FREEZING OF STRATOSPHERIC AEROSOL DROPLETS

Theoretical calculations are presented for homogeneous and heterogeneo us freezing of sulfuric acid droplets under stratospheric conditions, based on classical nucleation theory. In contrast to previous results it is shown that a prominent candidate for freezing, sulfuric acid tet rahydrate (SAT = H2SO4.4H2O), does not freeze homogeneously. The theor etical results limit the homogeneous freezing rate at 200 K to much le ss than 1 cm-3s-1, a value that may be estimated from bulk phase labor atory experiments. This suggests that the experimental value is likely to be a measure of heterogeneous, not homogeneous nucleation. Thus, u nder statospheric conditions, freezing of SAT can only occur in the pr esence of suitable nuclei; however, even for heterogeneous nucleation experimental results impose strong constraints. Since a nitric acid tr ihydrate (NAT) embryo probably needs a solid body for nucleation, thes e results put an important constraint on the theory of NAT formation i n polar stratospheric clouds.