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Chemists poke holes in ozone theory
News@Nature, 26 September 2007
http://www.nature.com/news/2007/070924/full/449382a.html
As the world marks 20 years since the introduction of the Montreal
Protocol to protect the ozone layer, Nature has learned of
experimental data that threaten to shatter established theories of
ozone chemistry. If the data are right, scientists will have to
rethink their understanding of how ozone holes are formed and how that
relates to climate change.
Long-lived chloride compounds from anthropogenic emissions of
chlorofluorocarbons (CFCs) are the main cause of worrying seasonal
ozone losses in both hemispheres. In 1985, researchers discovered a
hole in the ozone layer above the Antarctic, after atmospheric
chloride levels built up. The Montreal Protocol, agreed in 1987 and
ratified two years later, stopped the production and consumption of
most ozone-destroying chemicals. But many will linger on in the
atmosphere for decades to come. How and on what timescales they will
break down depend on the molecules' ultraviolet absorption spectrum
(the wavelength of light a molecule can absorb), as the energy for the
process comes from sunlight. Molecules break down and react at
different speeds according to the wavelength available and the
temperature, both of which are factored into the protocol.
So Markus Rex, an atmosphere scientist at the Alfred Wegener Institute
of Polar and Marine Research in Potsdam, Germany, did a double-take
when he saw new data for the break-down rate of a crucial molecule,
dichlorine peroxide (Cl2O2). The rate of photolysis (light-activated
splitting) of this molecule reported by chemists at NASA's Jet
Propulsion Laboratory in Pasadena, California1, was extremely low in
the wavelengths available in the stratosphere - almost an order of
magnitude lower than the currently accepted rate. "This must have
far-reaching consequences," Rex says. "If the measurements are correct
we can basically no longer say we understand how ozone holes come into
being." What effect the results have on projections of the speed or
extent of ozone depletion remains unclear.
The rapid photolysis of Cl2O2 is a key reaction in the chemical model
of ozone destruction developed 20 years ago2 (see graphic). If the
rate is substantially lower than previously thought, then it would not
be possible to create enough aggressive chlorine radicals to explain
the observed ozone losses at high latitudes, says Rex. The extent of
the discrepancy became apparent only when he incorporated the new
photolysis rate into a chemical model of ozone depletion. The result
was a shock: at least 60% of ozone destruction at the poles seems to
be due to an unknown mechanism, Rex told a meeting of stratosphere
researchers in Bremen, Germany, last week.
Other groups have yet to confirm the new photolysis rate, but the
conundrum is already causing much debate and uncertainty in the ozone
research community. "Our understanding of chloride chemistry has
really been blown apart," says John Crowley, an ozone researcher at
the Max Planck Institute of Chemistry in Mainz, Germany.
"Until recently everything looked like it fitted nicely," agrees Neil
Harris, an atmosphere scientist who heads the European Ozone Research
Coordinating Unit at the University of Cambridge, UK. "Now suddenly
it's like a plank has been pulled out of a bridge."
The measurements at the Jet Propulsion Laboratory were overseen by
Stanley Sander, a chemist who chairs a NASA panel for data evaluation.
Every couple of years, the panel recommends chemical kinetics and
photochemical data for use in atmosphere studies. Until the revised
photolysis rate has been evaluated, which won't be before the end of
next year, "modellers must make up their minds about what to do," says
Sander. One of the problems with checking the data is that the
absorption spectra of chloride compounds are technically challenging
to determine. Sander's group used a new technique to synthesize and
purify Cl2O2. To avoid impurities and exclude secondary reactions, the
team trapped the molecule at low temperatures, then slowly warmed it
up.
"Reactions in experimental chambers are one thing - the free
atmosphere is something else," says Joe Farman, one of the scientists
who first quantified the ozone hole over Antarctica3. "There's no
doubt that ozone disappears at up to 3% a day - whether or not we
completely understand the chemistry." But he adds that insufficient
control of substances such as halon 1301, used as a flame suppressor,
and HCFC22, a refrigerant, is a bigger threat to the success of the
Montreal Protocol than are models that don't match the observed
losses.
Hot topic
Meanwhile, atmosphere researchers have started to think about how to
reconcile observations of ozone depletion with the new chemical
models. Several thermal reactions, or combinations of reactions, could
fill the gap. Sander's group has started to study possible candidates
one by one - but so far without success.
Rex thinks that a chemical pathway involving a Cl2O2 isomer - a
molecule with the same atoms but a different structure - might be at
play. But even if the basic chemical model of ozone destruction is
upheld, the temperature dependency of key reactions in the process
could be very different - or even opposite - from thought. This could
have dramatic consequences for the understanding of links between
climate change and ozone loss, Rex says.
The new measurements raise "intriguing questions", but don't
compromise the Montreal Protocol as such, says John Pyle, an
atmosphere researcher at the University of Cambridge. "We're starting
to see the benefits of the protocol,
but we need to keep the pressure on." He says that he finds it
"extremely hard to believe" that an unknown mechanism accounts for the
bulk of observed ozone losses.
Nothing currently suggests that the role of CFCs must be called into
question, Rex stresses. "Overwhelming evidence still suggests that
anthropogenic emissions of CFCs and halons are the reason for the
ozone loss. But we would be on much firmer ground if we could write
down the correct chemical reactions."
Quirin Schiermeier
1. Pope, F. D., Hansen, J. C., Bayes, K. D., Friedl, R. R. & Sander,
S. P. J. Phys. Chem. A 111, 4322-4332 (2007).
2. Molina, L. T. & Molina, M. J. J. Phys. Chem. 91, 433-436 (1987).
3. Farman, J. C., Gardiner, B. G. & Shanklin, J. D. Nature 315,
207-210 (1985).
--
The object of life is not to be on the side of the majority but to
escape finding oneself in the ranks of the insane. -- Marcus Aurelius
Wherever I go it will be well with me, for it was well with me here, not
on account of the place, but of my judgments which I shall carry away
with me, for no one can deprive me of these; on the contrary, they alone
are my property, and cannot be taken away, and to possess them suffices
me wherever I am or whatever I do. -- EPICTETUS
Joseph R. Darancette
daranc@NOSPAMcharter.net
News@Nature, 26 September 2007
http://www.nature.com/news/2007/070924/full/449382a.html
As the world marks 20 years since the introduction of the Montreal
Protocol to protect the ozone layer, Nature has learned of
experimental data that threaten to shatter established theories of
ozone chemistry. If the data are right, scientists will have to
rethink their understanding of how ozone holes are formed and how that
relates to climate change.
Long-lived chloride compounds from anthropogenic emissions of
chlorofluorocarbons (CFCs) are the main cause of worrying seasonal
ozone losses in both hemispheres. In 1985, researchers discovered a
hole in the ozone layer above the Antarctic, after atmospheric
chloride levels built up. The Montreal Protocol, agreed in 1987 and
ratified two years later, stopped the production and consumption of
most ozone-destroying chemicals. But many will linger on in the
atmosphere for decades to come. How and on what timescales they will
break down depend on the molecules' ultraviolet absorption spectrum
(the wavelength of light a molecule can absorb), as the energy for the
process comes from sunlight. Molecules break down and react at
different speeds according to the wavelength available and the
temperature, both of which are factored into the protocol.
So Markus Rex, an atmosphere scientist at the Alfred Wegener Institute
of Polar and Marine Research in Potsdam, Germany, did a double-take
when he saw new data for the break-down rate of a crucial molecule,
dichlorine peroxide (Cl2O2). The rate of photolysis (light-activated
splitting) of this molecule reported by chemists at NASA's Jet
Propulsion Laboratory in Pasadena, California1, was extremely low in
the wavelengths available in the stratosphere - almost an order of
magnitude lower than the currently accepted rate. "This must have
far-reaching consequences," Rex says. "If the measurements are correct
we can basically no longer say we understand how ozone holes come into
being." What effect the results have on projections of the speed or
extent of ozone depletion remains unclear.
The rapid photolysis of Cl2O2 is a key reaction in the chemical model
of ozone destruction developed 20 years ago2 (see graphic). If the
rate is substantially lower than previously thought, then it would not
be possible to create enough aggressive chlorine radicals to explain
the observed ozone losses at high latitudes, says Rex. The extent of
the discrepancy became apparent only when he incorporated the new
photolysis rate into a chemical model of ozone depletion. The result
was a shock: at least 60% of ozone destruction at the poles seems to
be due to an unknown mechanism, Rex told a meeting of stratosphere
researchers in Bremen, Germany, last week.
Other groups have yet to confirm the new photolysis rate, but the
conundrum is already causing much debate and uncertainty in the ozone
research community. "Our understanding of chloride chemistry has
really been blown apart," says John Crowley, an ozone researcher at
the Max Planck Institute of Chemistry in Mainz, Germany.
"Until recently everything looked like it fitted nicely," agrees Neil
Harris, an atmosphere scientist who heads the European Ozone Research
Coordinating Unit at the University of Cambridge, UK. "Now suddenly
it's like a plank has been pulled out of a bridge."
The measurements at the Jet Propulsion Laboratory were overseen by
Stanley Sander, a chemist who chairs a NASA panel for data evaluation.
Every couple of years, the panel recommends chemical kinetics and
photochemical data for use in atmosphere studies. Until the revised
photolysis rate has been evaluated, which won't be before the end of
next year, "modellers must make up their minds about what to do," says
Sander. One of the problems with checking the data is that the
absorption spectra of chloride compounds are technically challenging
to determine. Sander's group used a new technique to synthesize and
purify Cl2O2. To avoid impurities and exclude secondary reactions, the
team trapped the molecule at low temperatures, then slowly warmed it
up.
"Reactions in experimental chambers are one thing - the free
atmosphere is something else," says Joe Farman, one of the scientists
who first quantified the ozone hole over Antarctica3. "There's no
doubt that ozone disappears at up to 3% a day - whether or not we
completely understand the chemistry." But he adds that insufficient
control of substances such as halon 1301, used as a flame suppressor,
and HCFC22, a refrigerant, is a bigger threat to the success of the
Montreal Protocol than are models that don't match the observed
losses.
Hot topic
Meanwhile, atmosphere researchers have started to think about how to
reconcile observations of ozone depletion with the new chemical
models. Several thermal reactions, or combinations of reactions, could
fill the gap. Sander's group has started to study possible candidates
one by one - but so far without success.
Rex thinks that a chemical pathway involving a Cl2O2 isomer - a
molecule with the same atoms but a different structure - might be at
play. But even if the basic chemical model of ozone destruction is
upheld, the temperature dependency of key reactions in the process
could be very different - or even opposite - from thought. This could
have dramatic consequences for the understanding of links between
climate change and ozone loss, Rex says.
The new measurements raise "intriguing questions", but don't
compromise the Montreal Protocol as such, says John Pyle, an
atmosphere researcher at the University of Cambridge. "We're starting
to see the benefits of the protocol,
but we need to keep the pressure on." He says that he finds it
"extremely hard to believe" that an unknown mechanism accounts for the
bulk of observed ozone losses.
Nothing currently suggests that the role of CFCs must be called into
question, Rex stresses. "Overwhelming evidence still suggests that
anthropogenic emissions of CFCs and halons are the reason for the
ozone loss. But we would be on much firmer ground if we could write
down the correct chemical reactions."
Quirin Schiermeier
1. Pope, F. D., Hansen, J. C., Bayes, K. D., Friedl, R. R. & Sander,
S. P. J. Phys. Chem. A 111, 4322-4332 (2007).
2. Molina, L. T. & Molina, M. J. J. Phys. Chem. 91, 433-436 (1987).
3. Farman, J. C., Gardiner, B. G. & Shanklin, J. D. Nature 315,
207-210 (1985).
--
The object of life is not to be on the side of the majority but to
escape finding oneself in the ranks of the insane. -- Marcus Aurelius
Wherever I go it will be well with me, for it was well with me here, not
on account of the place, but of my judgments which I shall carry away
with me, for no one can deprive me of these; on the contrary, they alone
are my property, and cannot be taken away, and to possess them suffices
me wherever I am or whatever I do. -- EPICTETUS
Joseph R. Darancette
daranc@NOSPAMcharter.net
