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Much uncertainty in the value of the imaginary part of the refractive indexof mineral dust contributes to uncertainty in the radiative effect ofmineral dust in the atmosphere. A synthesis of optical, chemical andphysical in-situ aircraft measurements from the DODO experiments duringFebruary and August 2006 are used to calculate the refractive index mineraldust encountered over West Africa. Radiative transfer modeling andmeasurements of broadband shortwave irradiance at a range of altitudes areused to test and validate these calculations for a specific dust event on 23August 2006 over Mauritania. Two techniques are used to determine therefractive index: firstly a method combining measurements of scattering,absorption, size distributions and Mie code simulations, and secondly amethod using composition measured on filter samples to apportion the contentof internally mixed quartz, calcite and iron oxide-clay aggregates, wherethe iron oxide is represented by either hematite or goethite and clay byeither illite or kaolinite. The imaginary part of the refractive index at550 nm (ni550) is found to range between 0.0001 i to 0.0046 i, andwhere filter samples are available, agreement between methods is founddepending on mineral combination assumed. The refractive indices are alsofound to agree well with AERONET data where comparisons are possible.ni550 is found to vary with dust source, which is investigatedwith the NAME model for each case. The relationship between both sizedistribution and ni550 on the accumulation mode single scatteringalbedo at 550 nm (ω0550) are examined and size distributionis found to have no correlation to ω0550, whileni550 shows a strong linear relationship with ω0550.Radiative transfer modeling was performed with differentmodels (Mie-derived refractive indices, but also filter sampling compositionassuming both internal and external mixing). Our calculations indicate thatMie-derived values of ni550 and the externally mixed dust wherethe iron oxide-clay aggregate corresponds to the goethite-kaolinitecombination result in the best agreement with irradiance measurements. Theradiative effect of the dust is found to be very sensitive to the mineralcombination (and hence refractive index) assumed, and to whether the dust isassumed to be internally or externally mixed

Using aircraft measurements to determine the refractive index of Saharan dust during the DODO Experiments