Simulations of Polar Stratospheric Clouds and Denitrification Using Laboratory Freezing Rates

Katja Drdla and Azadeh Tabazadeh

During the 1999-2000 Arctic winter, the SAGE III Ozone Loss and Validation Experiment (SOLVE) provided evidence of widespread solid-phase polar stratospheric clouds (PSCs) accompanied by severe denitrification. Previous simulations have shown that a freezing process occurring at temperatures above the ice frost point is necessary to explain these observations. Over the past year, the nitric acid freezing rates derived from laboratory measurements are used in the Integrated Microphysics and Aerosol Chemistry on Trajectories (IMPACT) model to calculate both PSC microphysical properties and their net effect on the denitirification process. A range of cases have been explored, including whether the solid-phase PSC particles are composed of nitric acid dihydrate (NAD) or trihydrate (NAT), whether the homogeneous freezing process occurs in the bulk or on the surface of stratospheric particles, and uncertainties in the derived freezing rates. Finally, a new theory to describe heterogeneous freezing has been developed and used to assess the possibility that meteoritic debris enhances solid-phase PSC production.

Homogeneous freezing processes have been found to be sufficiently fast to produce solid-phase particles and enhance denitrification. The details of the PSC concentration, particle size, and denitrification rate are dependent upon the assumed freezing rate. However, in all cases the extent of the modelled denitrification is less wide-spread than was observed. Furthermore, the observations of solid-phase particles in early December could not be reproduced by the model. Both of these discrepancies require freezing to occur at warmer temperatures than predicted by homogeneous freezing rates; specifically freezing must occur within 5 K of the NAT condensation point. Therefore, homogeneous freezing is found to be inherently limited in its ability to explain several aspects of the SOLVE observations.

The limitations of homogeneous freezing have led us to explore heterogeneous freezing in more detail. Classical nucleation theory has been used to derive the heterogeneous freezing rate from existing measurements of homogeneous freezing. Applying these equations to meteoritic debris, which has been measured in stratospheric aerosol, reveals that heterogeneous freezing is better able to explain the SOLVE measurements of denitrification and solid-phase PSC formation early in the winter. Meteoritic debris does not have to be an efficient nucleus for freezing; the m value, describing the compatibility of the nucleus, needs only be in the range —0.1 to —0.2.

Point of Contact: Katja Drdla, (650) 604-5663, katja@aerosol.arc.nasa.gov