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Glossary

Column ozone: ozone between the Earth's surface and outer space.
Ozone levels can be described in several ways. One of the most common measures is how much ozone is in a vertical column of air. The dobson unit is a measure of column ozone. Other measures include partial pressure, number density, and concentration of ozone, and can represent either column ozone or the amount of ozone at a particular altitude.
Dobson Unit (DU): a measurement of column ozone levels.
If 100 DU of ozone were brought to the Earth's surface, it would form a layer 1 millimeter thick. In the tropics, ozone levels are typically between 250 and 300 DU year-round. In temperate regions, seasonal variations can produce large swings in ozone levels. For instance, measurements in Leningrad have recorded ozone levels as high as 475 DU and as low as 300 DU. These variations occur even in the absence of ozone depletion, but they are well understood. Ozone depletion refers to reductions in ozone below normal levels after accounting for seasonal cycles and other natural effects. For a graphical explanation, see NASA's TOMS site.
Montreal Protocol: the international treaty governing the protection of stratospheric ozone.
The Montreal Protocol on Substances That Deplete the Ozone Layer and its amendments are the international treaties requiring that countries end production of ozone-depleting substances. Under the Montreal Protocol, several international organizations report on the science of ozone depletion, implement projects to help move away from ozone depleting substances, and provide a forum for policy discussions. In addition, the Multilateral Fund provides resources to developing nations to promote the transition to ozone-safe technologies. The full text of the Montreal Protocol is available online. Exit EPA Disclaimer
Nanometer: a length equal to one billionth of a meter.
The nanometer, or nm, is a metric unit of length commonly used to describe wavelengths of light or other electromagnetic radiation such as UV. For example, green light has wavelengths of about 500-550 nm, while violet light has wavelengths of about 400-450 nm. One billionth is a tiny number. One foot is about one billionth the distance of 48 round-trips between Los Angeles and Washington, DC.
Ozone: a molecule composed of three atoms of oxygen (O3).
Ozone is a bluish gas that is harmful to breathe. Nearly 90% of the Earth's ozone is in the stratosphere and is referred to as the ozone layer. Ozone absorbs a band of ultraviolet radiation called UVB that is particularly harmful to living organisms. The ozone layer prevents most UVB from reaching the ground.
Ozone depletion: Chemical destruction of the stratospheric ozone layer beyond natural reactions.
Stratospheric ozone is constantly being created and destroyed through natural cycles. Various ozone-depleting substances (ODS), however, accelerate the destruction processes, resulting in lower than normal ozone levels. Because ozone absorbs UVB, less ozone will allow more of this harmful solar radiation to reach the Earth's surface. The science section of EPA's ozone depletion Web site offers much more detail on the science of ozone depletion.
Ozone Depletion Potential (ODP): a number that refers to the amount of ozone depletion caused by a substance.
The ODP is the ratio of the impact on ozone of a chemical compared to the impact of a similar mass of CFC-11. The ODP of CFC-11 itself is defined to be 1.0. Other ozone-depleting substances have ODPs ranging from 0.02 to 10. Several classes of substitutes have zero ODP because they do not contain chlorine.
Ozone layer: the region of the stratosphere containing the bulk of atmospheric ozone.
The ozone layer lies approximately 15-40 kilometers (10-25 miles) above the Earth's surface, in the stratosphere. Depletion of this layer by several chemicals produced through human activities will lead to higher UVB levels, which in turn will cause increased skin cancers and cataracts and potential damage to some marine organisms, plants, and plastics. The science section of EPA's ozone depletion Web site offers much more detail on the science of ozone depletion.
Solar noon: the time of day when the sun appears to have reached its highest point in the sky.
Usually, solar noon is not the same time as "clock noon". The relationship between clock noon and solar noon depends on your location within your time zone and the time of year.
Stratosphere: the region of the atmosphere above the troposphere.
The stratosphere extends from about 10km to about 50km in altitude. Commercial airlines fly in the lower stratosphere. The stratosphere gets warmer at higher altitudes. In fact, this warming is caused by ozone absorbing ultraviolet radiation. Warm air remains in the upper stratosphere, and cool air remains lower, so there is much less vertical mixing in this region than in the troposphere.
Troposphere: the region of the atmosphere closest to the Earth.
The troposphere extends from the surface up to about 10 km in altitude, although this height varies with latitude. Almost all weather takes place in the troposphere. Mt. Everest, the highest mountain on Earth, is only 8.8 km high. Temperatures decrease with altitude in the troposphere. As warm air rises, it cools, falling back to Earth. This process, known as convection, means there are huge air movements that mix the troposphere very efficiently.
UV: ultraviolet radiation.
Ultraviolet radiation is a portion of the electromagnetic spectrum with wavelengths shorter than visible light. The sun produces UV, which is commonly split into three bands: UVA, UVB, and UVC. UVA is not absorbed by ozone. UVB is mostly absorbed by ozone, although some reaches the Earth under normal conditions and even more will reach the surface because of ozone depletion. UVC is completely absorbed by ozone and normal oxygen (O2). See the NASA Web site for more information.
UVA: a band of ultraviolet radiation with wavelengths from 320-400 nanometers produced by the Sun.
UVA is not absorbed by ozone. This band of radiation has wavelengths just shorter than visible violet light. See the NASA Web site for more information.
UVB: a band of ultraviolet radiation with wavelengths from 280-320 nanometers produced by the Sun.
UVB is a kind of ultraviolet light from the sun (and sun lamps) that has several harmful effects. It is particularly effective at damaging DNA, causing melanoma and other types of skin cancer. It has also been linked to damage to some materials, crops, and marine organisms. The ozone layer protects the Earth against most UVB coming from the sun. It is always important to protect oneself against UVB, even in the absence of ozone depletion, by wearing hats, sunglasses, and sunscreen. However, these precautions will become more important as ozone depletion worsens. See the NASA Web site for more information.
UVC: a band of ultraviolet radiation with wavelengths shorter than 280 nanometers.
UVC is extremely dangerous, but it is completely absorbed by ozone and normal oxygen (O2). See the NASA Web site for more information.

 


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