Optical Properties Of Mineral Dust Particles Mixed With Black Carbon In Indo-Ganges Basin, Northern India

Among all the aerosol species estimation of radiative forcing of dust particles is most uncertain because of the difficulties in modeling accurately their optical properties due to our limited knowledge of their mineralogy and morphology (Sokolik and Toon, 1999). There is large variation in their shapes and mineralogical composition across the global deserts and the data available is limited. The problem becomes further complicated when these highly non spherical shaped particles interact with other absorbing particles e. g. Black Carbon emitted from pollution sources. The dust from Indian desert, while being transported through the Indo Gangetic basin during the Pre Monsoon season (March-May), mixes with BC particles present in large concentration in the region (Tripathi et al., 2005). The accurate estimation of optical properties of these particles, which could be different from pure dust or BC particles, is important because of their suspected impact on the development of monsoon (Lau et al, 2006). In this work, we have made an attempt to model the optical properties of these complicated aerosol systems. We have used dust mineralogical composition (absorbing and non absorbing) from the recent work of Mishra and Tripathi (2008), who modeled the optical properties of pure dust particles over Indian desert. Mixing state of the two species i.e. dust and BC is an important issue (Clarke et al., 2004) while modeling these systems. We consider both external and internal mixing scenarios while modeling in absence of any observational information. Earlier study on mixing state of aerosols (assuming them to be spherical in shape), in IGB, have indicated that dust coated with BC is the most probable scenario for internal mixing (Dey et al., 2008). In the present study, optical properties (scattering and absorption cross-sections, phase functions and asymmetry parameter) have been modeled using Discrete Dipole Approximation method (Draine and Flatau, 2004) for a variety of non spherical shapes such as concentric spheres, spheroids, ellipsoids and layered rectangular bar, and various combinations of spheres and spheroids attached externally (maximum up to three). References Clarke, A. D., et al. (2004), Size distributions and mixtures of dust and black carbon aerosol in Asian outflow: Physiochemistry and optical properties, J. Geophys. Res., 109, D15S09. Dey, S., et al. (2008), On the mixing state of aerosols in the Indo-Gangetic basin, northern India, Geophys. Res. Lett., 35, L03808. Draine, B. T., and P. J. Flatau (2004), User Guide for the Discrete Dipole Approximation Code DDSCAT 6.1, http://arxiv.org/abs/astro-ph/0409262v2. Lau, K. M., et al. (2006), Asian summer monsoon anomalies induced by aerosol direct forcing: the role of the Tibetan Plateau., J. Climate, 26, 855-864. Mishra, S. K., and S. N. Tripathi (2008), Modeling optical properties of mineral dust over the Indian Desert, J. Geophys. Res., accepted. Sokolik, I. N., and O. B. Toon (1999), Incorporation of mineralogical composition into models of the radiative properties of mineral aerosol from UV to IR wavelengths, J. Geophys. Res., 104, 9423- 9444. Tripathi S. N., et al. (2005), Aerosol black carbon radiative forcing at an industrial city in Northern India, Geophys. Res. Lett., 32(8), L08802.

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