ON THE THEORIES OF TYPE-1 POLAR STRATOSPHERIC CLOUD FORMATION

Several mechanisms for the production of polar stratospheric cloud (PS C) particles are investigated using the classical theories of nucleati on and freezing and the multicomponent condensation theory. These mech anisms invoke particle compositions ranging from binary (H2SO4/H2O) so lution, solid sulfuric acid tetrahydrate (SAT) and ternary (HNO3/H2O/H 2SO4) solution to binary (HNO3/H2O) solution and solid nitric acid tri hydrate (NAT). Empirical relations, derived from classical nucleation studies, are used to calculate the surface energies required in calcul ations of nucleation and freezing. Using these data, we calculate that the nucleation of nitric acid solutions or solid phases onto SAT part icles is not efficient. Homogeneous freezing of SAT or NAT from ternar y solutions does not occur under stratospheric conditions. Homogeneous freezing of water ice can occur at temperatures near the frost point of pure water. Heterogeneous freezing is a strong function of the cont act parameter between the emergent crystal and the initiating seed par ticle. Heterogeneous freezing of the stratospheric aerosol to SAT and NAT at temperatures above the frost point is not ruled out by our calc ulation. If formed, NAT can deplete the gas phase nitric acid concentr ation, by condensational growth, more efficiently than ternary droplet s. We conclude that the most likely route to type 1 PSC particles is v ia condensational growth of ternary solution droplets followed by rapi d freezing to NAT, SAT, and water ice at temperatures near the ice fro st point. The particles formed are then stable and can reduce nitric a cid vapor pressures to the saturation vapor pressure over NAT at all t emperatures below the NAT point. Such a mechanism is consistent with o bservations.