Ozone Layer :
"The ozone layer" refers to the ozone within stratosphere, where over 90% of the earth's ozone resides. Ozone is an irritating, corrosive, colorless gas with a smell something like burning electrical wiring. In fact, ozone is easily produced by any high-voltage electrical arc (spark plugs, Van de Graaff generators, Tesla coils, arc welders). Each molecule of ozone has three oxygen atoms and is produced when oxygen molecules (O2) are broken up by energetic electrons or high energy radiation. For information on the history of the ozone layer for the layman, see the Short history of ozone depletion , National Oceanic and Atmospheric Administration's NOAA Ozone overview or NOAA on stratospheric ozone. For short and to-the-point answers, check out Robert Parson's Ozone overview, FAQ1
The Stratosphere
Variations in temperature and pressure divide the earths atmosphere into layers, shown below, and mixing of gases between the layers happens very slowly. 
- The altitudes on the diagram are logarithmic so an analogy in the glossary might give you a better idea of the relative thicknesses of these layers.
- Notice that the lowest 10% of the atmosphere holds 90% of the air. This is because gases are compressable. In a huge pile of feathers the bottom-most feathers become compressed under the weight of the feathers above them. Likewise the lower levels of the atmosphere are filled with compressed air while the upper levels, such as the stratosphere, contain very 'thin' uncompressed air. Although the stratosphere layer is over four times thicker than the lower atmosphere, the stratosphere holds so little gas that ozone is still considered one of the minor trace-gases of the overall atmosphere.
The ozone layer absorbs 97-99% of the sun's high frequency ultraviolet light , light which is potentially damaging to life on earth. Every 1% decrease in the earths ozone shield is projected to increases the amount of UV light exposure to the lower atmosphere by 2%. Because this would cause more ozone to form in the lower atmosphere, it is uncertain how much of UV light would actually reach the earths surface. Recent UV measurements from around the northern hemisphere indicate small UV increases in rural areas and almost no increase in areas near large cities.
Units used to measure ozone concentration
When describing the amount or concentration of gas, scientists resort to several different units:- Dobsin unit (DU)- the principle unit for measuring ozone concentration. One DU is about twenty-seven million molecules per square centimeter ( the palm of your hand covers an area of rougly a hundred square centimeters). The ozone concentration over the US is about 300 DU and the antarctic hole during the late spring can drop to 117 DU.
- Mixing ratios: within a specified volume, it is a fraction of the number of molecules of a particular gas divided by the total number of molecules in that given space. Terms of usually abreviated, like ppmv for parts-per-million or ppbv which is parts-per-billion . For example the concentration of HCl at 3 km is said to be about 0.1 ppbv; this means that if you selected a volume of air that contained 10 billion molecules of air, one of those molecules would be an HCl molecule.
Factors influencing Ozone concentrations
- Stratospheric sulfate aerosols: large explosive volcanoes are able to place a significant amount of aerosols into the lower stratosphere, as well as some chlorine. Because more than 90% of a volcanic plume is water vapor most of the other compounds, including volcanic chlorine, get ''rained-out'' of the stratosphere. The effects of a large volcano on global weather are significant, which in turn can affect localized weather patterns such as the antarctic ozone hole. Many observations have linked the 1991 Mt. Pinatubo eruption to a 20% increase in the ozone hole that following spring[Solomon et al. 1993]) . The effects of a large volcanic eruption on total global ozone are more modest (less than 3%) and last no more than 2-3 years.
- Stratospheric winds: every 26 months the tropical winds in the lower stratophere change from easterly to westerly and then back again, an event called the Quasi-biennial Ocillation (QBO). The QBO causes ozone values at a particular latitude to expand and contract roughly 3%. Since stratospheric winds move ozone, not destroy it, the loss of one latitude is the gain of another and globally the effects cancel out.
- Greenhouse gases: to the degree that greenhouse gases might heat the planet and alter weather patterns, the magnatude of the stratospheric winds will certainly be affected. Some of the more popular senarios of global warming predict cooler stratospheric temperatures, leading to more polar stratospheric clouds and more active chlorine in the area of the antarctic ozone hole.
- Sunspot cycle: ozone is created by solar UV radiation. The amount of UV radiation produced by the sun is not constant but varies by several percent in a rougly 11year cycle. This 11year cycle is related to magnetic changes within the sun which increase the solar UV output, and is heralded by an increase sunspots which appear on the surface of the sun. Comparisons of yearly ozone concentrations show a small 11 year variation in global ozone of about 2%. Episodes of unusual solar activity, solar storms and large solar flares, could certainly alter this value.
- Stratospheric chlorine, coming mostly from man-made halocarbons. Careful subtracting of other natural factors yields a net decrease of 3% per decade in global ozone,1978-1991; due most likely to catalytic degredation by stratospheric chlorine.
Decrease in global ozone The measurement period is from November 1978 through November 1987, and combines depletion due to natural and man-made causes. This analysis and graphic comes from the United Nations Environmental Protection Agency(UNEP).
Ultraviolet light and ozone
Although the concentration of the ozone in the ozone layer is very small, it is vitally important to life because it absorbs biologically harmful ultraviolet (UV) radiation coming from the Sun. UV radiation is divided into three categories, based on its wavelength; these are referred to as UV-A (400-315 nm), UV-B (315-280 nm), and UV-C (280-100 nm). UV-C, which would be very harmful to humans, is entirely screened out by ozone at around 35 km altitude. UV-B radiation can be harmful to the skin and is the main cause of sunburn; excessive exposure can also cause genetic damage, resulting in problems such as skin cancer. The ozone layer is very effective at screening out UV-B; for radiation with a wavelength of 290 nm, the intensity at the top of the atmosphere is 350 million times stronger than at the Earth's surface. Nevertheless, some UV-B reaches the surface. Most UV-A reaches the surface; this radiation is significantly less harmful, although it can potentially cause genetic damage.
Distribution of ozone in the stratosphere
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 towards the poles. It also varies with season, being in general thicker during the spring and thinner during the autumn in the northern hemisphere. The reasons for this latitude and seasonal dependence are complicated, involving atmospheric circulation patterns as well as solar intensity.
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 autumn, not winter in the northern hemisphere. 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. However in the southern hemisphere, owing to the ozone hole phenomenon, the lowest amounts of column ozone found anywhere in the world are over the Antarctic in the southern spring period of September and October.
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 very slowly. The time needed to lift an air parcel from the tropical tropopause near 16 km (50,000 ft) to 20 km is about 4–5 months (about 30 feet (9.1 m) 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.
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.
[edit] Ozone depletion
The ozone layer can be depleted by free radical catalysts, including nitric oxide (NO), nitrous oxide (N2O), hydroxyl (OH), atomic chlorine (Cl), and atomic bromine (Br). While there are natural sources for all of these species, the concentrations of chlorine and bromine have increased markedly in recent years due to the release of large quantities of manmade organohalogen compounds, especially chlorofluorocarbons (CFCs) and bromofluorocarbons.[3] These highly stable compounds are capable of surviving the rise to the stratosphere, where Cl and Br radicals are liberated by the action of ultraviolet light. Each radical is then free to initiate and catalyze a chain reaction capable of breaking down over 100,000 ozone molecules. The breakdown of ozone in the stratosphere results in the ozone molecules being unable to absorb ultraviolet radiation. Consequently, unabsorbed and dangerous ultraviolet-B radiation is able to reach the Earth’s surface.[citation needed] 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.
In 2009, nitrous oxide (N2O) was the largest ozone-depleting substance emitted through human activities. [4]
[edit] Regulation
In 1978, the United States, Canada and Norway, enacted bans on CFC-containing aerosol sprays that are thought to damage the ozone layer. The European Community rejected an analogous proposal to do the same. In the U.S., 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 CFCs.[5] 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. CFCs have very long atmospheric lifetimes, ranging from 50 to over 100 years, so the final recovery of the ozone layer is expected to require several lifetimes.
Compounds containing C–H bonds have been designed to replace the function of CFC's (such as HCFC), since these compounds are more reactive and less likely to survive long enough in the atmosphere to reach the stratosphere where they could affect the ozone layer. However, while being less damaging than CFC's, HCFC's also have a significant negative impact on the ozone layer. HCFC's are therefore also being phased out.[6]
