and this concentration ratio remains less than 10-4 for pH values less than 7.5. Therefore, for most atmospheric applications, the dissociation of dissolved hydrogen peroxide can be neglected. The equilibrium partitioning of H202 between the gas and aqueous phases can be calculated from (7.9) using the Henry's law coefficient //h2o, and is shown in Figure 7.12. H202 exists in appreciable amounts in both the gas and aqueous phases inside a typical cloud. For example, for a cloud liquid water content of 0.2 g m \ roughly 30% of the H202 will be dissolved in the cloudwater whereas the remaining 70% will remain in the cloud interstitial air.
Ozone Ozone is slightly soluble in water with a Henry's law constant of only 0.011M atm 1 at 298 K. For a cloud gas-phase mixing ratio of 100 ppb 03, the equilibrium ozone aqueous-phase concentration is 1.1 nM and practically all ozone remains in the gas phase.
Oxides of Nitrogen Nitric oxide (NO) and nitrogen dioxide (N02) are also characterized by small solubility in water (Henry's law coefficients 0.002 and 0.01 Matm 1 at 298 K). A negligible fraction of these species is dissolved in cloudwater, and their aqueous-phase concentrations are estimated to be on the order of 1 nM or even smaller.
Formaldehyde Formaldehyde, upon dissolution in water, undergoes hydrolysis to yield the gem-diol, methylene glycol
with a hydration constant A"hcho defined by
that has a value of 2530 at 298 K (Le Henaff 1968). This rather large value of ^hcho suggests that the hydration is essentially complete (>99.9%) and that practically all dissolved formaldehyde will exist in its gem-diol form.
Formaldehyde has a Henry's law constant //Hcno = 2.5 M atm at 298 K (Betterton and Hoffmann 1988), but its water solubility is enhanced by several orders of magnitude as a result of the diol formation. Combining the Henry's law equilibrium and the hydrolysis relationships, we can calculate the effective Henry's law coefficient for the total dissolved formaldehyde //]*[CHO as
with a value of 6.3 x 103 Matm^1. In the literature, the effective Henry's law coefficient for formaldehyde is often quoted instead of the intrinsic constant. Therefore, for thermodynamic equilibrium assuming that all dissolved formaldehyde exists as H2C(OH)2:
Most of the available formaldehyde remains in the gas phase inside a typical cloud (Figure 7.13).
Formic and Other Atmospheric Acids The most abundant carboxylic acids in the atmosphere are formic and acetic acid, although more than 100 aliphatic, olefinic, and aromatic acids have been detected in the atmosphere [Graedel et al. (1986); see also Chapter 14]. These acids are weak electrolytes, and their partial dissociation enhances their solubility.
Formic acid has a Henry's law coefficient of 3600 M atm-1 at 298 K and a dissociation constant Kf = 1.8 x 10 4 M. From the dissociation equilibrium
one can calculate the dissociated fraction of formic acid as
At pH = 3.74, 50% of the dissolved formic acid has dissociated. Above pH 4, most of the dissolved formic acid exists in its ionic form, while for pH values below 3 most of it is undissociated. The concentrations of HCOOH(aq) and HCOO" in equilibrium with 1 ppb
FIGURE 7.13 Equilibrium fraction of total formaldehyde in the aqueous phase as a function of cloud liquid water content at 298 K.
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