425
10
Processes in the Oceans
10.1 Photochemical Processes in Seawater
There has been increasing interest in the photochemical processes that occur in the surface
waters of the oceans and other natural waters. Much of this work has been stimulated by
the pioneering work of Zika and coworkers (at the University of Miami, 1981, 1982, 1985),
Zafiriou (at Woods Hole Oceanographic Institution [WHOI], 1983, 1990), and Zepp (at the
Environmental Protection Agency [EPA]), who applied state- of- the- art photochemistry to
ocean systems. This chapter briefly discusses the principles of photochemistry and the
application of these principles to some ocean systems.
10.1.1 Principles
A photochemical process is a chemical reaction that takes place only under the influence
of light. Photochemistry is concerned with the interaction of electromagnetic radiation
and matter; it is at the crossroads of physics and chemistry. The light for photochemical
processes includes wavelengths (λ) in the visible (400- to 700-nm), near- ultraviolet (UV)
(100- to 400-nm), and near- infrared (700- to 1000-nm) ranges. This light is considered to be
made up of individual photons of energy E = hν (where h is Planck’s constant, 6.63 × 10 –34 J;
ν is the frequency of light, ν = c/ λ; and c is the velocity of light).
Only the light that is absorbed can cause a photochemical effect (Grotthus–Draper law).
Although this may seem obvious, it has been known as the first law of photochemistry for
a long time. The primary photochemical reaction can be written in the form
M + hν → M*
(10.1)
where M is a molecule that can absorb light (a chromophore), hν is the photon of light, and
M* is the molecule in an “excited state.” The extra energy (hν) and the properties of M*
result in the photochemical process. The second law of photochemistry (Stark–Einstein
law) states that an atom or molecule undergoing a photochemical process absorbs only a
single photon.
Reactants + n hν → Products
(10.2)
It is necessary to consider the yield of a photochemical reaction in terms of the amount
of light absorbed by the sample. For long irradiations, it is often possible to obtain nearly
quantitative yields like common chemical reactions (which can be characterized in terms
10
Processes in the Oceans
10.1 Photochemical Processes in Seawater
There has been increasing interest in the photochemical processes that occur in the surface
waters of the oceans and other natural waters. Much of this work has been stimulated by
the pioneering work of Zika and coworkers (at the University of Miami, 1981, 1982, 1985),
Zafiriou (at Woods Hole Oceanographic Institution [WHOI], 1983, 1990), and Zepp (at the
Environmental Protection Agency [EPA]), who applied state- of- the- art photochemistry to
ocean systems. This chapter briefly discusses the principles of photochemistry and the
application of these principles to some ocean systems.
10.1.1 Principles
A photochemical process is a chemical reaction that takes place only under the influence
of light. Photochemistry is concerned with the interaction of electromagnetic radiation
and matter; it is at the crossroads of physics and chemistry. The light for photochemical
processes includes wavelengths (λ) in the visible (400- to 700-nm), near- ultraviolet (UV)
(100- to 400-nm), and near- infrared (700- to 1000-nm) ranges. This light is considered to be
made up of individual photons of energy E = hν (where h is Planck’s constant, 6.63 × 10 –34 J;
ν is the frequency of light, ν = c/ λ; and c is the velocity of light).
Only the light that is absorbed can cause a photochemical effect (Grotthus–Draper law).
Although this may seem obvious, it has been known as the first law of photochemistry for
a long time. The primary photochemical reaction can be written in the form
M + hν → M*
(10.1)
where M is a molecule that can absorb light (a chromophore), hν is the photon of light, and
M* is the molecule in an “excited state.” The extra energy (hν) and the properties of M*
result in the photochemical process. The second law of photochemistry (Stark–Einstein
law) states that an atom or molecule undergoing a photochemical process absorbs only a
single photon.
Reactants + n hν → Products
(10.2)
It is necessary to consider the yield of a photochemical reaction in terms of the amount
of light absorbed by the sample. For long irradiations, it is often possible to obtain nearly
quantitative yields like common chemical reactions (which can be characterized in terms