Abstract
Abstract. Dust and black carbon aerosol have long been known to exert potentially important and diverse impacts on cloud droplet formation. Most studies to date focus on the soluble fraction of these particles, and overlook interactions of the insoluble fraction with water vapor (even if known to be hydrophilic). To address this gap, we developed a new parameterization that considers cloud droplet formation within an ascending air parcel containing insoluble (but wettable) particles externally mixed with aerosol containing an appreciable soluble fraction. Activation of particles with a soluble fraction is described through well-established Köhler theory, while the activation of hydrophilic insoluble particles is treated by "adsorption-activation" theory. In the latter, water vapor is adsorbed onto insoluble particles, the activity of which is described by a multilayer Frenkel-Halsey-Hill (FHH) adsorption isotherm modified to account for particle curvature. We further develop FHH activation theory to i) find combinations of the adsorption parameters AFHH, BFHH which yield atmospherically-relevant behavior, and, ii) express activation properties (critical supersaturation) that follow a simple power law with respect to dry particle diameter. The new parameterization is tested by comparing the parameterized cloud droplet number concentration against predictions with a detailed numerical cloud model, considering a wide range of particle populations, cloud updraft conditions, water vapor condensation coefficient and FHH adsorption isotherm characteristics. The agreement between parameterization and parcel model is excellent, with an average error of 10% and R2~0.98. A preliminary sensitivity study suggests that the sublinear response of droplet number to Köhler particle concentration is not as strong for FHH particles.
Highlights
It is well established that atmospheric aerosols are often hydrophilic, and can serve as Cloud Condensation Nuclei (CCN), upon which cloud droplets are formed through the process of activation
Nenes and Seinfeld (2003), Fountoukis and Nenes (2005) and Barahona and Nenes (2007) have extensively evaluated the parameterization for aerosol composed of only Kohler particles. The focus of this evaluation is on the performance of the parameterization when FHH particles are externally mixed with Kohler particles, considering a wide range of AFHH, BFHH, water vapor accommodation coefficient, αc, and parcel updraft velocity, V
This study presents a new parameterization of cloud droplet formation for an external mixture of soluble particles that activate according to Kohler theory, and, completely insoluble, wettable particles that form droplets through adsorption activation
Summary
It is well established that atmospheric aerosols are often hydrophilic, and can serve as Cloud Condensation Nuclei (CCN), upon which cloud droplets are formed through the process of activation. The enhanced number of droplets is often accompanied by a reduction in their size, thereby affecting cloud precipitation efficiency This may result in increased cloudiness, which gives rise to the so-called “cloud lifetime” or “Albrecht” effect of aerosols (Albrecht, 1989). All physically-based parameterizations developed to date rely on Kohler theory (Kohler, 1936), which considers curvature and solute effects on the equilibrium vapor pressure of a growing droplet Most often, this equilibrium curve exhibits a maximum in supersaturation known as critical supersaturation, sc, at a critical wet droplet diameter, Dc. When a particle is exposed to supersaturation above sc for long enough to exceed Dc, it is in unstable equilibrium and can nucleate a cloud droplet.
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