Optical constants for Asian dust in midinfrared region

Asian dust aerosols are composed of quartz, plagioclase, K-feldspar, calcite, and phyllosilicates. The optical constants for mixtures of these minerals are important not only to understand the effects of Asian dust on climate but also to retrieve the properties of Asian dust. In this work, the optical constants for labradorite and orthoclase, representative minerals of plagioclase and K-feldspar, respectively, are determined for the spectral range of 500–2000 cm−1 using bidirectional reflectance data from the Advanced Spaceborne Thermal Emission and Reflection Radiometer. Assuming an internal mixture of component minerals, the effective refractive indices for Asian dust are calculated using Bruggeman's rule. The results are proposed as optical constants for Asian dust and differ from those for other dust aerosols, such as the refractive indices for “Saharan dust” derived from aerosol samples collected at Barbados. The imaginary refractive index for Asian dust is larger, indicating more absorptive, than the index for Saharan dust in the range of 1000–1300 cm−1. Using the optical constants derived in this study, the brightness temperatures of satellite measurements are simulated for typical loading scenarios of Asian dust aerosols. The simulated brightness temperatures exhibit a notable decrease with wave number in the region of 800–1000 cm−1. The results also corroborate the spectral features of the Atmospheric Infrared Sounder (AIRS) measurements for an Asian dust storm. AIRS brightness temperatures near 1223 cm−1, lower than the maximum near 830 cm−1, can also be simulated using the derived optical constants for Asian dust.