Aerosol Liquid Water Content

Water is an important component of atmospheric aerosols. Most of the water associated with atmospheric particles is chemically unbound (Pilinis et at. 1989). At very low relative

NH4HSO<

NH4HSO<

Relative Humidity, %

FIGURE 10.4 Diameter change of (NH4)2S04, NH4HS04, and H2S04 particles as a function of relative humidity. Dpo is the diameter of the particle at 0% RH.

humidities, atmospheric aerosol particles containing inorganic salts are solid. As the ambient relative humidity increases, the particles remain solid until the relative humidity reaches a threshold value characteristic of the particle composition (Figure 10.4). At this RH, the solid particle spontaneously absorbs water, producing a saturated aqueous solution. The relative humidity at which this phase transition occurs is known as the deliquescence relative humidity (DRH). Further increase of the ambient RH leads to additional water condensation onto the salt solution to maintain thermodynamic equilibrium (Figure 10.4). On the other hand, as the RH over the wet particle is decreased, evaporation of water occurs. However, the solution generally does not crystallize at the DRH, but remains supersaturated until a much lower RH at which crystallization occurs (Junge 1952; Richardson and Spann 1984; Cohen et al. 1987). This hysteresis phenomenon with different deliquescence and crystallization points is illustrated in Figure 10.4 for (NH4)2S04. The relative humidities of deliquescence for some inorganic salts, which are common constituents of ambient aerosols, are given in Table 10.1.

One should also note that some aerosol species do not exhibit deliquescent behavior. Species like H2S04 are highly hygroscopic, and therefore the water content associated with them changes smoothly as the RH increases or decreases (Figure 10.4).

For each relative humidity a single salt can exist in either of two states: as a solid or as an aqueous solution. For relative humidities lower than the deliquescence relative humidity the Gibbs free energy of the solid salt is lower than the energy of the corresponding solution and the salt remains in the solid state (Figure 10.5). As the relative humidity increases the Gibbs free energy of the corresponding solution state decreases, and at the DRH it becomes equal to the energy of the solid. When the RH increases further the solution represents the lower energy state and the particle spontaneously absorbs water to form a saturated salt solution. This deliquescence transition is accompanied by a

TABLE 10.1 Deliquescence Relative Humidities of Electrolyte Solutions at 298 K

Salt

DRH (%)

KCl

84.2 ±0.3

Na2S04

84.2 ±0.4

NH4CI

80.0

(NH4)2S04

79.9 ±0.5

NaCl

75.3 ±0.1

NaN03

74.3 ± 0.4

(NH4)3H(so4)2

69.0

NH4N03

61.8

NaHS04

52.0

NH4HS04

40.0

Sources: Tang (1980) and Tang and Munkelwitz (1993).

Sources: Tang (1980) and Tang and Munkelwitz (1993).

significant increase in the mass of the particle (Figure 10.4). For even higher RHs the solution state is the preferable one. When the RH decreases reaching the DRH the energies of the two states become once more equal. However, as the RH decreases further, for the particle to attain the lower energy state (solid), all the water in the particle needs to evaporate. This is physically difficult, as salt nuclei need to be formed and salt crystals to grow around them. In the atmosphere, where these salts are suspended in air, this transition does not occur at this point and the particle remains liquid. As the RH keeps decreasing the

Beer Lambert Law The Line Leveling Out
FIGURE 10.5 Gibbs free energy of a solid salt and its aqueous solution as a function of RH. At the DRH these energies become equal.

water in the particle keeps evaporating and the particle becomes a supersaturated solution. The solution eventually reaches a critical supersaturation, and nucleation (crystallization) takes place, forming at last a solid particle at RH significantly lower than the DRH (Figure 10.4).

10.2.1 Chemical Potential of Water in Atmospheric Particles

Water vapor exists in the atmosphere in concentrations on the order of grams per m3 of air while its concentration in the aerosol phase is less than 1 mg m 3 of air. As a result, transport of water to and from the aerosol phase does not affect the ambient vapor pressure of water in the atmosphere. This is in contrast to the cloud phase, where a significant amount of water exists in the form of cloud droplets (see Chapter 17). Thus the ambient RH can be treated as a known constant in aerosol thermodynamic calculations. Considering the equilibrium

and using the criterion for thermodynamic equilibrium and the corresponding chemical potentials

where pw is the water vapor pressure (in atm) and x„, is the water activity in solution. For pure water in equilibrium with its vapor, otm = 1 and pw = p°w (the saturation vapor pressure of water at this temperature); therefore

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