84
E. Pelizzetti . P. Calza
ponents of aquatic environments such as suspended sediments, aquatic humus
and algae also have an important effect on photoreactions in water (Zepp and Wolfe
1987; Pelizzetti et al. 1990). In some cases, xenobiotics that do not react on exposure to
sunlight in distilled water have been shown to rapidly react in natural water samples.
In the present chapter, after an overview on the principal light-induced processes
occurring in the water, the attention will be focused on the role of two important constituents of sea water: iron and chloride. Iron is an essential metal, involved in the main
redox reactions and electron transport, while chloride is an abundant constituent of
sea water, and both may strongly influence the photoinduced processes.
3.2
General Framework
The spectral irradiance at depth depends on three factors: atmosphere-transmitted
irradiance, which is incident on the water surface, transmission through the air-water
interface and optical properties of water and dissolved and suspended materials, which
determine the spectral attenuation properties of the water. The role of these three factors will be discussed below.
3.2.1
Solar Flux
The flux of solar energy that enters in the sea and is converted into other forms of
energy by interaction with molecules is enormous. The global average (as photons) is
roughly equal to the input of water in the form of rain (as molecules). These photons
have energies (14.0-33.5 x 10 3 cm -I) in excess of the activation energies for most chemical reactions (Lee et al. 1977).
Values for intensity of the radiation with wavelengths of 390-800 nm can be obtained
by calculations through data available for extraterrestrial solar irradiance, atmospheric
transmissivity in respect to scattering (Leighton 1961) and absorption coefficient for ozone.
3.2.2
Light Attenuation
The photoprocesses occurring in the euphotic zone may be schematically represented
as shown in Fig. 3-1.
Solar light can be reflected at the sea surface or can penetrate in sea water; in the
second case, absorption and scattering can occur. Absorption and scattering of radiation cause an attenuation of the intensity of solar radiation, and the extent of attenuation may vary from one water body to another. Attenuation coefficients are usually
wavelength-dependent, and the attenuation oflight intensity generally increases with
decreasing wavelength in the visible and ultraviolet region.
The incident light is greatly absorbed by various chromophores, such as dissolved
organic matter, living and dead inorganic and organic particulate matter, and water
itself; however, the water itself is an important absorber only under highly transparent conditions. Inland surface waters contain dissolved organic matter, which prevents
the sunlight from penetrating to large depths.
E. Pelizzetti . P. Calza
ponents of aquatic environments such as suspended sediments, aquatic humus
and algae also have an important effect on photoreactions in water (Zepp and Wolfe
1987; Pelizzetti et al. 1990). In some cases, xenobiotics that do not react on exposure to
sunlight in distilled water have been shown to rapidly react in natural water samples.
In the present chapter, after an overview on the principal light-induced processes
occurring in the water, the attention will be focused on the role of two important constituents of sea water: iron and chloride. Iron is an essential metal, involved in the main
redox reactions and electron transport, while chloride is an abundant constituent of
sea water, and both may strongly influence the photoinduced processes.
3.2
General Framework
The spectral irradiance at depth depends on three factors: atmosphere-transmitted
irradiance, which is incident on the water surface, transmission through the air-water
interface and optical properties of water and dissolved and suspended materials, which
determine the spectral attenuation properties of the water. The role of these three factors will be discussed below.
3.2.1
Solar Flux
The flux of solar energy that enters in the sea and is converted into other forms of
energy by interaction with molecules is enormous. The global average (as photons) is
roughly equal to the input of water in the form of rain (as molecules). These photons
have energies (14.0-33.5 x 10 3 cm -I) in excess of the activation energies for most chemical reactions (Lee et al. 1977).
Values for intensity of the radiation with wavelengths of 390-800 nm can be obtained
by calculations through data available for extraterrestrial solar irradiance, atmospheric
transmissivity in respect to scattering (Leighton 1961) and absorption coefficient for ozone.
3.2.2
Light Attenuation
The photoprocesses occurring in the euphotic zone may be schematically represented
as shown in Fig. 3-1.
Solar light can be reflected at the sea surface or can penetrate in sea water; in the
second case, absorption and scattering can occur. Absorption and scattering of radiation cause an attenuation of the intensity of solar radiation, and the extent of attenuation may vary from one water body to another. Attenuation coefficients are usually
wavelength-dependent, and the attenuation oflight intensity generally increases with
decreasing wavelength in the visible and ultraviolet region.
The incident light is greatly absorbed by various chromophores, such as dissolved
organic matter, living and dead inorganic and organic particulate matter, and water
itself; however, the water itself is an important absorber only under highly transparent conditions. Inland surface waters contain dissolved organic matter, which prevents
the sunlight from penetrating to large depths.