A MODEL DESCRIPTION FOR CIRRUS CLOUD NUCLEATION FROM HOMOGENEOUS FREEZING OF SULFATE AEROSOLS

Classical nucleation theory for homogeneous freezing as well as recent laboratory data are used to formulate an algorithm for ice nucleation from an aqueous sulfuric acid solution droplet A new parameterized fu nction is derived from recent thermodynamic data to express the variat ion of sulfuric acid solution composition with temperature and relativ e humidity. This function is then used to derive critical ice nucleati on parameters from recent laboratory data. The critical nucleation par ameters are used in a classical nucleation theory to derive the diffus ion activation energy of water molecules in a sulfuric acid solution f rom the measurements. The derived diffusion activation energy of water molecules in a sulfuric acid solution does not agree with the diffusi on activation energy calculated from a viscous flow formulation that i s commonly used in classical nucleation rate calculations. Our results show that ice nucleation in a sulfate droplet occurs when the interfa ce energy of ice against the sulfate solution is approximately 17 dyn cm(-1). We calculate that a supersaturation ratio of about 1.3 to 1.5 is required to nucleate ice from an aqueous sulfuric acid solution dro plet in the temperature range of about 185 to 240 K. This supersaturat ion corresponds to a supercooling of a sulfate solution to about 2 to 3 K below the equilibrium condensation point of ice. Simple functions are given for estimating the nucleation point of ice in the upper trop osphere. The differences between previous parameterizations and this w ork are discussed.