Ozone Depletion

I have been accused in the past of going to great length to explain the obvious. Then perhaps I have not, but I should have been. Anyhow, here is another go at explaining the obvious.

Science-OzoneDepletion

To be sure, I’m not doing original writing here. Most of what I’m putting out is straight from Wikipedia. If you want the full story, then you should go to the Wikipedia article. This is my Schaum’s Outline version of the ozone layer depletion story.

The image above, from Wikipedia, is an excellent condensation of the ozone depletion process. The story goes back a lot from there. It starts with the invention of chlorinated fluorocarbons, CFCs. CFCs have some useful properties:

  • Chemically inert (almost)
  • Low vapor pressure—boil within a useful temperature range: −51.7 (CF2H2) and −128 °C (CF4).
  • Complex molecular structure.

The complex molecular structure gives CFC molecules multiple quantum energy levels. Put another way, they are far from “ideal gases.” When you compress an ideal gas its temperature does not rise. The energy you use to compress the gas gets stored in the pressure-volume combination. When the pressure is released an ideal gas springs back, returning all the energy as mechanical energy. They make an ideal spring.

Complex molecules, such as CFCs, do not make for ideal gases. When you compress them, the energy of compression gets stored in the multiple quantum energy levels of the molecule. This is the definition of increased temperature. Absolute temperature of a system is proportional to the logarithm of the number of occupied quantum energy states. Compressing a CFC gas raises its temperature by storing energy in multiple quantum energy levels. With increased temperature, it is now possible to drain energy from the compressed gas by allowing heat to flow from the compressed gas into cooler surroundings. When the pressure is released the gas now contains less energy than before it was compressed and is colder. This makes CFCs useful in refrigeration systems. Freon-12 is a trade name for dichlorodifluoromethane by DuPont.

A problem was discovered with Freon-12 and other CFCs. First, they are very stable. When released into the atmosphere the substance does not readily break down. The molecule persists for a long time in the atmosphere. It persists long enough for the molecule to migrate into the stratosphere, where it begins to cause trouble. Refer to the diagram again.

From Wikipedia I get the following chemical reaction:

CCl3F → CCl2F. + Cl.

This cleaving of the CFC molecule is driven by ultraviolet radiation in the stratosphere. A product is the free chlorine atom. This chlorine atom interacts with ozone in the stratosphere and catalyzes its breakup. The chlorine is not consumed in the process, and a small amount of chlorine has the ability to do a lot of damage.

The damage done is the conversion of ozone to O2. Ozone absorbs ultraviolet radiation strongly. Very little ozone has the ability to block much of the ultraviolet coming from the sun. When this high-energy radiation reaches the surface it is a hazard to human health (skin cancer) and to other life forms. The depletion of the ozone layer was recognized in 1974 through research at the University of California at Irvine conducted by Frank Sherwood Rowland and Mario J. Molina. In 1995 Rowland and Molina, along with Paul Crutzen received the Nobel Prize in Chemistry “for their work on stratospheric ozone.”

Since it’s the chlorine that’s doing all the damage, many have questioned the culpability of CFCs in the ozone depletion quandary.  Why don’t other sources of chlorine reach the stratosphere and do the damage? The answer is they do, but to a much lower extent. Chlorine is a highly-reactive element, its strong bond with sodium being an example. It’s difficult for chlorine gas to make it into the stratosphere before it’s washed out by rain and other mechanisms. Massive injections of chlorine into the atmosphere by natural sources, such as the eruption of Pinatubo in 1991, have little effect compared to that produced by CFCs.

The good news is that, after years of political opposition, the climate changed when anti-science elements were replaced in office by more pragmatic elements. Production of CFCs has been curtailed world wide by law. The flow of CFCs into the atmosphere has been dramatically reduced, and these measures are bearing fruit. Recent measurements are indicating the science has been vindicated:

My observation over the past decades has been that political conservatives have not so much been troubled by science as they have been troubled by the consequences of scientific discoveries. When scientist observe a problem that can be addressed by government action, conservatives look toward discrediting the science rather than toward implementing a solution that will require government involvement. In the case of AGW, the government solution would be to impose measures to curb the release of greenhouse gases into the atmosphere. In the case of ozone layer depletion government action was to ban the production and sale of certain fluorinated hydrocarbons. This amounted to direct interference into people’s private lives and into the conduct of profitable business practices. In the case of ozone layer depletion, the scientist prevailed, governments worldwide stepped in, and the problemstarted to be resolved:

And that’s the ozone depletion story in just a few paragraphs.

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