Atmospheric Chemistry and Physics Using aircraft measurements to determine the refractive index of Saharan dust during the DODO Experiments

Abstract. Much uncertainty in the value of the imaginary part of the refractive index of mineral dust contributes to un-certainty in the radiative effect of mineral dust in the atmo-sphere. A synthesis of optical, chemical and physical in-situ aircraft measurements from the DODO experiments during February and August 2006 are used to calculate the refrac-tive index mineral dust encountered over West Africa. Ra-diative transfer modeling and measurements of broadband shortwave irradiance at a range of altitudes are used to test and validate these calculations for a specific dust event on 23 August 2006 over Mauritania. Two techniques are used to determine the refractive index: firstly a method combin-ing measurements of scattering, absorption, size distribu-tions and Mie code simulations, and secondly a method us-ing composition measured on filter samples to apportion the content of internally mixed quartz, calcite and iron oxide-clay aggregates, where the iron oxide is represented by ei-ther hematite or goethite and clay by either illite or kaolinite. The imaginary part of the refractive index at 550 nm (n550i) is found to range between 0.0001 i to 0.0046 i, and where filter samples are available, agreement between methods is found depending on mineral combination assumed. The refractive indices are also found to agree well with AERONET data where comparisons are possible. n550i is found to vary with dust source, which is investigated with the NAME model for each case. The relationship between both size distribution and n550i on the accumulation mode single scattering albedo at 550nm (ω5500) are examined and size distribution is found to have no correlation to ω5500, while