Chapter 3Earth’s Modern

Atmosphere

Robert W. ChristophersonCharlie Thomsen

© 2012 Pearson Education, Inc.

Earth’s Modern AtmosphereTopics in this chapter:

Atmospheric Composition, Temperature, and Function  

Atmospheric profile

Atmospheric composition

Atmospheric temperature

Atmospheric Components  

Air pollution

Ozone Depletion

Atmospheric Composition, Temperature, and Function  

The air in the atmosphere is a mix of various gases.

The earth’s principle atmosphere extends to approx 480 kms above the surface of the earth, but the depth varies at different latitudes.

Beyond that is the exosphere where the atmospheric gases are very rarified.  

Atmospheric Profile  Atmosphere extends to 32,000 km (20,000 mi) from surface

Thermosphere is at 480 km (300 mi)—top of the principal atmosphere

Three criteria to examine atmosphereComposition

Temperature

Function

Atmospheric Profile  In order to better understand the atmosphere and it’s composition we study the layers that make it up. These layers are defined according to their temperature, composition and function.

The atmosphere has mass (weight) and as a result exerts pressure on the earth. This is known as atmospheric pressure and is an important tool in understanding and predicting climate.

Average atmospheric pressure is 1kg/cm2

Atmospheric pressure decreases outwards from the earth’s surface.

Atmospheric Pressure

© 2012 Pearson Education, Inc. Figure 3.3

Atmospheric GasesNitrogen: 78%

Predominantly volcanic in origin.

It forms an important building block of organic matter. In animals it is consumed indirectly through compounds in food.

Oxygen: 21%

Respiration

Combustion

Decomposition

Carbon dioxide: 0.039% (increasing due to anthropogenic factors)

Natural greenhouse effect / atmospheric heat balance

Used by plants during photosynthesis

Argon: 0.934%

Inert

Trace gases eg. water vapour: amount variable

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Atmospheric Composition

Heterosphere – outer atmosphereFrom 80 km outward, to thermopause.

Layers of gases are not evenly mixed.

Homosphere – inner atmosphere Earth’s surface to 80 km.

Gases evenly blended

Consists of the inner three layers of the atmosphere.

Layers of the Atmosphere:Atmospheric Temperature

ThermosphereRoughly same as heterosphere

80 km (50 mi) outward

Gases rarified

Temperature increases sharply with altitude, but actual heat is low due to the rarified gases.

Upper limit is the thermopause which may vary in altitude, determined by solar activity from 480kms above the earth to 250 kms.

Layers of the Atmosphere:Atmospheric Temperature (contd.)

Mesosphere

Part of the homosphere

50 to 80 km above the earth

Temperature decreases with an increase in altitude, forming the coldest part of the atmosphere at the mesopause -90˚C

Can contain cosmic or meteoric dust, if ice crystals form on this it can create a noctilucent cloud.

Layers of the Atmosphere:Atmospheric Temperature (contd.)

Stratosphere

18 to 50 km

Contains the ozone layer.

This layer absorbs harmful ultra-violet radiation from the sun.

Temperatures increase with altitude throughout this layer due to the absorption of the ozone layer.

Atmospheric TemperatureTroposphere

Lowest layer of the atmosphere

Surface to average altitude of 18 km, although this differs dramatically between the equator and the poles.

Contains 90% mass of atmospheric gas

The average temperature at the top of this layer is -57˚C, but this also varies.

Normal lapse rate – average cooling (with altitude) at rate of 6.4 C°/km.

Environmental lapse rate – actual local lapse rate (this will be discussed more in chaps 7 and 8.

All biosphere processes occur in this layer.

All weather occurs in this layer.

The tropopause forms the upper boundary of water and water vapour in the atmosphere.

Profile of Atmosphere

© 2012 Pearson Education, Inc. Figure 3.2

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CO2 increase 1958–2010

Figure 3.4

Temperature Profile

© 2012 Pearson Education, Inc. Figure 3.5

Atmospheric FunctionIonosphere (mesosphere and thermosphere)

Absorbs cosmic rays, gamma rays, X-rays, some UV rays

Auroras occur in this layer.

Ozonosphere

Part of stratosphere

Ozone (O3) absorbs UV energy and converts it to heat energy

This process converts most of the harmful ultraviolet radiation to longer wavelengths effectively ‘safeguarding’ the earth from harmful radiation.

Protective Atmosphere

Figure 3.6© 2012 Pearson Education, Inc.

Ozone DepletionStratospheric ozone: a concentration of O3 gas in the stratosphere layer of the atmosphere.

This layer absorbs the harmful ultra-violet radiation from the sun, preventing it from reaching the earth.

Ozone loss in this layer is being caused by chloroflurocarbons (CFCs). This is a synthetic molecule composed of chlorine, fluorine and carbon. It was manufactured for use in refrigeration, solvents, aerosols and various other items. This molecule is stable under the earth’s surface conditions however when transported to the stratosphere the intense ultraviolet light breaks it down and liberates the chlorine.

Ozone Depletion (contd.)The chlorine molecule causes a complex set of chemical reactions that breaks down the O3 molecule leaving O2 molecules.

The effect is severe as one chlorine atom decomposes more than 100 000 ozone molecules. These chlorine atoms remain for between 40 and 100 years in the ozone and as a result the effects are extensive.

This has caused extensive loss of ozone, and what is commonly termed ozone holes particularly over the arctic and Antarctic.

Ozone Depletion (contd.)Depleted ozone causes an increase in UV radiation at the surface particularly dangerous to humans as it is cancer causing radiation.

Stratospheric ozone depletion was it’s worst ever above Antarctica in Sept.- Nov. 2006, and each year it widens and deepens although the chemicals which cause this depletion are not being produced or utilised much anymore.

The Montreal protocol was first signed in 1987 with initiatives to reduce and eliminate CFC production. This was successful, and has been amended and strengthened since, but the lifespan of the chlorine molecules in the ozone layer is still problematic.

Antarctic Ozone Hole2008

Figure 3.1.1© 2012 Pearson Education, Inc.

ClOand O3

Figure FS 3.1.2

Variable Atmospheric Components  

Air pollution can be caused by natural and anthropogenic factors:

Natural sources  

Natural factors that affect air pollution  

Anthropogenic pollution  

Natural Sources of Air PollutionLarge amounts of chemical and particulate matter from volcanoes. eg. 1991 Mt. Pinatubo eruption which ejected nearly 20 million tons of sulfur dioxide into the stratosphere.

Wild fires: soot, ash, and gasses (nitrogen oxides, carbon monoxide).

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Southern California Wildfires

Figure 3.7a© 2012 Pearson Education, Inc.

Anthropogenic Pollution  Anthropogenic air pollution tends to be worst in urban areas, although agriculture and human induced fires also cause air pollution in rural areas.

Many of these chemical pollutants are produced through combustion of fossil fuels in transport and electricity production.

Carbon monoxide: transportation, biomass burning.

Anthropogenic Pollution (contd.)  Photochemical smog: transportation, the outputs react with sunlight to produce dangerous air pollutants such as ozone, and nitric acid.

Industrial smog and sulfur oxides: energy production using coal, producing sulphur dioxide, sulphur trioxide and sulphate aerosols all of which are dangerous to health and corrode metals.

Particulates: haze, smoke and dust. Can cause scarring in lung tissue and fibrosis.

Air Pollution

Figure 3.14

Natural Factors That Affect Air Pollution  

Certain natural factors make the effects of pollutants worse:

Winds: transporting pollutants and dust.

Local and regional landscapes: mountains and valleys can trap and concentrate pollutants.

Temperature inversion: where normal temperature decrease with altitude is reversed and becomes a temperature increase with altitude pollution is trapped and concentrated below this layer.

Temperature Inversion

Figure 3.9© 2012 Pearson Education, Inc.

Photochemical Smog

Figure 3.10© 2012 Pearson Education, Inc.

Robert W. ChristophersonCharlie Thomsen

Geosystems 8eAn Introduction to Physical Geography

End of Chapter 3

© 2012 Pearson Education, Inc.