Sea Pollution
It seems that almost every day there is another story about pollution of one form or another, in the food we eat, the water we drink and the air we breathe. Very often our own actions lead to that pollution and in many cases we can do something about it. These notes explain how you can investigate sea pollution and advise on positive action to improve our seas and the beaches.
Polluting the seas The seas and oceans receive the brunt of human waste, whether it is by deliberate dumping or by natural run-off from the land. In fact over 80% of all marine pollution comes from land-based acivities and many pollutants are deposited in estuaries and coastal waters. Here the pollutants enter marine food chains, building up their concentrations until they reach toxic levels. It often takes human casualties to alert us to pollution and such was the case in Minimata Bay in Japan when many people died as a result of a pollutant building up in food chains. A factory was discharging waste containing methyl mercury in low concentrations into the sea and as this pollutant passed through food chains it became more concentrated in the tissues of marine organisms until it reached toxic levels. As a consequence 649 people died from eating fish and shellfish contaminated with mercury and 3500 people suffered from mercury poisoning. Investigating sea pollution Rubbish discarded at sea is often washed ashore onto our beaches polluting the coastline, but what kind of litter is thrown away and where does it come from? Very often the tide brings in such a lot of debris that it would be impossible to record each item of litter. A way of overcoming this problem is to examine selected areas of the beach and to set up a line transect enabling a survey of the litter to be carried out. Stretch a line, marked at regular intervals, from the sea across the beach and record the litter that you find at each point on the transect. Make a note of the composition of the litter stating whether it is made of plastic, wood, metal, glass, paper, rope or cardboard. Also record the identity of each item such as fishing line, nets, containers or wire. Containers of plastic and metal are the most frequently reported litter. Drinks containers are common, mostly of the pull-tab type probably thrown away by holidaymakers. There is more rubbish on the beach in the winter, probably because of the prevailing winds and large waves which force the litter ashore. Oil pollution Black tar-like oil is sometimes washed onto beaches not only causing a nuisance to holidaymakers but also killing many sea-birds. The oil mainly comes from tankers which wash out their holds while out at sea to save time in port. Enforcement of laws concerning the dumping of oil is difficult and responsibility rests with the captain of each tanker to obey the law. Once oil is in the sea and the tanker has sailed on, it is difficult to prove that an offence has been committed and unless the culprit can be identified the cost of clearing up is the responsibility of the local council. If you find that oil has been washed ashore report it at once to your local council who will arrange for the beach to be cleaned up. Local volunteers can also begin the task of trying to save the lives of sea-birds contaminated with oil. In 1992, more than 4 million tonnes of oil were released into the world's oceans. Recent research by The US National Science Foundation has found that only 2 per cent of hydrocarbon pollution finding its way into the sea each year comes from tanker accidents. Eleven per cent comes from natural sources - tar sands and oil seeps, 13 per cent comes from the atmosphere, 24 per cent from all forms of transport, and an astounding 50 per cent comes down drains and rivers to the sea from cities and industries. Anyone who has tipped old engine oil down the drain, or 'buried' it in the soil instead of taking it to a recycling point is responsible for some oil pollution at sea. Significant oil pollution is caused by tankers illegally cleaning their tanks while out at sea and dumping the dirty water overboard. Airborne surveillance spotted 64 vessels discharging their tanks in the Dutch sector of the North Sea during 1985. 72 per cent of oil pollution caused by shipping is estimated to be deliberate and illegal. Only 28 per cent is caused by tanker accidents. There is obviously a need for better monitoring of ships at sea by all countries, and the owners of ships illegally cleaning their tanks at sea should be prosecuted. Over 100,000 tonnes of oil is dumped in the North Sea by ships.
In 1992, there were 611 incidents of oil pollution in UK coastal waters alone. Many of the major oil spillages during the last 20 years have been caused, or made worse by human error. Human error can mean carelessness, but it also includes continuing to use old, unsafe ships and employing crews with inadequate training. In the last thirty years, there have been a number of serious oil spillages caused by oil tanker accidents. The first was in 1967, when the Torrey Canyon ran aground on the Seven Stones Rocks, off Land's End, leaking 106,000 tonnes of oil onto rocks and beaches on both side of the English Channel. British guillemots and razorbills were badly affected, and the population of puffins on the Sept Isles in France was virtually wiped out.
In 1978, The Amoco Cadiz was wrecked following engine failure on the coast of Brittany, releasing 223,000 tonnes of oil into the sea. Thousands of migrating seabirds were killed when they landed on the oily waters, and many French oyster fisheries and beaches were completely ruined. The Exxon Valdez was accidentally steered into a reef in Prince William Sound, Alaska in 1989. Emergency equipment did not arrive quickly, and this allowed the 37 million litres of spilled oil to form a slick covering 6,700 square kilometres. The delay in providing equipment, both by the oil company, Exxon, and by the State of Alaska, was unforgivable, and caused the needless death of thousands of animals, including 350,000 sea otters. More recently, the tanker Braer broke up off Shetland having run aground, and in February 1996, the Sea Empress spilled around 70,000 tonnes of crude oil off the Pembrokeshire coast in Wales. For further details, see the sheet 'Oil Pollution Case Study - the Sea Empress', also available from YPTENC. The effects of this oil pollution on wildlife can be terrible. Between January 1971 and June 1979, 36,000 birds were found dead around the British coast as a result of oiling. Migrating species like the puffin, guillemot and razorbill are especially at risk, as they look for areas of calm water on which to rest or catch fish. Oil-covered seas look calm, but if a bird land in a slick, the oil coats its feathers, affects its buoyancy and the insulating power of its feathers and makes it unable to fly away. Even slightly oiled birds sometimes die because they preen their feathers, and in doing so, ingest oily substances which are poisonous to them. An animal killed by oil may then be eaten by fish or birds, who in turn are poisoned by the oil. Cleaning up after an oil spill is a complicated business, and depends on weather conditions and water temperature. In calm waters, long floating booms can be used to help to contain oil, which can then be pumped off the surface of the sea. Chemical dispersants are often sprayed on oil to break up slicks into droplets which can be broken down by marine bacteria. Dispersants are quite toxic, however, so whilst they reduce damage to beaches and save seabird colonies from destruction by oil, they also add more poisons to the sea. Rough seas can break up slicks, but they can also spread oil right through the marine environment. Crude oil is made up of over 1,000 chemicals. Of these, the light hydrocarbons, which are used to make petrol and aviation fuel are the most toxic. In warm conditions, these usually evaporate quite quickly, making a foul smell, but reducing the danger to wildlife. In cold seas, however, the process of evaporation can be very slow, and this means the risk to wildlife lasts longer. To prevent this, the light hydrocarbons are often burned off the surface of the sea. In this country, we have a group of patrol aircraft whose job it is to search for oil floating on the surface of our seas. The spotter planes are able to distinguish different types of oil, and work alongside dispersant-spraying aircraft so that oil can be treated quickly and in the most effective way possible.
Sewage Many sewage pipelines were built years ago when little was known about the effects of pollution and it was thought that the sea would dilute the sewage. Since then the amount of sewage has increased considerably and very often organic matter is washed ashore by the incoming tide. The quality of the water around our coasts is assessed against standards set up by the European Community and over 350 British resorts are subject to inspection. At the moment these standards only apply to major resorts but public demand may eventually lead to other bathing beaches being inspected too. There are no official signs to warn the public of sewage pollution but there are some useful booklists available which give details of polluted beaches. The list of British Blue Flag beaches indicates the bathing waters which have passed certain standards of cleanliness. If you suspect that the water at a resort is unfit for bathing you should report the matter to the local council. If the public outcry is loud enough the authorities will be forced to consider further treatment of the sewage before releasing it into the sea. If you wish to do more about sea pollution the Marine Conservation Society would welcome your support.
Here are some addresses to write to for further information:-
Marine Conservation Society 9 Gloucester Road Ross-on-Wye Herefordshire HR9 5BU Tel: 01989 566017 Marine Information and Advisory Service Southampton Oceanography Centre, Empress Dock Southampton SO14 3ZH Tel: 01703 595000 Surfers Against Sewage Ltd The Old Counthouse Warehouse, Wheal Kitty St Agnes, Cornwall TR5 0RE Tel: 01872 553001 Further information produced by YPTENC on sea pollution can be found in the topic sheets 'Oil pollution case study - The Sea Empress' and 'Pollution'
Information supplied by the Young Peoples Trust for the Environment

Labels: ocean, water

Labels: viruses

The abnormal weather and climate patterns over Taiwan during the 1998 El Nino

Jau-Ming Chen¹, Fong-Ju Weng

Central Weather Bureau

In Taiwan, the annual-mean climate of the 1998 El Nino year is warmest and wettest since 1970. Some noticeable abnormal weather and climate patterns in Taiwan for the 1998 El Nino year are summarized as follows.

(1) For the entire year, each of the 12 monthly-mean temperatures of 1998 is warmer than the corresponding temperature climatology. Annual evolution of the above-normal temperature anomalies is rather consistent with the evolution of warm SST anomalies over the South China Sea and the western Pacific between Taiwan and Japan.

(2) For the spring season (February-April), the total precipitation amount over Taiwan is above normal when compared with climatology. Increase in precipitation amount is accompanied with the weakening of the local Hadley circulation around Taiwan area and the tropical Walker circulation over the equatorial Pacific. Changes in Hadley circulation and Walker circulation are in connection with the existence of significant warm SST anomalies in the eastern tropical Pacific.

(3) For the Mei-Yu season (May-June), the precipitation amount in Taiwan is also above normal when compared with climatology. Moreover, the precipitation evolution during this season shows a clear intraseasonal variation that is highly correlated with the intraseasonal variation of convection activities over the South China Sea.

(4) For the typhoon season (July-October), the precipitation in both July and August is below normal because only one weak typhoon passed Taiwan area in each month. In September, one typhoon (Typhoon Yanni) passed Taiwan area which brought in significant rainfall and made precipitation amount in this month about normal. In October, two typhoons (Typhoons Zeb and Babs) passed Taiwan area to result in above normal precipitation amount in this month.

¹E-mail: cjming@bingo.cwb.gov.tw

Labels: weather

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.

The European Ozone Reserach Coordinating Unit have full details of the THESEO programme. Visit their website to find out more about the missions planned, press releases and the latest report of the UK Stratospheric Ozone Review Group.

Chemical Modelling

Most of the research work here at the Centre for Atmospheric Science involves various computer models of the atmosphere. These models 'blow' (or advect) chemical species around the globe using known or computed weather patterns - winds, temperatures and pressures. The rates of various chemical reactions are dependent on temperature, pressure, and, in the case of photolytic processes, the position of the sun. At each step of the model, the computer code attempts to predict what chemical changes will occur by solving the equations representing each reaction.

The schematic figure below gives some idea of the different parts of such computer models and the sequence of events are the model executes on the computer. Such models can be, and often are, very complex with many man-years work behind them.

Anatomy of a Chemical Model

Different classes of model are used. These are:

Box Models consider just a single point in the atmosphere. Such models are comparatively cheap to develop and run on a PC or workstation. The advantage of such models is that very complex chemical reactions can be included since only the chemistry at a single point is simulated. This is very useful for comparing model simulations with measurements in idealised cases and also for developing less complex chemistry schemes which are used in multi-dimensional models.

Trajectory Models are the next step up from box models. Essentially a trajectory model is a 'box model that moves'. A trajectory of a point (or points) of air is calculated from known wind fields. The chemistry is then calculated for all points along the path that the parcel or air took. This type of model is very useful for determining the chemical properties of air reaching observation stations. By running very many chemical trajectory models, it is also possible to begin to develop a three-dimensional picture of the chemistry in the atmosphere.

Three-dimensional Models use the traditional technique of simulating the atmospheric system on a grid of latitude/longitude points and vertical levels (surfaces of constant Potential Temperature or Pressure). Such models have a realistic representation of the movement or meteorology of air as well as other processes such as clouds, solar radiation and so on. In a way, you can think of a 3D model as a grid of box models where the air it being moved through the boxes. As many points are being represented it becomes impossible to use the complex chemistry schemes found in box models as this would place too great a demand on computing power. As it is, these 3D chemical models of the atmosphere require the most powerful High Performance Computers around. In the UK we use the Cray supercomputer and Fujisu supercomputers at the Rutherford Appleton Laboratory in Oxford.

Models and Observations

The following graphics compare the output of the TOMCAT (grid-point) model with TOMS satellite data for the beginning of the Antartic 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. More information on TOMS is available here.

Comparison between Model Results and Actual Satellite Data
Day 20 (11 September '94)	Day 40 (1 October '94)	Day 56 (17 October '94)

			Inline movie of comparison between TOMS satellite data and TOMCAT model run (1.7 Mb)
			MPEG movie of comparison between TOMS satellite data and TOMCAT model run (366 Kb)

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.

If you want to find more links to Internet resources on Ozone, try our Ozone Data page.

If you are interested in finding out more about the Centre for Atmospheric Science (degrees on offer, current vacancies, current research), please visit our home page.

The Science of the Ozone Hole

Introduction

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. More on this in Part IV of the tour.

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. More about these international investigations in Part IV.

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.

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:

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.

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:

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)

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. Here is some more information on Polar Stratospheric Clouds.

This movie shows a 3D model simulation of how chlorine nitrate (ClONO2) changes during a northern hemisphere winter in the lower stratosphere. Remember that ClONO2 is destroyed when the PSCs form, so for a large part of the movie, you see nothing. But as sunlight returns to the polar night region over the Arctic we see the ClONO2 start to recover. This first happens around the edge of the polar vortex, and we the the now classic doughnut shape of the so-called 'chlorine nitrate collar'.
		Evolution of ClONO2 over the North Pole during winter 1994 (3.4 Mb)
		Evolution of ClONO2 over the North Pole during winter 1994 (small) (554 Kb)
		Evolution of ClONO2 over the North Pole during winter 1994 (large) (1.2Mb)

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

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 and 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.

Labels: hole, ozone, science

Recent Ozone Loss over Antarctica

Why 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 'halocarbons' 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.

Regular ozone measurement have been made from the Halley Bay Research Station for many years. Ozone depletion is most marked in the Antarctic Spring, around October.

TOMS Satellite Measurements

		Inline movie of TOMS ozone measurements from Nov 1978 to Jan 1992 (3.7 Mb)
		MPEG movie of TOMS ozone measurements from Nov 1978 to Jan 1992 (1 Mb)

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.

For more information, do visit the TOMS Home Page.

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.

You can obtain a larger image of a particular year by clicking on the appropriate globe.

Labels: ozone

The History behind the Ozone Hole

The Beginning ...

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.

The Halley Research Station - Information

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.

What Is Ozone And How Is It Formed?

Ozone (O₃ : 3 oxygen atoms) occurs naturally in the atmosphere.

The earth's atmosphere is composed of several layers. We live in the "Troposphere" where most of the weather occurs; such as rain, snow and clouds. Above the troposphere is the "Stratosphere"; an important region in which effects such as the Ozone Hole and Global Warming originate. Supersonic jet airliners such as Concorde fly in the lower stratosphere whereas subsonic commercial airliners are usually in the troposphere. The narrow region between these two parts of the atmosphere is called the "Tropopause".

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)

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 through 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. We'll learn about these in Part III of this tour of the ozone hole.

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.

A more detailed description of the chemistry will follow in Part III.

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.

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.

Labels: ozone

Tornado Variations

• Some tornadoes may form during the early stages of rapidly developing thunderstorms. This type of tornado is most common along the front range of the Rocky Mountains, the Plains, and the Western States.
• Tornadoes may appear nearly transparent until dust and debris are picked up.
• Occasionally, two or more tornadoes may occur at the same time.

Waterspout

• Waterspouts are weak tornadoes that form over warm water.
• Waterspouts are most common along the Gulf Coast and southeastern states. In the western United States, they occur with cold late fall or late winter storms, during a time when you least expect tornado development.
• Waterspouts occasionally move inland becoming tornadoes causing damage and injuries.

How Do Tornadoes Form?

Tornadoes Take Many Shapes and Sizes

Tornadoes Occur Anywhere

Labels: wind