Modeling optical properties of mineral dust over the Indian Desert

Abstract

[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.

The impact of atmospheric mineral aerosol deposition on the albedo of snow & sea ice: are snow and sea ice optical properties more important than mineral aerosol optical properties?

“…Bohren and Huffman (1983) proposed three equations for extreme forms of ellipsoids (spheres, needles and disks) allowing for the representation of the shapes found in mineral aerosols. Needles may correspond to glass shards found in volcanic ash , and dust particles take on a variety of shapes amongst which flat disks can be observed (Mishra and Tripathi, 2008). At 550 nm the absorption cross-section for the volcanic ash may increase by a factor of ∼ 1.20 for needles and ∼ 1.53 for disks.…”

Optical properties of accumulation mode, polluted mineral dust: effects of particle shape, hematite content and semi-external mixing with carbonaceous species

“…The optical constants for the pure mineral dust, at 0.550 mm wavelength, are taken from the study by Mishra and Tripathi (2008). They varied the hematite percentage to check its effect on the optical properties, as hematite is the major absorbing component in the pure mineral dust.…”

“…The sources of uncertainties in the radiative forcing estimation include mineral aerosol shape (Kalashnikova and Sokolik, 2002;Wang et al, 2003;Volten et al, 2005), proportion of hematite content in mineral dust (Mishra and Tripathi, 2008) and their mixing states with carbonaceous components (Bauer et al, 2008). The radiative forcing imposed by the atmospheric aerosols can be computed by summing over the entire aerosol population.…”

Bulk Level to Individual Particle Level Chemical Composition of Atmospheric Dust Aerosols (PM5) over a Semi-Arid Urban Zone of Western India (Rajasthan)

“…Mineral dust particles originating from the desert regions can potentially influence heterogeneous atmospheric chemistry and thus regional as well as global climate (Ravishankara, 1997;Buseck and Posfai, 1999;Tegen et al, 2003;Xu et al, 2004;Bauer et al, 2007). Besides, they can scatter/absorb incoming solar radiation influencing net radiative/optical forcing of the region Toon, 1996, 1999;Sokolik et al, 2001;Mishra et al, 2008aMishra et al, , b, 2012. There are considerable uncertainties in the regional radiative/optical forcings, mainly due to strong variability of the atmospheric dust burden and the lack of chemical composition data (Kandler et al, 2007).…”