For k

k\ (220 K) = 1.4 x 10"12 cm3 molecule"1 S"1 *i(298K} = 1.88 x 10"12 cm3 molecule-1 s"1

The termolecular rate coefficient is given by (3.25) with F = 0.6, n = 1, and m ~ 0. The atmospheric number concentrations are

Lower stratosphere [M] = 1.64 x 10,!i molecules cm"3 Surface [M] =2.46 x 1019 molecules cm"3

We obtain the following for the value of the pseudo-second order rate coefficient k2:

Lower stratosphere k2 = 1-09 x 10"12 cm3 molecule"'s-1 Surface k2 ~ 5.96 x 10"12 cm3 molecule"1 s"L

Under the lower stratospheric conditions, k2 is close to its low-pressure limit, whereas at the surface it is in the midrange. The ratio of the rates of reactions 1 and 2 is Rj/Ri — ki/kî-

Lower stratosphere R\/R2 = 1.2S Surface RjR2 = 0.31

Near the surface, where [M] is at its maximum, the second reaction predominates because a reasonable proportion of the excited H202 can collisionally de excite before it falls back to two OH radicals. In the lower stratosphere, [M] is almost an order of magnitude lower than at the surface, and a larger fraction of H^1 falls apart, so that the first reaction is favored somewhat over the second one.

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