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 300-350 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 (<1m) HNO3 rich particles. These have

a mass mixing ratio of about 10 ppbm.• Type II PSC are larger (from 10 m 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 isClO + ClO + M → (ClO)2 + M

(ClO)2 + hν → Cl + ClOOClOO + 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