17.airstructure

8/7/2019 17.AirStructure

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Environmental Chemistry

Atmosphere

Atmospheric Structure


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The atmosphere

The atmosphere is a thin blanket of gas thatenvelops the earth.

The gases that make up the atmosphere are

held close to the earth by the pull of gravity. With increasing distance from the earthssurface, the temperature, density, andcomposition of the atmosphere graduallychange

On the basis of air temperature, theatmosphere can be divided vertically into fourmajor layers.


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Atmospheric structure


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Troposphere

The troposphere is the layer from the earths surface tothe tropopause, which is at 10-15 km altitude dependingon latitude and time of year. (Mt. Everest 8.85km)

As altitude increases, air temperature decreases at a

rate of about 3.5o per 1000 ft. The tropopause has atemperature of about 57oC.

The lower part of the troposphere interacts directly withthe surface of the earththis part of the troposphere is

generally called air. The atmosphere in this layer is heated from below by

convection and radiation from the earths surface.

Most of our weather occurs in the troposphere.


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Stratosphere

The stratosphere is the layer above thetroposphere and extends to about 50km.

The temperature rises with increasing altitude,

reaching a maximum of about 1oC at thestratopause.

The ozone layer is in the stratosphere. Ozoneabsorbs UV, causing the rising temperature with

altitude in this layer. The temperature structure keeps the air calm in

this layer. (Thats why jet aircraft fly in the lowerstratosphere!)


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Mesosphere

The mesosphere extends from the top ofstratopause to ~80 km.

In the mesosphere, the temperature

decreases with altitude.


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Thermosphere

The layer of air above mesosphere is calledthermosphere.

In the thermosphere, temperature rises with

altitude, caused by absorption of UV solarradiation by N2 and O2.


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The lower atmosphere

The troposphere and the stratospheretogether are called the lower atmosphere.

The lower atmosphere account for 99.9% oftotal atmospheric mass

The lower atmosphere is the domain of maininterest from an environmental perspective.

Ozone depletion (stratosphere)

Air pollution (troposphere)


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Ionosphere

Ionosphere is a region where ions andelectrons are most abundant.

This region is located at altitude above 60km,therefore lie within the mesosphere and

above.


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Ionosphere

Ionosphere acts asa conducting layerin the upper

atmosphere thatwould allow atransmittedelectromagnetic

signal to bereflected backtoward the Earth.


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Ionosphere: Northern light (1)


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Energetic events on the Sun can turn the solarwind into an intense solar "gale which injectslarge numbers of energetic particles in the

Earth's magnetosphere.


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The charged particles travel along the fieldlines with so much energy that they

penetrate the ionosphere, where they hit gasatoms and molecules.

These collisions give off energy that we seeas colored light.


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atmospheric pressure

The atmospheric pressure is the weightexerted by the overhead atmosphere on aunit area of surface

A B

h

vacuum

Mercury barometer

ghP HgA V!


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Mass of the atmosphere

Ps: the global mean pressure at the surface,Ps=9.84x104Pa.

R=6400 km, the earth radius

g=9.8m2/s, acceleration of gravity

g

Rm sa

24T

!

Results: ma = 5.2x1018kg

24 R

gP as

T

!


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Pressure profile

0

10

20

30

40

50

60

70

80

0.01 0.1 1 10 100 1000

Pressure, hPa

Altitude(km

)


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Barometric law

Where Ma: the molecular weight of air

g: acceleration of gravity

R: gas constant

T: temperature

z: altitude

)exp()0()( zRT

gMzP

a!


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What fraction of total atmospheric mass is

in the troposphere?

The troposphere contains all of theatmospheric mass except for the fraction

P(tropopause)/P(surface) that lie above thetropopause.

)(

)(1

surfaceP

tropopausePF

trop !

P(tropopause) = 100 hPa

P(surface)=1000hPa

Resul

t: Ftrop =0.90


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Units for pressure

International System of Units: Pascal (N/m2)

Hectopascal (hPa)

mm Hg or Torr

Millibar (mbar)

psi (lb/in2)

1 atm = 1.01325 x 105 Pascal (Pa) = 1.01325 x 103hPa

1 atm = 760 mmHg = 760 Torr

1 atm = 1013.25 mbar

1 atm = 14. 7 psi

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