Characterization of the complex refractive index of a polluted dust storm over the Western Himalayas

Abstract. A polluted dust episode over the western Himalaya was characterized using in situ optical measurements, size-resolved aerosol sampling, single-particle analysis, and spectroscopic techniques. During the event, coarse-mode mass increased substantially, accompanied by elevated fine-mode particle number and black carbon concentrations. In situ observations revealed a threefold enhancement in the scattering coefficient and a 2–2.5-fold increase in the absorption coefficient relative to background conditions. Beyond these extrinsic property changes, intrinsic optical properties shifted markedly. Single scattering albedo and scattering Ångström exponent increased, and absorption Ångström exponent decreased, indicating compositional changes toward dust dominance with embedded anthropogenic absorbing aerosols. Single-particle analysis confirmed internal mixing of Fe-bearing mineral dust with carbonaceous material and secondary inorganic species. These compositional and morphological changes were reflected in the refractive index, which increased by approximately 4–6 % in the real part and 30–35 % in the imaginary part across visible wavelengths. The results demonstrate that polluted dust differs fundamentally from pure mineral dust and exhibits enhanced optical efficiency per unit mass and altered aerosol radiation interactions due to chemical transformation during transport. Accurate representation of such region-specific intrinsic properties is essential for quantifying aerosol radiative effects and boundary-layer interactions over the Himalayas.