Production Of Hydroxyl Radicals In The Troposphere

The key to understanding tropospheric chemistry begins with the hydroxyl (OH) radical. Because the OH radical is unreactive toward 02, once produced, it survives to react with virtually all atmospheric trace species. The most abundant oxidants in the atmosphere are 02 and 03, but these molecules have large bond energies and are generally unreactive, except with certain free radicals; this leaves the OH radical as the primary oxidizing species in the troposphere.

We learned in Section 3.6 and Chapter 5 that 03 photolysis (at wavelengths <319 nm) to produce both ground-state (O) and excited singlet (O('D)) oxygen atoms is important in both the stratosphere and troposphere:

The ground-state O atom combines rapidly with 02 to reform 03:

so reaction la followed by reaction 2 has no net chemical effect (a null cycle). However, when Of1D) is produced, since the spontaneous 0(' D) —> O transition is forbidden, it must react with another atmospheric species. Most often, 0('D) collides with N2 or 02, removing its excess energy and quenching 0(' D) to its ground state:

Since the ground-state O atom then just reacts with 02 by reaction 2 to replenish 03, this path, consisting of reactions lb, 3, and 2, is just another null cycle. Occasionally, however, 0(:D) collides with a water molecule and produces two OH radicals:

Reaction 4 is, in fact, the only gas-phase reaction in the troposphere able to break the H—O bond in H20.

Let us compute the rate of formation of OH radicals from reactions 1^1. 0(1D) is sufficiently reactive that the pseudo-steady-state approximation can be invoked for its concentration. Thus

At 298 K, the rate coefficient values are (Table B.l)

4.0 x 10"11 cm3 molecule"1 s"1 2.6 x 10"11 cm3 molecule"1 s"1 2.2 x 10"10 cm3 molecule"1 s"1

For the atmospheric N2/02 mixture, the value of k3 is 2.9 x 10 " cm3 molecule-1 s ~ 1. The first-order photolysis rate coefficient of O3 —> O('D) at the Earth's surface at solar zenith angle 0° is 6 x 10 5 s_l (Figure 4.15). Equation (6.1) can be written as iOf'Dll =

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