21 Is the middle-latitude ozone loss due to CFC emissions?
Description
This article is from the Ozone Depletion FAQ, by Robert Parson rparson@spot.colorado.edu with numerous contributions by others.
21 Is the middle-latitude ozone loss due to CFC emissions?
That's the majority opinion, although it's not a universal opinion.
The present trends are too small and the atmospheric chemistry and
dynamics too complicated to allow a watertight case to be
made (as _has_ been made for the far larger, but localized, depletion
in the Antarctic Ozone hole; see Part III.). Other possible causes
are being investigated. To quote from the 1991 Scientific Assessment
published by the World Meteorological Organization, p. 4.1 [WMO 1991]:
"The primary cause of the Antarctic ozone hole is firmly
established to be halogen chemistry....There is not a full
accounting of the observed downward trend in _global ozone_.
Plausible mechanisms include heterogeneous chemistry on sulfate
aerosols [which convert reservoir chlorine to active chlorine -
R.P.] and the transport of chemically perturbed polar air to middle
latitudes. Although other mechanisms cannot be ruled out, those
involving the catalytic destruction of ozone by chlorine and
bromine appear to be largely responsible for the ozone loss and
_are the only ones for which direct evidence exists_."
(emphases mine - RP)
The Executive Summary of the subsequent 1994 scientific assessment
(available on the Web at http://www.al.noaa.gov/WWWHD/pubdocs/WMOUNEP94.html)
states:
"Direct in-situ meaurements of radical species in the lower
stratosphere, coupled with model calculations, have quantitatively shown
that the in-situ photochemical loss of ozone due to (largely natural)
reactive nitrogen (NOx) compounds is smaller than that predicted from
gas-phase chemistry, while that due to (largely natural) HOx compounds
and (largely anthropogenic) chlorine and bromine compounds is larger
than that predicted by gas-phase chemistry. This confirms the key role
of chemical reactions on sulfate aerosols in controlling the chemical
balance of the lower stratosphere. These and other recent scientific
findings strengthen the conclusion of the previous assessment that the
weight of scientific evidence suggests that the observed middle- and
high-latitude ozone losses are largely due to anthropogenic chlorine and
bromine compounds." [WMO 1994]
For a contrasting view, see [Henriksen and Roldugin].
A legal analogy might be useful here - the connection between
_antarctic_ ozone depletion and CFC emissions has been proved beyond
a reasonable doubt, while at _middle latitudes_ there is only
probable cause for such a connection.
One must remember that there is a natural 10-20 year time lag
between CFC emissions and ozone depletion. Ozone depletion today is
(probably) due to CFC emissions in the 1970's. Present
controls on CFC emissions are designed to avoid possibly large
amounts of ozone depletion 30 years from now, not to repair the
depletion that has taken place up to now. [Prather et al. 1996]
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