Ozone in the Atmosphere

Ozone is very rare in our atmosphere, averaging about three molecules of ozone for every 10 million air molecules. In spite of this small amount, ozone plays a vital role in the atmosphere.

Ozone is a form of oxygen that comprises three atoms (O3) rather than the two atoms (O2) found in ordinary molecular oxygen.

Ozone is mainly found in two regions of the Earth's atmosphere. Most ozone (about 90%) resides in a layer that begins between 6 and 10 miles (10 and 17 kilometers) above the Earth's surface and extends up to about 30 miles (50 kilometers). This region of the atmosphere is called the stratosphere. The ozone in this region is commonly known as the ozone layer . Most of the remaining ozone is in the lower region of the atmosphere, which is called the troposphere. The figure (above) shows an example of how ozone is distributed in the atmosphere.

The level of maximum concentration is at about 25 km (15 mi) and approximately 10 ppm (parts per million); that is, for every one million molecules, 10 are ozone molecules.

The ozone molecules in the upper atmosphere (stratosphere) and the lower atmosphere (troposphere) are chemically identical, because they all consist of three oxygen atoms and have the chemical formula O₃. However, they have very different roles in the atmosphere and very different effects on humans and other living beings. Stratospheric ozone (sometimes referred to as "good ozone") plays a beneficial role by absorbing most of the biologically damaging ultraviolet sunlight (called UV-B), allowing only a small amount to reach the Earth's surface. The absorption of ultraviolet radiation by ozone creates a source of heat, which actually forms the stratosphere itself (a region in which the temperature rises as one goes to higher altitudes). Ozone thus plays a key role in the temperature structure of the Earth's atmosphere. Without the filtering action of the ozone layer, more of the Sun's UV-B radiation would penetrate the atmosphere and would reach the Earth's surface. Many experimental studies of plants and animals and clinical studies of humans have shown the harmful effects of excessive exposure to UV-B radiation.

Because ozone is toxic to humans and many other animals and plants, ozone near the ground surface is sometimes referred to as "bad ozone". Naturally, very little ozone is present near the Earth's surface. But, due to human activities, harmful concentrations can develop under certain conditions. The process that creates ozone near the ground is known as photochemical smog. It occurs when vehicle exhaust, other industrial chemicals, and abundant sunlight mix together in a complicated series of chemical reactions. This can be a very big problem in large cities in the summertime. Los Angeles, Phoenix, and Washington D.C. are examples of cities where ozone concentrations sometimes reach dangerously high levels. The national weather service will issue ozone warnings when ozone levels become dangerously high near the ground. The rest of this page concerns only stratospheric ozone loss.

Summary for Stratospheric Ozone Depletion

• We know that certain man-made chemicals, CFCs (and several other molecules), deplete the amount of ozone in the stratosphere. The reason is that these ozone-depleting chemicals contain chlorine and bromine. When chlorine and bromine atoms are released from these molecules in the stratosphere, they can destroy ozone. Because many of these molecules are generally unreactive, once released, they remain in the atmosphere for an average of 100 years. Eventually, they are lifted from the surface to the stratosphere. Once in the stratosphere, they are exposed to high levels of ultraviolet radiation. This breaks up the molecule and releases chlorine and bromine atoms, which then destroy ozone in a subsequent chemical reaction. Although the amount of chlorine and bromine in the stratophere is quite low, alarmingly each free chlorine and bromine atom may be capable of destroying up to 100,000 ozone molecules. A simplified chemical reaction pathway for how a CFC molecule relaeses chlorine leading to ozone destruction is shown in figure 1.
• Since 1979, the measured loss of stratospheric ozone has been:
• 10% at high latitudes
• (5-10)% at middle latitudes
• little change for tropics
• Except for the ozone hole described below, ozone is destoyed in homogeneous chemical reactions (gas phase chemical reactions) between chlorine (or bromine) radicals and ozone. CFCs provide the chlorine radicals. Depletion of ozone via homogeneous chemistry is relatively slow.
• Ozone "hole"
• large depletion of stratospheric ozone (up to 70%) that occurs ONLY over Antarctica from late September through early November (springtime in the Southern Hemisphere). The meteorological conditions and other factors required for this rapid destruction of ozone only happen in the Antarctic stratosphere in Southern Hemisphere spring.
• The Antarctic ozone hole has been observed every year since 1985. The geographical size and amount of the ozone depletion within the ozone hole vary from year and depends somewhat on specific atmospheric conditions in addition to the amount of free chlorine and bromine present.
• It has been proven that CFCs (and similar molecules) are responsible.
• Result of complex chemical reactions that can only happen under the extremely cold conditions that develop in the Antarctic stratosphere. It is a heterogeneous chemical reaction because some of the chemical reactions take place on the surfaces of ice crystals. These ice crystals make up polar stratospheric clouds, which only form when air temperatures drop below about -90°F. This chemical pathway leads to very rapid destruction of ozone.
• Once the Antarctic stratosphere warms by December, ozone levels return to near normal levels.
• Discovery of the ozone hole is what scared the developed world to take action on the statospheric ozone depletion problem.
• A much smaller and weaker ozone "hole" has been observed over the Arctic region, but conditions necessary for extreme ozone depletion do not happen in the arctic stratosphere as they do in the antarctic stratosphere.
• Montreal Protocol (1987) -- Developled countries agreed to phase out use of CFCs and other ozone-depleting chemicals in World treaty. Developed countries have almost completely substituted "ozone-friendly" chemicals for CFCs since mid 1990s; there are still a few ozone-depleting chemicals that are in the process of being phased out over the next decade or so in accordance with international agreements.

Stratospheric chlorine from various sources. Solid lines based on measurements through 2003, dashed lines based on future projections.

• It is expected that the ozone layer will fully recover, but it will take decades due to CFCs already in the atmosphere and leaking CFCs from old style refrigeration systems. (Keep in mind that the average lifetime of a CFC molecule in the atmosphere is about 100 years). In fact we are beginning to see evidence that chlorine levels in the stratosphere are decreasing (see figure above).

Size of the Antarctic Ozone hole based on satellite measurements through 2004 .

• We should expect to continue to observe an Antarctic ozone hole each year for at least decades. The size of the ozone hole and the amount of ozone destoyed varies quite a bit from year to year because the atmospheric conditions in the Antarctic winter and early spring are different from year to year. However, it is expected that the long-term trend will be toward a smaller ozone hole and less ozone depletion in the Antarctic. The figure above indicates that the size of the Antarctic ozone hole has stabalized recently. World-wide measurements are showing that the ozone layer is recovering, although full recovery to pre-industial levles of stratospheric ozone is not expected until at least the middle of this century.
• Despite all the doom and gloom articles that have been written about ozone depletion, most likely none of us has been exposed to dangerously high levels of ultraviolet radiation due to ozone depletion. Your risk of developing uv-related problems (such as skin cancer) depends more on your lifestyle than on the rather small ozone depletion that has occurred up until now.
• The international agreements do show that the world is capable of dealing with a global scale environmental threat, resulting from advanced technology. In this case, the cause of ozone destruction was easily proven and substitutes for CFCs were easily found without much additional cost. However, it remains to be seen if something similar will occur with greenhouse gas emissions and global warming, where we do not have a clear cause and effect mechanism nor a cheap alternative (to burning fossil fuel).

If you are interested, more detailed information about ozone depletion and the Antarctic ozone hole is provided by US Environmental Protection Agency Ozone Home Page.