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

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 A FHH , B FHH 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 Correspondence to: A. Nenes (nenes@eas.gatech.edu) of 10% and R 2 ∼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.

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

“…This effect is observed because FHH particles require less water uptake to reach the critical diameter than κ-KT particles and consequently deplete much less water vapor (Kumar et al, 2009a). Using FHH theory to describe activation of mineral dust will therefore bias estimates of the aerosol indirect effect when mineral dust act as CCN.…”

“…According to Kumar et al (2009a), using κ-KT instead of using FHH theory in model parameterizations changes the maximum supersaturations and sensitivity of the cloud droplet number to aerosol variations in parcel model simulations. This effect is observed because FHH particles require less water uptake to reach the critical diameter than κ-KT particles and consequently deplete much less water vapor (Kumar et al, 2009a).…”

Understanding the contributions of aerosol properties and parameterization discrepancies to droplet number variability in a global climate model

“…Equations (A1) and (A2) consider only Köh-ler theory for computation of CCN. The impact of water adsorption onto insoluble particles such as dust can also be treated with a similar formalism (Kumar et al, 2009). The second step is achieved by finding an approximate solution to the equation describing the supersaturation tendency in the ascending air parcel, which can be written as…”

“…The development of the adjoint of the Fountoukis and Nenes (2005) parameterization (FN), as well as that for the adsorption activation parameterization of Kumar et al (2009), is described in full detail in Karydis et al (2012b). Briefly, because the computation of N d in FN is achieved by iterative solution of Eq.…”

Online coupled regional meteorology chemistry models in Europe: current status and prospects

“…The potential of soot particles to act as cloud condensation nuclei (CCN) or ice nuclei (IN) is also modified by the aging process. The aging of mineral dust particles changes their capability to act as IN or CCN (Kumar et al, 2009). Therefore, the explicit treatment of the aging process is an important process that should not be neglected.…”