Ozone layer depletion
The ozone layer is found in the stratosphere, the segment of the atmosphere just above the troposphere. In the troposphere, which extends from the ground to about seven miles high, air temperature declines with increasing altitude. At the boundary between the troposphere and the stratosphere, the temperature of the (thin) air quits declining and begins to increase with altitude. The stratosphere extends to about 35 miles high. The area called the ozone layer is a portion of the stratosphere, from about 12 to16 miles high. This chart shows the layers of the atmosphere. Question: From the chart, what is the altitude for the tropopause (beginning/bottom of the stratosphere)? What is the altitude for the stratopause (end/top of the stratosphere)?

Ozone is formed in the ozone layer by oxygen molecules absorbing ultraviolet radiation. The O2 molecules split into oxygen atoms (O); then the atoms combine with molecules to form ozone: O + O2 --> O3; O3 molecules themselves absorb ultraviolet solar energy, causing them to split into O + O2. So the ozone layer contains a mixture of O, O2, and O3, combining and splitting. The result is the absorption of ultraviolet radiation and its conversion into infrared (heat) energy, which dissipates in the upper atmosphere. Without this ultraviolet absorption in the stratosphere the ultraviolet would penetrate to ground level. Life at ground level relies on this ultraviolet shield for survival. If there were no shield at all, little life could exist. If the shield performs less efficiently (as a consequence of ozone layer depletion) then adverse effects would begin. By the way the absorption of ultraviolet energy and its conversion to heat in the ozone layer is the reason why air temperatures in the stratosphere rise with altitude, as shown in the chart you just looked at.

Ultraviolet radiation
The ultraviolet part of the electromagnetic spectrum is divided into three segments.

Know: The names and properties of the three parts of the ultraviolet part of the electromagnetic spectrum.

It's UVB that is strongly absorbed in the ozone layer and it is UVB that would increase if the ozone layer contained a lower concentration of ozone molecules (ozone layer depletion).

Human threats to the ozone layer

The chlorofluorocarbons, a family of synthetic organic chemicals, were first identified as a threat to the ozone layer in a 1974 report in Nature, by Sherwood Rowland and Mario Molina (who, with Paul Crutzen, recently were given the Nobel Prize for their work). The CFCs contain carbon, fluorine, and chlorine atoms. They are (were, they are being phased out) used as refrigerants (in air conditioners, freezers, refrigerators), solvents (cleaners), foaming agents (to make pillows, automobile seats), and as aerosol propellants (the gas that carries the aerosol out of a spray can).  This and subsequent research proposed that the CFCs harmed stratospheric ozone by the following mechanism:

• This article also shows the mechanism of ozone depletion.

• An article from NASA showing the mechanism of ozone depletion.

• A recent article by Sherwood Roland on ozone depletion.

Good properties of CFCs: They are nonflammable, nontoxic, noncorrosive, nonreactive. Note, for example, that CFC refrigerants (brand name Freon) replaced ammonia and sulfur dioxide, which were used as refrigerants until the 1940s. They replaced highly toxic and irritating gases.

One of these good properties is a problem for CFCs as ozone layer depleters. Being nonreactive synthetic molecules, the CFCs have an life in the atmosphere of between 60 and 80 years. This means that they have plenty of time to mix upward in the troposphere and even find their way into the lower stratosphere to destroy ozone.

Adverse effects of significant ozone layer depletion

Note: None of these effects has been shown to be happening as a result of ozone depletion; instead, this is a list of known adverse effects of excessive exposure to ultraviolet radiation in general.

• Increased incidence of skin cancer in susceptible groups (light-skinned)

• Possible increased risk of cataracts

• Decreased crop yields

• Adverse effects on certain fish larvae, phytoplankton

• Weakening of plastic products

• More on adverse health effects of UVB.

• More on adverse environmental effects of UVB.

Solutions to the ozone layer depletion problem

1. International agreements to phase out ozone-depleting chemicals. A series of agreements began toward banning CFCs with the Vienna Convention for the Protection of the Ozone Layer in 1985 (full text). The first phaseout agreement was drafted in 1987, called the Montreal Protocol on Substances that Deplete the Ozone Layer. This was an extension of   the Vienna Convention for the Protection of the Ozone Layer. Subsequent changes to the Montreal Protocol  in 1990, 1992, 1995, and 1997 have accelerated the phaseout of ozone-depleted chemicals. Full text of the Montreal Protocol. Although the Vienna Convention is the original document and the Montreal Protocol has been changed many times, the shorthand term for the agreement to phase out CFCs and other ozone-depleting chemicals is the Montreal Protocol.

2. National bans on CFCs.
The United States banned the use of chlorofluorocarbons as a propellant in aerosol sprays in 1978. There are two important facts here: one the CFC is the propellant; it's not the aerosol. Also, no CFCs have been used as propellant in the US for 20 years. I still hear people say they won't use aerosol sprays because they want to protect the environment! By the way, the substitute for the propellant in aerosol sprays is quite commonly the flammable gas propane (read the label), so the trade did not eliminate risk but just changed its character from one of chronic worldwide increased ultraviolet exposure to acute personal fire hazard.

Following international and national bans and phaseouts of CFC production, smuggling of CFCs has become a major problem.

Another article on the CFC black market.

3. Substitutes for the CFCs

a.  Add a destabilizing hydrogen atom to the chlorofluorocarbon molecule to create a hydrochlorofluorocarbon (HCFC). For example:

CF2Cl2 --> CF2ClH (one of the two chlorine atoms is replaced by a hydrogen atom).

What this does is greatly shorten the atmospheric residence time of the molecule (it falls apart in the lower atmosphere before it has a chance to mix upward to the ozone layer).

So to replace CFC-12 (Freon 12), HCFC-22 is being used; one of the chlorine atoms on Freon 12 is replaced by a hydrogen atom.

b. Produce synthetic refrigerants that have no chlorine at all. These chemicals are call hydroflurocarbons (HFCs).

For example, HFC-134a is being used in new automobile air conditioners.

See the USEPA pages on substitutes.

Uncertainties

1. Ozone layer concentrations fluctuate naturally with the sunspot cycle and perhaps for other reasons. Our data set is fairly short and limited; therefore we do not know for certain that the ozone layer is being depleted. There is also the question of depletion by latitude and by time of year. (Recent depletion measurements using satellites have begun to show a more discernible downward trend.)

2. Ground level measurements of ultraviolet radiation have not shown a significant increase. If the ozone layer were getting thinner, then more ultraviolet should be getting through. One explanation is that ground level ozone (smog) is absorbing the ultraviolet before the UVB monitors can read it. See a United Nations statement on this uncertainty which agrees that the data aren't available. The USEPA says that ultraviolet radiation is increasing at ground level, but only shows that ultraviolet is stronger at the South Pole during the so-called ozone hole. The Toronto Study, which is the source cited by the USEPA to show increased ultraviolet levels at ground level, is inconclusive (see the Science article in the library; I also have a copy.)

3.  Fred Singer takes a contrarian view of ozone layer depletion and global warming. Question: What is Singer's answer to this question in the article, "Is There an Increasing Trend in Ultraviolet Radiation at the Earth’s Surface?" What did the reanalysis of the Kerr/McElroy data show?

Suggested uncertainties that probably aren't

1. Volcanoes put more chlorine in the atmosphere than CFCs.
A recent article in Nature contradicts this hypothesis.

2. CFCs are heavier than air; they can't get to the stratosphere.
The atmosphere is turbulent and will mix gases such as the CFCs readily.

See these ozone depletion documents, written by Robert Parson of the University of Colorado, which is in question and answer format, for an excellent overview.
Ozone Depletion FAQ Part I: Introduction to the Ozone Layer
Ozone Depletion FAQ Part II: Stratospheric Chlorine and Bromine
Ozone Depletion FAQ Part III: The Antarctic Ozone Hole
Ozone Depletion FAQ Part IV: UV Radiation and its Effects

Other Ozone layer depletion links
 Studies, outline, more links
Our Ozone Shield, from the National Oceanic and Atmospheric Administration

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