Water in the Atmosphere

providing a source of radicals for reactions (5-64) and (5-65). Dissociation of water into H2 and O may also be contributing in a minor way to absorption in this spectral region:

H2O ) H2 + O (ground state or excited ^2 at A < 178 nm) (5-67)

Between 140.5 and 124.5 nm the major reaction is still (5-66), but dissociations to excited OH* radicals

and to ground-state O atoms

also occur. Below 124.5 nm (to 500 nm), minor products are excited H atoms and OH+ ions in addition to the products from reactions (5-66), (5-67), (5-68), and (5-69).

Since water does not absorb dissociating radiation above 186 nm, it will not be photodissociated in the troposphere, which is protected from the high-energy photons by oxygen and the stratospheric ozone layer. Photodissociation does occur in the mesosphere and in the upper stratosphere as a result of partial transmission through the UV window (Figure 5-7). Absorption of nondissociating infrared radiation by water in the troposphere is of course an important contributor to the greenhouse effect (Section 3.1.2).

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