Background Information
From the Prezi, you should have learned that, much as scientists have broken the Earth up into layers (from innermost to outermost: inner core, outer core, lower mantle, upper mantle, and crust) that are distinguished by their composition and temperature, the atmosphere above the Earth is broken into layers (from innermost to outermost: troposphere, stratosphere, mesosphere, thermosphere, and exosphere). The image on the left below shows a profile of the Earth’s atmosphere. We live in the layer of the atmosphere known as the troposphere, which starts at the surface of the Earth and can extend up to 20 kilometers. The next layer is the stratosphere, which is the section found between twenty kilometers and fifty kilometers above the Earth’s surface.
Similar to the troposphere, the stratosphere is comprised almost entirely of nitrogen and oxygen. In contrast to the troposphere, it contains relatively high concentrations of ozone. In fact, the peak ozone concentration occurs between 30 and 35 kilometers above the Earth’s surface, and this is known as the “ozone layer.” Approximately 90% of the ozone in the atmosphere is found in the stratosphere. The middle image below illustrates how the concentration of ozone varies through the different layers of the atmosphere. Keep in mind, as you were informed in the Prezi, that even though the concentration of ozone is highest in the stratosphere, ozone still makes up a relatively small percentage of the gases in this layer: Only 0.0003 % of the total amount of gas in the stratosphere is ozone. The image on the right below shows you the percentages of several key gases in the stratosphere.
So why are we talking so much about the stratosphere and the ozone in it? To appreciate this, you need to think back to the previous lab when you learned that the radiant energy from the sun is composed of different types of electromagnetic (EM) radiation (visible, ultraviolet or UV, etc.). These different types of radiation are distinguished by their wavelengths and therefore their energies. With its shorter wavelengths (compared to visible light), UV radiation is energetic enough to cause damage to certain things that absorb it – such as your skin (leading to cancer), your eyes (leading to cataracts), and plant leaves (reducing their size). For example, ultraviolet radiation from sun exposure is the primary cause of skin cancer, and there are at least 2,000,000 new cases of skin cancer in the United States each year.
Just as EM radiation is separated into different types based on wavelength and energy, so is UV radiation: There is UV-A (longest wavelengths), UV-B, and UV-C (shortest wavelengths). When the ozone layer is in tact, it absorbs 50% of the UV-A radiation, 90% of the UV-B radiation, and all of the UV-C radiation coming from the sun. (See a graph of this on the left below.) Because of this, when you purchase a sunscreen (aka sunblock), you should look for one that offers UV-A and UV-B protection, so that the sunscreen will hopefully absorb the portions of the UV radiation coming from the sun that the ozone in the stratosphere does not. To help you visualize the important role of the ozone in absorbing (as opposed to reflecting) UV radiation, please take a look at the animation below the picture.
In the 1920’s, a team of researchers at General Motors Research Corporation lead by Thomas Midgley created the first chlorofluorocarbon (or CFC), which they called Freon (http://en.wikipedia.org/wiki/Thomas_Midgley,_Jr.). This team showed that the compound (made of carbon, hydrogen, chlorine, and fluorine) would be a safe alternative to the refrigerants available at the time. Later, other CFC’s were prepared, and the day found use in a wide variety of other places, including aerosol sprays, foams, and fire extinguishers. In the 1970’s, atmospheric chemists Mario Molina and Sherwood Rowland showed that reactions taking place when the normally stable CFC’s hit the lower temperatures and higher amounts of UV radiation in the stratosphere could break them down – and that once they were broken down, they could attack and destroy ozone molecules. An image from the first paper they published on this serious environmental issue appears below. The discovery that the CFC’s – even when present in extremely low concentrations in the stratosphere (as low as 1 in every 2,000 gas molecules) – could have this effect on the ozone layer was shocking to Sherwood and Rowland – and to the rest of the world as well. They describe the process by which CFCs break down ozone, and their realization of the consequences of this, in the video clip to the right below from the documentary Shattered Sky.
To say that it is unusual for countries across the world to reach agreement on anything is an understatement. So when the countries came together to sign the Montreal Protocol in September 1987 to phase out the production and use of CFCs (and related compounds), and then began executing this international agreement in January 1989, this was a momentous occasion. It demonstrated how serious the issue at stake was: doing nothing and allowing the concentrations of ozone-depleting chemicals to build up could have catastrophic affects on all the living things on this planet. While the Montreal Protocol was an unbelievably important step forward, it is important to note that it will take a while for this legislation to reverse the effect of the CFCs that were released before it was put into place. The table below – which shows the atmospheric lifetimes for a number of ozone-depleting chemicals, or how long it takes for the concentration of one of these compounds to decrease to their normal level – should help you understand why this is the case. (As you examine this table, you should note that CFC-12 – dichlorodifluoromethane – was the first CFC created and was produced in the largest amounts.)
