Spectral Absorption Characteristics of the Major Components of Dust Clouds (original) (raw)
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Absorption of sunlight by dust as inferred from satellite and ground-based remote sensing
Geophysical Research Letters, 2001
Dust absorption of solar radiation is not well known due to limitations in the accuracy of in situ measurements. Here we report two new independent remote sensing techniques that provide sensitive measurements of dust absorption. One uses satellite spectral measurements, the L second ground based sky measurements. Both techniques demonstrate that Saharan dust absorption of solar radiation is several times smaller than the current international standards. For example, at wavelength of 0.64 pm the dust single scattering albedo is reported here as 0.97kO.02 rather than 0.8710.04 in recent review.
Modeling optical properties of mineral dust over the Indian Desert
Journal of Geophysical Research, 2008
The direct radiative forcing (DRF) of dust particles is most uncertain among all the major aerosol species because of the large regional variation in their shapes and composition. The Indian Desert is known to be a source of natural mineral dust of nonspherical shapes. Particle shape and exact mineralogical information are essential for modeling dust optical properties as the latter governs their refractive indices. The realistic dust shapes, namely, sphere, spheroid, Chebyshev, and cylinder, based on Scanning Electron Microscope (SEM) images, have been used to model the mineral dust optics of the Indian Desert using the T-matrix method. The particle radius from 0.1 to 5.0 mm has been considered at wavelengths ranging from ultraviolet to near infrared (0.38-1.2 mm). Using Bruggman's effective medium mixing rule, the refractive index of composite dust particle has been calculated, accounting for both nonmetallic and metallic component (as hematite). Our calculations show that increasing the hematite percentage from 0% to 10% results in reduction of 0.477 and 0.013 in single scattering albedo (SSA) for cylindrical particle of radius 1 mm at 0.38 and 1.02 mm wavelengths, respectively, while the same for volume equivalent spherical particle were 0.484 and 0.022, respectively. The scattering signature of sharp-edged cylindrical particle showed the largest deviation to sphere compared with that of other relatively smooth particles (spheroid and Chebyshev). Changes in dust optical properties because of nonsphericity and varying hematite percentage were estimated for two cases: background dust and dust storm at visible wavelength. The change in SSA between the above two cases was insignificant for particles of radii <0.4 mm for each hematite percentage considered. On the other hand, for particles of size range 0.4-1 mm, the change in SSA increases with increasing hematite percentage. A 6% increase in hematite leads to an SSA reduction of more than 0.2 for particle radius of 1 mm for both background dust and dust storm cases. Optical properties of polydisperse dust distribution at visible wavelength suggest the likely hematite percentage as 0%-4% in the Indian mineral desert dust. The effect of hematite variation on SSA is found to be stronger than particle nonsphericity. The present work will lead to a better estimation of the radiative forcing imposed by dusts as well as their satellite retrieval over the Indian Desert region.
The necessity of ascertaining contamination by dust using soil spectral reflectance as its indicator
2021
Pollution of the urban environment by human disturbances and activities is a negative externality of urbanization, therefore becoming a great concern due to the serious problems associated with human health. The mobilization of heavy metals into the biosphere by human activities has become an important process in the geochemical recycling of these metals. Though the risks of exposure to road dust have been reported to be higher for individuals than those in soil, little attention has been paid to the occurrence characteristics of heavy metals in dust and its associated health risks to the population. In the present studies, the physical, chemical and spectral signatures gained from dust and soil constituents would be differentiated based on their reflectance in specific bands of the electromagnetic spectrum. It is expected that the analysis of road dust and soil samples will indicate spectral signatures exhibiting differences in specific wavelengths of the spectrum, hence, indicatin...
Modeling the mineralogy of atmospheric dust sources
Journal of Geophysical Research, 1999
The variability of atmospheric dust mineralogy influences the impact of desert dust on the Earth's radiative budget and biogeochemical cycles. Until now, atmospheric transport models have assumed that dust was a constant •--• •].u•mmu•,• m•uii. S mixture, hence neglecting this variability. The lack of mineralogical data in arid areas prevented a better description of the atmospheric dust composition, and we propose here a new formulation to estimate the mineral content of arid surfaces on a global scale. First, we collect a Database of Arid Soil Surface Mineralogy for eight major minerals: quartz, feldspar, calcite, gypsum, illite, kaolinite, smectite, and hematite, both for the clay and silt fraction. On the basis of this, we formulate a Mean Mineralogical Table that relates classical soil types to surface mineralogy. We use this table and the geographical distribution of soil types given in the Food and Agriculture Organization Soil Map of the World to obtain the mineralogy of arid surfaces globally. In order to validate these results, we present a compilation of measured mineralogical composition of dust samples with identified sources. The correlation between observed dust mineralogy and those inferred from soil types in corresponding areas is between 0.70 and 0.94. We then calculate the maps of the single scattering albedo and of the ratio of infrared extinction to visible extinction for the erodible fraction of arid areas. Mineralogical maps presented here will be used in future studies with an emission scheme in a global transport model.
Journal of Geophysical Research, 1999
We describe a technique to model the radiative properties of mineral aerosols which accounts for their composition. We compile a data set of refractive indices of major minerals and employ it, along with data on mineralogical composition of dust from various locations, to calculate spectral optical and radiative properties of mineral aerosol mixtures. Such radiative properties are needed for climate modeling and remote sensing applications. We consider external mixtures of individual minerals, as well as mixtures of aggregates. We demonstrate that an external mixture of individual minerals must contain unrealistically high amounts of hematite to have a single scattering albedo lower than 0.9 at 500 nm wavelength. In contrast, aggregation of hematite with quartz or clays can strongly enhance absorption by dust at solar wavelengths. We also simulate the daily mean net (solar + infrared) forcing by dust of varying compositions. We found that, for a given composition and under similar atmospheric conditions, a mixture of aggregates can cause the positive radiative forcing while a mixture of individual minerals gives the negative forcing. Paper number 1998JD200048 0148-0227/99/1998JD200048509.00 2. Mineralogical Composition of Dust at Various Locations Minerals are generally defined as naturally occurring elements or compounds formed by inorganic processes. The majority of 9423 9424 SOKOLIK AND TOON: DtJST RADIATIVE MODELS dust particles are lofted into the atmosphere by eolian (wind) erosion of arid and semi-arid lands. Human activities (agriculture, industry, construction, deforestation, etc.) can extend the geographical area of dust sources and increase dust loading into the atmosphere. This portion of dust is called anthropogenic dust, and it is of special interest in climate change studies. Because of the diversity of dust sources, the properties of dust originating from various types of land surfaces needs to be studied. There is a large body of data on mineralogical and elemental (or chemical) composition of the Earth's soils. These data demonstrate the complex spatial variability of soil composition. The observed variability is the result of the history of soil formation and weathering processes. It is common to compare elemental composition of soils from different parts of the world with the average composition of Earth's crust or of sedimentary rocks [Pye, 1987]. For instance, average sedimentary rocks have a Si/A1 ratio of 4.04. It turn, the Si/A1 ratio for dust from Sahelian region is 3.18-3.6, while for dust from northern Morocco is about 2.67-2.87 [Bergametti et al., 1989]. In contrast, dust in Arizona has the Si/A1 ratio of 4.02, and dust from Tadzhikistan (central Asia) has a ratio of about 3.06 [Gomes and Gillette, 1993]. Petrov [1976] showed that the Gobi desert contains 0.74-1.36% less Fe, but 5-12% more A1 and 8.4% more Ca that the average earth crust composition. These data are in agreement with measurements by Parungo et al. [1995] conducted at several locations in China. Variable elemental ratios reflect variable mineralogical composition of parent soils. For instance, dust in the Sahelian region is characterized by a high Fe/A1 ratio due to the abundance of ferralitic soils in the Sahelian region. In contrast soils in the semi-arid regions of central Asia contain lesser Fe. The variable composition of soils produces their variable color which is often used as a criteria in soil classification. Soil color can be controlled by organic material (the carbon content gives it a dark color) or by the inorganic compounds. The most important inorganic coloring agent is iron. Iron oxides occur, at least in small amounts, in nearly all soils (51 g/kg Fe in Earth's crust at average). However, large variations are observed. For instance, the soil of the Gobi is chiefly grayish brown, while dust collected in the Negev desert is typically light brown or tan, and Sahelian dust is bright red. Despite the wealth of dispersed data on soil chemical and mineralogical composition there is no readily available data set on the composition of parent soils on a global scale. Existing global data sets of soil properties currently include soil texture (three size classes: "clay", "silt" and "sand") and soil types with a resolution 1øxl ø but they do not provide information on sizeresolved mineralogical composition of the parent soil [Webb et al., 1991]. There are several systems of soil classification based on different criteria (type, composition, texture, etc.). For instance, the Food and Agriculture Organization (FAO) /UNESCO system and U.S. Soil Taxonomy are two often used classifications of soil types. The Soil Taxonomy system recognizes 11 orders and five categories below the order [Soil Survey Staff, 1996]. They are: suborder, great group, subgroup, family and series. Eleven orders, 47 suborders, about 195 groups, about 1200 subgroups, about 5000 families, and about 12,000 series are presently in the Soil Taxonomy system of the USA. It is at the family level that mineralogy enters into the classification for soils, although it is used as a criterion in a higher category for some soils. However, quantitative information on mineralogical composition of surface soils can not be extracted from these classification systems. There is a clear need for a new data set to provide information on mineralogical composition needed for dust modeling on global and regional scales. The main constituents found in dust derived from surface soils are quartz, feldspars, calcite, dolomite, gypsum, mica, kaolinite, illite, montmorillonite, palygorskite, chlorite and organic matter (such as bacteria, fungal spores, pollen grains, seeds, stem tissue, and ash) [Pye, 1987]. Although the chemical and mineralogical composition of airborne dust mimics the composition of the parent surface, it also depends on dust mobilization processes and compositional separation during dust transport. Changes in composition during dust transport have been reported by the numerous investigators. For instance, Rahn et al. [1979] demonstrated that the Si/A1 ratio changes during the long-range
Atmospheric Chemistry and Physics, 2006
In aerosol chamber experiments optical properties of resuspended mineral dust samples of defined size distributions were measured. Extinction coefficients (b ext) and mass specific extinction cross sections (σ ext) were determined for Saharan dust samples from different locations. The results for σ ext were not very sensitive to the type of dust and varied at λ=550 nm between 3.3±0.4 m 2 g −1 and 3.7±0.4 m 2 g −1. The absorption coefficients (b abs) and mass specific absorption cross sections (σ abs) were determined with a novel multiwavelength photo-acoustic absorption spectrometer (PAS). The single scattering albedo was close to 1 (0.98 to 0.99) at 532 nm and 1064 nm, but significantly lower (0.63 to 0.76) at 266 nm. Additionally the chemical and mineralogical composition of the dust samples were analysed with special regard to the iron oxide phases hematite and goethite. At λ=266 nm the mineral dust sample without any detectable iron oxides showed a significantly higher SSA compared to the sample with a hematite content of 0.6 wt-%.
Atmospheric Chemistry and Physics, 2014
Experimental estimations of the infrared refractive index of African mineral dust have been retrieved from laboratory measurements of particle transmission spectra in the wavelength range 2.5-25 µm. Five dust samples collected at Banizoumbou (Niger) and Tamanrasset (Algeria) during dust events originated from different Western Saharan and Sahelian areas have been investigated. The real (n) and imaginary (k) parts of the refractive index obtained for the different dust samples vary in the range 1.1-2.7 and 0.05-1.0, respectively, and are strongly sensitive to the mineralogical composition of the particles, especially in the 8-12 and 17-25 µm spectral intervals. Dust absorption is controlled mainly by clays (kaolinite, illite, smectite) and, to a lesser extent, by quartz and calcium-rich minerals (e.g. calcite, gypsum). Significant differences are obtained when comparing our results with existing experimental estimations available in the literature, and with the values of the OPAC (Optical Properties of Aerosols and Clouds) database. The different data sets appear comparable in magnitude, with our values of n and k falling within the range of variability of past studies. However, literature data fail in accurately reproducing the spectral signatures of the main minerals, in particular clays, and they significantly overestimate the contribution of quartz. Furthermore, the real and the imaginary parts of the refractive index from some literature studies are found not to verify the Kramers-Kronig relations, thus being theoretically incorrect. The comparison between our results, from western Africa, and literature data, from different locations in Europe, Africa, and the Caribbean, nonetheless, confirms the expected large vari-ability of the dust infrared refractive index. This highlights the necessity for an extended systematic investigation of dust properties at infrared wavelengths.
Quarterly Journal of the Royal Meteorological Society, 2011
This paper presents new results on the composition, size and shape of mineral dust particles from African sources which were obtained by analysis of bulk filter samples collected in June 2007 onboard the BAe-146 research aircraft of the Facility for Airborne Atmospheric Measurements (FAAM). The aircraft was operated over Mauritania, Mali and Niger during the Geostationary Earth Radiation Budget Intercomparisons of Longwave and Shortwave radiation (GERBILS) campaign. Dust sampled during the campaign originated from various sources, including locally in the Sahel as a result of large-scale convective activity. Regardless of origin, clays (illite, kaolinite) dominated the total volume (79-90%); the remainder was composed of quartz, calcium-rich minerals (calcite, dolomite, gypsum) and alkali feldspars. Iron oxides, measured using a selective chemical extraction method, accounted for 1-3% of the total dust mass. The dependence of particle number size and shape distribution on the origin of dust seems minor too, although our results might be slightly misleading due to the fact that those kinds of data have been gathered on flights when dust had comparable origins and residence time. Mineral dust is only weakly absorbing in the mid-visible wavelengths (single scattering albedo ω 0 > 0.95 at 550 nm), and ω 0 measured values can be reproduced by measuring the bulk fractions of the major minerals, i.e. clays, quartz, calcite and iron oxides. At this wavelength, knowledge of the nature of clays and iron oxides, or the state of mixing of the minerals, does not induce significant differences in the results. A more precise description of the nature of clays and iron oxides is necessary at lower wavelengths owing to larger differences in their spectral optical properties. In particular, knowledge of the nature of the dominant clay is important for determining light scattering in the backward hemisphere.
Modeling Optical Properties of Mineral Dust over
2007
1] The direct radiative forcing (DRF) of dust particles is most uncertain among all the major aerosol species because of the large regional variation in their shapes and composition. The Indian Desert is known to be a source of natural mineral dust of nonspherical shapes. Particle shape and exact mineralogical information are essential for modeling dust optical properties as the latter governs their refractive indices. The realistic dust shapes, namely, sphere, spheroid, Chebyshev, and cylinder, based on Scanning Electron Microscope (SEM) images, have been used to model the mineral dust optics of the Indian Desert using the T-matrix method. The particle radius from 0.1 to 5.0 mm has been considered at wavelengths ranging from ultraviolet to near infrared (0.38-1.2 mm). Using Bruggman's effective medium mixing rule, the refractive index of composite dust particle has been calculated, accounting for both nonmetallic and metallic component (as hematite). Our calculations show that increasing the hematite percentage from 0% to 10% results in reduction of 0.477 and 0.013 in single scattering albedo (SSA) for cylindrical particle of radius 1 mm at 0.38 and 1.02 mm wavelengths, respectively, while the same for volume equivalent spherical particle were 0.484 and 0.022, respectively. The scattering signature of sharp-edged cylindrical particle showed the largest deviation to sphere compared with that of other relatively smooth particles (spheroid and Chebyshev). Changes in dust optical properties because of nonsphericity and varying hematite percentage were estimated for two cases: background dust and dust storm at visible wavelength. The change in SSA between the above two cases was insignificant for particles of radii <0.4 mm for each hematite percentage considered. On the other hand, for particles of size range 0.4-1 mm, the change in SSA increases with increasing hematite percentage. A 6% increase in hematite leads to an SSA reduction of more than 0.2 for particle radius of 1 mm for both background dust and dust storm cases. Optical properties of polydisperse dust distribution at visible wavelength suggest the likely hematite percentage as 0%-4% in the Indian mineral desert dust. The effect of hematite variation on SSA is found to be stronger than particle nonsphericity. The present work will lead to a better estimation of the radiative forcing imposed by dusts as well as their satellite retrieval over the Indian Desert region.
Predicting infrared spectra of atmospheric dust samples of mixed composition particles
2016
A Mie-Bruggeman model is used to predict the shape and orientation-averaged infrared extinction and absorption spectra of a Saharan dust sample of mixed composition and size. The common minerals of atmospheric dust samples, clays in this case, have strong infrared transitions which match the particle size and probing wavelengths giving rise to interesting lineshape distortions. These distortions may need to be considered for quantitative analysis of infrared spectra measurements of atmospheric dust samples.