THE TURNBULL CORRELATION AND THE FREEZING OF STRATOSPHERIC AEROSOL DROPLETS

An empirical correlation that is important in the calculation of homog eneous freezing probabilities, the ''Turnbull correlation'' for interf acial tensions, has been reevaluated and applied to systems of interes t as possible components of polar stratospheric clouds (PSCs). The sys tems studied were: sulphuric acid solutions freezing to water ice and sulphuric acid tetrahydrate (SAT); and nitric acid solutions freezing to nitric acid trihydrate (NAT) and nitric acid dihydrate (NAD). The c alculations have been compared to experimental data: agreement is gene rally good, although aerosol freezing experiments, which would rigorou sly test the theory, have not been made for NAT. Of the three measurem ents of aerosol freezing to NAD, the calculations are closer to those measurements showing a lower freezing temperature, of about 175-177 K. The comparison substantially improves our confidence in our understan ding of the mechanisms of PSC formation. Freezing of stratospheric aer osol to water ice remains the most plausible first step in solid-parti cle PSC formation if homogeneous freezing is the mechanism by which so lid-particle PSC formation occurs.