Ph

FIGURE 7.10 Equilibrium fraction of total ammonia in the aqueous phase as a function of pH and cloud liquid water content at 298 K.

the nitric acid dissociates readily to nitrate

increasing further its solubility. The dissociation constant for nitrate is Kn\ = 15.4 M at 298 K, where by definition

- [HN03(aq)] (?"55) The total dissolved nitric acid [HNOj ] will then be

[HNO3] = [HN03 (aq)] + [N03] (7.56) and using Henry's law, we obtain

[HNO3 (aq)] = //hno3 Phno3 (7.57) Substituting this into the dissociation equilibrium equation, we have

The last three equations can be combined to provide an expression for the total dissolved nitric acid in equilibrium with a given nitric acid vapor concentration,

where //hno3 = ^hno, ( 1 + (^ni/[H+])) is the effective Henry's law coefficient for nitric acid.

Note that because the dissociation constant Kni has such a high value, it follows that

for any cloud pH of atmospheric interest. Therefore [NO3 ] [HN03(aq)] for all atmospheric clouds and one can safely assume that dissolved nitric acid exists in clouds exclusively as nitrate. In other words, nitric acid is a strong acid and upon dissolution in water it dissociates completely to nitrate ions.

The effective Henry's law constant for nitrate is then

FIGURE 7.11 Equilibrium fraction of total nitric acid in the aqueous phase as a function of pH and cloud liquid water content at 298 K assuming idea! solution.

FIGURE 7.11 Equilibrium fraction of total nitric acid in the aqueous phase as a function of pH and cloud liquid water content at 298 K assuming idea! solution.

Partitioning of Nitric Acid inside a Cloud Calculate the fraction of nitric acid that will exist in the aqueous phase inside a cloud as a function of the cloud liquid water content (L = 0.001-lg m~3) and pH, What does one expect in a typical cloud with liquid water content in the 0.1—1 gm 1 and pH in the 2—7 ranges?

Using (7.9) for the aqueous fraction and substituting H\ = ^hno3 = 3.2 x 106/[H~] (the Henry's law coefficient in that equation is the effective Henry's law coefficient for all the dissociating species), we can calculate the aqueous fraction of nitric acid as a function of pH for different cloud liquid water contents. The results are shown in Figure 7.11. For L= 1 g the aqueous fraction is practically 1 for all pH values of interest. For all clouds of atmospheric interest (L > 0.1 gm ) and for all pH values (pH>2), nitric acid at equilibrium is completely dissolved in cloudwatcr and its gas-phase concentration inside a cloud is expected to be practically zero. For example, for pH = 3, = 3.2x

109 M atm , and for a nitrate concentration of lOOpM, the corresponding equilibrium gas-phase mixing ratio is only 0.03 ppt(3 x 10"14 atm).

7.3.6 Equilibria of Other Important Atmospheric Gases

Hydrogen Peroxide Hydrogen peroxide is soluble in water with a Henry's law constant Hh2o2 = 1 x 10s M atm-1 at 298 K. It dissociates to produce H02

L, g m figure 7.12 Equilibrium fraction of total hydrogen peroxide in the aqueous phase as a function of cloud liquid water content at 298 K.

L, g m figure 7.12 Equilibrium fraction of total hydrogen peroxide in the aqueous phase as a function of cloud liquid water content at 298 K.

but is a rather weak electrolyte with a dissociation constant of i = 2.2 x 10 12 m at 298 k. Using the dissociation equilibrium

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