Abstract
Abstract. Soil dust aerosols created by wind erosion are typically assigned globally uniform physical and chemical properties within Earth system models, despite known regional variations in the mineral content of the parent soil. Mineral composition of the aerosol particles is important to their interaction with climate, including shortwave absorption and radiative forcing, nucleation of cloud droplets and ice crystals, heterogeneous formation of sulfates and nitrates, and atmospheric processing of iron into bioavailable forms that increase the productivity of marine phytoplankton. Here, aerosol mineral composition is derived by extending a method that provides the composition of a wet-sieved soil. The extension accounts for measurements showing significant differences between the mineral fractions of the wet-sieved soil and the emitted aerosol concentration. For example, some phyllosilicate aerosols are more prevalent at silt sizes, even though they are nearly absent at these diameters in a soil whose aggregates are dispersed by wet sieving. We calculate the emitted mass of each mineral with respect to size by accounting for the disintegration of soil aggregates during wet sieving. These aggregates are emitted during mobilization and fragmentation of the original undispersed soil that is subject to wind erosion. The emitted aggregates are carried far downwind from their parent soil. The soil mineral fractions used to calculate the aggregates also include larger particles that are suspended only in the vicinity of the source. We calculate the emitted size distribution of these particles using a normalized distribution derived from aerosol measurements. In addition, a method is proposed for mixing minerals with small impurities composed of iron oxides. These mixtures are important for transporting iron far from the dust source, because pure iron oxides are more dense and vulnerable to gravitational removal than most minerals comprising dust aerosols. A limited comparison to measurements from North Africa shows that the model extensions result in better agreement, consistent with a more extensive comparison to global observations as well as measurements of elemental composition downwind of the Sahara, as described in companion articles.
Highlights
Climate perturbations by soil dust aerosols created by wind erosion depend fundamentally upon the physical and chemical properties of the aerosol particles
Measurements of mineral fractions are based upon wet sedimentation techniques that disturb the soil sample, breaking aggregates that are found in the original, undispersed soil that is subject to wind erosion
Measurements are based on wet sedimentation (“wet sieving”) techniques that disturb the soil samples, breaking the aggregates that are found in the original, undispersed soil that is subject to wind erosion
Summary
Climate perturbations by soil dust aerosols created by wind erosion depend fundamentally upon the physical and chemical properties of the aerosol particles. Earth system models typically assume that soil dust aerosols have globally uniform composition, despite known regional variations in the mineral composition of the parent soil. Perturbations by dust to the energy and water cycles depend upon aerosol radiative forcing (Miller et al, 2004, 2014; Perlwitz and Miller, 2010), whose solar component is strongly related to the presence of iron oxides (Sokolik and Toon, 1996, 1999; Tegen et al, 1997; Redmond et al, 2010; Wagner et al, 2012; Moosmüller et al, 2012). The rates of heterogeneous chemical reactions on the dust particle surface that form coatings of sulfate, nitrate, chloride, or organics during atmospheric
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