Stratospheric Ozone


Mechanism of Ozone Formation

The Chapman Mechanism

As for many other species in the atmosphere, ozone in the stratosphere is kept by a dynamic equilibrium between production and destruction.

Ozone in the stratosphere is produced according to the reactions shown below and first described by S. Chapman in 1930. When UV radiation with a very high energy hits oxygen molecules, these photolize and produce two free oxygen atoms, that, being very reactive, combine immediately with other oxygen molecules to produce ozone. This is the only known reaction actually producing ozone in the stratosphere.

In order for a balance to be maintained without the concentration increasing indefinitely, other reactions must intervene to destroy ozone. The most important one, because of its consequences, is ozone photolisis. This provides a mechanism through which the troposphere is shielded from high-energy UV radiation. The electromagnetic energy absorbed in this way is then released as heat, which explains the high temperatures measured in the stratosphere.

Graphic sinthesis of Chapman mechanism.

Destructive cycles

The photolisis of ozone is not sufficient to offset ozone production in the stratosphere. Several other mechanisms intervene in its destruction and are referred to as cycles because the species of interest is continuously reformed and can catalytically convert ozone to oxygen. One of these is the HOx (meaning the OH and HO2 radicals) mechanism

Graphic sinthesis

the net effect of which is to convert one oxygen molecule and one oxygen atom to two oxygen molecules. In reality these cycles are more complex than what is presented here, so that more than one pathway is usually possible for each species, but the net result is basically the same.

Another similar cycle involving NO and NO2 is the following

Graphic sinthesis

Of particular importance are the cycles involving halogens like chlorine and bromine, because of the role human activities are playing in it, as it will be discussed in the section about the ozone hole. Two of the possible mechanisms are the following

Graphic sinthesis


Ozone Distribution

Areas of production

As explained for the Chapman mechanism, ozone production is dependent on the photolysis rate and it is therefore maximum where solar radiation is strongest. The top of the stratosphere in the equatorial region is in fact where the calculated rates of ozone formation are highest, about 5x106 molecules cm-3 s-1 at 40 km. Measured concentrations of atomic oxygen parallel indeed the same pattern. Since the UV radiation is absorbed effectively by the top layers, ozone formation rates drop off very quickly with decreasing altitude: at 10 km at the equator, the formation rate is over five orders of magnitude less than at 40 km and at ground level it is utterly negligible.

Moving towards higher latitudes formation rates decrease with the cosine of the latitude and, depending on the season, can reach zero in the areas experiencing polar nights. As for the seasonal dependence, of course, at the mid latitudes summer is the season with the highest production rates.

Areas of accummulation

Ozone concentrations, however, follow a pattern that is completely different from the production rates. At the equator, the highest concentrations are found at about 25 km. Moreover, concentrations peak in the northern polar regions around 20 km in winter and are lowest in the tropics. There is even an asimmetry between the northern and southern emisphere.

The explanation for this apparent paradox lies in the transport of ozone from the areas of production to areas of accumulation. As shown above (for example for OH)Where solar radiation is more intense, the destruction mechanisms are more effective as well, so that ozone molecules are very short-lived. The general stratospheric circulation moves poleward and takes about two years to cover the distance. Transport of ozone to the higher latitudes can therefore occur where the lifetime of ozone molecules is sufficient. At the equator at 40 km it is only about 1 day, but at 15 km it is three years. Therefore, below about 20 km at the equator and 40 at the poles ozone is imported rather than produced locally. For the same reason at the higher latitudes ozone concentration will peak in the winter months.

For a discussion of possible longitudinal variability consider this study.


Ozone "Hole"

Short historical summary

Total ozone concentrations in the atmosphere have been measured since the beginning of the century with specroscopic techniques. It became customary to express the total column amount in Dobson units (DU): 100 DU correspond to the amount of ozone that at 1 atm of pressure would produce a 1-mm-thick layer. Average values are about 300 DU.

Suspects that human activities could alter the composition of the stratosphere were advanced since the early '70s. It was not until 1985, however, that the observation of abnormally low ozone concentration over Anctartica during the local Spring were published a team led by J. Farman. These values, as low as 100 DU from the normal 350 DU, were obtained by traditional ground measurements. NASA had been measuring stratospheric ozone from the Nimbus 7 satellite as a part of the TOMS project but the decrease had gone unnoticed. As it turned out, concentrations were so low that the automatic analysis algorithm had discarded these data labelling them as invalid, being below what was considered the reasonable range. Reconsidering these data and taking more observations in the following years revealed a strong decreasing trend, with very serious episodes occurring about every other year around October, during the Anctartic Spring.

199119921993

To follow the evolution of the ozone hole follow the link.

Anthropogenic effects

Considerable research was then undertaken to determine the reason of this trend, in particular whether it could be due to anthropogenic effects. Although the Antarctic has some of the highest ozone concentrations during much of the year, a sharp decrease during the spring is natural for reasons explained below. Previous research by Crutzen, Rowland and Molina for example, had shown the ozone-destructive potential of some natural and man made chemicals, as already explained. This had led to abandon the US project of building a ultrasonic air fleet , because of the nitrogen oxides that this would have injected in the stratosphere. In situ observations allowed to determine without doubt that the species responsible of ozone depletion was clorine, as ClO. This was brought into the stratosphere in the form of chlorofluorocarbons (CFCs), widely used for many applications, notably refrigeration, because of their very low reactivity. This chemical inertia, as a matter of fact, allows them to reach undisturbed the stratosphere, where the high-energy UV radiation breaks them down and releases chlorine.

U.S. EPA has further information about the ozone depletion and the regulations dealing with it.

The special situation of the polar regions

The effect of ozone depletion showed up first in the Anctartic, because of the special meteorological conditions that take place there, namely the presence of a stable winter polar vortex that prevents the supply of ozone from other regions and the very low temperatures that favor the formation of polar stratospheric clouds. The latter host reactions that free chlorine atoms as the sunlight comes back in the spring but sequester compounds that may react with chlorine, such as NO3, and thus subtract it to the ozone depleting cycle.

In the arctic, on the other hand, the polar vortex is not as strong and breakes often, while average temperatures are somewhat higher so that PST are less frequent. An unmistakable trend toward lower concentrations has, however, been proven by staellite- and ground-based measurements also in the northern hemisphere.

NASA has made several animations available to follow the evolution of the antarctic hole over a period of several days.


The TOMS Project

In 1978 NASA launched the Nimbus 7 satellite equipped with a Total Ozone Mapping Spectrometer (TOMS). Several other satellites were launched subsequently. For more information about the specific satellites follow the link.

The principle of detection uses albedo as a ratio of backscattered Earth radiance to incident solar irradiance. Ozone retrieval is based upon comparison betweem measured radiances and radiances based on radiative transfer calculations for different amount of ozone in the atmosphere. In other words, as noted above, ozone absorbs strongly between 312 and 380 nm, in the UV region. Comparing what fraction of the incoming radiance in this band is reflected, it is possible to relate this value to the total ozone amount. To map total ozone, the instrument scans through the subsatellite point in a direction perpendicular to the orbital plane. According to the principles of sounding, the instruments can retrieve an ozone vertical profile by measuring the response in six bands with different absorbance. Data from the most absorbing band come primarily from the top of the stratosphere, whereas data from the least absorbing come from nearer the ground surface; it is therefore possible to get a profile at six different altitudes.

Although several algorithms are used to correct for the many possible interferences, several sources of uncertainty remain in the ozone determination. Total uncertainty has been estimated around 5.5%, mainly due to tropospheric ozone, retrieval errors and uncertainties in the ozone molecular cross section for the absorption. Other sources of interference, localized in space and time, include volcanic aerosol, PST, high terrain and solar eclipses.

NASA maintains a TOMS homepage, with extensive database and additional information about the project. It is also possible to retrieve the ozone concentration at any point and any time until the previous day.

A lot of information about stratospheric ozone is also available here


Send your comments or report improper links to Vito Ilacqua

Stratospheric Ozone/ Last updated April 29 1998/ Rutgers University - New Brusnwick, NJ