CHAPTER 3 . Photochemical Processes in the Euphotic Zone of Sea Water
91
III. ML n + + hv~ M(n+l) + L(3.12)
M(n-l) + O 2 ~ MV+ + .0;:
(3-13)
In all the considered pathways, the superoxide anion is a key intermediate in the
formation of H20 2• Thus, all the reactions that lead to 0;: will increase the concentration of H 2 0 2•
Studies by Zika et al. (1982) indicate that the formation of H20 2 is higher in coastal
waters with higher humic concentrations than in oligotrophic waters. The production
rates also appear to be directly related to the concentration of the humic in the water
as measured by absorbance at 300 nm. These results support the notion that abiotic
photochemical processes are responsible for the hydrogen peroxide in the surface
waters of the oceans. The biological and non-photochemical processes known to form
hydrogen peroxide are generally insignificant except in oligotrophic waters. The portion of the sunlight responsible for most of the formation of H 2 0 2 is below 400 nm.
Since steady-state levels of H20 2 are found at 10-200 nM in surface water, the decay of H20 2 must be slow. Studies also indicate that the lifetimes are much longer in
deep waters; these results indicate that particles or biological processes may control
the lifetime of the decay. The decay appears to be partly related to biological particles,
both living and dead cells. Nevertheless, further work is necessary to elucidate the decay
mechanism of H20 2 and the role that cells have on the formation and destruction of
H 2 0 2• The decomposition is not affected by light and occurs in a matter of days. Enzymes as well as particles may be important. Although abiotic decomposition processes
are small, they may be important in the open oceans.
3.3.2.2
Free Radicals
The involvement of radicals of various origins in natural waters has been suggested
repeatedly (Swallow 1969; Zafiriou 1983) but not on the basis of strong evidence. However, recent research continues to suggest a nearly ubiquitous formation of a variety
of radicals in natural waters. Many radicals live long enough to show a chemistry that
is independent of their immediate sources, and some generalizations can be made
about their chemical behaviours that are often interconnected.
Sources: Some studies (Sehgal et al. 1980) show that radical formation occur when ultrasound energy interacts with gas-satured aqueous solutions. The environmental impact of these processes has not been investigated at all, but is probably minor. A second
potential source of radicals in natural waters is supply from the atmosphere of OH, H0 2 ,
CH30 2 or precursor species such as 03' HOOH and peroxy acetyl nitrate. Although a
lack of evidence makes it impossible to estimate the fluxes of such radicals with confidence, approximate upper limits can be set by taking the expected concentrations of
these species either from atmospheric measurements or from models, and then assuming that they deposit to water surface rapidly because of their high reactivity and/or
high water solubility.
Biological emission of free radicals and their precursors is a little-understood
source. Free radicals play key metabolic roles in vivo and may be responsible for cel-
91
III. ML n + + hv~ M(n+l) + L(3.12)
M(n-l) + O 2 ~ MV+ + .0;:
(3-13)
In all the considered pathways, the superoxide anion is a key intermediate in the
formation of H20 2• Thus, all the reactions that lead to 0;: will increase the concentration of H 2 0 2•
Studies by Zika et al. (1982) indicate that the formation of H20 2 is higher in coastal
waters with higher humic concentrations than in oligotrophic waters. The production
rates also appear to be directly related to the concentration of the humic in the water
as measured by absorbance at 300 nm. These results support the notion that abiotic
photochemical processes are responsible for the hydrogen peroxide in the surface
waters of the oceans. The biological and non-photochemical processes known to form
hydrogen peroxide are generally insignificant except in oligotrophic waters. The portion of the sunlight responsible for most of the formation of H 2 0 2 is below 400 nm.
Since steady-state levels of H20 2 are found at 10-200 nM in surface water, the decay of H20 2 must be slow. Studies also indicate that the lifetimes are much longer in
deep waters; these results indicate that particles or biological processes may control
the lifetime of the decay. The decay appears to be partly related to biological particles,
both living and dead cells. Nevertheless, further work is necessary to elucidate the decay
mechanism of H20 2 and the role that cells have on the formation and destruction of
H 2 0 2• The decomposition is not affected by light and occurs in a matter of days. Enzymes as well as particles may be important. Although abiotic decomposition processes
are small, they may be important in the open oceans.
3.3.2.2
Free Radicals
The involvement of radicals of various origins in natural waters has been suggested
repeatedly (Swallow 1969; Zafiriou 1983) but not on the basis of strong evidence. However, recent research continues to suggest a nearly ubiquitous formation of a variety
of radicals in natural waters. Many radicals live long enough to show a chemistry that
is independent of their immediate sources, and some generalizations can be made
about their chemical behaviours that are often interconnected.
Sources: Some studies (Sehgal et al. 1980) show that radical formation occur when ultrasound energy interacts with gas-satured aqueous solutions. The environmental impact of these processes has not been investigated at all, but is probably minor. A second
potential source of radicals in natural waters is supply from the atmosphere of OH, H0 2 ,
CH30 2 or precursor species such as 03' HOOH and peroxy acetyl nitrate. Although a
lack of evidence makes it impossible to estimate the fluxes of such radicals with confidence, approximate upper limits can be set by taking the expected concentrations of
these species either from atmospheric measurements or from models, and then assuming that they deposit to water surface rapidly because of their high reactivity and/or
high water solubility.
Biological emission of free radicals and their precursors is a little-understood
source. Free radicals play key metabolic roles in vivo and may be responsible for cel-