Predicting the Mineral Composition of Dust Aerosols With an Earth
System Model (NASA GISS ModelE2)

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 soils. 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, coating by heterogeneous uptake 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 wet-sieved soils. The extension accounts for
measurements showing significant differences between the mineral
fractions of wet-sieved soils and the ones of dust aerosols after
emission. We reconstruct the mineral fractions of the dust aggegrates
that are emitted, by applying brittle fragmentation theory, which
accounts for dust emission by saltation (i.e., dust emission up to
about 16 um particle diameter). We complement this approach by
constraining the size distribution of the mineral fractions that are
emitted in the silt size range, with data derived from measurements to
account also for the emission of larger sized particles due to
aerodynamic entrainment. 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 sources, because
pure iron oxides are more dense and vulnerable to gravitational
removal than most minerals comprising dust aerosols. We use the above
methods to distinguish eight mineral phases of dust as individual
tracers in NASA GISS ModelE2: illite, kaolinite, smectite, carbonates,
quartz, feldspar, gypsum, and iron oxides as well as accretions
between iron oxides and the other minerals. Emission, transport, and
deposition of these mineral tracers are simulated. A global
compilation from nearly sixty measurement studies is used to evaluate
the mineral composition of the dust aerosols, which is predicted with
ModelE2.