MODELING THE COMPOSITION OF LIQUID STRATOSPHERIC AEROSOLS

There is extensive evidence to suggest that stratospheric aerosols can remain liquid to very low stratospheric temperatures, despite being h ighly supercooled. Even polar stratospheric clouds, which are a key fa ctor in the depletion of ozone in polar regions, can often consist of liquid rather than solid particles. It has been known since the 1960s that stratospheric aerosols are mostly concentrated sulfuric acid-wate r droplets, but the combination of recent laboratory measurements, fie ld observations, and thermodynamic model calculations has led to a rec ognition that many species other than water vapor can partition into t he aerosols, particularly at low temperatures. This has been shown to increase the aerosol size, to control their freezing properties, and t o affect the rates of important liquid phase reactions. This in turn i nfluences the formation of polar stratospheric clouds and the subseque nt extent and duration of seasonal ozone depletion in the polar region s. We review thermodynamic models of the liquid phase that enable the partitioning of gases such as HCl, HBr, HOCl, and HNO3 into sulfuric a cid aerosols to be calculated over the full range of stratospheric con ditions. Such models have been used to show that the uptake of nitric acid vapor can lead to a rapid transition from mainly sulfuric-acid- t o mainly nitric-acid-based liquid aerosols at low temperatures, a proc ess that has changed our view of how polar stratospheric clouds form. Liquid aerosol composition at these low temperatures is still known la rgely from predictions made by thermodynamic models, rather than from observations, and even laboratory data under these conditions are limi ted. This and other uncertainties in calculated aerosol composition ar e estimated, and their effect on the interpretation of particle observ ations and predictions made by chemical stratospheric models is descri bed.