Reevaluation of Mineral aerosol radiative forcings suggests a better agreement with satellite and AERONET data (original) (raw)
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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.
Uncertainties in assessing radiative forcing by mineral dust
Tellus B, 1998
The assessment of the climatic effects of an aerosol with a large variability like mineral dust requires some approximations whose validity is investigated in this paper. Calculations of direct radiative forcing by mineral dust (short-wave, long-wave and net) are performed with a single-column radiation model for two standard cases in clear sky condition: a desert case and an oceanic case. Surface forcing result from a large diminution of the short-wave fluxes and of the increase in down-welling long-wave fluxes. Top of the atmosphere (TOA) forcing is negative when short-wave backscattering dominates, for instance above the ocean, and positive when short-wave or long-wave absorption dominates, which occurs above deserts. We study here the sensitivity of these mineral forcings to different treatments of the aerosol complex refractive index and size distribution. We also describe the importance of the dust vertical profile, ground temperature, emissivity and albedo. Among these parameters, the aerosol complex refractive index has been identified as a critical parameter given the paucity and the incertitude associated with it. Furthermore, the imaginary part of the refractive index is inadequate if spectrally averaged. Its natural variability (linked to mineralogical characteristics) lead to variations of up to ±40% in aerosol forcing calculations. A proper representation of the size distribution when modelling mineral aerosols is required since dust optical properties are very sensitive to the presence of small particles. In addition we demonstrate that LW forcing imply a non-negligible sensitivity to the vertical profiles of temperature and dust, the latter being an important constraint for dust effect calculations.
Atmospheric Chemistry and Physics Discussions, 2009
Much uncertainty in the value of the imaginary part of the refractive index of mineral dust contributes to uncertainty in the radiative effect of mineral dust in the atmosphere. A synthesis of optical, chemical and physical in-situ aircraft measurements from the DODO experiments during February and August 2006 are used to calculate the refractive index mineral dust encountered over West Africa. Radiative transfer modeling and measurements of broadband shortwave irradiance at a range of altitudes are used to test and validate these calculations for a specific dust event on 23 August 2006 over Mauritania. Two techniques are used to determine the refractive index: firstly a method combining measurements of scattering, absorption, size distributions and Mie code simulations, and secondly a method using composition measured on filter samples to apportion the content of internally mixed quartz, calcite and iron oxideclay aggregates, where the iron oxide is represented by either hematite or goethite and clay by either illite or kaolinite. The imaginary part of the refractive index at 550 nm (n 550 i ) is found to range between 0.0001 i to 0.0046 i, and where filter samples are available, agreement between methods is found depending on mineral combination assumed. The refractive indices are also found to agree well with AERONET data where comparisons are possible. n 550 i is found to vary with dust source, which is investigated with the NAME model for each case. The relationship between both size distribution and n 550 i on the accumulation mode single scattering albedo at 550nm (ω 550 0 ) are examined and size distribution is found to have no correlation to ω 550 0 , while n 550 i shows a strong
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.
Modeling the radiative characteristics of airborne mineral aerosols at infrared wavelengths
Journal of Geophysical Research, 1998
We explore the importance of the composition of airborne mineral aerosols for assessments of their direct radiative forcing at infrared wavelengths. Our calculations employing Mie theory and data on spectral refractive indices show that the existing variations in refractive indices can cause large changes in the major aerosol optical characteristics. Calculations of IR radiative forcings at the top of the atmosphere and IR downward and upward fluxes, based on an one-dimensional radiation transfer code, give a wide range of results for varying optical models of the mineral aerosols. We estimate that for a "low dust loading" scenario the changes in IR downward flux at the surface relative to dust free conditions are in the range from 7 to 14 W/m 2 depending upon the mineral aerosol selected. Under "dry tropics" atmospheric conditions the IR forcing at the top of the atmosphere is in the range from 2 to 7 W/m 2. In turn, for a "high dust loading" scenario the calculated changes, relative to dust free conditions, in IR downward flux at the surface vary from 50 to 80 W/m 2, and the IR forcing at the top of the atmosphere varies from 15 to 25 W/m 2. Therefore, we conclude that incorporation of regionally and temporally varying dust mineralogical composition into general circulation models could be beneficial for decreasing the currently large uncertainties in the assessment of radiative forcing by the natural and anthropogenic components of the airborne mineral aerosols. Also the use of appropriate mineralogical data is required for remote sensing of the atmospheric aerosols using satellite infrared observations. Nickname Wavelengths, !xm Locations References "Sahara dust-Barbados" 2.5-40 Barbados, West Indies "Sahara dust-Niger" 4-40 Niamey, Niger 7.5-12 "Negev dust" "Clean" "Dust storm" "Afghan dust" 2.5-25 "Southwest USA dust" 1-16 Negev desert, Israel for background atmospheric conditions for dust storm Afghanistan-Tadzhikistan Whitehill, Texas, United States "Dust-like" 2.5-40 Germany Volz [1973] Volz cited in the work by Fouquart et al. [1987] Fisher [1976] Sokolik et al. [1993] Patterson [1981 ] Volz [19721 There were no data reported for China, India, Arabian Peninsula, and Australia. CHARACTERICTICS OF AIRBORNE MINERAL AEROSOLS 8821 Accumulation mode ("Afghan dsut") and coarse mode (Quartz)
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
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.
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.
Journal of Geophysical Research Atmospheres, 2005
We present a systematic theoretical study of atmospheric mineral dust radiative properties, focusing on implications for multiangle and multispectral remote sensing. We model optical properties of complex, nonspherical mineral dust mixtures in three visiblenear-infrared satellite channels: 0.550, 0.672, and 0.866 mm, accounting for recent field and laboratory data on mineral dust morphology and mineralogy. To model the optical properties of mineral dust, we employ the discrete dipole approximation technique for particles up to 2 mm diameter and the T matrix method for particles up to 12 mm. We investigate the impact of particle irregularity, composition, and size distribution on particle optical properties, and we develop optical models for representative natural mineral dust composition-size-shape types. Sensitivity studies with these models indicate that Multiangle Imaging Spectroradiometer (MISR) data should be able to distinguish platelike from grain-like dust particles, weakly from strongly absorbing compositional types, and monomodal from bimodal size distributions. Models containing grain-like, weakly absorbing, bimodal distributions of dust particles were favored for optically thick Saharan and Asian dust plume examples, whereas strongly absorbing and plate-like particles were rejected. We will present detailed, systematic MISR sensitivity studies and analysis of more complex field cases using the optical models derived here in a future paper.