Modeling Optical Properties of Mineral Dust over (original) (raw)
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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.
Geophysical Research Letters, 2008
The assessment of direct radiative forcing (DRF) of aerosol is uncertain, particularly where the natural dust particles mix with the anthropogenic components. One of the sources of such uncertainty is the assumption of morphology (size and shape) and composition of pure dust particles. Recently Mishra and Tripathi [2008] have computationally assessed the effect of particle morphology on optical properties over the Great Indian Desert. As a continuation of the previous study, in this paper, we have further examined the effects on dust radiative properties. Non-spherical pure dust particles show large variations in the optical and radiative properties from spherical pure dust particles, however, particle composition is found to have greater influence than particle shape on the radiative properties. Among the various shapes, sharp-edged particles show larger difference than smooth-shaped particles. Although the overall atmospheric absorption monotonically increases with increase in hematite content, maximum effect of particle non-sphericity at 4% hematite content implies that nonsphericity should be considered to minimize the uncertainty of regional estimates of aerosol DRF, as most of the global dusts contain that much hematite. However the difference in radiative properties for background dust and dust-storm cases due to particle morphology is low. Our results show that ignoring non-sphericity will lead to underestimation of the regional warming and dust-absorption efficiency.
Aerosol and Air Quality Research, 2015
The regional dust morphology and spectral refractive indices (RIs; governed by hematite, Fe 2 O 3 content at short wavelengths) are key elements for ascertaining direct radiative forcing of mineral dust aerosols. To provide morphological features of background mineral dust from a semi-arid zone in the vicinity of the Thar Desert, we carried out an expedition to the Jaipur city during late winter of 2012. Morphological analysis reveals the predominance of "Layered", "Angular" and "Flattened" particles, while the frequency distribution of a total of 235 dust particles shows the aspect ratio, AR and circularity parameter, CIR (measures of particle's non-sphericity) typically ~1.4 and ~0.8, respectively. Sensitivity analysis at 550 nm wavelength reveals the equivalent sphere model may underestimate Single Scattering Albedo, SSA for the dust with low (~1.1%) hematite by ~3.5%. Both underestimation (by ~5.6%) and overestimation (up to 9.1%) are probable in case of dust with high hematite content (~5.68%). In addition, the effect of AR on the dust scattering is significant in case of dust with high hematite content. More such regionally representative dust morphological data are required for better estimation of regional radiative forcing of mineral dust aerosols.
Atmospheric Chemistry and Physics Discussions, 2015
Mineral desert dust particles in general are no spheres and assuming spherical particles, instead of more realistic shapes, has significant effects on modeled optical dust properties and so on the belonging remote sensing procedures for desert dust and the derived radiative forcing. Thus in a new version of the data base OPAC (Optical 5 Properties of Aerosols and Clouds; Hess et al., 1998), the optical properties of the mineral particles are modeled describing the particles as spheroids with size dependent aspect ratio distributions, but with the size distributions and the spectral refractive indices not changed against the previous version of OPAC. The spheroid assumption strongly improves the scattering functions, but pays regard to the limited knowledge 10 on particle shapes in an actual case. The relative deviations of the phase functions of non-spherical mineral particles from those of spherical particles are up to +60 % at scattering angles of about 130 • and up to −60 % in the backscatter region, but the deviations are generally small for optical properties that are independent of the scattering angle. The improved version of OPAC (4.0) is freely available under www.rascin.net.
The radiative forcing estimation of the polluted mineral dust is limited due to lack of morphological analysis, mixing state with the carbonaceous components and the hematite content in the pure dust. The accumulation mode mineral dust has been found to mix with anthropogenically produced black carbon, organic carbon and brown carbon 5 during long range transport. The above features of the polluted dust are not well accounted in the optical models and lead the uncertainty in the numerical estimation of their radiative impact. The Semi-external mixing being a prominent mixing of dust and carbonaceous components has not been studied in details so for compared to coreshell, internal and external mixing studies. In present study, we consider the pure 10 mineral dust composed of non-metallic components (such as Quartz, Feldspar, Mica and Calcite) and metalic component like hematite (Fe 2 O 3 ). The hematite percentage in the pure mineral dust governs its absorbance. Based on this hematite variation, the hematite fraction in pure mineral dust has been constrained between 0-8%. The morphological and mineralogical characterization of the polluted dust led to consider 15 the three sphere, two sphere and two spheroid model shapes for polluted dust particle system. The pollution gives rise to various light absorbing aerosol components like black carbon, brown carbon and organic carbon (comprising of HUmic-Like Substances, HULIS) in the atmosphere. The entire above discussed model shapes have been considered for the mineral dust getting polluted with (1) organic carbon (especially 20 HULIS component) (2) Brown carbon and (3) black carbon by making a semi-external mixture with pure mineral dust. The optical properties (like Single Scattering Albedo, SSA; Asymmetry parameter, g and Extinction efficiency, Q ext ) of above model shapes for the polluted dust have been computed using Discrete Dipole Approximation, DDA code. For above model shapes, the SSA was found to vary depending on hematite con-25 tent (0-8%) and model shape composition. For the two sphere BC-mineral dust cluster, hematite was found to be dominating absorber compared to that of black carbon as the R BC /R dust decreases. (i.e. with increase of dust sphere size compared to black carbon 31254 sphere in the composite 2-sphere cluster). SSA was found to be very sensitivity for the hematite content when both of the spheres (i.e. mineral dust and BC) are nearly of same size. The two spheroid system composed of organic carbon and dust with 0% hematite (OCD -0) showed the maximum deviation of SSA (i.e. ∼5%) compared to the two sphere system of same composition and hematite content (OCD-0 ). Increase in 5 hematite from 0 to 8% caused maximum SSA deviation of ∼20% for two sphere organic carbon-dust system (OCD) while the same has been observed to be ∼18% for two spheroid organic carbon-dust system (OCD ). SSA was found to be more sensitive to hematite content than that of particle shape. Compared to SSA, Asymmetry parameter, g was found to be more sensitive towards particle shape. For three-sphere 10 model shapes with 0% hematite composed of black carbon-dust-dust (BCDD-0), brown carbon-dust-dust (BrCDD-0 ) and organic carbon-dust-dust (OCDD-0), the deviation of SSA and g relative to conjugate black carbon (BC), brown carbon (BrC) and organic carbon (OC) spheres are ∼68% and ∼31%, ∼83% and ∼31% and ∼70% and ∼33%, respectively. Thus modeled polluted dust optics will provide a better basis for radiative 15 forcing estimation and many sensitivity studies. and the global radiative forcing due to dust is expected to be negative (Diaz et al.,
Tellus B, 2012
A B S T R A C T The morphology, mixing state and hematite content of polluted mineral dust are not well accounted in the optical models and this leads to uncertainty in the radiative forcing estimation. In the present study, based on the morphological and mineralogical characterisation of polluted dust, the three-sphere, two-sphere and twospheroid model shapes are considered. The optical properties of the above model shapes are computed using Discrete Dipole Approximation code. The single scattering albedo, v 0 , was found to vary depending on hematite content (0Á6%) and model shape. For the two-sphere BC-mineral dust system, hematite was found to be a dominating absorber compared to that of black carbon as the R BC /R dust decreases. The v 0 of the polluted dust system is larger if polluted dust is considered as pure dust spheroid (with 4% hematite) while smaller value is observed for Q ext . Among all the systems, the v 0 of BCBCD (two BC spheres attached to one dust sphere) system showed the maximum departure (40 and 35% for polluted dust with 0 and 6% hematite, respectively) from that of pure dust spheroid with 0 and 6% hematite. For the Asian region (pollution-prone zone), the modelled polluted dust optics will help to trace the optical and radiative properties of dust.
Atmospheric Research, 2015
The objective of this work is to assess the variability of the size-distribution, real (n) and imaginary (k) parts of the refractive index, asymmetry parameter (g), and single scattering albedo (SSA) of desert dust events observed in the Sahara, Sahel, and Arabian Peninsula areas. For this we use the level-2 inversions of 14 AERONET sunphotometers representative of the area of study. In the dataset, the dust-dominated events are discriminated on the basis of their large optical depth and low (b0.3) Ångström exponent (α) calculated between 440 nm and 870 nm. In all the volume size-distributions a coarse mode (CM) of particles is observed but a fine mode (FM) of particles with radii b 0.2 μm is also present. The volume fraction represented by the FM is lower (3%) during the most intense dust storms than during moderate ones (12%). The inter-site variability of the characteristics of the CMdominated situations is found to be non-significant and at 440, 675, 870, and 1020 nm a common set of values can be adopted for n (1.
Atmospheric Chemistry and Physics, 2015
Mineral particles, in general, are not spheres and so the assumption of spherical particles, instead of more realistic shapes, has significant effects on modeled optical properties and therefore on remote-sensing procedures for desert aerosol and the derived radiative forcing. Thus, in a new version of the database OPAC (Optical Properties of Aerosols and Clouds; , the optical properties of the mineral particles are modeled describing the particles as spheroids with size dependent aspect ratio distributions, but with the size distributions and the spectral refractive indices not changed against the previous version of OPAC. The spheroid assumption is known to substantially improve the scattering functions but pays regard to the limited knowledge on particle shapes in an actual case. The relative deviations of the optical properties of non-spherical mineral particles from those of spherical particles are for the phase function in the solar spectral range up to +60 % at scattering angles of about 130 • and up to −60 % in the backscatter region, but less than 2 % for the asymmetry parameter. The deviations are generally small in the thermal infrared and for optical properties that are independent of the scattering angle. The improved version of OPAC (4.0) is freely available at www.rascin.net.
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.
Optical properties of mineral dust outbreaks over the northeastern
1] Ground-based aerosol optical measurements were conducted within the framework of the Aerosol Robotic Network (AERONET) program at the IMS-METU site at Erdemli (36°33 0 N, 34°15 0 E) along the Turkish coast of the northeastern Mediterranean from January 2000 to June 2001. The measurements were used to identify and define predominant regional aerosol optical properties, with an emphasis on mineral dust intrusion events. Dust storms affecting the region primarily originate from the central Sahara in spring, the eastern Sahara in summer, and the Middle East/Arabian peninsula in autumn. Summer and autumn dust intrusions usually occurred at higher altitudes (above 700 hPa), whereas urban-industrial aerosols from the north over the Balkan region, Ukraine, and Anatolia were transported to the region at lower altitudes. In addition to a drastic increase in the aerosol optical thickness, in some cases up to 1.8, the dust episodes were characterized by (1) a sharp drop in the Å ngstrom coefficient to values near zero, (2) a high-scattering with single-scattering albedo greater than 0.95 ± 0.03, and the real part of the refractive index around 1.5 ± 0.5, both of which acquire slightly higher values at longer wavelengths, (3) a lower absorption given by the imaginary part of the refractive index less than 0.002, and (4) an almost neutral spectral dependence of these parameters. Dust particles possessed a bimodal size distribution with typical volume mean radii of 2.2 mm and 0.08 mm for coarse and fine size fractions, respectively, and corresponding volume concentrations of about 1.0 and 0.1 mm 3 mm À2 of dust particles. It was apparent that the Saharan and Middle East desert dusts differ in their absorption index values (0.0015 and 0.0005, respectively). The difference is likely a result of their contrasting mineralogies. Citation: Kubilay, N., T. Cokacar, and T. Oguz, Optical properties of mineral dust outbreaks over the northeastern Mediterranean,