9.4.D - Atmospheric Chemistry


The ozone layer is a layer in Earth's atmosphere containing relatively high concentrations of ozone (O3). However, "relatively high," in the case of ozone, is still very small with regard to ordinary oxygen, and is less than 10 parts per million, with the average ozone concentration in Earth's atmosphere being only about 0.6 parts per million. The ozone layer is mainly located in the lower portion of the stratosphere from approximately 20 to 30 kilometres above Earth, though the thickness varies seasonally and geographically.

The ozone layer was discovered in 1913 by the French physicists Charles Fabry and Henri Buisson. Its properties were explored in detail by the British meteorologist G. M. B. Dobson, who developed a simple spectrophotometer (the Dobsonmeter) that could be used to measure stratospheric ozone from the ground. Between 1928 and 1958 Dobson established a worldwide network of ozone monitoring stations, which continue to operate to this day. The Dobson unit, a convenient measure of the columnar density of ozone overhead, is named in his honor.

The ozone layer absorbs 97-99% of the Sun's medium-frequency ultraviolet light (from about 200 nm to 315 nm wavelength), which potentially damages exposed life forms on Earth. It can be depleted by free radical catalysts, the most common of which is atomic chlorine (Cl) which is produced by the decomposition of chlorofluorocarbons (CFCs). These highly stable compounds are capable of surviving the rise to the stratosphere, where Cl radicals are liberated by the action of ultraviolet light. Each radical is then free to initiate and catalyse a chain reaction capable of breaking down over 100,000 ozone molecules.

The breakdown of ozone in the stratosphere results in the ozone molecules being unable to absorb ultraviolet radiation. Consequently, unabsorbed and dangerous ultraviolet-B radiation is able to reach the Earth's surface. Ozone levels over the northern hemisphere have been dropping by 4% per decade. Over approximately 5% of the Earth's surface, around the north and south poles, much larger seasonal declines have been seen, and are described as ozone holes.

In this topic students:

  • Describe the layered structure and composition of the atmosphere
  • Identy pollutants in the troposphere
  • Distinguish the properties of oxygen, ozone and the oxygen free radical
  • Name halogenated hydrocarbons using IUPAC nomenclature
  • Distinguish between isomers with the same molecular formulae but different structural formulae
  • Describe the depletion of ozone in the stratosphere and assess the effectiveness of steps taken to alleviate the problem


Atmopshere

Composition

The Earth's atmopshere is about 120 km thick and is divided into layers. The distinction between one layer and the next depends on temperature and not on the distance from the surface of the Earth.

The troposhere is 15 km thick and is the lowest layer. It contains about 75% of the gases in the atmosphere and all of the weather including clouds, lightning and rain. Temperature decreases as altitude increases to a low of about -60°C.

The stratosphere begins about 15 km up and ends about 50 km up. In this layer temperatures increase as altitude increases to a high of about 0°C. The ozone layer is found within the stratosphere at about 25 km above the Earth's surface.

The outer layer of the atmosphere extends to about 120 km above the Earth. Temperature falls as altitude increases to a low of about -100°C (the mesosphere) but then increases to about 1000°C (the
thermosphere
). Gaseous ions and free electrons exist in this layer.

Pollutants

The table below summarises the main pollutants found in the lower atmosphere and their sources. No natural sources are given as these are only considered as pollutants owing to man made sources. Carbon dioxide is not considered as a pollutant by the EPA as it is not toxic or carcinogenic, but you could argue for its inclusion in this list as it is produced by the burning of fossil fuels and it may cause problems for humans in the long term.

Pollutant
Source
Carbon monoxide (CO) Incomplete combustion of fossil fuels; bush fires
Nitrogen dioxide (NO2) Internal combustion engines; power stations
Sulfur dioxide (SO2) Coal burning power stations; smelting of sulfide ores
Ozone (O3 ) Photochemical smog
Hydrocarbons Unburnt vehical exhaust; factories
Soot Incomplete combustion of fossil fuels; bush fires

 

Ozone

Ozone in the Troposphere

Ozone is not found naturally in the troposphere but is produced by electrical discharges from high voltage devices and from photochemical smog. The reaction for the production of ozone during a lightning strike is shown below.

O2 (g) → 2O (g)

O (g) + O2 (g) → O3 (g)

Oxygen molecules are split into oxygen atoms, called radicals, by electricity. These highly reactive radicals combine with oxygen molecules by forming a coordinate covalent bond. This is a spceical type of covalent bond where a pair of electrons is shared between two atoms but both electrons originated from only one of these atoms.

The ozone molecule can be represented as containing a covalent double bond and a coordinate covalent single bond.

Measurements show that the bonds between the oxygen atoms in ozone are of equal length and strength and can be represented so:

This causes the lower stability of the ozone molecule. Each of the oxygen atoms in the diatomic oxygen provides their valence electrons equally to form the double bond.

Alternatively, ozone is produced by photochemical smog. Nitrogen monoxide is produced in internal combustion engines and immediately reacts to form brown nitrogen dioxide.

2NO (g) + O2 (g) → 2NO2 (g)

Nitrogen dioxide is decomposed by ultraviolet radiation to nitrogen monoxide and the oxygen free radical. These highly reaction oxygen atoms then react with oxygen to form ozone.

2NO2 (g) → 2NO (g) + O (g)

O (g) + O2 (g) → O3 (g)

This mixture of nitrogen dioxide and ozone is known as photochemical smog. Ozone is a dense gas and sits low to the ground in cities. It is toxic to humans as it readily oxidises living tissue.

Ozone Formation in the Stratosphere

Levels of ozone at various altitudes and related blocking of
several types of ultraviolet radiation. The width of the UV
band represents its intensity at various altitudes.

Ozone is found in a narrow layer 25 km above the surafce of the Earth within the stratosphere. The concentration of ozone here varies between 2 and 8 ppm. There is a natural balance in the stratosphere between the formation and decomposition of ozone which is facilitated by the presence of UV light from the sun.

O2 (g) + UV → 2O (g)

O (g) + O2 (g) →O3 (g)

O3 (g) + UV → O2 (g) + O (g)

The UV radiation absorbed by these reactions is high energy UV-C radiation (240-280 nm) and some UV-Bradiation (280-320 nm). Ozone does not absorb UV-A radiation which passes through the atmosphere to the Earth's surface where it is needed by plants for photosynthesis. The little UV-B radiation that does reach the Earth causes sunburn, melanoma and cataracts in humans.

Properties Oxygen and Ozone

Oxygen and ozone are allotropes. This means that they are different arrangements of the same atom. They have different properties because the bonding within the moelcules is slightly different as is the strength of the intermolecular forces. Ozone is more reactive than oxygen because it has a much lower bond energy,
355 kJ versus 498 kJ for oxygen. Ozone is more dense than oxygen due to the extra oxygen atom per molecule. Ozone has a higher melting and boiling point due to the stronger intermolecular forces in a bent molecule than in a linear molecule. Other properties are tabulated below.

Property
Ozone
Oxygen
Reactivity
greater
less
Boiling point (°C)
-111
-183
Density
1.61 g/mL
1.15 g/mL
Odour
pungent
odourless
Colour
pale blue gas
colourless gas
Shape
bent
linear

The single oxygen atom, known as a radical, can be found in the high energy atmosphere of the troposphere. These are very reactive and only exist for a short period of time before they react to form oxygen and ozone.

Measuring Ozone

High latitude ozone concentrations
measured over the South Pole using an
instrument such as TOMS.

The Total Ozone Mapping Spectrometer (TOMS), launched in July 1996 onboard an Earth Probe Satellite (TOMS/EP), continues NASA's long term daily mapping of the global distribution of the Earth's atmospheric ozone. TOMS/EP will again take high-resolution measurements of the total column amount of ozone from space that began with NASA's Nimbus-7 satellite in 1978 and continued with the TOMS aboard a Russian Meteor-3 satellite until the instrument stopped working in December 1994. This NASA-developed instrument, measures ozone indirectly by mapping ultraviolet light emitted by the Sun to that scattered from the Earth's atmosphere back to the satellite. The TOMS instrument has mapped in detail the global ozone distribution as well as the Antarctic "ozone hole," which forms September through November of each year.

Ozone, a molecule made up of three oxygen atoms, shields life on Earth from the harmful effects of the ultraviolet radiation of the Sun. The increased amounts of ultraviolet radiation that would reach the Earth's surface because of ozone depletion could increase the incidence of skin cancer and cataracts in humans, harm crops and interfere with marine life.

Researchers face two crucial problems in ozone studies: finding a slow, long-term trend among a variety of short-term trends, and ascertaining how much of the change in global ozone is due to human activities and how much is attributable to natural atmospheric processes. In order to separate these factors, scientists must record data over at least a complete solar cycle, 11 years. TOMS instruments aboard Nimbus-7 and Meteor-3 satellites have proved invaluable in meeting this requirement.

Measuring Techniques

Ozone concentrations over Arosa, Switzerland from
1927 to 2001.

The TOMS instrument is a second-generation backscatter ultraviolet ozone sounder. TOMS can measure "total column ozone" - the total amount of ozone in a "column" of air from the Earth's surface to the top of the atmosphere under all daytime observing and geophysical conditions. TOMS observations cover the near ultraviolet region of the electromagnetic spectrum, where sunlight is absorbed only partially by ozone.

TOMS/EP measures total ozone by observing both incoming solar energy and backscattered ultraviolet (UV) radiation at six wavelengths. "Backscattered" radiation is solar radiation that has penetrated to the Earth's lower atmosphere and is then scattered by air molecules and clouds back through the stratosphere to the satellite sensors. Along that path, a fraction of th UV is absorbed by ozone. By comparing the amount of backscattered radiation to observations of incoming solar energy at identical wavelengths, scientists can calculate the Earth's albedo, the ratio of light reflected by Earth compared to that it receives. Changes in albedo at the selected wavelengths can be used to derive the amount of ozone above the surface.

TOMS makes 35 measurements every 8 seconds, each covering 30 to 125 miles (50 to 200 km) wide on the ground, strung along a line perpendicular to the motion of the satellite. Almost 200,000 daily measurements cover every single spot on the Earth except areas near one of the poles, where the Sun remains close to or below the horizon during the entire 24-hour period.

Apart from TOMS, information on changing ozone levels comes from instruments on the ground and weather balloons. Ozone concentrations are measured in Dobson units. Dobson units (DU) are the standard way to express ozone amounts in the atmosphere. One DU is 2.7 x 1016 ozone molecules per square centimetre. One Dobson unit refers to a layer of ozone that would be 10 micrometres thick under standard temperature and pressure. For example, 300 Dobson units of ozone brought down to the surface of the Earth at 0 degrees celsius would occupy a layer only 3 mm thick. One Dobson unit is also one ozone moelcule per billion molecules of air (1 ppb).

 

Chloroflurocarbons

A chlorofluorocarbon (CFC) is an organic compound that contains only carbon, chlorine, hydrogen and fluorine, produced as a volatile derivative of methane and ethane. They are also commonly known by the DuPont brand name Freons. The most common representative is dichlorodifluoromethane (R-12 or Freon-12). Many CFCs have been widely used as refrigerants, propellants (in aerosol applications) and solvents. The manufacture of such compounds has been phased out gloablly by the Montreal Protocol because they contribute to ozone depletion in the upper atmosphere.

The main CFC's used in the 1980's and their formulas and uses are given below.

CFC
Name
Code
Uses
CCl3F Trichlorofluoromethane CFC-11 Aerosols; refrigerant
CCl2F2 Dichlorodifluromethane CFC-12 Aerosols; refrigerant
CCl2FCClF2 1,2,2-trichloro-1,1,2-trifluoroethane CFC-13 Dry cleaning
CClF2CClF2 1,2-dichloro-1,1,2,2-tetrafluoroethane CFC-14 Aerosols

Naming Halogenated Hydrocarbons

Halogenated hydrocarbons are named using the naming system you have already used for hydrocarbons with a few additional rules.

  1. Fluoro, chloro, bromo and iodo are used as prefixes to the main name of the molecule.

  2. The position of the halogen is identified using a number that corresponds to the carbon atom bonded to the halogen. The carbons are numbered so as to keep the numbers of the most electronegative halogen as low as possible.

  3. If more than one of a particular halogen is present, the prefixes, di, tri, tetra and penta are used.

  4. If more than one halogen is present, they are listed alphabetically.

  5. Halogen atoms are given before any alkyl branches.

Isomerism

Two constitutional isomers of propane.

More than one compound having a same molecular formula are called isomers. These compounds will have different physical properties and chemical properties.

Isomers of similar molecular formula may also be with different functional groups. Sometimes these isomers are different in the arrangement in a molecule. The position of a particular group in the molecule also causes isomerism.

CFCs exhibit isomerism and the IUPAC name is given to distinguish between isomers.

 

Ozone Depletion



A video with an overview of the issues associated with a hole in the ozone layer.

Chlorofluorocarbons were originally made for use as aerosols and refrigerants because they are inert and safe to use. However, because they are inert they travel up into the stratosphere unreacted and in this high energy environment they decompose due to the UV light present and a halogen atom is released.

CFC's tend to release the chlorine radical which is very reactive and reacts with ozone forming oxygen and chlorine oxide.

 

 

 

For example when CFC-11 decomposes the following occurs.

CCl3F (g) + UV → CCl2F (g) + Cl (g)

Cl (g) + O3 (g) → ClO (g) + O2 (g)

Chlorine oxide goes on to react with oxygen radicals in the stratosphere forming oxygen and releasing a chlorine radical which can go on and decompose more ozone molecules.

ClO (g) + O (g) → O2 (g) + Cl (g)

A single chlorine radical can go on to destroy hundreds of thousands of ozone molecules before it is eventually broken down or passes away into the upper atmosphere.



The mechanism of ozone depletion in the stratosphere whereby a single chlorine free radical can destroy billions
of ozone molecules.


Halons (eg CBrF3) are a group of compounds that contain carbon, hydrogen and bromine and may contain other halogens as well. They were used in fire extinguishers and have an even greater effect on ozone.

Steps Taken to Alleviate the Ozone Problem

Ozone concentrations and the effect of the Montreal
protocol.

It was discovered in the late 1970's that ozone levels were diminishing, particularly above the Antarctic. Scientists around the world lobbied extensively to bring this to the attention of the public. In 1987, a series of international agreements, starting with the Montreal Protocol, initiated the phase out of CFC's. It was recommended that industrialised countries cease the use of CFC's by 1995 and later for developing countries. CFC's were initially replaced by HCFC's (hydrochlorofluorocarbons) such as HCFC-22 (CHClF2) and HCFC-123 (C2HF3Cl2) and hydrofluorocarbons such as HFC-134 (C2H2F4). These compounds are much less damaging to the ozone layer than CFC's because they are more reactive and tend to react before they reach the ozone layer. Halons are also being phased out but on a longer timeline.

The Montreal Protocol was amended in London in 1990 and again in Copenhagen in 1992 following evaluation of ozone levels. The amendments included suggestions for alternative chemicals including the replacement of HCFC's with hydrocarbons. Hydrocarbons have been used as propellants in aerosol sprays in Australia since the mid 1990's.