Processing and Ageing in the Atmosphere - Mineral Dust: A Key Player in the Earth System - page 83

indicate that different source regions appear to have characteristic solubilities (see
Baker et al. 2006 ), for Fe, Al, P and Si, it would appear that solubility varies
strongly with total element concentration, as noted above. The solubility of Mn,
however, does not exhibit the same hyperbolic relationship with total concentration.
Mn solubility varies over a much more restricted range than the other elements and
appears to be much less dependent on total Mn concentration (Fig. 4.5 c, f). This
difference may be related to the association of Mn with different mineral phases
and to the tendency of Mn to form oxide coatings on other particles (Potter and
Rossman 1979 ; Guest et al. 2002 ).
The similarity of the hyperbolic solubility relationships for Fe, Al, P and Si imply
that the overall impact of atmospheric processing on solubility is similar for these
elements, even if the relationship itself does not give clear evidence for which, if
any, of the potential processing mechanisms dominate. However, redox changes can
probably be discounted as a major control on solubility (Baker and Croot 2010 ),
since there are no significant differences between redox-active Fe (Fig. 4.5 a) and
the non-redox-active elements Al, P and Si (Fig. 4.5 b, d, e).
If acid processing is a significant control on solubility, some significant dif-
ferences in dust-acid interactions might be expected in different regions of the
world. North African dust is uplifted into air influenced by European and North
African sulphur and nitrogen oxide emissions before transport over the Atlantic. In
contrast, dust from the great Asian deserts is transported significant distances before
encountering high concentrations of acid precursors emitted from the industrialised
regions of Southeast Asia. These two regimes imply rather different mechanisms
for the interaction of dust and anthropogenic acids, with direct uptake of gaseous
species likely to be more important for Asian dust.

4.3.3 Impacts on Atmospheric Composition

Heterogeneous chemistry on dust surfaces leads to uptake and release of several
atmospherically important trace gas species, as well as the formation or alteration
of secondary coatings on mineral particles. Controlled laboratory experiments on
model mineral phases and authentic dusts have demonstrated that these processes
can be sensitive to relative humidity, photolysis and the nature of the reactive
surface.

Nitric Acid and Nitrogen Oxides

Nitric acid reacts with mineral dust resulting in formation of surface nitrate and
gas-phase species including NO, NO 2 and N 2 O (Rubasinghege and Grassian 2009 ).
Studies performed on clays and authentic dust samples from Africa and Asia show
efficient uptake of nitric acid onto these samples (Hanisch and Crowley 2001 ).