On the theories of type 1 polar stratospheric cloud formation

Several mechanisms for the production of polar stratospheric cloud (PSC) particles are investigated using the classical theories of nucleation and freezing and the multicomponent condensation theory. These mechanisms invoke particle compositions ranging from binary (H 2 SO 4 /H 2 O) solution, solid sulfuric acid tetrahydrate (SAT) and ternary (HNO 3 /H 2 O/H 2 5O 4 ) solution to binary (HNO 3 /H 2 O) solution and solid nitric acid trihydrate (NAT). Empirical relations, derived from classical nucleation studies, are used to calculate the surface energies required in calculations of nucleation and freezing. Using these data, we calculate that the nucleation of nitric acid solutions or solid phases onto SAT particles is not efficient. Homogeneous freezing of SAT or NAT from ternary 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 contact parameter between the emergent crystal and the initiating seed particle. Heterogeneous freezing of the stratospheric aerosol to SAT and NAT at temperatures above the frost point is not ruled out by our calculations. If formed, NAT can deplete the gas phase nitric acid concentration, by condensational growth, more efficiently than ternary droplets. We conclude that the most likely route to type 1 PSC particles is via condensational growth of ternary solution droplets followed by rapid freezing to NAT, SAT, and water ice at temperatures near the ice frost point. The particles formed are then stable and can reduce nitric acid vapor pressures to the saturation vapor pressure over NAT at all temperatures below the NAT point. Such a mechanism is consistent with observations.

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