex. Atmospheric mineral dust can have very irregular shapes as well as be composed of many different kinds of minerals and other atmospheric components.
In this work, measurements of the visible scattering properties are measured and compared to model simulations. Different models are needed for different types of mineral dust which will have different compositions and particle shapes. Single component mineral samples are studied first. It is found that in order to simultaneously simulate the infrared extinction and visible scattering properties of illite and kaolinite, two silicate clay minerals, a model that allows particle shape to vary with particle size is required. Diatomaceous earth is also investigated because of its extreme particle shape. Authentic mineral dust samples are studied next because of the added complexity of having a mixture of different minerals which can each have different shape characteristics. Mineral dust samples are also processed with organic acids and humic material in order to better understand how the visible light scattering properties of mineral dust can change when it is atmospherically aged. Lastly, dynamic shape factors are explored and correlated to the spheroidal shape distributions derived in studies of the optical properties.
publicabstract, Aerosol, Light Scattering, Mineral Dust, T-matrix Method
Copyright 2015 Jennifer Mary Alexander