Mineral Dust and its Microphysical Interactions with Clouds - Mineral Dust: A Key Player in the Earth System - page 291

in water adsorption capacity arising from differences in molecular structure of the
minerals between the two particle types. Similar to dust, one set of adsorption
parameters ( A FHH D 0.12) was found to describe the

CCN activity of all volcanic ash samples considered, except for one (Mt. Redoubt
ash). The latter displayed comparable hydrophilicity to inorganic aerosol CCN, but
with x exp consistent with FHH-AT. The highly porous nature of Mt. Redoubt ash
particles, which increases their surface-to-volume ratio considerably, is postulated
to cause the observed high level of hydrophilicity; the lack of any considerable
soluble fraction upon these particles is consistent with this hypothesis. Lathem
et al. ( 2011 ) also noted that the combination of CCN activity and subsaturated
hygroscopicity growth measurements can also be used as proof of adsorption,
as the water uptake characteristics vary considerably between FHH-AT and KT
in each relative humidity regime. Hatch et al. ( 2014 ) directly measured water
vapor adsorption upon Na-montmorillonite and illite clay; FHH parameters derived
differed slightly from those reported by Kumar et al. ( 2011a ), although the CCN
activity inferred from the adsorption measurements is in excellent agreement with
wet-generated mineral particles.
Considerable work remains however to better constrain the values of the
adsorption parameters and the effect of chemical aging and presence of soluble
materials. To account for the CCN activity of dust containing soluble salt fraction,
Kumar et al. ( 2011b ) proposed a new framework of CCN activation that accounts
for concurrent effects of solute as well as water vapor adsorption, based on a core-
and-shell model with the core representing insoluble dust and shell consisting of a
layer of soluble salt. The model describes equilibrium supersaturation as a function
of adsorption parameters, hygroscopicity parameter of the soluble fraction, size of
the dry particle, and insoluble and soluble volume fractions as follows:

2.41 ˙ 0.93 and B FHH D 1.31 ˙

D P ! B FHH

" s D 3 dry
D P

" i 1=3 D dry
2D H 2 O

4 W M W
RT W D P

D

" i D dry

S A FHH (12.3)

where " i represents the insoluble volume fraction and " s the soluble volume fraction,
given by " s D " i . The new framework, referred to as the unified dust activation

framework, predicts that as " i decreases, x exp changes from

1

0.85 (FHH-AT limit) to

1.5 (KT limit) and predicts values of x exp consistent with published CCN activity
of playa salts that tend to contain a substantial soluble fraction (Fig. 12.3 ).
KT (and especially the -KT formulation) is the standard approach to describe
CCN activity; one might consider adopting an “effective” hygroscopicity parameter
for dust instead of invoking FHH-AT. Indeed all studies on dust CCN activity before
Kumar et al. ( 2009a , b ) and many after adopted this approach (see Table 12.2 ).
While using an effective hygroscopicity can certainly be used for predicting CCN
number, the consequences of adopting fundamentally different activation physics
for mineral dust in calculations of cloud droplet number (CDNC) are important.
FHH-AT particles require less water uptake to reach their critical diameter, com-
pared to KT particles with the same critical supersaturation (Kumar et al. 2009a , b ).