The Complex Refractive Index of Yellow Sand Inferred from the Brightness Temperature Difference in the Infrared Split Window

Abstract

The analysis of the brightness temperature differences (BTD) between 11 µm and 12 µm channels of Moderate Resolution Imaging Spectroradiometer (MODIS) aboard Aqua/NASA satellite for a Yellow Sand event on 8 April 2006 reveals that the BTD on this event cannot be explained by the theoretical simulation using the complex refractive index of ‘Sahara’ or ‘Afghan’ dust. The refractive indexes at 11 µm and 12 µm so determined as to fit the observed BTD are similar to those of quartz. In a visible wavelength, the observed reflectance of 0.65 µm channel of MODIS over the most dense portion of Yellow Sand on this day is much higher than that expected by the conventional complex refractive index of mineral dust. The weaker absorption, like the quartz refractive index, is required to explain the observed reflectance. The optical thickness at infrared and visible wavelengths using a small value of imaginary part of refractive index in visible wavelength show a smooth and monotonic relationship between them.