Ozone in the Atmosphere ; Chemical Destruction of Ozone in the Stratosphere

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 between 20 and 30 kilometers (12 and 18 miles) above the Earth's surface. This region of the atmosphere is part of the stratosphere. Within the stratosphere, the level of maximum concentration is at about 25 km (15 miles) where the average ozone concentration is 10 ppm (parts per million); that is, for every one million molecules, 10 are ozone molecules. The ozone in this region is commonly known as the ozone layer. It is important to realize that although we refer to this region of higher ozone concentrations as the ozone layer, ozone makes up only a small percentage the gas molecules in ozone layer (10 out of 1 million). The fact that there is so little ozone naturally in the stratosphere is one reason we are concerned about human activities that deplete stratospheric ozone, as this ozone is beneficial for life. The other region of elevated ozone concentration happens just above the ground surface and is mainly the result of human activity. This ozone can be harmful and is considered pollution. The figure above shows how ozone is typically distributed in the atmosphere.

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 and the issue of possible depletion of stratospheric ozone due to human activity.

Summary for Stratospheric Ozone Depletion

• We know that certain manmade chemicals, CFCs several other molecules, such as carbon tetrachloride, methyl bromide, methly chloroform, and halons, chemically deplete the amount of ozone in the stratosphere. CFC stands for chlorofluorocarbon. A CFC is a methane molecule (CH₄, which is a central carbon atom with 4 hydrogen atoms attached) that has one or more chlorine and or fluorine atoms substituted for the hydrogen atoms. More generally, ozone-depleting chemicals contain chlorine and/or bromine. When chlorine and bromine atoms break free from these molecules in the stratosphere, they can destroy ozone. Because these manmade molecules are chemically unreactive, once released, they remain in the atmosphere for an average of 100 years. Eventually, they are lifted from the lower troposphere into 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. Here is a link to a picture showing A simplified chemical reaction pathway describing how a CFC molecule relaeses chlorine leading to ozone destruction. The animation below shows the destruction of an ozone molecule by a chlorine atom, which was liberated from a CFC. Note that the chlorine atom is freed in a subsequent reaction, allowing it to chemically destroy more ozone molecules.
• Summary of the observed loss of stratospheric ozone for different latitude zones:
• Tropical Latitudes (30°North to 30°South): Little, if any, changes observed
• Middle Latitudes (30° to 60° in both Northern and Southern Hemisphere): 6% decrease from 1980 to 1996, with slight increase after 1996
• Arctic Latitudes (60°North to North Pole): Reduction of ozone highly variable from year to year. Can spike to 30% reduction during cold periods in winter and spring
• Antarctic Latitudes (60°South to South Pole): Region where seasonal ozone "hole" occurs. The ozone hole is described in the next section
• Except for the ozone hole described below (and during extremely cold periods in the Arctic), ozone is destoyed in homogeneous chemical reactions (gas phase chemical reactions) between chlorine (or bromine) radicals and ozone. CFCs and other manmade chemicals provide the chlorine or bromine 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 December (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.
• Although often referred to as the ozone 'hole', it is really not a hole but rather a thinning or a depletion of a large amount of the ozone in the stratosphere. The term 'hole' in used in reference to the large seasonal loss of stratospheric ozone over the southern pole, not complete removal of all ozone.
• 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. While it was previously known that certain manmade chemicals were capable of destroying stratospheric ozone, it was not realized that destruction of stratopheric ozone could take place so quickly under the conditions of the Antarctic stratosphere.
• A much smaller and weaker ozone "hole" has been observed over the Arctic region over short periods of time in years when it is cold enough for the formation of extensive polar stratospheric clouds, but the conditions necessary for rapid, sustained ozone depletion do not happen in the arctic stratosphere as they do in the antarctic stratosphere.
• Montreal Protocol (International Treaties to Save Ozone)
• The Montreal Protocol, signed on Sept. 16 1987, began a gradual phaseout of production and use of CFCs, halons, and other ozone depleting substances (ODS). It is considered by many to be the most successful international agreement in history. September 16 is now known as the International Day for the Preservation of the Ozone Layer, or, more simply, World Ozone Day.
• CFCs have been replaced with HCFCs (hydrochlorofluorocarbons), and HFCs (hydrofluorocarbons). HCFCs still contain chlorine but are more reactive than CFCs and are therefore less likely to make it into the stratosphere. HFCs don't contain chlorine at all and are not a threat to the ozone layer. Both HCFCs and HFCs are both greenhouse gases (as are CFCs) and thus might contribute to global warming. This will be further discussed when we cover global warming and greenhouse gases.
• Because of the Montreal Protocol and many subsequent revisions (see Wikipedia Page on the Montreal Protocal for a complete list) the ozone hole is now beginning to recover.
• The figure below shows the sharp decline in the consumption of known ozone-depleting substances from 1989 to 2009.

The worldwide consumption (use) of ozone depleting products (ODP) in millions of tonnes from 1989 to 2009. There has been a sharp decrease after the Montreal Protocol.

• 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 blue line in figure below on the left side, which corresponds with the axis on the right side of the graph). The figure on the left also indicates that the total loss of ozone each year (during the ozone hole period) has peaked and is trending down. The figure on the right shows that the size of the ozone hole each year has stabalized since the late 1990s and is showing signs of getting smaller. The size is defined as the area of the region that has ozone levels below a minimum threshold.

Red line is observed ozone mass deficit ("missing ozone") and green line is the projected ozone mass deficit over south polar region at time of minimum ozone. The blue line (axis on right) shows the measured and projected changes in stratospheric chlorine content.	Maximum size of ozone hole over south pole each year from 1980 to 2012.

• 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 around the middle of this century as indicated by the green line in the figure on the left side above. Recent measurements indicate that the size of the ozone hole continues to show signs that it is slowly getting smaller, although there is quite a bit of variability from year to year as the size and duration of the ozone hole depends on the specific atmospheric conditions in the Antarctic stratosphere each year. Here is a link to a NASA video that claims large ozone holes over Antarctica will stop forming by 2040, Big Ozone Holes Headed For Extinction By 2040.

• 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, i.e., the amount of sun exposure and use of sunscreens, than on the rather small ozone depletion that has occurred up until now. In fact a 2010 report by the science advisors to the Montreal Protocal, report that in middle latitudes, surface ultraviolet radiation has been about constant over the previous decade from 2001 to 2010. However, keep in mind that this is for the Northern Hemisphere mid-latitudes where most of us live. In other parts of the world, there are seasonally higher amounts of uv radiation at the surface than were present before the advent of CFCs. This is particularly true for the Antarctic region and parts of the higher latitudes in the Southern Hemisphere that are in or close to the ozone hole region.
• In spite of all that is reported on this page, the incidence of both melanoma and melanoma skin cancers has been increasing over the past decades in the United States and throughout much of the world. Occasionally you will see someone trying to link that increase with human caused stratospheric ozone depletion. However, this has little if anything to do with stratospheric ozone depletion. Again, the main factors that predispose to the development of melanoma seem to be connected with excessive exposure to the sun and a history of sunburn. These factors lie within each individual's own responsibility. There are other contributing reasons for the increase in reported cases of skin cancers. One is simply due to the fact that people live longer today and this increases the chance that they will develop skin cancer. Another reason is that we have better detection and screening for skin cancer as well as public education campaigns about the importance of early detection, which increases the number or reported cases.
• 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.