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Comments on “Influence of dust composition on cloud droplet formation” by Kelly et al. 2007.

July 17th, 2007 by Jamison A. Smith, Ph. D. · No Comments

Kelly, J. T., C. C. Chuang, and A. S. Wexler, “Influence of dust composition on cloud formation,” Atmos. Environ. 41, 2904-2916, 2007.

This paper addresses the issue of dust particles serving as cloud condensation nuclei (CCN). Usually, dust is thought of as insoluble in water. Therefore, dust should not be able to serve as a CCN. In this paper, the authors address the role of slightly water-soluble components of dust. They use an expression to calculate Kohler curves for dust containing slightly soluble compounds. In addition, they use these Kohler curves in a parcel model to evaulate the effects of the dust CCN on cloud properties. Some of the interesting conclusions from this paper are:

1) Dust particles with a dimeter greater than 2 um activate regardless of their composition (um is my shorthand for micrometers because I don’t have a mu character handy)

2) For most conditions, the activation of dust particles does not affect the total number of cloud particles

This paper shows that dust can act as CCN, but the effect of the presence of dust CCN is unlikely to affect cloud properties. There are special conditions where dust CCN can have a significant role, and it will be interesting to look for these special conditions such as:

- fine dust aerosol with highly soluble compounds present on them

- large number concentration of dust aerosol could reduce the size of cloud droplets and inhibit precipitation

- dust aerosol particles with a large diameter that activate could accelerate precipitation by being highly active in coalescing with small cloud droplets

The conclusion that dust particles with diameters greater than 2 um activate regardless of composition is based on a highly idealized assumption of the particles’ morphology. For the Kohler curves, the authors assume that the slightly soluble material is a surface coating on the insoluble core. This morphology maximizes the ability of a slighly soluble compound to activate by reducing the Kelvin effect (i.e. the enhancement of vapor pressure over a curved surface) as much as possible. It is possible that this morphology could result from heterogenous reactions on the particle surface. Another likely scenario is that dust particles consist of little grains attached to one another. Such a morphology reduces the abililty of a slightly soluble compound to activate because small grains have a large Kelvin effect. If slightly soluble compounds are present as small grains on and within large dust particles, they may never be able to activate.

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