Stratospheric Ozone Depletion Research

✅ Paper Type: Free Essay	✅ Subject: Chemistry
✅ Wordcount: 3541 words	✅ Published: 28th Nov 2017

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A) Effects of Ozone on the Lower Atmosphere

The lower atmosphere (Troposphere) includes 75% by mass of the atmosphere (concentrated). Natural sources of Ozone in the troposphere includes lightning. Approximately 10% of all atmospheric ozone is present in the troposphere. If ozone levels reach 20ppm, they are very poisonous to humans, animals and plants. It oxidises organic tissue which disrupts the normal biochemical reactions in the body, irritates the eyes and causes breathing difficulties. It can be detrimental to plants and agriculture, as it oxidises much more readily then oxygen, killing/spoiling the agriculture and destroying it.

Sources of ozone in the troposphere include diffusion from the stratosphere, internal combustion engines, petrochemical smog, naturally from lightning and photochemically when nitrogen dioxide in polluted air is decomposed by sunlight.

NO_2(g) NO_(g) + O_(g)

O_2(g) + O_(g) O_3(g)

Positive effects of ozone include that it can kill bacteria and viruses in water and thus is useful in purifying water supplies.

B) Effects of Ozone in the Stratosphere

Contrastingly to ozone in the troposphere, Ozone in the stratosphere is essential to life on earth, as it absorbs ultraviolet (UV) radiation which can be harmful to living cells on earth as they can damage living tissues and cause skin cancers. Ozone in the stratosphere is commonly referenced to as “the ozone shield” as it protects living organisms on earth from UV rays.

Ozone Reactions in the Stratosphere and their Beneficial Effects on Living Organisms

Formation of Ozone in the Stratosphere

O_2(g) O_(g) + O_(g)

O_(g) + O_2(g) → O_3(g)

Stratospheric oxygen absorbs UV light to form free oxygen radicals
The oxygen radicals are highly reactive and combine with oxygen molecules to form an energized ozone molecule.

Decomposition of Ozone in the Stratosphere

O_3(g) O_2(g) + O_(g)

Due to the absorption of UV radiation, the Ozone layer acts as a radiation shield by absorbing medium and high energy UV rays. This protects living organisms as UV radiation can have very harmful effects on them including:

Can cause sunburn on skin which can lead to skin cancer caused by a mutation in DNA
Can form cataracts on eyes
It can kill cells due to DNA readily reacting with UV rays
It can impair photosynthesis in plants
Many more…

Thus without the ozone shield in the stratosphere, life in the biosphere would be dramatically impaired and destroyed by harmful UV rays.

A) Functional Groups and General Structure of Compounds Classified as CFC’s

CFC’s – Chlorofluorocarbons are haloalkanes in which the hydrogen atoms have been replaced by fluorine or chlorine atoms.

Haloalkanes are the products when alkanes react with halogens (members of group 7 of the periodic table). CFC’s generally contain “chloro” and “flouro” functional groups and no hydrogen atoms.

The general structure of compounds classified as CFCs are haloalkanes whose hydrogens have been replaced by chlorine or fluorine atoms.

E.g.

B) Main Uses of CFC’s

CFC’s were used as refrigerants and as propellants in aerosol spray cans. They have a variety of uses as demonstrated below.

Coded Compound	Formula	Use
CFC-11	CCl₃F	Refrigerant, Aerosol, Foams
CFC-12	CCl₂F₂	Refrigerant, Aerosol, Air Conditioning, Foams
CFC-113	CCl₂FCClF₂	Electronics, Dry cleaning, Fire Extinguishers
CFC-14	CClF₂CClF₂	Aerosols

However due to the harmful effects of CFC to the environment and the ozone shield, CFCs are not used for these uses anymore.

C) Reactions between CFC’s and Ozone that Result in the Destruction of Ozone in the Stratosphere. Effects of Small Concentrations of CFC’s that can Damage Large amounts of Ozone

Reactions between CFCs and Ozone

Synthetic CFCs are responsible for the destruction of the ozone shield, natural CFCs such as CH₃Cl and HCl rarely reach the stratosphere as they readily oxidise in the troposphere. However, synthetic CFCs slowly diffuse from the troposphere into the stratosphere, where they undergo photodissociation (due to UV rays) to produce chlorine and bromine radicals that attack and destroy ozone molecules.

e.g. CFC-11 Trichloroflouromethane (CFCl₃) (Lifetime of 70 years)

1. In the stratosphere, the CFC comes into contact with short wavelength UV

CFCl_3(g) + UV → CFCl₂·_(g) + Cl·_(g)

2. The chlorine free radical then reacts with the ozone molecule

Cl + O₃ → ClO + O₂

3. The ClO molecule reacts with free oxygen atoms which exist naturally from UV breakdown of O₂

ClO + O → O₂ + Cl

This Cl Is then regenerated and able to attack more Ozone (Step 2) thus further demonstrating the harmful effects of even one CFC

This reaction causes destruction of ozone in the stratosphere, due to the (previously used) synthetic CFCs for refrigeration, dry cleaning etc.

[Schematic of the ozone depletion process, steps 1-6]

Small Amounts of CFCs can still do harm:

Evidence has shown that even small amounts of CFCs can damage large amounts of ozone. Firstly, CFCs generally have a long lifespan, ranging from approximately 57 (CFC-11) years to 333 years (CFC-12), and due to the fact that each Chlorine radical can be responsible for the breakdown of tens of thousands of ozone molecules, and due to their lifespan once released, even a small amount, will be around for many decades to come.

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In addition, most CFCs will almost definitely make their way up to the stratosphere as they cannot be destroyed at low altitudes as they are unreactive and they are insoluble in water and therefore cannot be washed out of the atmosphere by rain.

Alternative Compounds for CFCs.

Name of the Compounds	General Molecular Formula	Effectiveness as CFC Substitutes
Hyrdochloroflourocarbons (HCFCs)	Example: HCFC-22 Chlorodifluoromethane Or CHClF₂	These compounds contain C-H bonds which are susceptible to attack by reactive radicals and atoms in the troposphere and thus the majority of them do not reach the stratosphere. However, this CFC substitute is not effective as there is still damage to the ozone layer due to the small amount of HCFCs that reach the stratosphere.
Hydroflourocarbons (HFCs)	Example: HFC-134a 1,1,1,2-tetraflouroethane or CH₂FCF₃	Due to the lack of chlorine HFCs are remarkable better than HCFCs in replacing CFCs, whereby they do not deplete the ozone layer. They are useful refrigerants and in air conditioners and therefore are effective in substituting CFCs. However, HFCs are expensive and are not as good refrigerants as CFCs.
Hydrocarbons (HCs)	C_nH_2n+2 Example: Propane C₃H₈	Hydrocarbons have replaced CFCs as propellants in spray cans. They are effective in the way that they have no ozone depletion potential, they are of low toxicity and generally (with the exception of methane) have little impact on global warming.They are effective propellants with one significant problem, like other aerosols in that they are flammable. Overall they are good alternatives to CFCs

A) Ozone Monitoring Instruments

Name of Instrument	Period of Use	Type of Data Collected	Explanation of Use	Advantages of Instrument	Limitations
Ground Based U.V Spectrophotometers	1940-Now	Intensity of light wavelengths, provides a measure of the total amount of ozone per unit of earth where the spectrophotometer is located.	The instrument is directed directly vertical through the atmosphere to measure the intensity of light that ozone absorbs as well as the intensity of the surrounding light rays.	The total ozone in the atmosphere per unit area of the earth’s surface is created. It provided the first means of observing the ozone hole.	Due to limitations, helium balloons have been used to carry instruments up into the stratosphere, however there is no control over where these balloons go.
Total Ozone Mapping Spectrometer (TOMS)	1985 – 2007	Measures the ozone concentration as a function of altitude and geographical position.	It determines amounts of ozone in the stratosphere (ozone layer) by reading backscattered UV light which is the UV light the earth emits back into space (like a reflection)	Produces accurate coloured maps of the extent of ozone holes, thus allowing for a visual way to determine the decrease/increase of ozone in the stratosphere. Provided a means of determining when the ozone hole would be fixed, in 2002 an Australian study based on TOMS maps estimated that the Ozone hole would be gone by 2050.	Due to limitations, helium balloons have been used to carry instruments up into the stratosphere, however there is no control over where these balloons go.
Ozonesonde	1970 – Still used today	Electrical Current from ozone when reacted with potassium iodide to record amount of ozone present.	A balloon carries the instrument up higher than 33.8 kilometre high to the stratosphere where it sucks in and holds air, testing the amount of ozone gas using an electrochemical concentration cell(ECC). The ECC uses potassium iodide, which reacts with ozone to create anelectrical current, to measure the amount of ozone present.	The ozonesonde is relatively cheap, and the use of a helium balloon means that no aircraft or satellite is required to measure amount of ozone. It transmits the results by radio, so if there is a loss of the physical instrument the results are still obtained.	The balloon pops at high altitudes, and despite having parachutes attached often the machine is broken upon impact which causes the ozonesonde break. The ozonesode can only record amount of ozone present in specific locations, and due to the limitation of not being able to send one up in all locations, the results cannot be representative of the overall ozone concentration.
Global Ozone Monitoring by Occulation of Stars (GOMOS)	2002-2010	Global distribution of ozone with a high vertical resolution and with high and long-term stable accuracy. Measures spectral range and resolution	GOMOS uses the stellar occultation measurement principle in monitoring of ozone in the Earth’s stratosphere. The wavelength region is in ultraviolet and in visible, 250-675 nm, and there are also two infrared channels at 756-773 nm and at 926-952 nm. Measurements cover altitude region 120-15 km	It retrieved ozone profiles, algorithm details and data quality to help understand the distribution of ozone in the stratosphere.	The data is collected on a satellite, and on a variety of instances there have been issues with the GOMOS technology that took weeks to fix, and therefore make some data unavailable.

Ozone Concentrations in the Stratosphere

total ozone

Source: Earth System Research Laboratory, 2012, Viewed 07.06.14,

http://www.esrl.noaa.gov/gmd/dv/spo_oz/spototal.html

A) Analysis of Trends

There are a variety of trends that can be interpreted from the above diagram.

Based on the data above, before the 1980’s, the total ozone concentration was VERY high, at approximately 194 DU (Dobson Units), however there was a very rapid decrease in this concentration from 1980 – 1999, whereby in this 19 year period sees a 56% decrease in the amount of total ozone, a remarkably concerning figure.
On a year round basis globally, total ozone concentration have caused a 3-8% decrease in the amount of ozone, this increased in the years between 1995-200, where there was a low of total ozone concentration. However, in more recent years, 2010 to 2014 there has been a general increase in total ozone concentration, which can be inferred from the replacement of CFCs finally starting to impact (slightly) on the total concentration of ozone, this increase based on the data is 31%.
The general pattern is that the total column ozone decreases during spring time, it can be inferred that the overall concentration of ozone decreases during this time of the year. This is because in an Antarctic winter, there is no U.V light to convert the chlorine molecule Cl₂ into a Cl radical, which then destroy ozone molecules, and thus the concentration of ozone is higher in winter. In spring, the U.V light converts the Cl₂ into Cl which then destroys ozone in a chain reaction, thus decreasing the total ozone concentration
There are various peaks in the graph, in the years of 1988, 2003, 2011 and 2013, which may be due to limitation so of the instruments used.

B) Montreal Protocol Effectiveness

The Montreal protocol occurred in 1987, which the main aim was to control the production of ozone depleting substances (CFCs) worldwide. A number of amendments have been adopted to further ride ozone depleting substances. The protocol is applied in 193 countries.

The main aims of the original agreement is as follow:

Halt the use of Halons by late 1994
By the early 21^st century, phase out the use of HCFCs
Stop manufacture of CFCs by 1996
Allow for leeway with less developed countries but still get them to rid the use of these substances

The Montreal Protocol (and amendments) has been effective as by 2006, the consumption of ozone depleting substances has been reduced globally by 96%. However, due to the long lives of the previously used ozone depleting substances, the total concentration will take hundreds of years to be completely down. However, the total concentration in the troposphere has generally been declining since the mid-1990s.

Bibliography

Thickett, G 2006, Chemistry 2 HSC course, John Wiley and Sons, Queensland, Australia.

Role of Ozone, 2013, viewed 05.06.14,

http://www.easychem.com.au/monitoring-and-management/the-atmosphere/roles-of-ozone

Allen, J, 2001, Ultraviolet Radiation – How it Affects life on Earth, viewed 05.06.14,

http://earthobservatory.nasa.gov/Features/UVB/

Environmental Protection Agency, 2010, The Process of Ozone Depletion, Viewed 07.06.14,

http://www.epa.gov/ozone/science/process.html

Clean Air Strategic Alliance, 2013, Chlorofluorocarbons (CFCs) and Halons, Viewed 07.06.13,

http://dwb.unl.edu/teacher/nsf/c09/c09links/www.casahome.org/chlorofl.htm

Welch, C 2014, The Ozone Hole, Viewed 07.06.14,

http://www.theozonehole.com/cfc.htm