Mineral Dust and its Microphysical Interactions with Clouds - Mineral Dust: A Key Player in the Earth System - page 310

20 ı C. The IN
concentration is based on the modeled K-feldspar content of airborne mineral dust (This figure is
reproduced from Atkinson et al. ( 2013 ))

Fig. 12.9 Annual average mineral dust IN concentrations at 600 hPa active at

A limitation in these modeling studies was the lack of fundamental quantitative
understanding of what causes ice nucleation and how efficiently it does so. Recently,
Atkinson et al. ( 2013 ) published the global distribution of IN from mineral dust
aerosol based on the laboratory discovery that K-feldspar is the most efficient
mineral IN in mineral dust. They used the laboratory-derived ice-active site densities
together with a global aerosol model (GLOMAP) to predict the mineral dust IN
concentration globally. In GLOMAP, the dust emissions were based on AeroCom
recommendations (Dentener et al. 2006 ), together with the dust mineralogy of
Nickovic et al. ( 2012 ). Figure 12.9 shows the resulting IN concentrations at 600 hPa
which are active at 20 ı C and at water saturation. This figure shows a clear peak

in IN concentrations near in vicinity of the dust belt with less dust in remote
locations. It should be borne in mind that aging and all scavenging processes
were not represented; hence, the IN concentrations far from source were probably
overestimated. Another general feature is the strong hemispheric divide, with dust
IN concentrations much lower in the Southern Hemisphere.
We now turn our attention to the much colder upper troposphere where hetero-
geneous IN have a profound impact on the properties of cirrus clouds (DeMott
et al. 1997 ;Krämeretal. 2009 ; Cziczo et al. 2013 ; Barahona et al. 2010 ). In the
absence of heterogeneous IN, in situ cirrus (not including anvil outflow cirrus)
forms via the homogeneous nucleation of ice in aqueous haze particles. The number
of ice crystals which form is, in part, determined by the competition between
the rate of increase in saturation driven by adiabatic expansion and ice crystal