The retrieval of optical properties from terrestrial dust devil vortices

The retrieval of the optical properties of desert aerosols in suspension within terrestrial dust devils is presented with possible future application for martian dust devils. The transmission of light through dust devil vortices was measured in situ to obtain the wavelength-dependent attenuation by the aerosols. A Monte Carlo model was applied to each dust devil with the retrieved optical properties corresponding to the set of parameters which lead to the best model representation of the observed transmission spectra. The retrieved optical properties agree well with single scattering theory and are consistent with previous studies of dust aerosols. The enhanced absorption observed for dust devils with a higher tangential wind speed, and in comparison to atmospheric aerosol studies, suggests that larger dust particles are lofted and suspended around dust devil vortices. This analysis has shown that the imaginary refractive indices (and thus the optical properties of the suspended dust) are generally overestimated when these larger dust grains entrained by dust devils are neglected. This will lead to an overestimation of the amount of solar radiation absorbed by the small particles that remain in suspension after the dust devil terminates. It is also demonstrated that a 10% uncertainty in the particle size distribution of the dust entrained in the dust devils can result in a 50% increase in the predicted amount of incident solar radiation absorbed by the dust particles once the dust devil has terminated. The method used here provides the capability to retrieve the optical properties of the dust entrained in martian dust devils by taking advantage of transits over surface spacecraft which are capable of making optical measurements at ultraviolet and visible wavelengths. Our results suggest that we would observed higher absorption at all wavelengths for dust particles entrained in dust devil vortices compared to the ubiquitous dust haze.