Aerosol: small droplet or particle suspended in the atmosphere, typically containing sulfur

Aerosols are emitted naturally (e.g., in volcanic eruptions) and as the result of human activities (e.g., by burning fossil fuels). There is no connection between particulate aerosols and pressurized products also called aerosols (see below).

Aerosol: a product that relies on a pressurized gas to propel substances out of a container

Consumer aerosol products have not used ozone-depleting substances (ODS) since the late 1970s because of voluntary switching followed by federal regulation. The Clean Air Act and EPA regulations further restricted the use of ODS for non-consumer products. All consumer products, and most other aerosol products, now use propellants that do not deplete the ozone layer, such as hydrocarbons and compressed gases.

Carbon Tetrachloride (CCl4): a compound consisting of one carbon atom and four chlorine atoms

Carbon tetrachloride was widely used as a raw material in many industrial uses, including the production of CFCs, and as a solvent. Solvent use ended when it was discovered to be carcinogenic. It is also used as a catalyst to deliver chlorine ions to certain processes. Its ozone depletion potential is 1.2.

Chlorofluorocarbon (CFC): a compound consisting of chlorine, fluorine, and carbon

CFCs are very stable in the troposphere. They are broken down by strong ultraviolet light in the stratosphere and release chlorine atoms that then deplete the ozone layer. CFCs are commonly used as refrigerants, solvents, and foam blowing agents. The most common CFCs are CFC-11, CFC-12, CFC-113, CFC-114, and CFC-115. Their ozone depletion potentials are, respectively, 1, 1, 0.8, 1, and 0.6.

Class I Substance: one of several groups of chemicals with an ozone-depletion potential of 0.2 or higher

Class I substances listed in the CAA include CFCs, halons, carbon tetrachloride, and methyl chloroform. EPA later added HBFCs and methyl bromide to the list by regulation. Lists of class I substances with their ODPs and CAS numbers are available.

Class II Substance: a chemical with an ozone-depletion potential of less than 0.2

Currently, all of the HCFCs are class II substances. Lists of class II substances with their ODPs and CAS numbers are available.

Clean Air Act (CAA): law amended by Congress in 1990

Title VI of the CAA directs EPA to protect the ozone layer through several regulatory and voluntary programs. Sections within Title VI cover production of ozone-depleting substances (ODS), the recycling and handling of ODS, the evaluation of substitutes, and efforts to educate the public.

Dobson Unit (DU): a measurement of ozone levels

The unit of measure for total ozone. 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.

Federal Register (FR): the daily publication containing all federal government actions

The Federal Register is the formal method of communication for any Notice, Notice of Proposed Rulemaking (NPRM), or Final Rulemaking (FRM). Once published in the FR, rules are collected in the Code of Federal Regulations. FR publications represent U.S. law in written form. The FR is available at many libraries.

Global Warming Potential: a number that refers to the amount of global warming caused by a substance

The GWP is the ratio of the warming caused by a substance to the warming caused by a similar mass of carbon dioxide. Thus, the GWP of CO2 is defined to be 1.0 . CFC-12 has a GWP of 8,500, while CFC-11 has a GWP of 5,000. Various HCFCs and HFCs have GWPs ranging from 93 to 12,100. Water, a substitute in numerous end-uses, has a GWP of 0.

Halon: a compound consisting of bromine, fluorine, and carbon

The halons are used as fire extinguishing agents, both in built-in systems and in handheld portable fire extinguishers. Halon production in the U.S. ended on 12/31/93 because they contribute to ozone depletion. They cause ozone depletion because they contain bromine. Bromine is many times more effective at destroying ozone than chlorine. At the time the current U.S. tax code was adopted, the ozone depletion potentials of halon 1301 and halon 1211 were observed to be 10 and 3, respectively. These values are used for tax calculations. Recent scientific studies, however, indicate that the ODPs are at least 13 and 4, respectively. Note: technically, all compounds containing carbon and fluorine and/or chlorine are halons, but in the context of the Clean Air Act, "halon" means a fire extinguishing agent as described above.

Hydrobromofluorocarbon (HBFC): a compound consisting of hydrogen, bromine, fluorine, and carbon

Although they were not originally regulated under the Clean Air Act, subsequent regulation added HBFCs to the list of class I substances.

Hydrocarbon (HC): a compound consisting of carbon and hydrogen

Hydrocarbons include methane, ethane, propane, cyclopropane, butane, and cyclopentane. Although they are highly flammable, HCs may offer advantages as ODS substitutes because they are inexpensive to produce and they have zero ozone depletion potential, very low global warming potential (GWP), and low toxicity.

Hydrochlorofluorocarbon (HCFC): a compound consisting of hydrogen, chlorine, fluorine, and carbon

The HCFCs are one class of chemicals being used to replace the CFCs. They contain chlorine and thus deplete stratospheric ozone, but to a much lesser extent than CFCs. HCFCs have ozone depletion potentials (ODP)ranging from 0.01 to 0.1. Production of HCFCs with the highest ODPs will be phased out first, followed by other HCFCs.

Hydrofluorocarbon (HFC): a compound consisting of hydrogen, fluorine, and carbon

The HFCs are a class of replacements for CFCs. Because they do not contain chlorine or bromine, they do not deplete the ozone layer. All HFCs have an ozone depletion potential of 0. Some HFCs have high GWPs.

Methyl Bromide (CH3Br): a compound consisting of carbon, hydrogen, and bromine

Methyl Bromide's chemical formula is CH3BR. An effective pesticide, this compound is used to fumigate soil and many agricultural products. Because it contains bromine, it depletes stratospheric ozone and has an ozone depletion potential of 0.6. Production of methyl bromide will end in the U.S. on 12/31/2000. Much more information is available.

Methyl Chloroform (CH3CCl3): a compound consisting of carbon, hydrogen, and chlorine

Methyl chloroform is used as an industrial solvent. Its ozone depletion potential is 0.11.

Montreal Protocol: the international treaty governing the protection of stratospheric ozone

The Montreal Protocol on Substances That Deplete the Ozone Layer and its amendments control the phase-out of ODS production and use. Under the MP, several international organizations report on the science of ozone depletion, implement projects to help move away from ODS, 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 MP is available online and it is part of the OzonAction Information Clearinghouse database.

Nanometer: a distance of one billionth of a meter

The nanometer, or nm, is a common unit 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

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. It is also a very reactive compound, which makes it harmful down here at the surface, because it can damage (i.e. react with) the lung tissue of those who breathe it.

Ozone-Depleting Substance(s) (ODS): a compound that contributes to stratospheric ozone depletion

ODS include CFCs, HCFCs, halons, methyl bromide, carbon tetrachloride, and methyl chloroform. ODS are generally very stable in the troposphere and only degrade under intense ultraviolet light in the stratosphere. When they break down, they release chlorine or bromine atoms, which then deplete ozone. A detailed list of class I and class II substances with their ODPs and CAS numbers are available.

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. The science page 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

DP is the ratio of the impact on ozone of a chemical compared to the impact of a similar mass of CFC-11. Thus, the ODP of CFC-11 is defined to be 1.0. Other CFCs and HCFCs have ODPs that range from 0.01 to 1.0. The halons have ODPs ranging up to 10. Carbon tetrachloride has an ODP of 1.2, and methyl chloroform's ODP is 0.11. HFCs 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. As electromagnetic radiation comes in from the sun and hits the earth's atmosphere, certain wavelengths in the ultraviolet range (180-240 nanometers) are absorbed by and break apart oxygen molecules (which are made of two oxygen atoms). Some of the resulting unattached pairs of oxygen atoms then recombine into triplets to form ozone. A different range of wavelengths of ultraviolet (290-300 nm, UVB) are strongly absorbed by ozone, which breaks down as a result and reforms into molecular oxygen again. Depletion of this layer by ODS 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.

Perfluorocarbon (PFC): a compound consisting of carbon and fluorine

PFCs have extremely high global warming potentials (GWPs) and very long lifetimes. They do not deplete stratospheric ozone, but EPA is concerned about their impact on global warming.

Photolysis: Chemical decomposition induced by light.

This is also sometimes called photodissociation or photochemical decomposition. Photolysis takes place when UV light is absorbed by ozone creating diatomic and monoatomic oxygen. Photolysis plays a major role in a number of chemical reactions that affect the balance of ozone destruction and creation.

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.

Total Ozone: the total amount of ozone in a column of air stretching from the earth's surface to space.

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. UVC is completely absorbed by ozone.

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.

UVB: a band of ultraviolet radiation with wavelengths from 280-320 nanometers produced by the Sun

UVB is a kind of ultraviolet light that is particularly effective at damaging DNA. It is a cause of 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. An EPA fact sheet explains more about the health effects of increased UVB.

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).


**NOTE: This glossary is based on the glossary at the U.S. Environmental Protection Agency's internet site. Only minor changes have been made.

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