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Uppe r A tm osphe r e

Basics

Unit 1Understanding and observation of the midatmosphere

The region of the atmosphere above the tropopause is called the stratosphere. Inthis unit, we look at how the stratosphere differs from the troposphere. We alsoinvestigate why there are other distinct layers in the atmosphere and how these

layers are defined.

We look at how the physical and

meteorological parameters of theatmosphere change with altitude andinvestigate how the chemicalcomposition changes withheight. We also look at how modern

measuring techniques, using satellitesand lasers, have been used to provideus with this infomation.

LIDAR in Davis / Antarctica with aurora in the

background

Photo: David Correll - Australian Antarcticivision - http://www.antdiv.gov.auD

Part 1: Layers

ESPERE Climate Encyclopaedia www.espere.net - Upper Atmosphere Basics - page 1

English offline version

supported by the International Max Planck Research School on Atmospheric Chemistry and Physics


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Part 1: Players

The laye rs o f the a tm osphere

The d if fe ren t l aye rs w e see i n th e a tm osphe re have d i f fe ren t ph ys i ca l

p roper t ies . As th e a l t i t ude increases, a tm ospher ic p ressure decreases.Th is is because the dens i ty o f th e a i r decreases - th e h igher w e go , theless a ir m o lecu les w e f i nd i n the sam e vo lume o f space. Tem pera tu re ,hum id i t y and w ind speed al so change w i th a l t i t ude .

1. Blue sky above the clouds.

ource:www.freefoto.coms

If we look up into the sky fromthe ground we can't see thelayers of the atmosphere, weeither see a clear blue sky orclouds. However we get an idea

that the properties of theatmosphere change with altitude

if we travel by aeroplane.Regardless of the weather on theground, we see blue sky with noclouds above us once we reach analtitude of 10 - 11 km. At thisheight we are in the tropopauseor even the lower stratosphere.

There are no clouds this high upsimply because there isn't enoughwater in the air to allow them toform.

W h y d o es t h e t e m p e r atu r e

change?

Small scale temperature changesare seen in the atmosphere whichoccur as a result of local changesin conditions, for example, theland cools down and heats upmore quickly than the sea.

There are two main reasons whylarge scale changes intemperature are seen in theatmosphere:

a) the surface of the Earthabsorbs sunlight and heats up.As we move away from the warmsurface of the Earth, the coolerthe air becomes. This leads to a

decrease in temperature withaltitude.

2. Profiles of temperature, air pressure and air density

with increasing altitude. adapted from: Schirmer -

Wetter und Klima - Wie funktioniert das? Please clickto enlarge! (120 K)

Why does the tem pera tu re change?

Small scale temperature changes are seen in the atmosphere which occur as a

result of local changes in conditions, for example, the land cools down and heats

up more quickly than the sea.There are two main reasons why large scale changes in temperature are seen in





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the atmosphere:

a) the surface of the Earth absorbs sunlight and heats up. As we move away fromthe warm surface of the Earth, the cooler the air becomes. This leads to adecrease in temperature with altitude.

b) the temperature of the atmosphere is also governed by the chemicals the aircontains. Some chemicals are able to absorb sunlight themselves and heat up theair around them. Ozone (O3) molecules in the s t ra tosphe re are able to absorbultra-violet radiation from the Sun and warm the surrounding air. This leads to anincrease in the temperature in the stratosphere. The temperature increases withaltitude until a local maximum is reached. This temperature maximum defines theborder between the stratosphere and the next layer of the atmosphere above.

This border is known as the s t ra topause. The layer above the stratosphere isknown as the mesosphe re and here temperature decreases with altitude.Another temperature increase takes place in the the rmosphe re , where nitrogenand oxygen absorb extremely energetic short wavelength ultra-violet radiationfrom the Sun and are partially converted into charged ions. This layer is,

therefore, also known as the i onosphe re.

2. Profiles of temperature, air pressure and air density with increasing altitude. adapted from:

Schirmer - Wetter und Klima - Wie funktioniert das?





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3. Like a pillow tower:

How air is compressed ...

by Elmar Uherek

Wh y does th e p ressure decrease?

The difference between air and water is that air iscompressible and water is not. If you are diving in thesea and have 10 metres of water above you, the

pressure is 1 bar, if you have 20 metres of water aboveyou it's 2 bar simply because the amount of water isdoubled. However, air is different. Just imagine youhave a tower of very light pillows. As the height of thetower increases, the pillows on the bottom of the towerbecome flatter due to the weight of the ones above.They can be compressed because they have a lot of freespace in them. So at the end, you may have 10 pillows

in the first 30 cm layer of your tower and only one inthe 8th layer even though each pillow weighs the same.This is the same in the atmosphere. Therefore,meteorologists very often use pressure rather thanheight in metres to define the altitude of the

atmosphere. The amount the air compresses depends abit on the temperature but roughly we can divide thepressure by a factor of 2 for every 5.5 km increase inheight.

Click here for more detailed information on howatmospheric pressure is calculated.

I s t h e t h e r m o sp h e r e r ea l ly t h a t h o t?

Temperatures recorded in the thermosphere, 200 - 500km up in the atmosphere, reach 500 - 1000 oC. Is itreally that hot? The problem here is our definition of

temperature. In the thermosphere the molecules havea huge amount of energy so the temperatures are

correct. However, the number of molecules per volumeof space is about one millionth of the number ofmolecules near the surface of the Earth. This meansthat the probability that the molecules will collide,transfer their energy and cause heating is extremelylow. Therefore, the temperatures recorded in thethermosphere are good measures of molecular energy

but not compable to temperatures measured with athermometer on the ground.



http://www.atmosphere.mpg.de/enid/50164413c1f8d088621ec6ca26951418,0/1__Understanding_the_stratosphere/pressure___altitude_1z4.htmlhttp://www.atmosphere.mpg.de/enid/50164413c1f8d088621ec6ca26951418,0/1__Understanding_the_stratosphere/pressure___altitude_1z4.htmlhttp://www.atmosphere.mpg.de/enid/50164413c1f8d088621ec6ca26951418,0/1__Understanding_the_stratosphere/pressure___altitude_1z4.htmlhttp://www.atmosphere.mpg.de/enid/50164413c1f8d088621ec6ca26951418,0/1__Understanding_the_stratosphere/pressure___altitude_1z4.html


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4. a) Weather map at ground level. From:

Schirmer - Wetter und Klima - Wieunktioniert das?f

4. b) The same weather map at 300 hPa

(about 9 km in altitude). Please note thewind speed symbols! From: Schirmer -

Wetter und Klima - Wie funktioniert das?

4. c) Have alook at thefigure on the rightand compare thewind speeds at theground (dark blue,

below) and at 9 kmaltitude (light blue,above) at the

same places. Whatis the wind speedin km h-1 at thethree marked

locations?

5. Wind speed is often measured in knots where

knot = kn = nautical mile h-1 or in km h-1.

The correct unit is m s-1.1 m s-1 = 3.6 km h-1

1 knot = 1.852 km h-1

The symbols in the weather map tell us the winddirection (where the wind comes from) and the wind

speed in knots. As the example shows, a full sized tick

mark represents a wind speed of 10 knots, a half sizedtick mark represents a wind speed of 5 knots.

How does the w ind change?

The figure above shows that wind

speeds are much greater in theupper troposphere than they are

lower in the atmosphere. So anormal wind speed at thetropopause is equivalent to asevere storm at ground level. Asa result, air traffic uses a

special weather forecastingsystem to take these changes in

wind speed into account. Oncewe reach the stratosphere,however, wind speed decreasessignificantly.





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6. Wind speed vertical profile. Data from a balloon experiment of the US national weather service.

Published at Exploring Earth.

7. Comparisons of wind speed and temperature.





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Part 2: Composition

Compos i t i on o f t he s t r a tosphe re

Most o f t he compounds r e leased a t the Ear t h ' s su r face do no t r each t hes t ra tosphe re , i nstead t hey a re :

decomposed by t he ma in t r oposphe r i c ox idan ts ( hyd rox y l r ad i cal s- OH , n i t r a t e r a di ca ls - NO , ozon e - O )3 3

b r o ke n d o wn b y su n l ig h t

deposi ted back to t he sur face o f the Ear t h in ra in o r as par t ic les

t r apped in t he co ld t r opopause.

Because the tem pera tu re t r end be tw een th e t r oposphe re and thes t ra tosphe re reve rses , the re i s a lmos t n o a i r exchange be tw een thesetw o laye rs . M ix ing o f ai r i n the t r oposphe re takes hou rs to days w he reas

m ix ing i n the s t r a tosphe re takes m on ths to yea rs .

One of the consequences of thislack of mixing between the

troposphere and the stratosphereis that the water vapour contentof the stratosphere is very low.Typical mixing ratios (see belowfor definition) are in the range of2 - 6 ppm (parts per million)compared to 100 ppm in the

upper troposphere and 1,000 -

40,000 ppm in the lowertroposphere, close to the surfaceof the Earth. This means thatstratospheric clouds form veryrarely and only if temperaturesare so low that ice crystals grow.

These conditions generally onlyoccur in the polar regions.

However, increasing water vapourconcentrations due to emissionsfrom aeroplanes and highertemperatures due to tropospheric

warming below may lead to morepolar stratospheric clouds beingformed in the future.

1. Polar stratospheric clouds over Kiruna / Sweden.

source: MPI Heidelberg.

I no rgan ic compounds i n the s t r a tosphe re

Stratospheric chemistry is dominated by the chemistry of ozone. Between 85 and90% of all the ozone in the atmosphere is found in the stratosphere. Ozone isformed when sunlight breaks down molecular oxygen (O2) in the

stratosphere into oxygen atoms (O). The highly reactive oxygen atoms thenreact with more molecular oxygen to form ozone (O3). Most of the other gases

in the stratosphere are either really long lived compounds emitted originally intothe troposphere (such as the chlorofluorocarbons - CFC's) or are brought in by





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severe volcanic eruptions (generally sulphur containing compounds andaerosols). So inorganic compounds such as ozone, nitrogen oxides, nitric acid,sulphuric acid, halogens and halogen oxides from CFC's are the dominantchemicals in the stratosphere.

2. Eruption of Mt. Pinatubo Philippines in June 2001.

source: Cascades Volcano Observatory USGS Photo by

ick Hoblitt.R

Volcan ic erup t ion s

Severe volcanic eruptions caninject large quantities of gases

and particles directly into thestratosphere. These gases include

the halogen containting acids,hydrochloric acid (HCl) andhydrofluoric acid (HF) and sulphurdioxide (SO2) which is convertedto sulphuric acid (H2SO4), one ofthe compounds responsible forcloud formation. The particles

emitted include silicates andsulphates and these absorb

sunlight in thestratosphere. Volcanic eruptionscan, therefore, lead to atemporary warming in thestratosphere and a temporary

cooling in the troposphere. Theseeffects on temperature can last

around 1 - 2 years. If theeruption is large enough, such theeruption of Mt. Pinatubo in the

Philippines in June 1991, theeffect can be seen over the wholehemisphere.

Understand ing concen t r a t i ons and m ix ing ra t i os

We can express the amount of a compound in the atmosphere in two ways,relative and absolute:a) mixing ratio = the fraction of the compound as a proportion of all the airmolecules present. If there are 40 ozone molecules in 1 million air molecules the

mixing ratio is 40 ppm (parts per million). This is relative.b) concentration = the concentration of the molecules of the compound in a

certain volume of air. If there are 100 molecules of ozone in one cubic meter ofair, the concentration is 100 molecules m-3. This is absolute.If you know the air pressure, it is possible to convert between the two units.Pressure decreases with altitude, i.e. the higher we go in the stratosphere,the fewer molecules there are in each unit volume of air. This means that if the

absolute amount of ozone remains the same as the altitude increases, the mixingratio for ozone also increases.

We can explain this general principle very simply. In a certain volume (light bluebox) there is a certain number of air molecules (blue) and a certain number ofozone molecules (red). The number of air molecules decreases with altitude.





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3. Here the number of ozone molecules

remains constant with altitude. As the total

number of air molecules decreases withaltitude, the ozone mxing ratio increases with

ltitude (see below).a

3. b) Here the absolute number of ozone

molecules decreases in parallel with the

decrease in the number of air molecules. Asa result, the mixing ratio remains constant as

he altitude increases.tIn reality, there is only around 1 molecule of ozone for every million molecules ofair!

3. a) Simple ozone profile for the example above. The total concentration of air is given in blue, theozone concentration in red and the ozone mixing ratio (% ozone) is shown in green. Since the

number of ozone molecules stays constant but the total air concentration decreases, the mixing ratio

i creases with altitude.n





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3. Simple ozone profile for the example above. The total concentration of air molecules is given in

blue, the ozone concentration in red and the ozone mixing ratio (% ozone) in green. As the ozone

concentration decreases in parallel with the decrease in the total concentration of air molecules, theozone mixing ratio is constant with altitude.

Between the ground and the lower stratosphere, ozone mixing ratios tend toincrease with altitude as ozone concentrations remain nearly constant but air

becomes thinner. In the lower stratosphere, ozone concentrations increase withaltitude (the example below shows an increase of a factor of eight) increasing

ozone mixing ratios further. It is only above the ozone layer that mixing ratiosare approximately constant with altitude.

4. Figure showing how the ozone mixingratio and ozone concentration changes

with altitude.

source: adapted from IUP Bremen.





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Part 3: Observation

Measu remen ts i n the S t ra tosphe re

The s t ra tosphe re beg ins at an a l t i t ude o f be tw een 8 and 15 km and thein te res t i ng reg ions a re h igher than no rm a l p lanes can f l y . So how do w eknow abou t the chem is t r y o f the s t r a tosphe re?

In order to study the chemistry of the stratosphere we can either:

1. send measurement instruments into the stratosphere on special aircraft oron balloons.

2. use the characteristic way in which a specific chemical compound interactswith light to study the stratosphere from the ground or from space usingsatellites.

Aerop lanes

Uniquemeasurementshave been madepossible withspecial

aeroplanes, suchas the former

Russian highaltitude spy plane.This plane, nowcalled"Geophysica", hasbeen convertedinto an airborne

laboratory andsuch planes canreach altitudes ofaround 20 km.The flights are veryexpensive and, asa result, this

method is not usedoften.

1. Geophysica - high altitude research aircraft.source:MDB Design Bureau

Bal loons

A more common alternative is to take measurements using

meteorological balloons. Weather balloons can reach altitudes of between 30 and35 km before they burst. They carry sensors to measure, for example, ozone andsend the information back to Earth via a radio signal. As the balloon travels upthrough the air it sends continuous information back to Earth. Balloons are,therefore, a very useful and relatively inexpensive way of finding out about the

vertical structure of the atmosphere.





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2. a) Start of an ozone balloon ascent atHohenpeissenberg Observatory, Germany.

hoto courtesy of Ulf Khler.P

2. b) Ozone probe for balloonmeasurements. Photo courtesy of

lf Khler, DWD Hohenpeissenberg.UI n te rac t i on o f mo lecu les w i th l i gh t

The way in which different chemicals interact with light is really complicated. Invery simple terms, something happens when light and matter interact. The lightcan be absorbed completely by the compound. It can be reflected or scattereddirectly back into space or can be taken up and re-emitted at a different

energy (as a different wavelength).

Its easy to see the impact of lightabsorption by clouds, water andlarge particles- direct sunlight isblocked by clouds, as we dive intothe sea it becomes darker as

more light is lost and a duststorm makes the sun look pale.Smaller molecules do the same.They can also absorb orreflect light, they can scatter the

light back to Earth or absorb thelight and re-emit less energeticlight of a different wavelength.Examples of this are

phosphorescence andfluorescence. These effectshappen when chemicals take updaylight and emit different energylight which we can see in thedark. The sort of light re-emittedtells us something about the typeof chemical and the intensity ofthe light tells us something about

its concentration.

3. Phosphorescence takes place if light is absorbed and

reemitted again at an other wavelength. source:composed from web-advertisements.





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4. How does a LIDAR work?Please press reload in order

to restart the animation!by EU

Interaction of light

with molecules in

the stratosphere canbe observed fromthe groundor measured from

space usinginstruments

mounted onsatellites.

LI DAR

Lidar (LI ght

Detection AndRanging) is one

technique which canbe used from theground. A shortpulse of veryintensive laserlight is sent into thesky. After a while,

light returns toEarth and is

measured. This lightgives us informationabout thecompounds in theatmosphere (fromthe wavelength ofthe returning light)

and at whatconcentration theyoccur (the intensityof the returninglight). But how dowe know how highup in the

atmosphere thesecompounds are?

Light has a certainvelocity and thelonger the lighttakes to come backto Earth, the higherthe compounds are.

5. LIDAR measurements. Imagesource: University of Western Ontario.

The animation on the left showsa laser pulse (light blue) whoselight is scattered back to Earthat three different altitudes by airmolecules (green) and arrives atthe detector (light green) atthree different times.





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6. SODAR - wind speed measurements.

icture source: Meteotestp

RADAR and SODAR

Different variations of the wave detectionand ranging technique can also beused. The best known is RADAR (RAdio

Detection And Ranging), which is used tomeasure particles in the air and theproperties of clouds. RADAR allows us totrack thunderstorms over several hundredkilometers. If sound is used instead oflight, the technique is known asSODAR (SOund Detection And Ranging)and this gives us a powerful tool for the

measurements of wind speed anddirection.

Sate l l i tes

Satellites observe our planet from space.Some of them observe just one area of theEarth and are known as geostationarysatellites whereas others orbit the Earth atan altitude of between 500 and 1000 km

and can circle the Earth in about 1.5 to 2hours. Some of these satellites haveinstruments known as spectrometersaboard and these can detect differentwavelengths of light and give usinformation on the chemical compositionof the atmosphere.

1. Using satellites wecan measure the amountof sunlight scattered byclouds or air molecules.

2. Satellites can carryspectrometers which workin the infra-red region ofthe spectrum and measurelong wave radiationcoming directly from theEarth.

3. For certain positions of theSun and

the Earth, sunbeamspass through just air tothe detector on thesatellite. This can give usinformation on howconcentrations of differentmolecules changethroughout the

atmosphere.

7. Different techniques of satellite measurements.

scheme by Elmar Uherek.




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