Geology Reference
In-Depth Information
both direct photolysis and photooxidant chemistry to determine reac-
tion rates to establish atmospheric half-lives and hence atmospheric
transport potential - an important feature with respect to the contam-
ination of remote environments by persistent chemicals (see Table 2).
For the atmospheric chemist, much more emphasis is placed on photo-
chemistry of VOCs, which play a major role in influencing air quality, by
driving the formation of ground-level O
3
40
as well as initiating particle
formation and growth.
41
In the following sections focus will be on semi-
volatile organic contaminants and their aqueous photochemistry.
6.6.1 Light Absorption and the Beer-Lambert Law
For the environmental photodegradation of organic chemicals, direct
photolysis is an important removal process for those pollutants that
absorb light at wavelengths above 290 nm; the cut-off of solar irradia-
tion at the earth's surface. Visible light includes those wavelengths that
cover the spectral range of 400-760 nm, whereas shorter wavelengths i.e.
290-400 nm are in the ultraviolet or UV region, and posses greater
energy than light at longer wavelengths. Hence light in the UV-region is
of most concern with respect to sunburn and skin cancers, and can
be further divided into different wavelength regions, including UV-B
(290-320 nm) and UV-A (320-400 nm).
The degree of absorption of UV and visible wavelengths (UV/vis) by
organic compounds is related to the molecular structure of the chemical,
in particular, by the presence of chromophores; structural moieties that
exhibit a characteristic UV/vis absorption spectrum. Aromatic rings and
conjugated double bonds are good examples of chromophores, but
others include double bonds containing, for example, nitrogen and
oxygen and other heteroatoms present within the carbon skeleton of
the molecule. Light absorption by organic compounds at the wave-
lengths of environmental relevance is usually associated with the delo-
calised p-electron system of double bonds (i.e. those electrons involved
in the multiple bonding of C
Q
C and C
Q
O groups, for example), rather
than electrons in sigma orbitals as part of single bonds, as the absorp-
tion maxima of these electrons generally lie at much shorter wavelengths
than the solar wavelength minimum of 290 nm. Nearly all environmen-
tal photochemistry involves the excitation of p or non-bonding electrons
(n) to higher-energy, antibonding p* orbitals, thereby destabilising the
molecule. Examples of n electrons are the non-bonding electron pairs
associated with oxygen atoms of carbonyl groups (CQO) or nitrogen
atoms (CQN). Chromophores consisting of a series of conjugated
carbon double bonds (C
Q
C-C
Q
C) are readily found in nature, and
