The Attenuation of Sunlight by Airborne Dust Particles on Earth and Mars

This thesis details an investigation into how dust and ice aerosols in the atmosphere of Earth and Mars affect the solar spectrum from the ultraviolet to the near infrared, allowing the characterisation of the aerosols using broad band surface measurements. A Monte Carlo Light Scattering Model (MCLSM) was developed to predict the optical signature of terrestrial and martian dust devils. The MCLSM was applied to measurements taken in the Eldorado Valley, U.S.A. Transit signatures were found to be dependent on the method of observation. The transit signature measured in scattered light depends on the dust concentration and distribution in contrast to the total light transit signature, which depends primarily on the column-integrated dust optical depth. On Mars the high diffuse irradiance provides better definition of the vortex interior within the transit signature, with wavelengths between 600-750 nm optimal for detecting a transit. To determine the vortex size and dust concentration, both the total and the scattered light must be measured. Retrieval of the dust optical properties showed that spectral measurements and the calculated mass concentration were sensitive to the presence of small particles (0.5-5.0 μm). A comparison of the downward irradiance for two distinct dust components suggests that dust with a higher absorption at visible wavelengths causes increased heating at higher altitudes, leading to a more statically-stable atmosphere, which may result in more rapid decay of large-scale dust storms. The investigations have also shown that variations in single scattering properties can lead to -20% difference in the daily UV-C dose. A method was devised to distinguish compositional changes in the ubiquitous martian dust haze and validated against wind tunnel experiments. This technique is applicable to determine the size of water-ice crystals in clouds and the optical depth of the dust haze at the time of the cloud passage