Q

FIGURE 12.23 Temperatures (K) at which SAT deliquesces at different gas-phase pressures of H20 and HNO, typical of the stratosphere (from Koop and Carslaw, 1996).

sive analysis. For example, Pueschel et al. (1994) collected and analyzed aerosols after the Mount Pinatubo eruptions and identifed ash particles at lower altitudes that contained Si, Al, Mg, and Na that were coated with H2S04. Particles at higher altitudes were shown to be predominantly H2S04-H20 mixtures.

Another method of probing sulfuric acid aerosols is to heat the sample intake sufficiently to vaporize sulfuric acid-water aerosols but not other particles such as those containing ash minerals; the difference between the measured particles with and without intake heating provides a measure of the contribution of sulfuric acid-water. Using this technique, Deshler et al. (1992), for example, have shown that more than 90% of the stratospheric particles above Laramie, Wyoming, after the Mount Pinatubo eruption were composed of sulfuric acid-water mixtures.

Light scattering and absorption techniques have also been used, for example to obtain the index of refraction of the particles and then to compare these atmospheric measurements to laboratory measurements of NAT, NAD, etc. determined in laboratory studies. Adriani and co-workers (1995), for example, using light scattering in the visible, report four types of particles over McMurdo Station, Antarctica, in 1992: ice, volcanic aerosol particles, and two types of nitric acid-water PSCs. The characteristics of these four types are shown in Table 12.3. The index of refraction measured at 532 nm for particles identified as ice, 1.32 + 0.01, is consistent with the known index of refraction for ice. Two types of nitric acid containing particles were observed, which had indices of refraction in the range of 1.39-1.42; one substantially depolarized the light beam and one did not. These are consistent with laboratory measurements of the refractive indices of thin films of amorphous, or possibly crystalline, nitric acid trihydrate or with higher amorphous hydrates of HNO, (Berland et al., 1994; Middlebrook et al., 1994; Niedziela et al., 1998). The value of 1.43 ± 0.04 observed for the volcanic aerosol in this and earlier studies (e.g., Grainger et al., 1993; Santer et al., 1988) is consistent with sulfuric acid-water solutions (Russell et al., 1996; Luo et al, 1996). Refractive indices of ternary solutions of HN03-H2S04-H20 are now available for use in identifying particle compositions in such studies (Luo et al., 1996).

Similar measurements of particles and PSCs have been made in the Arctic region (e.g., see Pueschel et al., 1992b; Dye et al., 1992; Wilson et al., 1992; Deshler et al., 1994; and Brogniez et al., 1997).

Baumgardner et al. (1996) measured the refractive index of particles at altitudes from 4 to 20 km at locations from 43.5°S to 37.5°N. While the measured refractive indices were consistent with sulfuric acid-water mixtures at lower temperatures (around 195 K), at higher temperatures the values were smaller than predicted. Furthermore, a significant decrease in the average refractive index of the particles to 1.34 was observed at lower altitudes, from approximately 4 to 9 km, which Baumgardner and co-workers suggest is due to to the presence of nonspherical or light-absorbing particles.

Other light scattering techniques, e.g., using several different scattering wavelengths, have also been used to probe PSCs and aerosols (e.g., Larsen et al., 1995;

FIGURE 12.23 Temperatures (K) at which SAT deliquesces at different gas-phase pressures of H20 and HNO, typical of the stratosphere (from Koop and Carslaw, 1996).

107Phno3 (mbar)

107Phno3 (mbar)

TABLE 12.3 Characteristics of Particles over McMurdo Station, Antarctica, in 1992"

Nitric acid-water PSC

TABLE 12.3 Characteristics of Particles over McMurdo Station, Antarctica, in 1992"

Nitric acid-water PSC

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