Abstract

Abstract. Much uncertainty in the value of the imaginary part of the refractive index of mineral dust contributes to uncertainty in the radiative effect of mineral dust in the atmosphere. 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 refractive index mineral dust encountered over West Africa. Radiative 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 combining measurements of scattering, absorption, size distributions and Mie code simulations, and secondly a method using 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 either hematite or goethite and clay by either illite or kaolinite. The imaginary part of the refractive index at 550 nm (ni550) 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. ni550 is found to vary with dust source, which is investigated with the NAME model for each case. The relationship between both size distribution and ni550 on the accumulation mode single scattering albedo at 550 nm (ω0550) are examined and size distribution is found to have no correlation to ω0550, while ni550 shows a strong linear relationship with ω0550. Radiative transfer modeling was performed with different models (Mie-derived refractive indices, but also filter sampling composition assuming both internal and external mixing). Our calculations indicate that Mie-derived values of ni550 and the externally mixed dust where the iron oxide-clay aggregate corresponds to the goethite-kaolinite combination result in the best agreement with irradiance measurements. The radiative effect of the dust is found to be very sensitive to the mineral combination (and hence refractive index) assumed, and to whether the dust is assumed to be internally or externally mixed.

Highlights

  • Mineral dust is an important component of the Earth’s atmosphere, affecting climate principally through the direct radiative effect and through the deposition of dust to the ocean

  • Imaginary refractive indices for dust by 0.001i to 0.0046i for the accumulation mode have been presented. They have been derived from DODO aircraft measurements of scattering, absorption and size distribution using Mie code simulations, assuming spherical particles

  • These measurements agree well with refractive indices calculated from filter samples collected during DODO, covering the accumulation mode and coarse mode, based on the methodology of Lafon et al (2006) and assuming internal mixing, depending on the mineral combination assumed

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Summary

Introduction

Mineral dust is an important component of the Earth’s atmosphere, affecting climate principally through the direct radiative effect and through the deposition of dust to the ocean. Recent estimates of the dust anthropogenic net (shortwave plus longwave) radiative forcing from the Intergovernmental Panel on Climate Change (IPCC) range from −0.3 to +0.1 Wm−2 (Forster et al, IPCC, 2007), putting a greater emphasis on the negative forcings than the previous IPCC 2001 report estimates of −0.6 to +0.4 Wm−2 (Penner et al, IPCC, 2001). This change has largely been due to more recent models using higher single scattering albedo (ω0) values, of approximately 0.96 at 670 nm, than previous simulations. The morphology and mixing state of different mineral constituents can have an impact on the optical properties (Mishchenko et al, 1997; Sokolik and Toon, 1999)

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