Biology 381, Pollution Biology

Department of Biological Sciences,
Faculty of Science,
University of Alberta,
Edmonton, Alberta.

9. Ozone: The ups and downs of a simple molecule.

9.1 Required reading and other announcements.

The following "landmark papers" are optional reading. They provide an interesting view of the history of the subject.

Molina, M.J. and Rowland, F.S., 1974. Stratospheric sink for chlorofluoromethanes: chlorine atomic-atalysed destruction of ozone. Nature 249: 810-812.

Farman, J.C., Gardiner, B.G., and Shanklin, J.D., 1985. Large losses of total ozone in Antarctica reveal seasonal ClO_x/NO_x interaction. Nature 315: 207-210.

Gleason, J.F., et al., 1993. Record low global ozone in 1992. Science 260: 523-526.

9.2 Background on ozone (O₃).

Ozone formation - The Chapman Reactions.

3O₂ + hv ^{_______________} 2O₂ + 2O^{. _______________} 2O₃

Ozone destruction - the Chapman Reactions.

2O₃ + hv ^{_______________} O^. + O₂ + O₃ ^{_______________} 3O₂

These reactions occur both in the stratosphere and the troposphere, but UV penetration to the troposphere (and to the lower stratosphere) is dramatically reduced by absorption of UV radiation by stratospheric ozone.

The dynamic equilibrium, a small but constant amount of O₃ in the stratosphere.

9.3 The distribution of ozone in the atmosphere.

Most ozone is concentrated in the lower 50 km of the atmosphere (troposphere and stratosphere).

In the stratosphere, maximum ozone concentrations occur at about 20 to 25 kilometers - the ozone layer.

One Dobson Unit (DU) is the amount of O₃ required to produce 0.01 mm thickness at standard atmospheric temperature and pressure.

Concentrations of ozone are normally greatest near the poles, but ozone molecules drift and swirl around the stratosphere in changing concentrations.

In this figure, we have captured a ozone depletion event over Antarctica, and to a lesser extent over the Arctic.

Depending on where ozone resides, it can protect or harm life on Earth.

In the stratosphere, ozone acts as a shield to protect Earth's surface from the sun's harmful ultraviolet radiation.

In contrast, ozone in the troposphere is a harmful pollutant.

The amounts of ozone in both the stratosphere and the troposphere depend on a balance between processes that create ozone and those that destroy it.

This balance is being altered by man's activities.

9.4 Ozone in the troposphere.

The term "smog" originally referred to heavy layers of SMoke and fOG over London, but use of the term has evolved.

Today's photochemical smog is characterized by high concentrations of ozone (O₃) and peroxyacetyl nitrates (PANs).

The concentration of ground level ozone is usually used as an indicator of the overall severity of smog.

Ozone in the troposphere, the so-called bad ozone, typically accounts for about 10% of total column ozone.

Tropospheric ozone occurs naturally as a result of a limited intrusion from the stratosphere.

However, ozone levels over many populated regions of the Northern Hemisphere have increased by more than 50%.

These increases appear to reflect photochemical production from anthropogenic precursors.

Generally, O₃ is not emitted directly into the air.

It is formed as a secondary product of combustion from nitrogen oxides (NO_x) and volatile organic compounds (hydrocarbons), components of automobile exhaust.

Reactions accounting for ozone formation and accumulation.

• In the absence of volatile organic compounds (VOCs or hydrocarbons, H_xC_x).

  NO₂ + hv ^{_______________} NO + O^.
  O^. + O₂ ^{_______________} O₃
  O₃ + NO ^{_______________} NO₂ + O₂
  ^{___________________________________}
  No net change...

• In the presence of volatile organic compounds.

  NO₂ + hv ^{_______________} NO + O^.
  O^. + O₂ ^{_______________} O₃
  NO + H_xC_x ^{_______________} PANs
  ^{_______________________________________________}
  NO₂ + O₂ + H_xC_x + hv ^{_______________} O₃ + PANs

Insert figure

Figure 13-6, Nebel (1987)

• Nitrogen dioxide (NO₂) absorbs energy from sunlight and is converted to nitrogen oxide (NO) and a free oxygen atom (O^.).

• Free atomic oxygen (O^.) can react with O₂ to form ozone.

• In the presence of VOCs, nitrogen oxide formed as a result of photo-oxidation, reacts with VOCs to form peroxyacetyl nitrates (PANs).

• Because nitrogen oxide is tied up in the reaction with VOCs, ozone accumulates in the atmosphere.

Ground-level ozone forms readily in the atmosphere in hot, sunny weather.

Insert figure.

Figure 13-2, Nebel (1987).

Areas downwind of cities can also be subject to high ozone exposure, as winds carry VOCs and NOx from their original sources.

What evidence do we have that these downwind problems are anthropogenic?

In remote areas, ozone concentrations are correlated with carbon monoxide, an unreactive "tracer" of man-made pollution.

9.5 Health and environmental effects of ground-level ozone.

As an extremely reactive molecule, ozone has a powerful effect on biological systems.

• It attacks long chain fatty acids which are important parts of biological membranes.

• It attacks proteins, often resulting in changes in both structure and function.

Ozone can cause chest pain, coughing, nausea, throat irritation, eye irritation and congestion. It may also worsen bronchitis, heart disease, emphysema, and asthma and reduce resistance to colds and other infections.

The American Lung Association estimates that the health costs associated with air pollution in the U.S. are $40-50 billion a year.

Ozone interferes with the production and storage of starches within plants, reducing their growth rates.

By weakening plants, ozone can make plants more susceptible to disease, insect attacks, and harsh weather (cold).

Agricultural crop losses in the US has been estimated at $2-3 billion dollars per year.

9.6 Controlling the release of tropospheric ozone precursors.

Many governments have established maximum ozone standards for the atmosphere.

What are some of the weaknesses of using air quality standards for controlling a pollutant such as ozone?

What other control options might be available?

The Canadian government has made use of alternative strategies for control of ozone pollution.

Cars are getting cleaner, but people are driving more, thus offsetting progress in pollution control.

9.7 Ozone in the stratosphere.

A review of Chapman Reactions (ozone formation and destruction).

• 3O₂ + hv ^{_______________} 2O₂ + 2O^{. _______________} 2O₃

• 2O₃ + hv ^{_______________} O^. + O₂ + O₃^{_______________} 3O₂

Other reactions are affecting stratospheric ozone as well.

Ozone is a highly unstable molecule which can donate its "extra" oxygen molecule to nitrogen, hydrogen, and chlorine found in natural compounds.

Because the concentrations of N, H and Cl change as part of regular natural cycles (changing seasons, sun cycles and winds), concentrations of ozone change as well.

Human activities are disrupting the balance of ozone formation and ozone destruction.

9.8 Anthropogenic ozone depletion.

The discovery of the ozone depletion phenomenon is worthy of a Nobel Prize.

Nitrous oxide and compounds containing chlorine or bromine have the potential to catalyze ozone destruction.

Ozone depleting chemicals include the chlorofluorocarbons, hydrochlorofluorocarbons, methyl chloroform, carbon tetrachloride, halons, and methyl bromide.

These compounds have atmospheric lifetimes long enough to allow them to be transported by winds into the stratosphere.

Nitrous oxides were the first ozone-depleting catalysts to be characterized.

NO + O₃ ^{_______________} NO₂ + O₂
NO₂ + O^{. _______________} NO + O₂
O₃ + hv ^{_______________} O₂ + O^{.
___________________________________}
Net result: 2O₃ ^{_______________} 3O₂

Quantitatively, the most important ozone depleting compounds are CFCs, stable compounds made up of chlorine, fluorine and carbon.

• CFCs do not react easily with other chemicals in the lower atmosphere.

• In the troposphere, CFCs are protected from ultraviolet radiation by the ozone layer.

• Once in the stratosphere, CFC molecules are bombarded by ultraviolet energy (UV-C) which causes them to break up and release chlorine atoms.

• Cl^- + O₃ ^{_______________} ClO + O₂

  

• Chlorine acts as a catalyst for ozone destruction.

• ClO + O^{. _______________} Cl^- + O₂

  

• Fortunately, chlorine atoms do not remain in the stratosphere forever.

• Bromine (halons and methyl bromide) also acts as a catalyst for ozone depletion, but is 40 times more effective than chlorine.

9.9 The ozone hole.

Catalytic ozone depletion does not proceed at a constant rate with time.

Reactions occur on the surface of ice particles that accelerate ozone destruction catalyzed by stratospheric chlorine.

Thus, periods of extreme cold can lead to ozone depletion "events" - the ozone hole.

These periods of extreme cold are normally associated with the formation of a tight polar vortex.

The ozone hole has now been detected on an annual basis and measures in the range of 8.2 million square miles, an area bigger than United States and Canada combined.

Total O₃ concentrations over Halley Bay, Antarctica.

Animation of the 1995 ozone hole.

Ozone depletion is generally worse in the Antarctic, although evidence for an Arctic ozone hole is emerging.

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9.10 Declines in global ozone levels.

There are now significant decreases in ozone concentrations during the spring and summer in both the northern and southern hemispheres at both high and middle latitudes.

Ozone levels have declined by 5 (summer) to 10% (winter) in the middle latitudes since 1979.

Further details on global ozone levels

Total ozone over Arosa, Switzerland.

Ozone depletion over North America.

Worldwide ozone losses in 1992 and 1993 were the largest ever recorded.

The eruption of Mt. Pinatubo in the Philippines was not likely a major factor.

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9.11 Predicting the impact of stratospheric ozone depletion.

The primary concern related to ozone depletion is the expectation that more potentially damaging UV light will reach the earth's surface as the ozone layer deteriorates.

The sun emits a wide variety of electromagnetic radiation, including light in the UV-A (320-400 nm), UV-B (290-320 nm), and UV-C (<280 NM) portions of the spectra.

Ozone strongly absorbs light in the UV-B and UV-C portion of the spectra.

UV spectrum at Lauder, N.Z. and the action spectra for damage to human skin (erythema).

UV spectrum at Lauder, N.Z. weighted by the action spectra for damage to human skin (erythema).

For each 1% drop in ozone levels, about 1-2% more UVB will reach the Earth's surface.

UV radiation at Halley Bay, Antarctica.

The impact of UV-B radiation on biological systems stems from the high energy of these light rays.

Increased UV-B radiation leads to increased incidence of skin cancer (melanoma and non-melanoma).

Increased UV-B also leads to cataracts and other eye damage, and possible inhibition of the immune system in humans and other animals.

Many crop plants and forest species are adversely affected by ultraviolet light.

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9.12 Controlling the release of ozone depleting chemicals.

How do we devise an environmental policy, in the face of incomplete information, for a pollutant which does not exert an effect for 5-20 years, and yet has the potential for extreme consequences?

What factors must we take into account in making such a policy decision?

The Montreal Protocol (1987) set strict limits on the production and use of CFCs.

Furthermore, the protocol provided for an ongoing process, so that the phase-outs could be accelerated

In 1990 (London), treaty's signatories adopted a deadline for phasing out the most damaging chemicals by the year 2000.

The chemicals that are currently replacing CFCs are either hydrochlorofluorocarbons (HCFCs ) or hydrofluorocarbons (HFCs).

Changes in stratospheric chlorine given three future scenarios:

a) without restrictions on release of ozone-depleting chemicals,

b) according to the original Montreal Protocol of 1987, and

c) according to the revised Montreal Protocol

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9.13 Is ozone depletion related to global warming?

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9.14 Additional world wide web information.

The University of Cambridge, Centre for Atmospheric Studies Tour- A multimedia extravaganza on ozone depletion. Slow but impressive.

The U.S. Environmental Protection Agency's web page on the science of ozone depletion. - A wealth of useful data and information.

The U.S. Environmental Protection Agency's ozone depletion glossary. - Do you need any definitions?

WMO/UNEP Scientific Assessment of Ozone Depletion: 1994. - An executive summary to the bible of ozone depletion.

NASA's Facts about ozone- A good review. You'll recognize some of the figures from this site.

Global Resources frequently asked questions about ozone depletion.- A few tasty morsels.

Global 2000 view of a "critical issue." - A few more tasty morsels.

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Last updated, January 29, 1997.

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