Processing and Ageing in the Atmosphere - Mineral Dust: A Key Player in the Earth System - page 77

Fig. 4.2 ( a ) Heterogeneous chemistry of mineral dust measured with scanning electron
microscopy and energy dispersive X-ray analysis. These data show that carbonate particles “c” and
“d” have unique reactivity with gas-phase nitric acid to form nitrate particles that take up water as
shown by the change in particle size and shape ( spherical ). ( b ) Infrared extinction spectra at 2 %
RHs for carbonate and nitrate particles showing that nitrate particles scatter light to a greater extent
at higher relative humidity due to the size change and increased water content of the particles. ( c )
Increase in particle size for a 0.1 micron-sized dry particle as a function of relative humidity for
carbonate compared to nitrate particles. The growth factor (ratio of particle diameter at a given
relative humidity to the dry particle diameter) for carbonate particles is one at all % RH, whereas
the growth factor of nitrate particles is greater than one and increases with relative humidity
Several potential mechanisms which might lead to changes in the chemical
properties of dust during atmospheric transport have been proposed (see Baker and
Croot 2010 ). The interaction of dust with both mineral and organic acids has been
suggested to enhance dissolution of trace metals (Pehkonen et al. 1993 ; Spokes
et al. 1994 ; Spokes and Jickells 1996 ; Desboeufs et al. 2001 ; Mackie et al. 2005 ;
Shi et al. 2011 ) and phosphorus (Nenes et al. 2011 ). Although this enhancement
appears to be largely reversible when pH is raised (Spokes et al. 1994 ; Spokes and
Jickells 1996 ; Mackie et al. 2005 ; Shi et al. 2011 ), it appears that newly precipitated