طبقة الأوزون (Ozone)

بسم الله الرحمن الرحيم

جامعة القدس


Ozone


Prepared by:
Alaa Al Shawamri

By Doctor :
Saed Khayat


Years:
2009/2010

Ozone
Definition :(O3 : 3 oxygen atoms) is a gas that have blue color occurs both in the Earth's upper atmosphere and at ground level. It can be at 30-50 Km and can be found at less than 30Km .It have length from 2-8Km. Ozone can be "good" or "bad" for people's health and for the environment, depending on its location in the atmosphere.
Ozone forms a layer in the stratosphere, thinnest in the tropics (around the equator) and denser towards the poles. The amount of ozone above a point on the earth's surface is measured in Dobson units (DU) - typically ~260 DU near the tropics and higher elsewhere, though there are large seasonal fluctuations. It is created when ultraviolet radiation (sunlight) strikes the stratosphere, dissociating (or "splitting") oxygen molecules (O2) to atomic oxygen (O). The atomic oxygen quickly combines with further oxygen molecules to form ozone:
O2 + *hv -> O + O (1)
O + O2 -> O3 (2)
*(1/v = wavelength < ~ 240 nm)
O3 + hv -> O2 + O (3)
O + O2 -> O3 (2) as above
Ozone is also destroyed by the following reaction:
O + O3 -> O2 + O2 (4)
* Ozone Hole
The Ozone Hole often gets confused in the popular press and by the general public with the problem of global warming. Whilst there is a connection because ozone contributes to the greenhouse effect, the Ozone Hole is a separate issue. However it is another stark reminder of the effect of man's activities on the environment.


Over Antarctica (and recently over the Arctic), stratospheric ozone has been depleted over the last 15 years at certain times of the year. This is mainly due to the release of manmade chemicals containing chlorine such as

CFC's (ChloroFluoroCarbons), but also compounds containing bromine, other related halogen compounds and also nitrogen oxides (NOx). CFC's are a common industrial product, used in refrigeration systems, air conditioners, aerosols, solvents and in the production of some types of packaging. Nitrogen oxides are a by-product of combustion processes, e.g. aircraft emissions.
The current levels of depletion have served to highlight a surprising degree of instability of the atmosphere, and the amount of ozone loss is still increasing. GreenPeace have documented many of the concerns that this raises.
*What Is Being Done?
The first global agreement to restrict CFCs came with the signing of the Montreal Protocol in 1987 ultimately aiming to reduce them by half by the year 2000. Two revisions of this agreement have been made in the light of advances in scientific understanding, the latest being in 1992. Agreement has been reached on the control of industrial production of many halocarbons until the year 2030. The main CFCs will not be produced by any of the signatories after the end of 1995, except for a limited amount for essential uses, such as for medical sprays.
It was anticipated that these limitations would lead to a recovery of the ozone layer within 50 years of 2000; the World Meteorological Organization estimated 2045 (WMO reports #25, #37), but recent investigations suggest the problem is perhaps on a much larger scale than anticipated.
*The Science Of The Ozone Hole
Evidence that human activities affect the ozone layer has been building up over the last 20 years, ever since scientists first suggested that the release of chlorofluorocarbons (CFCs) into the atmosphere could reduce the amount of ozone over our heads.

The breakdown products (chlorine compounds) of these gases were detected in the stratosphere. When the ozone hole was detected, it was soon linked to this increase in these chlorine compounds. The loss of ozone was not restricted to the Antarctic – at around the same time the first firm evidence was produced that there had been an ozone decrease over the heavily populated northern mid-latitudes (30-60N). However, unlike the sudden and near total loss of ozone over Antarctica at certain altitudes, the loss of ozone in mid-latitudes is much less and much slower – only a few percentage per year. However, it is a very worrying trend and one which is the subject of intense scientific research at present.
So, we have the first few ingredients for our 'ozone loss recipe'. We must have:
1. Polar winter leading to the formation of the polar vortex which isolates the air within it.
2. Cold temperatures; cold enough for the formation of Polar Stratospheric Clouds. As the vortex air is isolated, the cold temperatures persist.
*Chemical Processes Leading To Polar Ozone Depletion
It is now accepted that chlorine and bromine compounds in the atmosphere cause the ozone depletion observed in the `ozone hole' over Antarctica and over the North Pole because countries have put their chemical litter in it for example chlorine and bromine compounds.

The figure above shows a schematic illustrating the life cycle of the CFCs; how they are transported up into the upper stratosphere/lower mesosphere, how sunlight breaks down the compounds and then how their breakdown products descend into the polar vortex.
The main long-lived inorganic carriers (reservoirs) of chlorine are hydrochloric acid (HCl) and chlorine nitrate (ClONO2). This form from the breakdown products of the CFCs. Dinitrogen pentoxide (N2O5) is a reservoir of oxides of nitrogen and also plays an important role in the chemistry. Nitric acid (HNO3) is significant in that it sustains high levels of active chlorine (as explained soon).
*Production Of Chlorine Radicals
One of the most important points to realize about the chemistry of the ozone hole is that the key chemical reactions are unusual. They cannot take place in the atmosphere unless certain conditions are present: our first two ingredients in our recipe for ozone loss.
The central feature of this unusual chemistry is that the chlorine reservoir species HCl and ClONO2 (and their bromine counterparts) are converted into more active forms of chlorine on the surface of the polar stratospheric clouds. The most important reactions in the destruction of ozone are:
HCl + ClONO2 -> HNO3 + Cl2 (1)
ClONO2 + H2O -> HNO3 + HOCl (2)
HCl + HOCl -> H2O + Cl2 (3)
N2O5 + HCl -> HNO3 + ClONO (4)
N2O5 + H2O -> 2 HNO3 (5)
It's important to appreciate that these reactions can only take place on the surface of polar stratospheric clouds, and they are very fast. This is why the ozone hole was such as surprise. Heterogeneous reactions (those that occur on surfaces) were neglected in atmospheric chemistry (at least in the stratosphere) before the ozone hole was discovered. Another ingredient then, is these heterogeneous reactions which allow reservoir species of chlorine and bromine to be rapidly converted to more active forms.


The nitric acid (HNO3) formed in these reactions remains in the PSC particles, so that the gas phase concentrations of oxides of nitrogen are reduced. This reduction, 'denoxification' is very important as it slows down the rate of removal of ClO that would otherwise occur by the reaction:
ClO + NO2 +* M -> ClONO2 +* M (6)
*(where M is any air molecule)
… And so helps to maintain high levels of active chlorine
*The Return Of Sunlight
Lastly note that we have still only formed molecular chlorine (Cl2) from reactions (1)-(5). To destroy ozone requires atomic chlorine.
Molecular chlorine is easily photodissociated (split by sunlight):
Cl2 + hv -> Cl + Cl
This is the key to the timing of the ozone hole. During the polar winter, the cold temperatures that form in the 'vortex' lead to the formation of polar stratospheric clouds. Heterogeneous reactions convert the reservoir forms of the ozone destroying species, chlorine and bromine, to their molecular forms. When the sunlight returns to the polar region in the southern hemisphere spring (northern hemisphere autumn) the Cl2 is rapidly split into chlorine atoms which lead to the sudden loss of ozone. This sequence of events has been confirmed by measurements before, during and after the ozone hole.
There is still one more ingredient for our recipe of ozone destruction. We have most of it but we have still not explained the chemical reactions that the atomic chlorine actually takes part in to destroy the ozone. We'll discuss this next.
*Catalytic Destruction of Ozone
Measurements taken of the chemical species above the pole show the high levels of active forms of chlorine that we have explained above. However, we still have many more atoms of ozone than we do of the active chlorine so how it is possible to destroy nearly all of the ozone?
The answer to this question lies in what are known as 'catalytic cycles'. A catalytic cycle is one in which a molecule significantly changes or enables a reaction cycle without being altered by the cycle itself.
The production of active chlorine requires sunlight, and sunlight drives the following catalytic cycles thought to be the main cycles involving chlorine and bromine, responsible for destroying the ozone:
(I) ClO + ClO + M -> Cl2O2 + M
Cl2O2 + hv -> Cl + ClO2
ClO2 + M -> Cl + O2 + M
then: 2 x (Cl + O3) -> 2 x (ClO + O2)

net: 2 O3 -> 3 O2
and
(II) ClO + BrO -> Br + Cl + O2
Cl + O3 -> ClO + O2
Br + O3 -> BrO + O2

net: 2 O3 -> 3 O2
The dimer (Cl2O2) of the chlorine monoxide radical involved in Cycle (I) is thermally unstable, and the cycle is most effective at low temperatures. Hence, again low temperatures in the polar vortex during winter are important. It is thought to be responsible for most (70%) of the ozone loss in Antarctica. In the warmer Arctic a large proportion of the loss may be driven by Cycle (II).
*The Recipe For Ozone Loss
To summarise then, we have looked at the 'ingredients' or conditions necessary for the destruction of ozone that we see in Antarctica. The same applies more or less to the loss of ozone in the Arctic stratosphere during winter. Although in this case the loss is not nearly so severe.
To recap then, the requirements for ozone loss are:
• The polar winter leads to the formation of the polar vortex which isolates the air within it.
• Cold temperatures form inside the vortex; cold enough for the formation of Polar Stratospheric Clouds (PSCs). As the vortex air is isolated, the cold temperatures and the PSCs persist.
• Once the PSCs form, heterogeneous reactions take place and convert the inactive chlorine and bromine reservoirs to more active forms of chlorine and bromine.
• No ozone loss occurs until sunlight returns to the air inside the polar vortex and allows the production of active chlorine.
• Initiates the catalytic ozone destruction cycles. Ozone loss is rapid. The ozone hole currently covers a geographic region a little bigger than Antarctica and extends nearly 10km in altitude in the lower stratosphere.
*The Ozone Hole - Current Research Work
Where Does All The Ozone Go?
A major European campaign, the European Arctic Stratospheric Ozone Experiment (EASOE) was organised to study the Polar Regions during the winter of 1991/92. Much new information was gained, but many questions still remained:
• What caused the mid-latitude loss?
• How were the losses over the poles linked to those at mid-latitudes?
While CFCs and the bromine-containing compounds known to destroy ozone over the poles are strongly implicated in the mid-latitude loss, many uncertainties remain.
In 1994 and 1995 European scientists conducted SESAME, the Second European Stratospheric Arctic and Mid-latitude Experiment. They investigated the processes occurring at both high and mid-latitudes and how they are linked. At the same time a US-led expedition considered similar processes in the southern hemisphere.
The latest European campaign is called THESEO (THird European Stratospheric Experiment on Ozone) which takes places from 1997-1999. Scientists from many European countries, including some of this site, are collaborating on a wide range of experiments to determine the processes responsible for depleting ozone in the lower stratosphere but at mid-latitudes over the northern hemisphere.
*Models And Observations
Comparison of model results with observations both helps confirm our understanding of the processes responsible for ozone depletion, and can highlight those processes that require further study. A model of chemistry and transport has been used extensively in recent observational campaigns in the Arctic and Antarctic.
The following graphics compare the output of the TOMCAT (grid-point) model with TOMS satellite data for the beginning of the Antarctic spring - the ASHOE Campaign. TOMCAT was run on a resolution of approximately 5 deg x 5 deg. Further studies have used far higher resolutions.
The TOMS instrument relies on backscattered sunlight for its measurements; hence for the Antarctic winter, data tends to be sparse and incomplete. This data came from the Meteor 3 Satellite
Comparison between Model Results and Actual Satellite Data




Day 20 (11 September '94) Day 40 (1 October '94) Day 56 (17 October '94)

The model column ozone is very similar to that observed by satellite. Over the Antarctic continent there are low amounts of ozone, where there has been chemical destruction. Around the edge of the vortex, between 30S and 60S, there are higher amounts of ozone. These high amounts result from the transport of ozone from the region of production in the tropics.
*Why Hole Of Ozone Very Dangerous?
1-It caused many disease same cancer and eyes disease.
2-Increase of temperature on the world.
3-Died different plant on world.
4-Fish and plant in the water doesn’t stable in the Environment.
5-Pollution in the air cause hole ozone makes air many increase in its temperature.
6-Burn the tree and increase sea level.

*How can we stop hole ozone?
1-Don’t buy any thing that contain CFCs in it.
2-Farm number of trees in world.
3-Less using Petrol in our life cause it have when it burn very toxic gas that stand to air to ozone and make hole bigger.
4-Don’t use toxic chemical same that use in war.































References:*
1-Book of Health and the Environment of the tenth grade curriculum, as the Palestinian.
2-www.atm.ch.cam.ac.uk/tour/
3-www.epa.gov/ozone/
4-http://www.feedo.net/Environment/OzoneLayer/OzoneLayer.htm
5-Dotto.L.and Schiff. H .the ozone ware .Douhledy (1978.
6- Brum. C.; Mckane. L. and karp. C. Biology- Exploring Life. Second Edition. John Wiley .and Sons. Inc. New York (1994.
7- عبد الحميد غزي بن حسن. التلوث البيئي: الهم الكبير لسكان الأرض مجلة القافلة. المجلد (41) العدد (. صفحة: 42 - 47 8- عبد الله النعنيش. طبقة الأوزون: عشرات الملايين يصابون بسرطان الجلد وإعتام العين إذا استمر تآكل طبقة الأوزون. منبر البيئة. المجلد (6). العدد (2) صفحة 7 (يونيو 1993).
9- حماية الأوزون: البدائل لمركبات الفلوروكربون. منبر البيئة. ملحق خاص. العدد (2): صفحة 1 - 8 (سبتمبر 1994).