PH

FIGURE 7.8 Concentrations of S(IV) specics expressed as S(iV) mole fractions. These fractions are independent of the gas-phase S02 concentration.

7.3.4 Ammonia-Water Equilibrium

Ammonia is the principal basic gas in the atmosphere. Absorption of NH3 in H20 leads to

with

where at 298 K, H^ = 62Matm_1 and Ka] = 1.7 x 10~5 M. NH4OH is an alternative notation often used instead of NH3 H20. The concentration of NH4 is given by

The total dissolved ammonia [NH3 ] is simply

[NH3] = [NH3 • H20] + [NH+] = tfnh3jpnh3 (l + ) (7.51)

and the ammonium fraction is given by

[NH4] + Note that for pH values lower than 8, iTai[H+] Kw and

So under atmospheric conditions practically all dissolved ammonia in clouds is in the form of ammonium ion. The aqueous-phase concentrations of [NH4 ] in equilibrium with 1 ppb of NH3(g) are shown in Figure 7.9. The partitioning of ammonia between the gas and aqueous phases inside a cloud can be calculated using (7.9) and the effective Henry's law coefficient for ammonia, //¿,hi = //nh:, ka] [H '' ]/AT„,. If the cloud pH is less than 5 practically all the available ammonia will be dissolved in cloudwater (Figure 7.10).

7.3.5 Nitric Acid-Water Equilibrium

Nitric acid is one of the most water-soluble atmospheric gases with a Henry's law constant (at 298 K) of//hno3 = 2.1 x 105Matm-1. After dissolution

1 ppb NH3

FIGURE 7.9 Ammonium concentration as a function of pH for a gas-phase ammonia mixing ratio of 1 ppb at 298 K.

1 1

' 1 '

1 1 1 \>°

\o.i\ -

-

L=0.01 g m~3

\ \ V

! 1

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