McKay, Donald. "Front matter" Multimedia Environmental Models ...

to Zepp and Cline (1977) for the original work in this area; to Leifer (1988) for an
update; and to Calvert and Pitts (1966), Mill (2000), and Schwarzenbach et al.
(1993) for more details and examples of photochemical reactions and computer
programs.
For our purposes, it is sufficient to appreciate that, knowing the absorbance
properties of the molecule, the quantum yield and the local insolation conditions, it
is possible to calculate a rate constant and a half-life for direct photolysis.
Relatively simple experiments can be conducted in which the chemical is dissolved
in distilled or natural water in a suitable container and exposed to natural
sunlight or to artificial light for a period of time, and the concentration decay is
monitored. Test methods have been described by Svenson and Bjarndahl (1988),
Lemaire et al. (1982), and Dulin and Mill (1982).
The issue is complicated by the presence of photosensitizing molecules or
substances. These substances absorb light then pass on the energy to the chemical
of interest, resulting in subsequent chemical reaction. It is therefore not necessary
for the chemical to absorb the photon directly. It can receive it second hand from a
photosensitizer. This is a troublesome complication, because it raises the possibility
that chemicals may be subject to photolysis due to the unexpected presence of a
photosensitizer. Of particular interest are the naturally occuring organic matter photosensitizers
that are present in water and give it its characteristic brown color,
especially in areas in which there is peat and decaying vegetation.
6.6.4 Atmospheric Oxidation Reactions
A chemical present in the atmosphere may react with oxygen, an activated form
of oxygen such as singlet oxygen, ozone, hydrogen peroxide, or with various radicals,
notably OH radicals. Fortunately, we live in a world with an abundance of oxygen,
and it is not surprising that a suite of oxygen compounds exists that are eager to
oxidize organic chemicals. The rates of these reactions can be estimated by conducting
conventional chemical kinetic experiments in which the substance is contacted
with known concentrations of the oxidant, the decay of chemical is followed,
and a kinetic law and rate constant established.
The most important oxidative process is the reaction of hydroxyl radicals with
chemical species in the atmosphere. The concentration of sunlight-induced hydroxyl
radicals is exceedingly small, averaging only about 1 million molecules per cubic
centimetre. Peak concentrations approach 8 million per cm 3 in urban areas. Concentrations
in rural or remote areas are much lower. They are extremely reactive and
are responsible for the reaction of many organic chemicals in the environment that
would otherwise be persistent.
Ozone is produced by UV radiation in the stratosphere and by certain hightemperature
and photolytic processes in the troposphere. The average mixing ratio,
i.e., the ratio of ozone to non-ozone molecules, is in the range of 10 to 40 ¥ 10 –9 .
Oxides of nitrogen produced at high temperature include NO, NO 2, and the
reactive NO 3 radical. The latter has an average concentration of about 500 million
molecules per cm 3 and peaks in concentration at night.
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