An Attempt to Characterize a Relationship between Supersaturation Spectrum, Size Spectrum and Solubility of CCN Observed over a Tropical Rain Forest

Abstract

A demonstration of the Twomey's relation N = CSk is presented; it is significantly different from the earlier demonstration and takes into account the very small size and solubility of cloud condensation nuclei (CCN) activated at supersaturations between 0.3 and 1.0%. A simple expression for the distribution function f(r)=?dN/d logr as a function of CCN concentrations and supersaturation spectrum curve slope k is found. This enables us to predict in what case the size spectrum f(r) = ?dN/d logr is increasing or decreasing with increasing radius. An expression of the slope s of the size spectrum curve is then proposed. The theoretical discussion indicates two opposite cases: convex supersaturation spectrum, increasing size spectrum, aged and soluble aerosol or concave supersaturation spectrum, decreasing size spectrum, young and less soluble aerosol. These two cases were really observed in the tropical wet forest during quite different meteorological situations; the first one occurred during the dry season at the level of the Intertropical Convergence Zone (ITCZ), the second during the rainy season.