Box 37 Acidification of rain droplets

In Box 3.4 we saw the way reactions affect the solubility of gases. It is possible for some gases to undergo more complex hydration reactions in water, which influence its pH (see Box 3.5). The best known of these is the dissolution of carbon dioxide (CO2), which gives natural rainwater its characteristic pH.

aH+ .aHCO-aH2CO3

aH2CO3

aH2CO3 pCO2

which defines aH2CO3 as KH.pCO2, which can now be substituted in equation 4:

Rearranging gives:

Equation 3 is not important in the atmosphere, so the pH of a droplet of water in equilibrium with atmospheric CO 2 can be determined by combining the first two equilibrium constant equations that govern the dissolution (i.e. Henry's law, as discussed in Box 3.4) and dissociation. If carbonic acid (H2CO3) is the only source of protons, then aH+ must necessarily equal aHCO-. Thus the equilibrium equation for equation 2 can be written:

Substituting the appropriate values of the equilibrium constants (Table 1) and using a CO2 partial pressure (pCO2) of 360 ppm, i.e. 3.6 x 10-4atm, will yield a hydrogen ion (H+) activity of 2.4 x 10-6mol l-1 or a pH of 5.6.

Sulphur dioxide (SO2) is at much lower activity in the atmosphere, but it has a greater solubility and dissociation constant. We can set equations analogous to those for CO2:

aH2SO

and once again rearranging gives:

If a small amount of SO2 is present in the air at an activity of 5 x 10-9atm (not unreasonable over continental land masses), we can calculate a pH value of 4.85. So even low activity of SO2 can have a profound effect on droplet pH.

aHSO3(

eqn. 10

The Henry's law constant defined by equation 1 is:

Table 1 Henry's law constants and first dissociation constants for atmospheric gases that undergo hydrolysis (25°C).

Kh .pCO2

Table 1 Henry's law constants and first dissociation constants for atmospheric gases that undergo hydrolysis (25°C).

Gas

Kh (mol l-1 atm-1)

K (mol l-1)

Sulphur dioxide

2.0

2.0 x 10-2

Carbon dioxide

0.04

4.0 x 10-7

still termed 'dry deposition'. It is really a bit of a misnomer because the surfaces available for dry deposition are often most effective when wet.

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