Stratospheric Ozone: Production, Destruction, and Trends.

Take away ideas and understandings:

1. Understand how the atmosphere interacts with incoming solar radiation
2. Why is the sky blue and why are sunsets red?
3. Understand the absorption of UV radiation by stratospheric O₂ and O₃.
4. Understand the processes of natural ozone production and destruction
5. Understand the life cycle of CFC's in the stratosphere, and their role in ozone destruction.
6. Understand why the Antarctic ozone hole is larger than the arctic, and why there is not "hole" over the tropics.
7. How large is the ozone hole? How much lower are the ozone concentrations today?
8. What are the effects on UV radiation at the ground?

1.0 Effects of the atmosphere on incoming solar radiation?

Radiant energy can interact with matter in three extreme modes. Most often its behavior is a combination of two or more of these modes, but for the sake of explanation we will look at them one at a time. If matter does not interact with the incident radiation i.e. there is no change in the matter because of the radiant energy that strikes it and it does not let the energy pass through it i.e. it is opaque to the radiant energy, then it reflects the energy. Reflection only changes the direction of the beam of radiant energy not its wavelength or amplitude. If matter allows radiant energy to pass through it unchanged, the matter is described as transparent to the incident radiation. Again, as with reflection, there is no change in any of the properties of the radiant energy. On the other hand, if there is some interaction between the incident radiation and the matter e.g. some energy is transferred from the radiant beam to the matter resulting in an increase in molecular energy of the matter, then we describe this transfer of energy from the radiant beam to the matter as absorption.

In terms of the incoming solar radiation budget:

Solar energy...	percentage
reflected	35.0
absorbed by atmosphere	17.5
scattered to the Earth from blue sky	10.5
scattered to the Earth from clouds	14.5
radiation going directly to Earth's surface	22.5
	100.0

Why is the sky blue? Why are sunsets red?

This lecture on stratospheric ozone addresses the absorption(and transmission) of high energy ultraviolet solar radiation by (decreasing) concentrations of ozone molecules in the stratosphere.

1.1 Effects of the atmosphere on incoming solar radiation

The absorption is the process by which radiant energy is transferred to matter. If the matter is a gas, radiation can effect it in a number of ways. The ways it can absorb energy depends on the size and complexity of the gas molecule. The gas molecule can be rotated and a variety of vibratory modes can be excited depending on the nature of the molecule. If the energy is strong enouigh the molecule can be broken apart.

Each mode of energy absorption occurs at a specific narrow band of the solar spectrum. Gases, therefore, are not like black bodies that absorb equally and completely at all wavelengths. Rather, they absorb only at specific, often narrow ranges of wavelengths. Diatomic molecules such as nitrogen and oxygen (most of our atmosphere) can absorb energy by increasing the vibration of the bond between the two atoms. At lower energies the molecules can rotate due to UV absorption, at higher energies the molecules can break up, or photolyze. If the energy absorbed is great enough it may break the bond resulting in two free wheeling oxygen or nitrogen atoms traveling at high speeds.

In the case of atmospheric oxygen (O₂), the precursor to ozone formation, UV radiation (~0.1-0.2µm wavength) is sufficiently energetic to break up, or photodissociate molecular oxygen into individual O-atoms. This is a first step toward natural ozone (O₃) production.

O₂ + ultraviolet light = O + O

1. The shortest wavelengths of the solar spectrum, the ultraviolet < .12 microns are absorbed by oxygen and nitrogen in the upper atmosphere above 100 kilometers.
   
2. UV radiation between between .12 and .18 microns are absorbed by oxygen above 50 kilometers.
   
3. UV radiation between .18 and .34 microns are absorbed between 50 kilometers and 10 kilometers by ozone.
   
4. The atmosphere is nearly transparent to visible radiation between .34 and .7 microns.
   
5. The infrared part of the solar spectrum is partially absorbed by water vapor in the troposphere below 10 kilometers.

1.2 Where does absorption of UV radiation occur?

This occurs in the uppermost regions of the atmosphere, the thermosphere and stratosphere (see below). Here the most energetic (shortest wavelength) part of the solar spectrum is involved in this process. The highest energy (highest frequency) UV radiation is absorbed by atmospheric O₂ and N₂ in the thermosphere. Nitrogen absorbes only in the extreme ultraviolet of which there is very little in the sun's radiation.

Oxygen absorbs more strongly than nitrogen and over a wider range of wavelenths in the ultraviolet. Oxygen molecules are therefore broken into oxygen atoms in the highest regions of the atmosphere. By an altitude of about 100 kilometers much of the radiation that is energetic enough to do this breaking of molecular bonds is used up and this process diminishes. Hence there is heating of the uppermost atmosphere (fast moving atoms of nitrogen and oxygen) and as the altitude decreases to about one hundred kilometers the atmosphere cools. For some distance above and below 80 kilometers there is little absorption of solar energy and consequently little heating of the atmosphere so the temperature reaches a minimum.

Descending below eighty kilometers the atmosphere is heated by another process. Here as the atmosphere gets denser (thicker) with decreasing altitude the molecules of oxygen and nitrogen are closer together. Now if the bond of an oxygen molecule is broken and the two atoms go flying off, there is a higher likelihood that one of these atoms will strike an oxygen molecule. If it does it may form an ozone molecule.

1.3 Natural Ozone Production.

The two most abundant gasses in the Earth's atmosphere are oxygen and nitrogen. Of these oxygen is most effected by solar radiation. Earth's atmospheric oxygen is mostly molecular oxygen (O₂). The ultraviolet part of the solar spectrum can break the bond between the two oxygen atoms (photodissociation). This results in the formation of O (atomic oxygen) and O₃ (ozone).

Three factors control the rate of photodissociation:

1. The wavelength of light.
   The wave length must be short enough so the wave has sufficient energy to break the bond between the two atoms in the oxygen molecule. The most efficient wave lengths for photodissociation occur in the ultraviolet.
   
2. Variation of oxygen density.
   As altitude increases oxygen density decreases (Chapman Profile). The higher the oxygen density the greater the likely hood of having an interaction between an oxygen molecule and a photon.
   
3. Variation of photon flux (Figure 1C).
   Photon flux decreases with decreasing altitude because of photon absorption by the atmosphere. Rate of photodissociation of oxygen is greatest at an altitude of about 100 km:

   O + hn -> O + O

   Consequently atomic oxygen is abundant at high altitudes not low altitudes. (Figure 2)
   There is a natural process whereby atomic oxygen recombines to form molecular oxygen. (Figure 3)

O + O + M -> O₂ + M

M represents any other atmospheric molecule.

Why are other compounds [M] (catalysts) needed for photodissociation?

Chemical energy is released when two oxygen atoms combine to form O₂. If the oxygen molecule is unable to get rid of the excess energy it will break apart. If in the other hand it can get rid of its excess energy through a collision with another atmospheric molecule then it will not come apart. The M represents such an atmospheric molecule.

The rate of the above reaction depends on the density of the constituents that take part on the reaction. the rate of any chemical reaction is proportional to the densities of the species that participate in the reaction.

For the above reaction the rate of the reaction is proportional to the (density of the atomic oxygen) x (density of the atomic oxygen) x (the density of other atmospheric molecules [M]).

So, in summary:

Above 50 kilometers the heating is primarily due to the break up of oxygen molecules by ultraviolet radiation with wavelengths between .12 and .18 microns, while between 50 kilometers and 10 kilometers the heating is due to the absorption by ozone of ultraviolet radiation with wavelengths between .18 and .34 microns.

O + O₂ = O₃ (Ozone Production)

Ozone can in turn be broken up by ultraviolet light resulting in this reaction:

O₃ + ultraviolet light = O₂ + O (Ozone Destruction)

Here's an illustration and a movie of ozone production and destruction by UV radiation.

Both the breaking up of oxygen molecules above fifty kilometers and ozone molecules at fifty kilometers and below causes heating of the atmosphere that peaks at about 50 kilometers (the stratopause). Between fifty and ten to fifteen kilometers (the stratosphere) the solar energy energetic enough to break up ozone (ultraviolet radiation) is used up and the atmosphere cools.

1.4 Measuring Atmospheric Ozone: The Dobson Unit (from NASA).

A dobson unit is the most basic measure used in ozone research. The unit is named after G.M.B. Dobson, one of the first scientists to investigate atmospheric ozone (~1920 - 1960). He designed the 'Dobson Spectrometer' - the standard instrument used to measure ozone from the ground. The Dobson spectrometer measures the intensity of solar UV radiation at four wavelengths, two of which are absorbed by ozone and two of which are not.

If all the ozone in a column of atmosphere were to be compressed to stp (0°C and 1 atmosphere pressure) and spread out evenly over the area, it would form a slab approximately 3mm thick.

1 Dobson Unit (DU) is defined to be 0.01 mm thickness at stp. The ozone layer over Antarctica has decreased from ~300 to <150 DU between 1956-2000.

1.5 The seasonality of Ozone inventories over Antarctica: Role of CFC's in ozone depletion.

Ozone production and destruction exhibits a very large seasonal range over Antarctica due to several factors, natural and anthropogenic. Its polar location and the extremely cold conditions that develop there in (austral) winter time allow the development of a Polar Vortex which isolates the atmosphere over Antarctica, an area larger than North America. The confined, cylindrical circulation inhibits mixing across the polar vortex (here's an animation of the Artic polar vortex).

The atmospheric release of CFC's - chlorinated compounds used for air conditioning, propellants, cleaning solutions - are the catalyzing agents responsible for the dramatic loss of protective stratospheric ozone over the past several decades. The following list of CFC compounds also reveals a fundamental problem - CFC compounds have very long (~20-200 year) residence times in the atmosphere.

Two conditions develop over Antarctica during austral winter which lead to the strong ozone depletion there. First is the onset of darkness during winter so that UV heating (by ozone absorption) is reduced. In the extremely cold winter conditions the polar vortex temperatures fall low enough so that clouds of Nitric acid trihydrate and ice can form despite the dryness of the stratosphere (2pm mixing ratio of H2O). These clouds are called Polar Stratospheric Clouds (PSC). Pure ice clouds form at -88°C at 50 mb pressure (about 20 kilometers), while clouds of Nitric acid trihydrate form at temperature roughtly 10 degrees warmer and probably account for the bulk of the PSC's. The PSC's are the key ingredient in the spring time destruction of the polar ozone layer. They are the sites for a set of reactions that destroy ozone.

M

ClONO₂ + HCl --> HNO₃ + Cl₂

Where M is a third body molecule or particle which acts as a catalyst. The molecular chlorine (Cl₂) is photolyzed in the spring sunlight and atomic chlorine (Cl^-) quickly reacts with O₃ to form the chlorine monoxide radical (ClO). Substantial catalytic O₃ destruction at rates of 0.5% perday begining with the formation of (ClO)₂ and the reaction of ClO with bromine monoxide (BrO).

To prevent the conversion of ClO back to unreactive chlorine nitrate through the reaction of ClO with NO₂, active nitrogen compounds must be suppressed. The formation of PSC's sequesters HNO₃ in the trihydrate particles by condensation and also denitrifies the vortex air. The observed ozone destruction in both hemispheres is therefore dependent on the stratosphere remaining denitrified during the period of ozone loss and this means that mid latitude air containing reactive nitrogen compounds cannot be moved into the polar vortex.

1.6 Ozone depletion by CFC photolysis:

1. UV radiation strikes CFC molecule (here's an animation of the CFC photolysis process).
2. Chlorine atom is split from the CFC.
3. Chlorine atom reacts with ozone molecule (here's an animation of the Cl atom - ozone reaction).
4. Forms ClO and O₂.
5. Free oxygen collides with ClO (O + ClO).
6. Forms O₂ + Cl.
7. Chlorine atom can destroy another O₃ molecule in step 2.

The important ozone destroying reactions are the following:

  Cl + O₃ --> ClO + O₂
  ClO + O --> Cl + O₂
Net: O₃ + O --> O₂ + O₂

One CFC molecule can dissociate and recombine over 100,000 times before being removed from the stratosphere, and ozone molecules are consumed each time. Cl is eventually removed from the stratosphere by reaction to form HCl and can be mixed back down to the troposphere.

An example of the life cycle of the contents of a spray can: 1) Spray starch aerosol can used in New York; 2) The CFC is rapidly dispersed throughout the entire troposphere (one year); 3) After a few years, some of the CFC mixes into the stratosphere; 4) At a sufficiently high altitude (~30 km), the available UV light can photolyze the CFC, liberating chlorine; 5) Each atom of chlorine participates in the catalytic destruction of many thousands of molecules of ozone; 6) Eventually the chlorine atom reacts with methane to produce HCl, a molecule of hydrochoric acid; 7) Some of the HCl reacts with OH to liberate Cl again, but a small fraction of it mixes down into the troposphere where it can dissolve in rainwater and be lost to the atmosphere through precipitation.

1.7 The Ozone Hole.

The Antarctic ozone hole was first recognized in 1980s: ozone depletion was recorded by satellites (Total Ozone mapping Spectrometer (TOMS) instrument aboard Nimbus 7). January through December satellite images of the ozone inventories shown above.

The ozone hole begins to develop in August (early Spring), peaks in October (Summer), and closes again by November (late Fall). The monthly progression of the Ozone hole is shown above (dark blue = lowest Dobson unit values; ~90DU!). The ozone inventory has fallen from peak values of ~210 DU in 1980 to values near 90 DU last year (below).

Some facts:

1. The Antarctic Ozone inventory has been reduced by 60-70% over the last 50 year. It continues to deepen and widen every year, although not monotonically.
2. The size of the ozone hole over Antarctica has increased from "zero" in the mid-1970's to an area slightly larger than North America
3. An Arctic ozone "dent" is appearing, reductions of ~15% over northern Europe. Over the US, ozone reductions are about a few percent.
4. No significant reductions in tropical ozone have been detected

1.8 Projected ozone trends (from NASA).

Scientific evidence shows that ozone depletion caused by human-made chemicals is continuing and is expected to persist until chlorine and bromine levels are reduced. Worldwide monitoring has shown that stratospheric ozone has been decreasing for the past two decades or more. Globally averaged losses have totaled about 5% since the mid-1960s, with cumulative losses of about 10% in the winter and spring and 5% in the summer and autumn over locations such as Europe, North America, and Australia. Since the late-1970s, an ozone "hole" has formed in Antarctica each Southern Hemisphere spring (September / October), in which up to 60% of the total ozone is depleted. The large increase in atmospheric concentrations of human-made chlorine and bromine compounds is responsible for the formation of the Antarctic ozone hole, and the weight of evidence indicates that it also plays a major role in midlatitude ozone depletion.

During 1992 and 1993 ozone in many locations dropped to record low values: springtime depletions exceeded 20% in some populated northern midlatitude regions, and the levels in the Antarctic ozone hole fell to the lowest values ever recorded. The unusually large ozone decreases of 1992 and 1993 are believed to be related, in part, to the volcanic eruption of Mount Pinatubo in the Philippines during 1991. This eruption produced large amounts of stratospheric sulfate aerosols that temporarily increased the ozone depletion caused by human-made chlorine and bromine compounds. Recent observations have shown that as those aerosols have been swept out of the stratosphere, ozone concentrations have returned to the depleted levels consistent with the downward trend observed before the Mount Pinatubo eruption.

In 1987 the recognition of the potential for chlorine and bromine to destroy stratospheric ozone led to an international agreement (The United Nations Montreal Protocol on Substances that Deplete the Ozone Layer) to reduce the global production of ozone-depleting substances. Since then, new global observations of significant ozone depletion have prompted amendments to strengthen the treaty. The 1992 Copenhagen Amendments call for a ban on production of the most damaging compounds by 1996.

The figure below shows past and projected future stratospheric abundances of chlorine and bromine: (a) without the Protocol; (b) under the Protocol's original provisions; and (c) under the Copenhagen Amendments now in force. Without the Montreal Protocol and its Amendments, continuing human use of CFCs and other compounds would have tripled the stratospheric abundances of chlorine and bromine by about the year 2050. Current scientific understanding indicates that such increases would have led to global ozone depletion very much larger than observed today. In contrast, under current international agreements, which are now reducing and will eventually eliminate human emissions of ozone-depleting gases, the stratospheric abundances of chlorine and bromine are expected to reach their maximum within a few years and then slowly decline.

All other things being equal, the ozone layer is expected to return to normal by the middle of the next century.
In summary, record low ozone levels have been observed in recent years, and substantially larger future global depletions in ozone would have been highly likely without reductions in human emissions of ozone-depleting gases. However, worldwide compliance with current international agreements is rapidly reducing the yearly emissions of these compounds. As these emissions cease, the ozone layer will gradually improve over the next several decades. The recovery of the ozone layer will be gradual because of the long times required for CFCs to be removed from the atmosphere.

1.9 Ozone depletion and UV radiation.

Reductions in the concentration of stratospheric ozone necessarily imply a reduction in the absorption of incoming solar UV radiation. This relationship is shown below from data collected at the South Pole, Antarctica. In essence, the data show that a 50% decrease in ozone content results in a greater (~100%, not non-linear curve) increase in UV radiation levels at the ground.

More information?:

NOAA maintains a web site that addresses many questions about ozone depletion.

NASA images and movies of TOMS satellite monitoring of ozone inventories.

Text updated: February 13, 2002 after Jim Hays and Peter deMenocal (2002).