CHAPTER 3 . Photochemical Processes in the Euphotic Zone of Sea Water
87
3.3
Main Photo processes Occurring in Water and Air
Solar radiation is fundamental to all the photochemical and photobiological processes
of natural waters. The amount, spectral quality and spatial-temporal distribution of
sunlight are key variables at the surface and within the water.
Various photoreactions occur in the environment; hydrogen peroxide forms in fresh
and sea water, the free radical NO reaches detectable steady state levels in the equatorial Pacific, a photochemical rearrangement product is found in shallow-water encrusting corals (Look and FenicaI1984), and so on. On the one hand, solar radiation promotes photosynthesis processes and influences phytoplankton growth; on the other,
light activated abiotic processes, through absorption by chromophores, can occur both
by direct and indirect photoreactions.
3.3.1
Direct Photolysis
The direct photolysis of known dissolved molecules with known chromophores is the
simplest type of photochemical reaction. The degree of spectral overlap between the
electronic absorption spectrum of a chemical species and the emission spectrum of
the light source is one of the primary determinant factors of direct photolysis rates.
Compounds that strongly absorb at wavelengths greater than 320 nm have the potential to rapidly undergo direct photolysis in sunlight, especially if absorption extends
into the visible region. On the other hand, if absorption is weak or non-existent at
wavelengths greater than 300 nm, direct photolysis is usually negligible. Most xenobiotics
absorb in the ultraviolet spectral region, but there are notable exceptions, such as dyes
and certain nitroaniline herbicides, that absorb visible light. Electronic absorption spectra
can be employed to compute direct photolysis rate constant in sunlight (Zepp 1978).
The simple inorganic components of natural waters are also generally transparent
to sunlight. In the domain of particle-free water without dissolved organic chromophores, water absorbs nearly all the light. The rarity of direct photoreactions of
inorganic chromophores is underscored by the fact that nitrite and nitrate are the only
well-studied cases. Nitrogen oxides and OH radicals are photoproducts; nitrate is quite
unreactive, while at the sea surface nitrite shows a net loss of about 10% per day. A few
additional weak chromophores with some known photochemistry are iodate, uranyl
ion, hydrogen peroxide and ferrous ions (Braterman et al. 1984).
It is not possible to derive much useful information about direct photolysis from
theory, although the literature may indicate the most probable products and mechanisms. For these reasons, direct reactions are best studied experimentally in the medium of interest by monitoring starting material disappearance as well as product
appearance. Simpler experiments may fail to reveal the true quantum yield because
of unexpected products, or they may give unrealistic rates and product distributions
ascribable to medium effects. The photolysis of complex organic chromophore molecules of unknown structures is particularly important in natural waters because
simple natural chromophores are rare (Zika 1981; Zepp 1980; Zafiriou 1983). The most
important chromophores seem to be diverse organic molecules of unknown structure
and they may be partially colloidal. Harvey (1983) have extracted weakly coloured
87
3.3
Main Photo processes Occurring in Water and Air
Solar radiation is fundamental to all the photochemical and photobiological processes
of natural waters. The amount, spectral quality and spatial-temporal distribution of
sunlight are key variables at the surface and within the water.
Various photoreactions occur in the environment; hydrogen peroxide forms in fresh
and sea water, the free radical NO reaches detectable steady state levels in the equatorial Pacific, a photochemical rearrangement product is found in shallow-water encrusting corals (Look and FenicaI1984), and so on. On the one hand, solar radiation promotes photosynthesis processes and influences phytoplankton growth; on the other,
light activated abiotic processes, through absorption by chromophores, can occur both
by direct and indirect photoreactions.
3.3.1
Direct Photolysis
The direct photolysis of known dissolved molecules with known chromophores is the
simplest type of photochemical reaction. The degree of spectral overlap between the
electronic absorption spectrum of a chemical species and the emission spectrum of
the light source is one of the primary determinant factors of direct photolysis rates.
Compounds that strongly absorb at wavelengths greater than 320 nm have the potential to rapidly undergo direct photolysis in sunlight, especially if absorption extends
into the visible region. On the other hand, if absorption is weak or non-existent at
wavelengths greater than 300 nm, direct photolysis is usually negligible. Most xenobiotics
absorb in the ultraviolet spectral region, but there are notable exceptions, such as dyes
and certain nitroaniline herbicides, that absorb visible light. Electronic absorption spectra
can be employed to compute direct photolysis rate constant in sunlight (Zepp 1978).
The simple inorganic components of natural waters are also generally transparent
to sunlight. In the domain of particle-free water without dissolved organic chromophores, water absorbs nearly all the light. The rarity of direct photoreactions of
inorganic chromophores is underscored by the fact that nitrite and nitrate are the only
well-studied cases. Nitrogen oxides and OH radicals are photoproducts; nitrate is quite
unreactive, while at the sea surface nitrite shows a net loss of about 10% per day. A few
additional weak chromophores with some known photochemistry are iodate, uranyl
ion, hydrogen peroxide and ferrous ions (Braterman et al. 1984).
It is not possible to derive much useful information about direct photolysis from
theory, although the literature may indicate the most probable products and mechanisms. For these reasons, direct reactions are best studied experimentally in the medium of interest by monitoring starting material disappearance as well as product
appearance. Simpler experiments may fail to reveal the true quantum yield because
of unexpected products, or they may give unrealistic rates and product distributions
ascribable to medium effects. The photolysis of complex organic chromophore molecules of unknown structures is particularly important in natural waters because
simple natural chromophores are rare (Zika 1981; Zepp 1980; Zafiriou 1983). The most
important chromophores seem to be diverse organic molecules of unknown structure
and they may be partially colloidal. Harvey (1983) have extracted weakly coloured