4. The ozone layer and the ozone hole

Ozone⁷ is a compound of three oxygen atoms, O_3, rather than the usual oxygen molecule of two atoms, O₂. In high concentrations it is a bluish green gas that is strongly oxidizing and irritating. It is quite toxic, and while it makes the air smell fresh after thunderstorms, where it occurs transiently in small concentrations, it is a common component of air pollution. Its natural concentration on the Earth’s surface is only a few parts per million (Ozone Hole).

In 1879 Cornu suggested the sharp drop off in transmission of light from the Sun below 300 nm wavelengths was due to atmospheric absorption. In 1880 Hartley postulated the existence of a layer above the lowest region of the atmosphere in which ozone absorbed Solar ultraviolet radiation at wavelengths between 200 and 300 nm. The lowest region of the atmosphere is now called the troposphere⁸, and the region postulated by Hartley is now known as the stratosphere. In 1921 Dobson⁹ and Linderman discovered the temperature in the stratosphere increased with height in contrast to the troposphere where increasing altitude leads to a decrease in temperature (Austr. Gov, 2006). They concluded radiative processes dominate in the stratosphere and the source of energy was from the absorption of Solar ultraviolet radiation by ozone. Figure 7 shows the relationship of the troposphere and the stratosphere.

Figure 7. The relationship between the troposphere, the stratosphere, and the ozone layer. From: http://amap.no/acia/Files/Ozone-Atmosphere_150.jpg

Ozone is created in the stratosphere by the photo-dissociation of O₂, liberating free oxygen atoms, O, which can combine with another oxygen molecule to produce ozone, O₃. To dissociate an oxygen molecule requires 5.08 eV (Pauling, 1949). This is equivalent to the energy of an ultraviolet photon with a wavelength of approximately 243 nm. Ultraviolet photons from the Sun with wavelengths shorter than ~ 240 nm thus are involved in the dissociation of oxygen molecules in the stratosphere where effectively all the shortwave length UVR is absorbed. The free oxygen atoms liberated quickly combine with oxygen molecules to form ozone.

Ozone itself is dissociated by light with wavelengths less than 1100 nm into O₂ and O, and it thus absorbs much of the remaining UVR not involved in the dissociation of oxygen molecules themselves. The oxygen atom freed up by the dissociation of ozone quickly finds another O₂ molecule to pair up forming an ozone molecule anew. The final result is absorption of most of the Sun’s UVR with its energy being transformed into thermal energy in stratosphere. This mechanism thereby protects the plants and animals on the Earth’s surface from dangerous or lethal levels of ultraviolet radiation.

Dobson designed and built a number of precise ultraviolet spectrographs to study the stratospheric ozone. His measurements of ozone’s seasonal and latitude variations beginning in the 1920’s helped established the basic patterns of stratospheric ozone recognized today. Also, he quantified the vertical column density of ozone in the atmosphere. Today, the thickness of the atmospheric ozone layer is measured in Dobson units (DU), one unit of which is defined to be 0.01 mm thickness of ozone at standard temperature and pressure (STP: 0⁰ Celsius; one atmosphere or 1013.25 milibars). It is the thickness of the ozone layer if it were fully compressed in the Earth’s atmosphere. A normal range is 300-500 DU, which translates to a compressed thickness of 3 to 5 mm (Ozone Hole).

Atmospheric ozone is spread from an altitude of 10-12 km to 40 km with the maximum at 17-25 km. This region is known as the ozone layer which is now monitored extensively from the ground and from space. Even in the ozone layer, ozone is not very common existing as only one part in 100,000. A global network of Dobson spectrophotometers was established during the International Geophysical Year in 1956-1957. Today, at least 150 Dobson Spectrophotometers are in use worldwide making daily observations. These measure the intensity of the Solar UV radiation at two different wavelengths, one of which is absorbed by the ozone layer and one of which is not.

The ozone in the ozone layer is known as “good ozone” while ozone created near the ground is sometimes called “bad ozone.” The latter is formed when various compounds, particularly nitrogen oxides, emitted by cars, power plants, industrial plants, refineries, chemical plants, and many other sources react in the atmosphere in the presence of sunlight to produce ozone. Bad ozone is particularly a concern in summer when high temperatures and much sunlight provide the ideal conditions for its formation. Ozone in more than minimal amounts is damaging to the lungs (Ozone Hole).

Chlorofluorocarbons (CFCs) are a large family of compounds containing hydrogen, carbon, chlorine, and fluorine (figure 8). They were first introduced in the 1930’s as safe non-flammable and non-toxic refrigerants and spray can propellants. CFCs are very stable in ordinary circumstances and have been extensively used as aerosol propellants, refrigerants, solvents, foam blowing agents, and the like. They can be readily converted from the liquid to a gaseous form. One common CFC is Freon.

Figure 8. Typical chlorofluorocarbons. From: http://www.ucar.edu/learn/images/cfcmole.gif

Unfortunately, despite their safety record on the ground, CFCs are extreme environmental hazards, because CFCs released into the atmosphere make their way to the stratosphere where the ultraviolet radiation can decompose them into their base elements. The chlorine reacts with ozone to form O₂ and ClO, the latter of which can react with free oxygen atoms to form more chlorine:

CFCl₃ + UVR -> CFCl₂ + Cl
Cl + O₃ -> O₂ + ClO
ClO + O -> Cl + O₂

These reactions not only destroy ozone, but they compete with the formation of ozone itself. Chlorine stays in the atmosphere for many years, and one atom of chlorine can destroy thousands of atoms of ozone (NOAA, 1998). Bromine containing chemicals act in a similar fashion (Ozone Hole).

In 1995, the Nobel Prize for chemistry was awarded to F. Sherwood Rowland, Mario Molina, and Paul Crutzen for their 20 plus years of studying the ozone layer. The ozone layer is critical to life because it absorbs the majority of the Sun’s UVR. In 1970 Crutzen showed nitrogen oxides NO and NO₂ react with stratospheric ozone to hasten its destruction. These oxides are common air pollutants and occur naturally as soil microorganisms produce NO₂ as part of their natural metabolic processes. In 1974 Molina and Rowland proposed that CFCs could be gradually transported through normal air circulation to the stratosphere where their breakdown would catalyze ozone destruction (Molina, 1974). This produced an intense reaction from users and producers of CFCs, but in 1985 a drastic seasonal depletion of stratospheric ozone over Antarctica was discovered, the so-called ozone hole (figure 9) (Science Updates; Ozone Hole).

The ozone hole forms seasonally over Antarctica, because in the Antarctic spring stratospheric clouds form in exceptionally cold temperatures. These clouds allow chemical reactions that transform various chlorine species into forms that are particularly destructive of ozone. The rare stratospheric clouds are formed by water and nitric acid at extremely low temperatures, mainly over Antarctica. The amount of harmful reactive chlorine over the Antarctica becomes much higher than at middle latitudes leading to a faster destruction of ozone and the formation of an ozone hole (NOAA FAQ).

Figure 9. The ozone hole over Antarctica October 5, 1987. From Science Updates and NASA.

Is the ozone hole a real effect, and is ozone depletion a concern that should be addressed? Yes. If atmospheric ozone were to be depleted, high levels of UVR would reach the ground. For example, McKenzie and colleagues (1999) found that long term decreases in summertime ozone over Lauder, New Zealand, at 45⁰ south latitude led to substantial increases in peak ultraviolet radiation. In the summer of 1998-1999, the peak sun burning ultraviolet radiation was 12% more than a decade earlier, whereas UVA radiation which is not absorbed by the ozone layer, showed no increase. It is estimated a 1% decrease in ozone leads to up to a 3% increase in non-melanoma skin cancers (McKenzie, 1999).

The possibility supersonic aircraft and supersonic travel (SST) might be a threat to the ozone layer was noted by the American researcher Harold Johnson in 1971. These aircraft would be capable of releasing nitrogen oxides directly into the ozone layer at altitudes of 20 km (Science Updates). The United Nations became involved in the threat to the ozone layer and the Montreal Protocol¹⁰ was signed in 1987. This called for the complete elimination of CFCs, halons, carbon tetrachloride, and methyl chloroform by 2000 (2005 for methyl chloroform)¹¹ (EPA).

The first comprehensive satellite monitoring of the ozone layer was started in 1978 with the Nimbus-7 satellite which carried a Total Ozone Mapping Spectrometer (TOMS) (figure 9). Today, stratospheric ozone is monitored by Dobson spectrophotometers throughout the world, by ozone sondes, balloon borne instruments that continuously measure ozone concentrations as they ascend through the atmosphere as high as 30 km, and by satellite observations. Satellites have provided a nearly continuous record of ozone observations since October 1978. For example, the Australian Government Bureau of Meteorology Atmosphere Watch Section routinely monitors total ozone data from the Tiros Operational Vertical Scanner (TOVS) and other information from the U.S. National Oceanic and Atmospheric Administration (NOAA) polar orbiting satellites as well as from China’s FY-1D satellite (Austr. Gov, 2006). The currently active NOAA polar orbiting satellites are NOAA 12, 15, 16, and 17.

There is a confusing array of satellites launched by the United States, the European Space Agency (ESA), Russia, Japan, China, and India which monitor the Earth’s weather. One of the most advanced satellites is the Aura mission which was launched on July 15, 2004 as part of NASA’s Earth Observing System (EOS). A prime instrument of Aura is the Ozone Monitoring Instrument (OMI) contributed by the Netherlands’s Agency for Aerospace Programs (NVR) in collaboration with the Finnish Meteorological Institute (FMI). OMI observes Solar backscatter radiation in the visible and ultraviolet. Aura provides daily global coverage of the Earth with its 14 orbits a day. OMI is the latest contributor to the several decades old nearly continuous satellite ozone monitoring that began with the Nimbus-4 in 1970 and the Nimbus-7 TOMS instrument in 1978 (Aura). A daily ozone hole watch is available from NASA's Ozone Hole Watch. Figure 10 shows in graphical form the severity of the ozone hole since 1979.

Figure 10. Comparative values for the ozone concentration and the ozone hole size from 1979 to 2013. From Ozone Hole Watch .

There is now speculation that the stratospheric ozone layer will recover by 2050 to 2070 (Ozone Hole). However, the effect of ozone on the weather and the effect of ozone on global warming are unpredictable. Ozone generates heat in the stratosphere by absorbing the Sun’s ultraviolet radiation and by absorbing infrared radiation from the troposphere. Falling ozone levels may somewhat mitigate the effects of global warming, or they may start a vicious cycle as falling ozone levels decrease stratospheric temperatures producing conditions that favor even more ozone depletion (Ozone Hole)

| Back | | Next |

Notes

[7] In 1839 Christian Schonbein (1799-1868) smelled a previously unknown component of air during electrolysis of water, ascribing this smell to a new compound he called “ozone” which means ill-smelling in Greek.  Jacques-Louis Soret (1827-1890) in 1864 recognized the structure of ozone (the dioxide of the O atom).  From Crutzen (2000).

[8] The troposphere is the lowest level of the Earth’s atmosphere.  It extends up to a height of approximately 10 to 15 km, and its temperature decreases steadily with altitude.  The troposphere and the stratosphere were named by Leon-Philippe Teisserenc de Bort (1855-1913) in 1902 after years of atmospheric measurements with unmanned balloons. He found that at altitudes above 8-10 km the temperature did not decline further.  He called the region of more constant temperature maintained by the ozone absorption of Solar UVR the “stratosphere” and the region below it the “troposphere.”  The tropopause is the region where the temperature remains constant before increasing in the stratosphere.  Teisserenc’s balloons only reached the tropopause and did advance into the stratosphere.  From: Crutzen (2000).

[9] At Oxford G.M.B. Dobson (1889-1976) together with F.A. Lindemann (1886-1957) (later Lord Cherwell) studied meteor trails from which they deduced the temperature profile above the troposphere was not constant.  Dobson followed on earlier work of Charles Fabry (1867-1945) and Henri Buisson (1873-1944) and measured ozone by observing its absorption in the Solar ultraviolet spectrum.  He was conferred the title of Oxford Professor in 1945, and his work with Alan Brewer led to the Brewer-Dobson Circulation model for the stratosphere.  From: http://www-atm.physics.ox.ac.uk/user/barnett/ozoneconference/dobson.htm. and other sources.

[10] The official name for this treaty is The Montreal Protocol on Substances That Deplete the Ozone Layer.  It was originally signed in 1987 and substantially amended in 1990 and 1992.  The Vienna Convention for the Protection of the Ozone Layer (1985) established the framework for the later Montreal Protocol.  From: http://www.ciesin.org/TG/PI/POLICY/montpro.html

[11] Halons are hydrocarbon compounds containing a combination of fluorine, chlorine, or bromine substituted for some or all of the hydrogen atoms.  They are mainly used in fire extinguishers.  Carbon tetrachloride was formally used as an aerosol can propellant, a solvent, a cleaning fluid, and in fire extinguishers. It is carcinogenic and can cause liver, kidney, and nervous system damage.  Methyl chloroform is used as a solvent for adhesives and for metal degreasing as well as a dry cleaning agent.