One of the most important aqueous-phase reactions involving organic species is attack of the OH radical on hydrated formaldehyde
H2C(OH)2 + OH(aq) HCOOH(aq) + H02(aq) + H20 (7A.32)
with a rate constant k1A32 = 2 x 109M_1 1 at 298K (Bothe and Schulte-Frohlinde, 1980). Assuming [OH(aq)] = 10"'2 M and that aqueous-phase formaldehyde is in Henry's law equilibrium with t,HCHO = 1 ppb, [H2C(OH)2] = 6.3 pM, and the reaction proceeds with a rate of 36pMh~'. Using Figure 7.14 and a typical cloud liquid water content of 0.1 gm"3, the equivalent rate in gas-phase concentrations is 0.1 ppbh-1. Therefore for typical continental clouds this reaction has the potential to consume formaldehyde at a rate of 10% h_1 and also to produce formic acid.
The produced formic acid reacts rapidly with dissolved OH
with reaction rate constants of ¿7.A.33 = 2.5 x 109 and £7. a.34 = 2 x 10a M 1 s_l, respectively (Anbar and Neta 1967). The formic acid produced is therefore converted to carbon dioxide by the same radical that led to its production.
The ratio of the rates of formic acid production to destruction r/ is given by
¿7.a.32 [H2C(OH)2] f ¿7.a33[HCOO-]+fc7.a.34[HCOOH(aq)] K-- )
Assuming Henry's law equilibrium for formic acid and formaldehyde, we obtain
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