Mineral Dust and its Microphysical Interactions with Clouds - Mineral Dust: A Key Player in the Earth System - page 292

CD dry

Fig. 12.3 Derived theoretical exponent, x , in the critical supersaturation relationship S c D

for dust coated with material of hygroscopicity characteristic of organic aerosol ( D 0.1),

ammonium sulfate ( D 0.61), and sea salt ( D 1). Results computed for an aerosol with FHH-AT

parameters A FHH D 1.20. Also shown are data points for Canary Islands dust and

Owens Lake dust that are known to contain a large fraction of soluble salts (figure obtained from
Kumar et al. 2011b ). Aging of dust corresponds to going from right to left (i.e., in the direction of
decreasing insoluble fraction)

2.25 and B FHH D

This means KT particles can deplete considerably more water vapor than FHH-AT
particles in cloudy updrafts (even if both theories are applied so that the CCN
spectra computed are identical). This can affect the maximum supersaturation
that develops, which would affect both the droplet number that forms and its
sensitivity to aerosol variations. The latter point was demonstrated by Kumar et al.
2009b with parcel simulations and by Karydis et al. ( 2011 ) in a global modeling
study; the latter showed that droplet number in regions of dust emissions can be
affected by up to 60 % when dust hygroscopicity transitions from an FHH-type
to KT-type behavior. Recently, Morales Betancourt and Nenes ( 2013 ) showed that
large particles (following KT) had a disproportionately strong impact on CDN, its
sensitivity to aerosol variations, and cloud forcing; such supersaturation depletion
is largely mitigated when FHH-AT is used instead.

12.3 The IN Activity of Mineral Dust

The nucleation of ice crystals in the atmosphere can occur though a number of
distinct pathways, only some of which are likely to be important in any one
cloud type. For mineral dust there are two distinct temperature ranges where
the nucleation behavior is distinct. These ranges approximately correspond to the
conditions of mixed-phase and cirrus-type clouds and are illustrated in Fig. 12.4 .