Lecture #4 - Stratospheric Ozone Depletion

Stratospheric Ozone Depletion

Ozone is a pale blue gas made up of oxygen and it has a strong odor. Ninety percent of all atmospheric ozone is found in the stratosphere, the layer of the atmosphere from 15 to 50 km above the earth. Most of the upper level ozone is concentrated about 25 km from the planets surface in the "ozone layer".

Ozone is extremely important to all forms of life on Earth as it is the only gas that absorbs lethal ultraviolet radiation from the Sun.

1995 Nobel Prize in Chemistry

Chlorine levels are expected to peak over the next few years and then slowly decline over several decades.

We have gained an increaed understanding of stratospheric ozone through a combination of field observations, laboratory experiments, and computer modeling.

Formation and Destruction of Ozone in the Stratosphere

Ozone Depletion Process
CFCs and halons break down when exposed to intense ultraviolet light in the upper atmosphere.

Antarctic and Arctic Ozone Holes

Each year since the late 1970s, much of the stratospheric ozone above Antarctica has disappeared during September, creating what is popularly called the ozone hole.
The hole is caused by chlorine and bromine pollution in the atmosphere. It normally peaks in mid-October and usually closes in November, when ozone-rich air from the north moves over Antarctica.
In 1993, ozone dipped to 91 Dobson units (DU), the record low. In 1995, it was 98 DU.
Size of the Antarctic Ozone Hole
The stratosphere over Antarctica in winter is isolated by a circle of strong wind called the polar vortex, so ozone concentrations there are normally at a stable minimum before the ozone hole begins to form.
The dramatic ozone depletion occurs as the Sun rises in the early spring.
The Arctic polar vortex is much weaker and the ozone concentrations in the north polar regions are constantly changing. The concentrations increase in late winter and early spring, bolstered by waves of ozone-rich air from the tropics.
The decline of ozone in a given air parcel is proportional to the time it spends in sunlight and this is consistent with photochemical ozone depletion catalyzed by hologens.
The chemical ozone loss coincides with, and slightly lags, the occurrence of temperatures low enough (below 195 K) for polar stratospheric clouds.
Polar stratospheric clouds (PSCs) provide surfaces for reactions that convert chlorine compounds from relativly inert forms to reactive species that destroy ozone in sunlight.
It is the presence of PSCs that makes ozone in the polar regions so much more vulnerable than it is in the temperate regions.
The total amount of chlorine and bromine compounds is roughly uniform throughout the stratosphere.
In most seasons and regions, the halogen atoms are tied up in so-called reservoir molecules that do not react with ozone - hydrogen chloride and chloride nitrate (ClONO2), for examlpe.
PSCs condense in the frigid cold of the stratospheric polar vortices and provide heterogeneous surfaces for reactions that convert the reservoir species to more reactive ones.
The most important reaction is between the two chlorine reserviors: ClONO2 + HCL -> Cl2 + HNO3
The molecular chlorine produced is in the gas phase and is photolyzed by even weak sunlight to give chlorine radicals - active chlorine - that can catalyze ozone destruction.
Equally important is the fate of the nitric acid (HNO3). It remains within the PSCs, effectively sequestering the nitrogen family of compounds that would otherwise react with the active chlorine to reform chlorine nitrate.
This process, called denitrification, allows the photochemical reactions that destroy ozone to run effectively for a long time without termination.
Changes in ozone could result from the constant motion of the atmosphere tranporting air with different ozone concentrations from one place to another. It is difficult to tell if the variations are chemical or dynamical. One technique is to measure the amount of ozone in a parcel of air, track the air mass as it travels around the polar vortex, and the measure its ozone content some days later.
TOMS Images of North America and the Arctic 1979-1994

Ozone Myths

Montreal Protocol on Substances That Deplete the Ozone Layer

The nations of the world have joined in the Montreal Protocol on Substances That Deplete the Ozone Layer to phase out the production of chlorofluorocarbons (CFCs) and halons, the major sources of ozone-depleting halogen compounds in the stratosphere.
The Montreal Protocol ban the production of the ozone-depleting pesticide methyl bromide by 2010.