Enve- 307
12 Temmuz 2007
Enve- 307
Air pollution
Term-project
Ozone depletion
What is ozone
Types of ozone
Good ozone
Bad ozone
How ozone is created
The Chapman reactions
The missing reactions
Ozone measurements
Ozone depletion process
What is the ozone hole
The history behind the ozone hole
The science behind the ozone hole
The recipe for ozone loss
The special features of polar meteorology
Chemical processes leading to polar ozone depletion
Production of chlorine radicals
The return of sunlight
Catalytic destruction of ozone
Example: Antarctic
Halley bay
TOMS satellite measurements
Monthly averages for October
Conclusion
Appendix
What is Ozone:
Ozone is a molecule that contains three atoms of oxygen and thus has the formula O3.The name ozone is derived from a Greek word meaning “to smell”.
Ozone is a relatively simple molecule, consisting of three oxygen atoms bound together. Yet it has dramatically different effects depending on where ozone resides, it can protect or harm life on Earth. High in the atmosphere about 15 miles (24 km) up ozone acts as a shield to protect Earth’s surface from the sun’s harmful ultraviolet radiation. Without this shield, we would be more susceptible to skin cancer, cataracts, and impaired immune systems.
Closer to Earth, in the air we breathe, ozone is a harmful pollutant that causes damage to lung tissue and plants. Near Earth’s surface, where ozone comes into direct contact with life forms, it primarily displays a destructive side. At ground level, ozone is a health hazard it is a harmful pollutant that causes damage to lung tissue and plants- it is a major constituent of smog.
Types of ozone:
In the absence of this gaseous shield in the stratosphere, the harmful radiation has a perfect portal through which to strike Earth.
The amounts of “good” and “bad” ozone in the atmosphere depend on a balance between processes that create ozone and those that destroy it. An upset in the ozone balance can have serious consequences for life on Earth. Scientists are finding evidence that changes are occurring in ozone levelsthe “bad” ozone is increasing in the air we breathe, and the “good” ozone is decreasing in our protective ozone shield.
Good Ozone.
Ozone occurs naturally in the Earths upper atmosphere10 to 30 miles above the Earths surfacewhere it forms a protective layer that shields us from the suns harmful ultraviolet rays. This beneficial ozone is gradually being destroyed by manmade chemicals. An area where ozone has been significantly depletedfor example, over the North or South poleis sometimes called a hole in the ozone.
Bad Ozone.
In the Earths lower atmosphere, near ground level, ozone is formed when pollutants emitted by cars, power plants, industrial boilers, refineries, chemical plants, and other sources react chemically in the presence of sunlight. Ozone at ground level is a harmful pollutant. Ozone pollution is a concern during the summer months, when the weather conditions needed to form itlots of sun, hot temperatures normally occur.
How Ozone is created:
Ozone is produced naturally in the stratosphere when highly energetic solar radiation strikes molecules of oxygen, O2, and cause the two oxygen atoms to split apart in a process called photolysis.
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:
Graphic: Emily Shuckburgh Cambridge University
O2 + hv
->
O + O
(1)
+ O2
->
O3
(2)
It’s ironic that at ground level, ozone is a health hazard - it is a major constituent of photochemical smog. However, in the stratosphere we could not survive without it. Up in the stratosphere it absorbs some of the potentially harmful ultra-violet (UV) radiation from the sun (at wavelengths between 240 and 320 nm) which can cause skin cancer and damage vegetation, among other things.
Although the UV radiation splits the ozone molecule, ozone can reform the following reactions resulting in no net loss of ozone:
O3 + hv
->
O2 + O
(3)
O + O2
->
O3
(2) as above
Ozone is also destroyed by the following reaction:
O + O3
->
O2 + O2
(4)
The Chapman Reactions
The reactions above, labelled (1)-(4) are known as the “Chapman reactions”. Reaction (2) becomes slower with increasing altitude while reaction (3) becomes faster. The concentration of ozone is a balance between these competing reactions. In the upper atmosphere, atomic oxygen dominates where UV levels are high. Moving down through the stratosphere, the air gets denser, UV absorption increases and ozone levels peak at roughly 20km. As we move closer to the ground, UV levels decrease and ozone levels decrease. The layer of ozone formed in the stratosphere by these reactions is sometimes called the ‘Chapman layer’.
The Missing Reactions..
But there was a problem with the Chapman theory. In the 1960s it was realised that the loss of ozone given by reaction (4) was too slow. It could not remove enough ozone to give the values seen in the real atmosphere. There had to be other reactions, faster reactions that were controlling the ozone concentations in the stratosphere.
If a freed atom collides with another O2, it joins up, forming ozone O3. Most of the ozone in the stratosphere is formed over the equatorial belt, where the level of solar radiation is greatest. The circulation in the atmosphere then transports it towards the pole . So, the amount of stratospheric ozone above a location on the Earth varies naturally with latitude, season, and from day-to-day.
Under normal circumstances highest ozone values are found over places such as Canada and Siberia, whilst the lowest values are found around the equator. The ozone layer varies naturally with season. Over Canada is normally about 25% thicker in winter than summer. Weather conditions can also cause considerable daily variations.
Ozone is also naturally broken down in the stratosphere. In an unpolluted atmosphere there is a balance between the amount of ozone being produced and destroyed and so the total concentration remains relatively constant. At different temperatures and pressures (i.e. varying altitudes), there are different production and destruction reaction rates leading to a variation in concentration. The highest ozone concentrations are in the lower stratosphere, between about 18 and 26 km.
Ozone also occurs in very small amounts in the troposphere. It is produced at ground level through a reaction between sunlight and, e.g., gases emitted from cars. As a pollutant it should not be confused with the separate problem of stratospheric ozone depletion.
Ozone Measurement:
1 Dobson Unit (DU) is defined to be 0.01 mm thickness at STP (standard temperature and pressure). Ozone layer thickness is expressed in terms of Dobson units, which measure what its physical thickness would be if compressed in the Earth’s atmosphere. In those terms, it’s very thin indeed. A normal range is 300 to 500 Dobson units, which translates to an eighth of an inch-basically two stacked pennies. In space, it’s best not to envision the ozone layer as a distinct, measurable band. Instead, think of it in terms of parts per million concentrations in the stratosphere (the layer six to 30 miles above the Earth’s surface).
The unit is named after G.M.B. Dobson, one of the first scientists to investigate atmospheric ozone . In 1923 he produced the first Dobson Ozone Spectro-meter, to be succeeded in 1931 by his Spectrophotometer - a device which is still in use worldwide, with a network of over 150 instruments making daily observations. The Dobson spectrometer measures the intensity of solar UV radiation. A single measurement uses two wavelengths of UV, but for normal operation pairs of readings are taken at two different wavelength settings for a total of four wavelengths, two of which are absorbed by ozone and two of which are not. (see appendix)
THE OZONE DEPLETION PROCESS:
The ozone depletion process begins when CFCs and other ozone-depleting substances (ODS) are emitted into the atmosphere(1). Winds efficiently mix the troposphere and evenly distribute the gases. CFCs are extremely stable, and they do not dissolve in rain. After a period of several years, ODS molecules reach the stratosphere, about 10 kilometers above the Earth’s surface (2).
Strong UV light breaks apart the ODS molecule. CFCs release chlorine atoms, and halons release bromine atoms (3). It is these atoms that actually destroy ozone, not the intact ODS molecule. It is estimated that one chlorine atom can destroy over 100,000 ozone molecules before finally being removed from the stratosphere (4).
Ozone is constantly being produced and destroyed in a natural cycle, as shown in the above picture, courtesy of NASA GSFC. However, the overall amount of ozone is essentially stable. This balance can be thought of as a stream’s depth at a particular location. Although individual water molecules are moving past the observer, the total depeth remains constant. Similarly, while ozone production and destruction are balanced, ozone levels remain stable. This was the situation until the past several decades.
Large increases in stratospheric chlorine and bromine, however, have upset that balance. In effect, they have added a siphon downstream, removing ozone faster than natural ozone creation reactions can keep up. Therefore, ozone levels fall.
Since ozone filters out harmful UVB radiation, less ozone means higher UVB levels at the surface. The more depletion, the larger the increase in incoming UVB. UVB has been linked to skin cancer, cataracts, damage to materials like plastics, and harm to certain crops and marine organisms. Although some UVB reaches the surface even without ozone depletion, its harmful effects will increase as a result of this problem.
What Is The 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, eg 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. Green Peace have documented many of the concerns that this raises.
The History behind the Ozone Hole
Dramatic loss of ozone in the lower stratosphere over Antarctica was first noticed in the 1970s by a research group from the British Antarctic Survey (BAS) who were monitoring the atmosphere above Antarctica from a research station much like the picture to the right.
BAS research stations in the Antarctic
Folklore has it that when the first measurements were taken in 1985, the drop in ozone levels in the stratosphere was so dramatic that at first the scientists thought their instruments were faulty. Replacement instruments were built and flown out, and it wasn’t until they confirmed the earlier measurements, several months later, that the ozone depletion observed was accepted as genuine.
Another story goes that the TOMS satellite data didn’t show the dramatic loss of ozone because the software processing the raw ozone data from the satellite was programmed to treat very low values of ozone as bad readings! Later analysis of the raw data when the results from the British Antarctic Survey team were published, confirmed their results and showed that the loss was rapid and large-scale; over most of the Antarctica continent.
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.
Many of these findings have since been reinforced by a variety of internationally supported scientific investigations involving satellites, aircraft, balloons and ground stations, and the implications are still being quantified and assessed.
The Recipe For Ozone Loss
In trying to understand how the ozone loss occurs and the things that need to happen to destroy so much ozone, it helps to think of it as a ‘recipe’. We need several ingredients to make the ozone loss occur. We’ll now look at these ‘ingredients’ one at a time.
The Special Features of Polar Meteorology
We start by looking at the way the atmosphere behaves over the poles - the features of the meteorology in the stratosphere.
The figure to the right shows schematically what happens over Antarctica during winter. During the winter polar night, sunlight does not reach the south pole. A strong circumpolar wind develops in the middle to lower stratosphere. These strong winds are known as the ‘polar vortex’. This has the effect of isolating the air over the polar region.
Since there is no sunlight, the air within the polar vortex can get very cold. So cold that special clouds can form once the air temperature gets to below about -80C. These clouds are called Polar Stratospheric Clouds (or PSCs for short) but they are not the clouds that you are used to seeing in the sky which are composed of water droplets. PSCs first form as nitric acid trihydrate. As the temperature gets colder however, larger droplets of water-ice with nitric acid dissolved in them can form. However, their exact composition is still the subject of intense scientific scrutiny. These PSCs are crucial for ozone loss to occur.
So, we have the first few ingredients for our ‘ozone loss recipe’. We must have:
Polar winter leading to the formation of the polar vortex which isolates the air within it.
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. However, the relative importance of chlorine and bromine for ozone destruction in different regions of the atmosphere has not yet been clearly explained.
Nearly all of the chlorine, and half of the bromine in the stratosphere, where most of the depletion has been observed, comes from human activities.
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). These 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 realise 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:
(where M is any air molecule)
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)
… 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).
EXAMPLE: The Antarctic
There are now many measurements and observations of the changes in ozone that occur over Antarctica. Such measurements come from ground based instruments at the Antarctica research stations, from aircraft during scientific missions and from satellites.
Ozone loss was first detected in the stratosphere over the Antarctic (see Part I). Although mid-latitude and Arctic depletion has also been observed, the loss is most dramatic in the lower stratosphere over the Antarctica continent, where nearly all the ozone is destroyed over an area the size of Antarctica within a layer in the lower stratosphere that’s many km thick.
Halley Bay, Antarctica
The graph to the right shows the measured total ozone above the Halley Bay station in Antarctica. Each point represents the average total ozone for the month of October. Note the sudden change in the curve after about 1975. By 1994, the total ozone in October was less than half its value during the 1970s, 20 years previous. This dramatic fall in ozone was caused by the use of man-made chemicals known as ‘halogens’ which include the well-known CFCs commonly used in fridges and so on. These CFCs had made their way into the upper atmosphere where the much stronger UV radiation from the Sun had broken them down into their component molecules, releasing the potentially damaging chlorine (and bromine) atoms, which, given the right conditions, could destroy ozone. We’ll learn more about the chemistry behind the loss of ozone in Part III of this tour.
TOMS Satellite Measurements
The TOMS (Total Ozone Mapping Spectrometer) is a satellite-borne instrument used to gain a global picture of ozone levels. The TOMS instrument measures ozone levels from the back-scattered sunlight, specifically in the ultra-violet range. It measures wavelength bands centred at 312.5, 317.5, 331.3, 339.9, 360.0 and 380.0 nanometres. The first four wavelengths are absorbed to greater or lesser extents by ozone; the final two are used to assess the reflectivity. The ozone levels computed are ‘column ozone’ (i.e. Dobson Units or DU for short).
During the Antarctic winter (May - July), data is unavailable near the pole, which is in total darkness.
Monthly Averages for October
It is important to appreciate that the atmosphere behaves differently from year to year. Even though the same processes that lead to ozone depletion occur every year, the effect they have on the ozone is altered by the meteorology of the atmosphere above Antarctica. This is known as the ‘variability’ of the atmosphere. This variability leads to changes in the amount of ozone depleted and the dates when the depletion starts and finishes. To illustrate this, the monthly averages for October, from 1980 to 1991, are shown below.
CONCLUSION:
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.
The countries of the European Community have adopted even stricter measures than are required under the Montreal Protocol agreements. Recognising their responsibility to the global environment they have agreed to halt production of the main CFCs from the beginning of 1995. Tighter deadlines for use of the other ozone-depleting compounds are also being adopted.
It was anticipated that these limitations would lead to a recovery of the ozone layer within 50 years of 2000; the World Meteorological Organisation estimated 2045 (WMO reports #25, #37), but recent investigations suggest the problem is perhaps on a much larger scale than anticipated.
APPENDIX:
EPA Air Quality Index
The AQI is an index for reporting daily air quality. It tells you how clean or polluted your air is, and what associated health concerns you should be aware of. The AQI focuses on health effects that can happen within a few hours or days after breathing polluted air. EPA uses the AQI for five major air pollutants regulated by the Clean Air Act: ground-level ozone, particulate matter, carbon monoxide, sulfur dioxide, and nitrogen dioxide. For each of these pollutants, EPA has established national air quality standards to protect against harmful health effects.
Air Quality Index (AQI): Ozone
Index
Values
Levels
of Health
Concern
Cautionary Statements
0-50
Good
None
51-100*
Moderate
Unusually sensitive people should consider limiting prolonged outdoor exertion.
101-150
Unhealthy for Sensitive Groups
Active children and adults, and people with respiratory disease, such as asthma, should limit prolonged outdoor exertion.
151-200
Unhealthy
Active children and adults, and people with respiratory disease, such as asthma, should avoid prolonged outdoor exertion; everyone else, especially children, should limit prolonged outdoor exertion.
201 300
Very Unhealthy
Active children and adults, and people with respiratory disease, such as asthma, should avoid all outdoor exertion; everyone else, especially children, should limit outdoor exertion.
301 500
Hazardous
Everyone should avoid all outdoor exertion.
Each category corresponds to a different level of health concern. For example, when the AQI for a pollutant is between 51 and 100, the health concern is Moderate. Here are the six levels of health concern and what they mean:
Good The AQI value for your community is between 0 and 50. Air quality is considered satisfactory and air pollution poses little or no risk.
Moderate The AQI for your community is between 51 and 100. Air quality is acceptable; however, for some pollutants there may be a moderate health concern for a very small number of individuals. For example, people who are unusually sensitive to ozone may experience respiratory symptoms.
Unhealthy for Sensitive Groups Certain groups of people are particularly sensitive to the harmful effects of certain air pollutants. This means they are likely to be affected at lower levels than the general public. For example, children and adults who are active outdoors and people with respiratory disease are at greater risk from exposure to ozone, while people with heart disease are at greater risk from carbon monoxide. Some people may be sensitive to more than one pollutant. When AQI values are between 101 and 150, members of sensitive groups may experience health effects. The general public is not likely to be affected when the AQI is in this range.
Unhealthy AQI values are between 151 and 200. Everyone may begin to experience health effects. Members of sensitive groups may experience more serious health effects.
Very Unhealthy AQI values between 201 and 300 trigger a health alert, meaning everyone may experience more serious health effects.
Hazardous AQI values over 300 trigger health warnings of emergency conditions. The entire population is more likely to be affected.
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