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Sensitivity of direct radiative forcing by mineral dust to particle characteristics

School of Geographical Sciences, University of Bristol, University Road, Bristol BS8 1SS, UK, Department of Earth Sciences, University of Bristol, Wills Memorial Building, Bristol BS8 1RJ, UK

Sandy P. Harrison

School of Geographical Sciences, University of Bristol, University Road, Bristol BS8 1SS, UK, sandy.harrison{at}bristol.ac.uk

I. Matthew Watson

Department of Earth Sciences, University of Bristol, Wills Memorial Building, Bristol BS8 1RJ, UK

Y. Balkanski

LSCE/IPSL, Laboratoire CEA-CNRS-UVSQ, L'Orme des Merisiers, Bat. 712, 91191 Gif-sur-Yvette Cedex, France

Airborne dust affects the Earth's energy balance — an impact that is measured in terms of the implied change in net radiation (or radiative forcing, in W m^-2) at the top of the atmosphere. There remains considerable uncertainty in the magnitude and sign of direct forcing by airborne dust under current climate. Much of this uncertainty stems from simplified assumptions about mineral dust-particle size, composition and shape, which are applied in remote sensing retrievals of dust characteristics and dust-cycle models. Improved estimates of direct radiative forcing by dust will require improved characterization of the spatial variability in particle characteristics to provide reliable information dust optical properties. This includes constraints on: (1) particle-size distribution, including discrimination of particle subpopulations and quantification of the amount of dust in the sub-10 µm to <0.1 µm mass fraction; (2) particle composition, specifically the abundance of iron oxides, and whether particles consist of single or multi-mineral grains; (3) particle shape, including degree of sphericity and surface roughness, as a function of size and mineralogy; and (4) the degree to which dust particles are aggregated together. The use of techniques that measure the size, composition and shape of individual particles will provide a better basis for optical modelling.

Key Words: atmospheric dust loading • climate uncertainties • dust-particle properties • mineral dust • radiative forcing.

Progress in Physical Geography, Vol. 33, No. 1, 80-102 (2009)
DOI: 10.1177/0309133309105034


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