An adsorption model of insoluble particle activation: Application to black carbon

Abstract

[1] We present a model of insoluble particle activation based on a modification of the Köhler equation in which we introduce a term based on the activity of water adsorbed on the particle surface. We illustrate the model by application to activation data from black carbon (BC) particles. We parameterize the model using a free energy of adsorption that reflects the relative affinity for water vapor adsorbing on either the BC surface or the adsorbed water layer. This enables the parameterization of either chemically modified (hydrophilic) or graphitic (hydrophobic) BC. Several features of a suite of carbon activation data are captured by the model. In particular, upper and lower bounding curves are predicted for activation supersaturation as a function of diameter. We show a large body of recent activation data that all fall within these bounds. The model also predicts that activation of BC aerosol leads to activation diameters from 3 to 10 times smaller than activation of soluble particles of identical dry diameter. The activation of smaller particles may be expected to impact the size distribution of resulting cloud droplets and thus the aerosol first indirect effect on climate. Finally, we compare activation of BC aerosol as calculated with this model to activation of mixed particles of BC and ammonium sulfate. It is shown that for some range of the adsorption free energy a hydrophilic BC aerosol is predicted to activate at lower supersaturation than comparable mixed aerosol of low mass fraction in the soluble component, indicating the utility of a model of mixed particle activation based on the adsorption of water to form an interfacial solution.

Cloud condensation nucleus activity comparison of dry- and wet-generated mineral dust aerosol: the significance of soluble material

“…In this treatment of FHH theory, the water activity is related to the of thickness of the adsorbed water via the BET multilayer equation of state for the adsorbed water layer (Henson et al, 2007;Kumar et al, 2009b). Therefore, the equilibrium saturation ratio for a droplet depends on the thickness of the adsorbed water layer via the activity of water.…”

Parameterization of cloud droplet formation for global and regional models: including adsorption activation from insoluble CCN

“…The threshold of nucleation substantially decreases when water interacts (adsorbs) onto slightly soluble particles giving rise to the process of adsorption activation (Sorjamaa and Laaksonen, 2007;Henson, 2007). Henson (2007) showed that a number of existing adsorption models (e.g., Fletcher, 1958;Wexler and Ge, 1998) for slightly soluble and insoluble particles can be successfully applied to represent droplet formation from adsorption activation.…”

“…The threshold of nucleation substantially decreases when water interacts (adsorbs) onto slightly soluble particles giving rise to the process of adsorption activation (Sorjamaa and Laaksonen, 2007;Henson, 2007). Henson (2007) showed that a number of existing adsorption models (e.g., Fletcher, 1958;Wexler and Ge, 1998) for slightly soluble and insoluble particles can be successfully applied to represent droplet formation from adsorption activation. Similarly, Sorjamaa and Laaksonen (2007) used the Frenkel-Halsey-Hill (FHH) multilayer physical adsorption model to describe water uptake as a function of relative humidity (i.e., water activity) and applied the theory to describe the activation of perfectly wettable and insoluble hydrophilic CCN.…”

“…Similarly, Sorjamaa and Laaksonen (2007) used the Frenkel-Halsey-Hill (FHH) multilayer physical adsorption model to describe water uptake as a function of relative humidity (i.e., water activity) and applied the theory to describe the activation of perfectly wettable and insoluble hydrophilic CCN. As with Köhler theory, adsorption of water can result in equilibrium curves with a critical supersaturation of atmospheric relevance (Henson, 2007;Sorjamaa and Laaksonen, 2007).…”

Meeting Summaries

“…Detailed models of inorganic and organic aerosols and the effect of cirrus clouds on radiation were developed (Amundson et al 2007). Novel process-level models to capture activation of black carbon (Henson 2007) and a probability distribution function method to improve the description of subgrid details and to improve cloud treatment in climate models and treat aerosol indirect effects were developed (Jeffery 2007). Cloud-resolving models, which treat aerosol-cloud processes in detail, are used to incorporate laboratory and field observations to develop parameterizations for coarse global climate models (Tao et al 2007).…”