Ozone in the Atmosphere

Discussion Questions

Ozone in the Atmosphere

Most of the ozone in the atmosphere is in the stratosphere of the atmosphere, with about 8% in the lower troposphere. As mentioned there, the ozone is formed due to photo reaction. Information regarding a current state of stratospheric ozone, check the environment Canada Internet pages.

The ozone level is measured in Dobson Unit (DU), named after G.M.B. Dobson, who investigated the ozone between 1920 and 1960. One Dobson Unit (DU) is defined to be 0.01 mm thickness of ozone at STP when all the ozone in the air column above an area is collected and spread over the entire area. Thus, 100 DU is 1 mm thick. The animated picture shown here is from this link, which shows the ozone hole in the artic region. Often, the observed thickness is 300 DU.

What is UV?

In the electromagnetic radiation spectrum, the region beyond the violet (wavelength ~ 400 nanometer nm) invisible to eye detection is called ultraviolet (UV) rays. Its wavelength is shorter than 400 nm.

UV is divided into three regions:

UV A, wavelength = 400 - 320 nm
UV B, wavelength = 320 - 280 nm
UV C, wavelength = < 280 nm
Obviously, photons of UV C are the most energetic.

UV-A radiation is needed by humans for the synthesis of vitamin-D; however, too much UV-A causes photoaging (toughening of the skin), suppression of the immune system and, to a lesser degree, reddening of the skin, and cataract formation.

Ozone strongly absorbs UV B and C, but the absorption decreases as the wavelength increases to 320 nm. Very little UV C reaches the Earth surface due to ozone absorption.

How is ozone produced in the atmosphere?

When an oxygen molecule receive a photon (h v), it dissociates into monoatomic (reactive) atoms. These atoms attack an oxygen molecule to form ozone, O3. O2 + h v ® O + O
O2 + O ® O3
The last reaction requires a third molecule to take away the energy associated with the free radical O and O2, and the reaction can be represented by O2 + O + M ® O3 + M*
The over all reaction between oxygen and ozone formation is: 3 O2 « 2 O3

The absorption of UV B and C leads to the destruction of ozone

O3 + h v ® O + O2
O3 + O ® 2 O2
A dynamic equilibrium is established in these reactions. The ozone concentration varies due to the amount of radiation received from the sun.

Example 1

The enthalpy of formation of ozone is 142.7 kJ / mol. The bond energy of O2 is 498 kJ / mol. What is the average O=O bond energy of the bent ozone molecule O=O=O?

Solution
The overall reaction is

3 O2 ® 2 O3   DH = 286 kJ
Note that 3 O=O bonds of oxygen are broken, and 4 O-O bonds of ozone are formed. If the bond energy of ozone is E, then E = (3*498 + 286) kJ / 4 mol
      = 445 kJ / mol

Discussion
The ozone bonds are slightly weaker than the oxygen bonds. The average bond energy is not the bond energy for the removal of one oxygen from ozone.

O3 + h v ® O + O2 Can the energy to remove one oxygen be estimated from the data given here?

The techniques used in this calculation is based on the principle of conservation of energy.

Example 2

The bond energy of O2 is 498 kJ / mol. What is the maximum wavelength of the photon that has enough energy to break the O=O bond of oxygen?

Solution
The energy per O=O bond is:

(498000 J/mol) / (6.022x1023 bonds/mol) = 8.27x10-19 J/bonds Thhe wavelength wl of the photons can be evaluated using E = h c / wl wl = (6.626x10-34 J s)*(3x108 m/s) / (8.27x10-19 J)
      = 2.403-7 m

Discussion
The visible region range from 300 nm to 700 nm, and radiation with a wavelength of 240 nm is in the ultraviolet region. Visible light cannot break the O=O bond, and UV light has enough energy to break the O=O bond.

What are CFCs?

Chemist Roy J. Plunkett discovered tetrafluoroethylene resin while researching refrigerants at DuPont. Known by its trade name, Teflon, Plunkett's discovery was found to be extremely heat-tolerant and stick-resistant. After ten years of research, Teflon was introduced in 1949. His continued research led to the usage of chlorofluorohydrocarbons known as CFCs or freon as refrigerants.

CFCs are made up of carbon, hydrogen, fluorine, and chlorine. DuPont used a number system to distinguish their product based on three digits. The digits are related to the molecular formulas.

For example, CFC (or freon) 123 should have a formula C2HF3Cl2. The number of chlorine atoms can be deduced from the structural formula of saturated carbon chains. CFC's containing only one carbon atom per molecule has only two digits. Freon 12 used for fridge and automobil air conditioners has a formula of CF2Cl2.

The nontoxic and nonflammable CFCs have been widely used as refrigerants, in aerosol spray, and dry cleaning liquid, foam blowing agents, cleansers for electronic components in the 70s, 80s and early 90s.

In 1973, James Lovelock demonstrated that all the CFCs produced up to that time have not been destroyed, but spread globally throughout the troposphere. (Lovelock's report was later published: J. E. Lovelock, R.J.Maggs, and R.J. Wade, (1974); Nature, 241, 194) In the article, concentrations of CFCs at some parts per 1011 by volume was measured, and they deducted that with such a concentration, CFCs are not destroyed over the years. In 1974, Mario J. Molina published an article in Nature describing the ozone depletion by CFCs. (see M.J. Malina and F.S. Rowland, (1974); Nature, 249, 810) NASA later confirmed that HF was present in the stratosphere, and this compound had no natural source but from the decomposition of CFCs. Molina and Rowland suggested that the chlorine radicals in CFCs catalyze the decomposition of ozone as discussed below.

How do CFCs help depleting ozone?

A relatively recent concern is the depletion of ozone, O3 due to the presence of chlorine in the troposphere, and eventually their migration to the stratosphere. A major source of chlorine is Freons: CFCl3 (Freon 11), CF2Cl2 (Freon 12), C2F3Cl3 (Freon 113), C2F4Cl2 (Freon 114). Freons decompose in the troposphere. For example, CFCl3 ® CFCl2 + Cl
CF2Cl3 ® CF2Cl + Cl.
The chlorine atoms catalyze the decomposition of ozone,

Cl + O3 ® ClO + O2 and ClO molecules further react with O generated due to photochemical decomposition of ozone:

O3 + h v ® O + O2,
ClO + O ® Cl + O2
O + O3 ® O2 + O2.
The net result or reaction is
2 O3 ® 3 O2
Thus, the use of CFCs is now a world wide concern. In 1987, one hundred and forty nine (149) nations signed the Montreal Protocol. They agreed to reduce the manufacturing of CFCs by half in 1998. They also agree to phase out CFCs.

How is ozone depletion in the polar region different from other regions?

The debate of ozone depletion often involves the North and South Poles. In these regions when temperatures drop to 190 K, ice cloud is formed. The ice crystals act as heterogeneous catalyst converting HCl and ClONO2 into HNO3 and Cl2, HCl + ClONO2 ® HNO3 + Cl2
H2O + ClONO2 ® HNO3 + HOCl.
Both Cl2, and HOCl are easily photolyzed to Cl atoms, which
catalyze the depletion of ozone. This has just been discussed in the previous section.

What has been done and what can be done to reduce ozone depletion?

The U.S. and Canadian governments have banned the use of Freons in aerosol sprays, but their use in air conditioner and cooling machines continue. In order to eliminate Freon in the atmosphere, international concerted effort and determination is required. However, sound and reliable scientific information is required.

The banning of CFCs opens a research opportunity for another invention to find its substitute. Who knows what other problems will the new product bring?

Ozone Information on the Internet

Stratospheric Ozone -- An electronic textbook gives you a better understanding of the stratospheric ozone. It contains 12 chapters each written by an authoritative scientist for people receiving a science and engineering program in a university.

Total Ozone Mapping Sepctrometer (TOMS) instruments, the NASA site for ozone mapping.

Confidence Building Questions

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