The 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.

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:

1. 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.
2. 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

1. 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.
2. 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.

3. 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.
4. 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.
5. 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).