Biogeochemical Cycles - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 165

The direct photolysis rate depends on a number of factors related to the char-
acteristics of the compound and the environment: 45

• Solar radiation (intensity and wavelength)—Depending on the time of the
year, the weather, and the geographical position of the water body, dif-
ferent levels of incoming solar radiation will be delivered to the surface
of water.
• Light attenuation in the water—Suspended matter, surface films, color,
and other factors influence the penetration and attenuation of light. Pho-
tolysis in quiescent, turbid water bodies might be limited to a thin surface
layer, whereas it could extend to great depths in relatively clear water.
• Absorption spectrum of the chemical—Due to its chemical structure, each
compound will absorb light energy to different degrees from various
wavelengths.

• Quantum yield—This refers to the fraction of absorbed photons that result

in a chemical reaction.

When light strikes a pollutant molecule, the energy content of the molecule is
increased and the molecule is elevated into an excited electron state. This excited
state is unstable, and the molecule returns to its normal (lower) energy level by one
of the two paths: 131

• It loses its “extra” energy through energy emission; that is, fluorescence

or phosphorescence.

or
• It is converted to a different molecule through the new electron distribution

produced in the excited state. Usually the organic chemical is oxidized.

Photolysis rate constants can be measured in the field with sunlight or under
laboratory conditions. Some representative photolysis rate constants, quantum yields,
and wavelengths at which they were measured are given in Table 4.42. 131
Photolysis will not be an important fate process unless sunlight is absorbed in
the visible or near-ultraviolet wavelength ranges (above 290 nm) by either the organic
chemical or its sensitized agent.
The intensity of incident light varies over the depth of the water column and
may be modeled by

I z = I 0 e −Ke.z (4.43)

where
I z = intensity at depth z

I 0 = intensity at the water surface

Ke = extinction coefficient for light disappearance

Indirect, or sensitized, photolysis occurs when a nontarget molecule is trans-
formed directly by light, which, in turn, transmits its energy to the pollutant molecule.
Transformation then occurs in the pollutant molecule as a result of the increased
energy content.