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Boiling Point,

FIGURE 10.14 Vapor pressure of sulfuric acid as a function of temperature using the Ayers et al. (1980) and Kulmala and Laaksonen (1990) estimates. (Reprinted with permission from Kulmala, M. and Laaksonen, A. Binary nucleation of water sulfuric acid system. Comparison of classical theories with different H2S04 saturation vapor pressure, J. Chem. Phys. 93, 696-701. Copyright 1990 American Institute of Physics.)

Temperature, K

FIGURE 10.14 Vapor pressure of sulfuric acid as a function of temperature using the Ayers et al. (1980) and Kulmala and Laaksonen (1990) estimates. (Reprinted with permission from Kulmala, M. and Laaksonen, A. Binary nucleation of water sulfuric acid system. Comparison of classical theories with different H2S04 saturation vapor pressure, J. Chem. Phys. 93, 696-701. Copyright 1990 American Institute of Physics.)

The saturation vapor pressure of pure sulfuric acid, /?h2so4 > ^een an elusive quantity. Roedel (1979), Ayers et al. (1980), and Kulmala and Laaksonen (1990) have argued that />h2so4 = 1-3 ± 1.0 x 10"8 atm (13 ± lOppb) at 296 K. The temperature dependence of the saturation vapor pressure as a function of temperature is shown in Figure 10.14.

The variation of the vapor pressure of H2SO4 in a mixture with water over a flat surface as a function of composition at ambient temperatures is shown in Figure 10.15. Information on Figures 10.13, 10.14, and 10.15 can be combined to obtain most of the required information about sulfuric acid properties in the atmosphere. For example, let us calculate the expected H2SO4 concentrations, focusing only on the H2SO4—H2O system. If the relative humidity exceeds 50%, the H2SO4 concentration in solution will be less than 40% by mass, and the H2SC>4 mole fraction is less than 0.1 (Figure 10.13). For a temperature equal to 20°C, this corresponds to equilibrium vapor pressures less than 10"12 mm Hg. Under all conditions the concentration of H2SO4 in the gas phase is much less than the aerosol sulfate concentration.

The role of the Kelvin effect on the composition of atmospheric H2S04 —H2O droplets is illustrated in Figure 10.16. If the effect were negligible the equilibrium aerosol composition would not be a function of its size. This is the case for particles larger than 0.1 pm in diameter. For smaller particles the H2SO4 mole fraction in the droplet is highly dependent on particle size. Also notice that for a fixed droplet size, the water concentration increases as the relative humidity increases.

The sulfuric acid-water system reactions are shown in Table 10.7. Note that the reaction H2S04(g) — H2S04(aq) is not included as H2S04(aq) can be assumed to completely dissociate to HSO4 in the atmosphere for all practical purposes.

H2S04 Mole Fraction

FIGURE 10.15 Equilibrium vapor pressure of H2S04 in a mixture with water over a flat surface as a function of composition and temperature.

FIGURE 10.15 Equilibrium vapor pressure of H2S04 in a mixture with water over a flat surface as a function of composition and temperature.

Particle Diameter, (im

FIGURE 10.16 Equilibrium concentration of H2S04 in a spherical droplet of H2S04 and H20 as a function of relative humidity and droplet diameter.

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