19 And how about about Iodine?
Description
This article is from the Ozone Depletion: Stratospheric Chlorine and Bromine FAQ, by Robert Parson rparson@spot.colorado.edu with numerous contributions by others.
19 And how about about Iodine?
Since Chlorine and Bromine radicals both enter into ozone-destroying
catalytic cycles, it comes as no surprise that Iodine can do so as well.
One possible mechanism is:
ClO + IO -> Cl + I + O2
Cl + O3 -> ClO + O2
I + O3 -> IO + O2
_______________________
Net: 2 O3 -> 3 O2
Note that this is precisely analogous to the Bromine/Chlorine cycle
given in section 3.2; the Iodine acts in concert with Chlorine. There
are also cycles in which Iodine and Bromine, and Iodine and OH, act
together.
At present it is not known whether there is enough Iodine in the
stratosphere to make these reactions important for the overall ozone
balance. The principle source of atmospheric iodine is methyl iodide,
produced in large quantities by marine biota. Methyl iodide, like methyl
chloride and bromide, is insoluble in water and is thus not "frozen out"
at the tropopause; however it has a much shorter atmospheric lifetime
so only a small fraction survives long enough to reach the stratosphere.
It has recently been suggested [Solomon et al. 1994a,b] that this small
fraction may nevertheless be large enough to influence ozone depletion
in the lowest part of the stratosphere. (Current models using only
chlorine and bromine chemistry predict significantly less ozone loss in
these regions than has been observed.) More measurements will be needed
to resolve this issue.
Anthropogenic sources of stratospheric iodine are negligible.
Trifluoromethyliodide, CF3I, has been suggested as a substitute for
halons, since unlike halons, CF3I has a short atmospheric lifetime.
[Solomon et al. 1994b] estimate its ozone depletion potential (ODP) to
be less than 0.008 and probably less than 0.0001; CF3Br, in contrast,
has an ODP of 7.8. Iodine may be accelerating the rate at which
(mostly) anthropogenic chlorine and (partly) anthropogenic bromine
destroy ozone, but iodine in itself is not an anthropogenic influence.
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