Geoscience Reference
In-Depth Information
4.3
Chemical Processing
4.3.1
Impacts on Physical Properties of Dust
Heterogeneous chemistry and photochemistry of mineral dust aerosol is extremely
important as these reactions change the properties of the particles and their direct
and indirect climate forcing, thus impacting climate (see Chap.
13
for a detailed
discussion of the impact of dust on radiative forcing). There are still many
uncertainties associated with the role of mineral dust aerosol on climate, in part
due to its inherent heterogeneity and in part due to the incomplete understanding
of the influence of ageing processes on climate-related properties of mineral dust.
Processing may alter the chemical composition of particles and, ultimately, may
change the optical properties, hygroscopicity, cloud condensation nuclei (CCN) and
For example, sulphate coatings on mineral dust particles are known to have a
cooling effect by effectively scattering radiation and to promote CCN activity (Yin
et al.
2002
); at the same time, the presence of sulphuric acid leads to a decrease in
the dust particles' ice nucleation ability (Sullivan et al.
2010
). Cloud processing of
dust particles with sea-salt species and anthropogenic pollutants is an important
pathway in the formation of hygroscopic species (Kim and Park
2012
). Calcite
particles reacted with nitric or oxalic acids show an increase in the hygroscopicity
due to the formation of hygroscopic species (Gierlus et al.
2012
). Coating of mineral
particles with soluble materials such as sulphate, nitrate and chloride also increases
cloud condensation nuclei activity (Crumeyrolle et al.
2008
); however, the exact
effect (enhancement or inhibition) on cloud formation depends on the particular
chemical interactions (Sullivan et al.
2009
). Coated mineral particles may accelerate
the formation of precipitation particles in continental clouds (Yin et al.
2002
).
For calcium carbonate (CaCO
3
), reaction with HNO
3
in the presence of water
leads to the formation of calcium nitrate droplets. This causes a significant change
in particle morphology, size, scattering properties and hygroscopic growth relative
to calcium carbonate (Gibson et al.
2006
), as shown in Fig.
4.2
.
4.3.2
Impacts on Dust Reactivity
Chemical processing has some potentially important impacts on the reactivity of
dust in the environment. The most significant of these is the potential to increase
the solubility of nutrients, such as iron (Fe) and phosphorus (P) contained in dust,
and thus to enhance their bioavailability once deposited onto marine and terrestrial
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