Nature of Aerosols and PSCs

Sulfate aerosol particles with diameters typically in the 0.1- to 0.3-jum range are well known to be formed 

However, the eruption of large volcanoes also injects large quantities of S02 into the stratosphere, increasing the concentration of SSA significantly. For example, typical number concentrations of SSA are about 1-10 particles cm-3 under nonvolcanically perturbed conditions the number density increases by 1-2 orders of magnitude after major volcanic eruptions (e.g., see Russell et al., 1996). 

The concentration of sulfuric acid in SSA is typically 50-80 wt% under mid- and low-latitude stratosphere conditions. However, as the temperature drops, these particles take up increasing amounts of water, which dilutes the particles to as low as 30 wt% H2S04. Gaseous nitric acid is also absorbed by these solutions, forming ternary H2S04-H20-HN03 solutions with as much as 30 wt% in each acid. 

Sulfuric acid tetrahydrate (SAT) also ultimately freezes out of these ternary solutions (Molina et al., 1993 Iraci et al., 1995). At higher temperatures found at higher altitudes in the middle and low latitudes, sulfuric acid monohydrate (SAM) may also be stable (Zhang et al., 1995). 

The nitric acid concentration may be a major determinant of the extent of supercooling that occurs for these ternary mixtures (Molina etal., 1993 Song, 1994). For example, Molina et al. (1993) observed that HNO, did not affect the supercooling of H2S04-H20 mixtures at temperatures above 196 K, but below this temperature, the presence of HN03 rapidly promoted freezing. In addition, the availability of seed crystals to promote crystallization appears to be a critical issue. As discussed in detail by MacKenzie et al. (1995), a variety of potential seed crystals and/or surfaces that 

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