Ozone layer

The ozone layer is that part of the Earth's stratosphere which contains relatively high concentrations of ozone (O₃). "Relatively high" means a few parts per million, much higher than the concentrations in the lower atmosphere but still small compared to the main components of the atmosphere. The ozone layer was discovered in 1913 by the french physicists Charles Fabry and Henri Buisson. Its properties were explored in detail by the British meteorologist G.M.B. Dobson, who developed a simple spectrophotometer that could be used to measure stratospheric ozone from the ground. Between 1928 and 1958 Dobson established a worldwide network of ozone monitoring stations which continues to operate today. The "Dobson unit", a convenient measure of the total amount of ozone in a column overhead, is named in his honor.

Origin of ozone

The photochemical mechanisms that give rise to the ozone layer were worked out by the British physicist Sidney Chapman in 1930. Ozone in the earth's stratosphere is created by ultraviolet light striking oxygen molecules containing two oxygen atoms (O₂), splitting them into individual oxygen atoms (atomic oxygen); the atomic oxygen then combines with unbroken O₂ to create ozone, O₃. The ozone molecule is also unstable (although, in the stratosphere, long-lived) and when ultraviolet light hits ozone it splits into a molecule of O₂ and an atom of atomic oxygen, a continuing process called the ozone-oxygen cycle, thus creating an ozone layer in the stratosphere. Tropospheric ozone is created in smaller amounts through different mechanisms.

About 90% of the ozone in our atmosphere is contained in the stratosphere, the region from about 10 to 50km (32,000 to 164,000 feet) above Earth's surface. Ten percent of the ozone is contained in the troposphere, the lowest part of our atmosphere where all of our weather takes place. Ozone concentrations are greatest between about 15 and 40 km, where they range from about 2 to 8 parts per million.If all of the ozone were compressed to the pressure of the air at sea level, it would be only a few millimeters thick.

Ultraviolet light and ozone

While the ozone concentrations are very small, it is vitally important to life because it absorbs biologically harmful ultraviolet radiation from the Sun. There are three different types of ultraviolet (UV) radiation, based on the wavelength of the radiation. These are referred to as UV-A, UV-B, and UV-C. UV-C is entirely screened out by ozone around 35 km altitude. On the other hand, most UV-A reaches the surface, but it is not as genetically damaging. It is the UV-B radiation that can cause sunburn and that can also cause genetic damage, resulting in things like skin cancer, if exposure to it is prolonged. Ozone screens out most UV-B, but some reaches the surface. Were the ozone layer to decrease, more UV-B radiation would reach the surface, causing increased genetic damage to living things. Because of ozone, it is virtually impossible for solar ultraviolet to penetrate to Earth's surface. For radiation with a wavelength of 290 nm, the intensity at Earth's surface is 350 million times weaker than at the top of the atmosphere.

DNA sensitivity to UV

To appreciate how important this ultraviolet radiation screening is, we can consider a characteristic of radiation damage called an action spectrum. An action spectrum gives us a measure of the relative effectiveness of radiation in generating a certain biological response over a range of wavelengths. This response might be erythema (sunburn), changes in plant growth, or changes in molecular DNA. There is much greater probability of DNA damage by UV radiation at various wavelengths. Fortunately, where DNA is easily damaged, such as by wavelengths shorter than 290 nm, ozone strongly absorbs UV. At the longer wavelengths where ozone absorbs weakly, DNA damage is less likely. If there was a 10% decrease in ozone, the amount of DNA damaging UV increases, in this case, by about 22%. Considering that DNA damage can lead to maladies like skin cancer, it is clear that this absorption of the Sun's ultraviolet radiation by ozone is critical for our well being.

Amount of ozone

The "thickness" of the ozone layer - that is, the total amount of ozone in a column overhead - varies by a large factor worldwide, being in general smaller near the equator and larger as one moves towards the poles. It also varies with season, being in general thicker during the spring and thinner during the fall. The reasons for this latitude and seasonal dependence are complicated, involving atmospheric circulation patterns as well as solar intensity.

Distribution of ozone in the stratosphere

Since stratospheric ozone is produced by solar UV radiation, one might expect to find the highest ozone levels over the tropics and the lowest over polar regions. The same argument would lead one to expect the highest ozone levels in the summer and the lowest in the winter. The observed behavior is very different: most of the ozone is found in the mid-to-high latitudes of the northern and southern hemispheres, and the highest levels are found in the spring, not summer, and the lowest in the fall, not winter. During winter, the ozone layer actually increases in depth. This puzzle is explained by the prevailing stratospheric wind patterns, known as the Brewer-Dobson circulation. While most of the ozone is indeed created over the tropics, the stratospheric circulation then transports it poleward and downward to the lower stratosphere of the high latitudes.

The ozone layer is higher in altitude in the tropics, and lower in altitude in the extratropics, especially in the polar regions. This altitude variation of ozone results from the slow circulation that lifts the ozone-poor air out of the troposphere into the stratosphere. As this air slowly rises in the tropics, ozone is produced by the overhead sun which photolyzes oxygen molecules. As this slow circulation bends towards the mid-latitudes, it carries the ozone-rich air from the tropical middle stratosphere to the mid-and-high latitudes lower stratosphere. The high ozone concentrations at high latitudes are due to the accumulation of ozone at lower altitudes.

The Brewer-Dobson circulation moves at a literal snail's pace. The time needed to lift an air parcel from the tropical tropopause near 16 km (50,000 feet) to 20 km is about 4-5 months (about 30 feet per day). Even though ozone in the lower tropical stratosphere is produced at a very slow rate, the lifting circulation is so slow that ozone can build up to relatively high levels by the time it reaches 26 km (85,000 feet).

Ozone amounts over the continental United States (25°N to 49°N) are highest in the northern spring (April and May). These ozone amounts fall over the course of the summer to their lowest amounts in October, and then rise again over the course of the winter. Again, wind transport of ozone is principally responsible for the seasonal evolution of these higher latitude ozone patterns.

The total column amount of ozone generally increases as we move from the tropics to higher latitudes in both hemispheres. However, the overall column amounts are greater in the northern hemisphere high latitudes than in the southern hemisphere high latitudes. In addition, while the highest amounts of column ozone over the Arctic occur in the northern spring (March-April), the opposite is true over the Antarctic, where the lowest amounts of column ozone occur in the southern spring (September-October). Indeed, the highest amounts of column ozone anywhere in the world are found over the Arctic region during the northern spring period of March and April. The amounts then decrease over the course of the northern summer. Meanwhile, the lowest amounts of column ozone anywhere in the world are found over the Antarctic in the southern spring period of September and October, owing to the ozone hole phenomenon.

Ozone depletion

The ozone layer can be depleted by free radical catalysts, including nitric oxide (NO), hydroxyl (OH), and atomic chlorine and bromine (see ozone depletion). While there are natural sources for all of these species, the concentrations of Cl and Br have increased markedly in recent years due to the release of large quantities of manmade organohalogen compounds, especially chlorofluorocarbons (CFCs). These highly stable compounds find their way to the stratosphere, where Cl and Br atoms are liberated by the action of ultraviolet light on them. Each chlorine atom is capable of breaking down approximately one hundred thousand ozone molecules during the time that it resides in the stratosphere, and bromine is even more efficient. Ozone levels, over the northern hemisphere, have been dropping by ~4% per decade. Over approximately 5% of the Earth's surface, around the north and south poles, much larger (but seasonal) declines have been seen; these are the ozone holes.

Regulation

On January 23, 1978 Sweden became the first nation to ban CFC-containing aerosol sprays that are thought to damage the ozone layer. The United States and other developed nations followed suit soon after. Chlorofluorocarbons continued to be used in other applications, such as refrigeration and industrial cleaning, until after the discovery of the antarctic ozone hole in 1985. After negotiation of an international treaty (the Montreal Protocol), CFC production was sharply limited beginning in 1987 and phased out completely by 1996.

On August 2, 2003, scientists announced that the depletion of the ozone layer may be slowing down due to the international ban on chlorofluorocarbons. [1] (http://news.independent.co.uk/world/environment/story.jsp?story=429802) Three satellites and three ground stations confirmed that the upper atmosphere ozone depletion rate has slowed down significantly during the past decade. The study was organized by the American Geophysical Union. Some breakdown can be expected to continue due to CFCs used by nations which have not banned them, and due to gases which are already in the stratosphere.

External links

• Stratospheric ozone: an electronic textbook (http://www.ccpo.odu.edu/SEES/ozone/oz_class.htm)
• http://www.unep.org/ozone/Public_Information/4Aii_PublicInfo_Facts_OzoneLayer.asp
• NASA. Studying Earth's Environment From Space. June 2000. (accessed April 14, 2005) http://www.ccpo.odu.edu/SEES/index.html.

See also

• Ozone depletion

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