Mineral Dust and its Microphysical Interactions with Clouds - Mineral Dust: A Key Player in the Earth System - page 288

and water adsorption on hydrophilic insoluble enclosures within the aerosol particle.
When the size of a droplet decreases, its curvature is enhanced; this promotes the
equilibrium vapor pressure of water at the surface, as there are fewer molecules
available for interaction in the condensed phase (this is known as the “Kelvin effect”;
Pruppacher and Klett 1997 ). Furthermore, water equilibrium vapor pressure may
be depressed from the dissolution of solutes in the aqueous phase that forms on
the CCN. This depression is known as the “Raoult effect” (Pruppacher and Klett
1997 ). The solutes can preexist in the particle phase (e.g., ammonium sulfate, sea
salt, organics) or partition from the gas phase (e.g., HNO 3 ,NH 3 , volatile organic
acids; Laaksonen et al. 1998 ; Nenes et al. 2002 ; Topping et al. 2013 ). Combination
of the Kelvin and Raoult effects yields the total equilibrium vapor pressure of a
wet aerosol particle, and how it varies as its wet size changes due to fluctuations
in environmental conditions. When the ambient relative humidity (the ratio of
ambient water vapor pressure over the equilibrium vapor pressure of pure water
over a flat surface) approaches 100 %, the CCN continuously absorbs water vapor
but remains in stable equilibrium with the environment. Even when the system
becomes supersaturated, the water tends to condense from the gas phase onto the
particle, but remains in stable equilibrium owing to the Kelvin effect. However, once
the supersaturation exceeds a characteristic (or “critical”) value, the wet aerosol
particle cannot remain in stable equilibrium with its environment and experiences
unconstrained growth. It is this point and on where the particle is said to act as a
CCN and activates into a cloud droplet.
The above theory was first introduced by Köhler in the early twentieth century
(Köhler 1936 ) to describe the activation of soluble (such as sea salt) aerosol particles
into cloud droplets. With appropriate extensions to account for the multicomponent
nature of global aerosol, Köhler theory (KT) remains the theoretical framework
used in models to link aerosol with CCN and cloud droplet formation. The theory
determines the lowest-level (or “critical”) ambient water vapor supersaturation, S c ,
required for particles to activate into cloud droplets (Pruppacher and Klett 1997 ).
The chemical complexity of ambient aerosol can be addressed by parameterizing
the solute effects in terms of a hygroscopicity parameter, (Petters and Kreidenweis
2007 ). can be thought as an “equivalent volume fraction” of solute with hygro-
scopicity similar to sea salt (Lance 2007 ) and allows a direct comparison of the
water uptake properties of aerosol over a wide composition range;

! 0represents

completely insoluble, wettable material, while 1.4 for NaCl (effectively, the

most hygroscopic of all atmospheric aerosol species). For dust-relevant range of
(i.e., <0.05; Kumar et al. 2009a , b ), the critical supersaturation of a particle with
dry diameter D dry is computed from the maximum of the following equation:

!

D dry

4 W M W
RT W D P

S D (12.1)

D p D dry .1 /

where S is the particle equilibrium supersaturation (equal to the fractional relative
humidity minus one), D p is the droplet diameter, w is the CCN surface tension at