Physico-Chemistry of Polar Stratospheric Clouds

Edited by John S.Wettlaufer, J. Gregory Dash and Norbert Untersteiner Springer-Verlag Berlin Heidelberg 1999 

Besides the direct effects of heterogeneous reactions, equally important indirect effects of PSCs on gas phase chemistry were revealed. It was suggested that HNO3 condenses onto the aerosol particles allowing them to grow at temperatures above those at which pure ice crystals can exist  

From lidar measurements there is clear evidence for both solid and liquid PSCs in the polar stratospheres. The volume depolarization of backscat-tered light in aerosol-free air is 1.4 % (due to the polarizability of the air molecules), and data below this value indicate liquid type-lb clouds. Ternary solution droplets in such clouds yield substantial backscatter, but only in the parallel polarization plane. In contrast, type-la clouds have large depolarization ratios but small backscatter. Toon et al, referring to lidar measurements of Browell et al, were the first to describe these characteristics. They showed that type-la clouds consist of only a few very large aspherical particles, but the identification of type-lb as ternary liquids was made only later when the composition and size of stratospheric droplets were calculated. 

A glaring deficiency in our current knowledge of PSCs is the description of phase transitions like freezing nucleation or deposition nucleation. Results from laboratory freezing studies using solutions with stratospheric 

The thermodynamic properties of stratospheric ternary solutions have been described in detail in a recent review by Carslaw et al . Here only some important points are summarized. The thermodynamics of ternary solutions now appear to be reasonably well determined and widely accepted, in contrast to the metastable solid hydrates of potential importance for PSCs. This is due to the employment of thermodynamic models which, based on measurements at higher temperatures, allowed extrapolation to the relevant conditions. 

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Peter T (1999) Physico-chemistry of polar stratospheric clouds. In Ice Physics and the Natural Environment. Wettlaufer JS (ed) p 143-167. Berlin Springer-Verlag Peters SJ, Ewing GE (1997a) Thin film water on NaCl(lOO) imder ambient conditions An infrared study. Langmuir 13 6345-6348... 

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