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Radiative forcing on the atmosphere due to mineral dust aerosols plays a key role in climate change. This results from dust optical properties, more specifically the Single Scattering Albedo (SSA), which is a key aerosol optical parameter and determines sign and magnitude of radiative forcing due to aerosols. In order to get a better understanding of how mineral dust aerosols have an impact on climate, it is of great importance the study of the variability of the mineralogical composition of airborne dust. Here, we revise two data sets that provide mineralogical information for several soil units of the clay and the silt fraction of the soil based on the relative abundance of selected minerals. Furthermore, this paper is focused on the study of the variability of the relative abundance of iron oxide minerals and mean iron content of soils, which are key to determine the SSA of mineral dust in the short-wave spectrum. A regional analysis is conducted in the Sahara desert, located in northern Africa, to study the variability in the relative abundance of hematite and goethite according to each data set. These analyses are conducted by statistical analyses and developing adequate MATLAB codes in order to obtain the desired results. Results show a high variability and large uncertainties on the definition of iron oxide minerals in the erodible fraction of the soil, presenting high coefficients of variance for both data sets. This uncertainty on the resulting values is due to lack of mineralogical data and hypothesis that have been assumed in order to provide more geographical coverage. Mineralogical maps in a 0.5' x 0.5' grid, have been constructed from data extracted from the databases in order to display geographical distribution of the mineralogical composition at a global or regional scale. Finally, the SSA of mineral dust, at two wavelengths 405 nm and 870 nm, basing the calculations on the linear relationship established in Moosmüller et al. (2012), that relates the iron content to the SSA. This optical parameter calculated for 12 textural soil classes and for the region of the Sahara desert. Results present a high SSA in both case analyses, but when calculating the SSA for the Sahara region, the SSA is relatively higher to those presented in the theoretical calculations for the soil textural classes. Nevertheless, as we obtain a high values, as expected, it is confirmed that iron content in mineral dust affects significantly the values of the Single Scattering Albedo.
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.
A special challenge posed by mineral dust aerosols is associated with their predominantly nonspherical particle shapes. In the present study, the scattering and radiative properties for nonspherical mineral dust aerosols at violet-to-blue (0.412, 0.441, and 0.470 [mu]m) and red (0.650 [mu]m) wavelengths are investigated. To account for the effect of particle nonsphericity on the optical properties of dust aerosols, the particle shapes for these particles are assumed to be spheroids. A combination of the T-matrix method and an improved geometric optics method is applied to the computation of the single-scattering properties of spheroidal particles with size parameters ranging from the Rayleigh to geometric optics regimes. For comparison, the Mie theory is employed to compute the optical properties of spherical dust particles that have the same volumes as their nonspherical counterparts. The differences between the phase functions of spheroidal and spherical particles lead to quite different lookup tables (LUTs) involved in retrieving dust aerosol properties. Moreover, the applicability of a hybrid approach based on the spheroid model for the phase function and the sphere model for the other phase matrix elements is demonstrated. The present sensitivity study, employing the Moderate Resolution Imaging Spectroradiometer (MODIS) observations and the fundamental principle of the Deep Blue algorithm, illustrates that neglecting the nonsphericity of dust particles leads to an underestimate of retrieved aerosol optical depth at most scattering angles, and an overestimate is noted in some cases. The sensitivity study of the effect of thin cirrus clouds on dust optical depth retrievals is also investigated and quantified from MODIS observations. The importance of identifying thin cirrus clouds in dust optical depth retrievals is demonstrated. This has been undertaken through the comparison of retrieved dust optical depths by using two different LUTs. One is for the dust only atmosphere, and the other is for the atmosphere with overlapping mineral dust and thin cirrus clouds. For simplicity, the optical depth and bulk scattering properties of thin cirrus clouds are prescribed a priori. Under heavy dusty conditions, the errors in the retrieved dust optical depths due to the effect of thin cirrus are comparable to the assumed optical depth of thin cirrus clouds. With the spheroidal and spherical particle shape assumptions for mineral dust aerosols, the effect of particle shapes on dust radiative forcing calculations is estimated based on Fu-Liou radiative transfer model. The effect of particle shapes on dust radiative forcing is illustrated in the following two aspects. First, the effect of particle shapes on the single-scattering properties of dust aerosols and associated dust direct radiative forcing is assessed, without considering the effect on dust optical depth retrievals. Second, the effect of particle shapes on dust direct radiative forcing is further discussed by including the effect of particle nonsphericity on dust optical depth retrievals.
Light scattering from particles in the nanometric and micrometric size range is relevant in several research fields, such as aerosol science and nanotechnology. In many applications, the description of the optical properties of non-spherical, inhomogeneous particles is still inadequate or requires demanding numerical calculations. Lorenz–Mie scattering and effective medium approximations represent currently the main theoretical tools to model such particles, but their effectiveness has been recently called into question. This work examines how the morphology of a particle affects its scattering parameters from an experimental standpoint, supporting findings with extensive simulations. The dust content of Antarctic, Greenlandic, and Alpine ice cores is analysed with a particle-by-particle approach. Moreover, a study on colloidal aggregates shows that correlations among the fields radiated by primary particles are responsible for the poor agreement of effective medium approximations with experimental results. On the theoretical side, an interpretation in terms of the structure factor is given, which satisfactorily describes the data. The insights of this thesis are relevant for quantifying the contribution of mineral dust to the radiative energy balance of the Earth.
A thorough and up-to-date treatment of electromagnetic scattering by small particles.
This new text offers experienced students a comprehensive review of available techniques for the remote sensing of aerosols. These small particles influence both atmospheric visibility and the thermodynamics of the atmosphere. They are also of great importance in any consideration of climate change problems. Aerosols may also be responsible for the loss of harvests, human health problems and ecological disasters. Thus, this detailed study of aerosol properties on a global scale could not be more timely.
Aerosols are important atmospheric constituents that impact the Earth's radiative balance and climate. The detailed knowledge of the aerosol optical properties is required for a comprehensive analysis of the impacts of aerosols on climate. Mie theory is often used in satellite and ground-based retrieval algorithms to account for atmospheric mineral dust. However, the approximations used in Mie theory are often not appropriate for mineral dust and can lead to errors in the optical properties modeling. Analytic models based on Rayleigh theory that account for particle shapes can offer significant advantages when used to model infrared (IR) extinction of mineral dust. Here, the IR optical properties of some components of mineral dust, authentic dust samples and minerals processed with organic acids were investigated. Detailed characterization of the particles through online and offline methods of analysis that include IR extinction spectroscopy, micro-Raman spectroscopy and scanning electron microscopy was performed. Analysis of the IR extinction spectra and spectral simulations showed that the positions of the peaks and the shapes of the bands of the IR characteristic features are not well simulated by Mie theory in any of the samples studied. The resonance peaks were consistently shifted relative to the experimental spectrum in the Mie simulation. Rayleigh model solutions derived for different particle shapes better predicted the peak positions and band shapes of experimental spectra. To fill the gaps in the refractive index data for atmospherically relevant organic compounds in the IR region optical properties of atmospherically relevant carboxylic acids and humic-like substances using the IR extinction spectra and size distributions measured in the laboratory were determined. In addition to properties of mineral dust this dissertation focuses on properties of sea spray aerosol. Chemical and elemental composition of individual sea spray aerosol particles were studies using micro-Raman spectroscopy, mass-spectrometry and X-ray spectroscopy to provide insights into the biochemical processes that give rise to classes of organic molecules that make up these aerosol particles. The results suggested that degradation of biota (bacteria and diatoms) present in sea water led to lipopolysaccharides and extracellular polymeric substances that further degraded down to carbohydrates and fatty acids. Solubility of the resulting organic species seemed to play a role in their transfer to the aerosol phase. Furthermore, water uptake and hygroscopic growth of multi-component particles were studied. Understanding the interactions of water with atmospheric aerosols is crucial for determining their size, physical state, reactivity, and therefore for aerosol interactions with electromagnetic radiation and clouds. It was determined that particles composed of ammonium sulfate with succinic acid and of mixture of chlorides typical for marine environment show size dependent hygroscopic behavior. Microscopic analysis of the distribution of components within the aerosol particles showed that the observed size dependence is due to the differences in the mixing state. The composition and water uptake properties of sea spray aerosol particles were also measured during a phytoplankton bloom. The results showed that water uptake properties were directly related to the chemical composition of the particles and hygroscopicity decreased with increase in the fraction of water insoluble organic matter emitted during phytoplankton bloom. Finally, multiple methods of particle size, phase and shape analysis were compared and the results showed that the techniques that operate under ambient conditions provide the most relevant and robust measurement of particle size. Additionally, several storage methods for substrate deposited aerosol particles were evaluated and it was determined that storing samples at low relative humidity led to irreversible changes due to sample dehydration while sample freezing and thawing leads to irreversible changes due to phase changes and water condensation. Therefore it is suggested that samples used for single-particles analysis should be stored at ambient laboratory conditions, or near conditions which they were collected, in order to preserve the sample phase and hydration state. The results presented in this dissertation provide insight into physicochemical properties of atmospheric aerosols and help us better understand the role of aerosol particles in the Earth's atmosphere.
This volume is a collection of review articles by scientists who have pioneered many of the recent advances in studies of the optical effects of small particles. The book begins with a review of the multitude of sharp dielectric resonances which exist in all optical spectra as a result of particle size and shape. Latest advances in absorption and fluorescence spectroscopy of a single particle and/or an ensemble of particles are also discussed, as well as advances in the energy transfer mechanisms for molecules embedded in the particle. The effects of laser-induced heating on a single particle are reviewed in terms of the hydrodynamics and thermodynamics of the liquid droplet and its ambient gas surrounding. The limits of applying bulk optical constants to small particles which lie between the bulk substance and the quantum-sized substance are also presented.
Modeling the single-scattering properties of nonspherical particles in the atmosphere (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 electromagnetic 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 dependence 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 computational 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 calibration algorithm for 1.064-[mu]m 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.