Reactions In Solution

1. Interactions of Gaseous Air Pollutants with Atmospheric Aqueous Solutions

Because of the gaseous nature of many of the important primary and secondary pollutants, the emphasis in kinetic studies of atmospheric reactions historically has been on gas-phase systems. However, it is now clear that reactions that occur in the liquid phase and on the surfaces of solids and liquids play important roles in such problems as stratospheric ozone depletion (Chapters 12 and 13), acid rain, and fogs (Chapters 7 and 8) and in the growth and properties of aerosol particles (Chapter 9). We therefore briefly discuss reaction kinetics in solution in this section and "heterogeneous" kinetics in Section E.

The aqueous phase that serves as a reaction medium in the atmosphere is present in the form of clouds, fogs, rain, and particulate matter consisting of either an aqueous solution containing pollutants or a film of water surrounding an insoluble core (see Chapter 9). For example, at typical relative humidities, ~ 30-50% of the aerosol mass is due to water (Graedel and Weschler, 1981). However, many of the species that are believed to react in such atmospheric solutions, for example, S02, 03, H202, and NOx, are emitted or formed in the gas phase. Before reactions can occur in solution, then, several steps illustrated in Fig. 5.12 must first take place:

(1) Diffusion of the gas to the surface of the droplet

(2) Transport of the gas across the air-water interface

(3) Diffusion of the solvated species into the bulk phase of the droplet

(4) Reaction of the species in the aqueous phase or at the interface itself (see Section E.l).

Diffusion of gases is fast relative to diffusion in the aqueous phase; i.e., step 1 is fast relative to step 3. Thus diffusion coefficients for gases at f atm pressure are ~ 0.1-1 cm2 s"1, whereas in liquids they are ~ 10"5 cm2 s"1 for small molecules. As discussed in detail by Schwartz and Freiberg (1981), gas-phase diffusion, in most (but not all) cases, will not be the slowest (i.e., rate-determining) step.

Gases dissolve in aqueous solution to various extents, depending on the nature of the gas. At sufficiently long times, an equilibrium can be established between the gas- and liquid-phase concentrations, which is described by Henry's law:

where [X] is the equilibrium concentration of X in solution (in M = mol L"1), px is the gas-phase equilibrium pressure (in atm), and hx is the Henry's law constant (in M atm-1). Table 5.6 shows some values of h for some species of interest dissolving in aqueous solutions at 25°C (Schwartz, 1984a). They range from ~f0~3 M atm-' for relatively insoluble gases such as 02 to ~ 105 M atm-1 for highly soluble gases such as H202 and HNO,.

Henry's law can be applied to predict solution concentrations only if certain conditions are met. Thus it

Gas Phase

Gas phase diffusion

Gas Phase

Gas phase diffusion

Products

FIGURE 5.12 Schematic diagram of uptake and reaction of gases in liquids.

Evaporation

Products

FIGURE 5.12 Schematic diagram of uptake and reaction of gases in liquids.

TABLE 5.6 Henry's Law Coefficients ( H ) of Some Atmospheric Gases Dissolving in Liquid Water at 25°C

Gas Phase

TABLE 5.6 Henry's Law Coefficients ( H ) of Some Atmospheric Gases Dissolving in Liquid Water at 25°C

Gas

H (mol L"1 atm ~ ' )

Reference''

o2

1.3 x 10-3

Loomis, 1928

no

1.9 x 10-3

Loomis, 1928

c2h4

4.9 x 10 - 3

Loomis, 1928

no/

1 x 10-2

Schwartz and White, 1983

O,

(0.82-1.3) x 10-2

Briner and Perrottet, 1939

n2o

2.5 x 10-2

Loomis, 1928

co/

3.4 x 10-2

Loomis, 1928

so/

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