Supercooled ternary solution

Supercooled ternary solution h2so4/hno3/h2o (STS) liquid droplets T < 192 K strong... 

Polar stratospheric clouds have been classified into two broad types, so-called Type I and Type II (Table 4.1). Type I PSCs have been further subdivided into Type la and Type Ib. Type la PSCs have traditionally been identified as crystals of nitric acid trihydrate, HNO, 3 H2O, denoted NAT, that form once temperatures fall below about 195 K. Type lb PSCs consist of supercooled ternary solutions of HNO3/H2SO4/H2O, also forming at about the same temperature threshold. Type II PSCs are largely frozen water ice, nonspherical crystalline particles, that form at temperatures below the ice frost point. The ice frost point, for example, at 3 X 10 Torr H O is 191 K. Despite the above classification, the composition of PSCs is still uncertain (Toon and Tolbert, 1995). 

Solid ice Solid NAT nitrogen trihydrate Solid SAT sulfuric acid tetrahydrate Liquid SSA stratospheric sulfate aerosols Liquid STS supercooled ternary solution... 

Toon and Tolbert (1995) suggest that if Type I PSCs are primarily ternary solutions rather than crystalline NAT, the higher vapor pressure of HN03 over the solution would in effect distill nitric acid from Type I to Type II PSCs, assisting in denitrification of the stratosphere. This overcomes the problem that if Type II PSCs have nitric acid only by virtue of the initial core onto which the water vapor condenses, the amount of HN03 they could remove may not be very large. The supercooled H20-HN03 liquid layer observed by Zondlo et al. (1998) clearly may also play an important role in terms of the amount of HNO, that can exist on the surface of these PSCs. 

Figure J. Possible pathways for stratospheric aerosol formation Left path the conventional 3-stage model ( SAT, NAT, [ice) the aerosol remains solid. Right path the aerosols remains liquid, takes up HNOj forming ternary supercooled solution, and freezes out below the frost point [15,28].

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