Meto 637
Lesson 11
The Ozone Hole
Antarctic total ozone
Decline in mean October ozone levels over
Halley Bay
Antarctic Ozone Hole
• First discovered in October 1984 by scientists at
the British Antarctic Survey, and by the TOMS
team at NASA.
• It is obvious that a dramatic ozone loss began in
the mid 1970’s.
• The TOMS and GOME data show that the
depleted region has grown much deeper and
covers a much larger area of the earth since
then.
• Pre-hole had average ozone amounts of 300350 DU at Hadley Bay, now they were 220 DU
or less.
Antarctic Ozone Hole
• The Antarctic ozone hole is a seasonal
phenomena beginning ion September and
ending in late November to early December.
• The use of the word ‘hole’ may sound dramatic,
but between 12 and 20 km the ozone is reduced
by as much as 80%
• The smoking gun.
• The amount of the depletion could not be
accounted for with the current chemistry at that
time.
• Also, the seasonal nature of the depletion was a
mystery.
Ozone concentrations over Syowa Station
Polar meteorology
• In the winter months the poles are in perpetual
darkness. This causes extremely cold
temperatures in the stratosphere (-80oC).
• These cold temperatures favor the formation of
ice clouds known as polar stratospheric clouds
(PSC).
• It is significant that the years when the
temperature was lowest corresponded to the
years when the ozone depletion was largest.
• In addition a vortex forms around the pole as the
cold air descends. Wind speeds of 100 meters
per second or more have been observed
Polar meteorology
• The strength of the downward moving air is such
that the air inside the vortex is almost sealed off
from the air at lower latitudes.
• A giant reaction vessel has been created
• As the vortex forms in early austral wintger, and
only finally breaks down in November, there is
plenty of time even for slow reactions to be
effective.
• In reality the vortex is a leaky vessel, but the
leaks are small, about 1% per day.
Antarctic polar vortex
Comparison of the land
mass for the Northern
and Southern
hemispheres
Ozone Hole
ClO and O3 in mid-September
Partitioning of Chlorine
Changes in species concentration near
the vortex boundary
Formation and composition of polar
stratospheric clouds
Polar Stratospheric Clouds
• There are two main classes of PSC.
• Type 1 PSC are small (<1mm) HNO3 rich particles. These have
a mass mixing ratio of about 10 ppbm.
• Type II PSC are larger (from 10 mm to about 1 mm) composed
primarily of H2O-ice with minor amounts of HNO3 as
hydrates.
• They can constitute up to 1000 ppbm of the stratosphere.
• As noted before, the primary reaction that can be induced on
the surface of the PSC is
ClONO2 + HCl → Cl2 + HNO3
• The HNO3 is then retained in the PSC.
Perturbed Chemistry
• Most of the chlorine in the stratosphere is bound up in
two species, hydrogen chloride and chlorine nitrate:
Cl + CH4 → CH3 + HCl
ClO + NO2 + M → ClONO2 + M
• Normally homogeneous reactions only slowly convert
these reservoir species back to chlorine.
• However these two species can react on the surfaces of
PSC’s:
ClONO2 + HCl → Cl2 + HNO3
• The molecular chlorine is released as a gas, and the
nitric acid is retained within the PSC (as nitrates – NAT).
• The chlorine molecule can then be dissociated easily by
visible radiation.
Perturbed Chemistry
• Other surface reactions that have been shown to occur
in the laboratory are;
ClONO2 + H2O → HOCl + HNO3
N2O5 + H2O → 2HNO3
• As noted, the Cl2 dissociates as soon as light penetrates
to the poles. The atoms immediately react with ozone to
give ClO.
• However the simple catalytic cycle that was introduced
before requires atomic oxygen to complete the cycle
• The alternative cycle is
ClO + ClO + M → (ClO)2 + M
(ClO)2 + hν → Cl + ClOO
ClOO + M → Cl + O2 + M
2(Cl + O3 → ClO + O2)
2O3 + hν → 3O2
Perturbed Chemistry
• The reactions that occur on the surface of the
PSC’s, and the formation of the ClO dimer are
all favored at low temperatures.
• Hence these reactions are only important at the
low temperatures found in the polar
stratosphere.
• We noted earlier that NOX can interfere with the
chlorine catalytic cycle. This would also be true
in the polar regions, except that the NOX is
converted to nitric acid, which is sequestered in
the PSC’s which are transported to the
troposphere.
Chemistry of the ClO/PSC system
Photochemistry & dynamics in the polar
stratosphere