Emissivities of quartz and Sahara dust powders in the infrared region (7–17 μ)

Abstract

Emissivities of quartz and Sahara dust powders are computed in the infrared wavelength region (7–17 μm) by the doubling method, together with those of a plane surface. The effect of particle size on emissivities is evaluated. A cloudy atmosphere model is used for radiative transfer in condensed powders, where a size distribution of powders is given. Particle sizes are enlarged 2, 20, and 200 times for a comparison purpose. Computational results show that emissivities of quartz powders exhibit very small values in the region 8.3–9.3 μm with peak value at the center near 8.6 μm. Another small drop exists at 12.5 μm. With the increase of particle size, a monotonical increase of emissivity is noted in the wavelength regions 10–12 μm, 13–14 μm, and 15–17 μm. With the increase of zenith angle of emergent direction, emissivity decreases monotonically throughout the wavelength region considered. Sahara dust powders show rather small emissivity change with particle size in comparison with that of quartz powders. These results may be used for the further development of remote sensing techniques applied to surface emissivity measurements. In the wavelength regions of NOAA-AVHRR radiometer in space, emissivity of quartz in Channel 5 (11.5–12.5 μm) is higher than that in Channel 4 (10.5–11.5 μm). The corresponding apparent temperature difference ( ΔT) between Channel 5 and Channel 4 decreases with the increase of particle size. Thus ΔT may be large enough to be noted from space under favorable conditions.