Dr Mark Cresswell

The Atmosphere

69EG5513 – Climate & Climate Change

Lecture Topics• General Points

• A history of the atmosphere

• Composition of the atmosphere

• Structure of the atmosphere

• Cycles (nitrogen and oxygen)

• Greenhouse Gases

• The atmosphere behaves like a fluid

• The atmosphere is a mixture of different gases, aerosols and particles

• The atmosphere remains around the earth as an envelope because of gravity

• Much of the observed motion in the atmosphere results from solar radiation

General Points

The atmosphere is like a skin

• The Earth’s original atmosphere (4.6 BYA) was most likely hydrogen and helium - as well as methane and ammonia

• Speculation suggests that much of this early atmosphere escaped to space from the hot surface

• A more dense atmosphere enveloped the Earth as molten rock from volcanoes and steam vents escaped from the hot interior

History of the atmosphere

• Such volcanic activity produces mostly water vapour (about 80%), carbon dioxide (about 10%) and a few percent of nitrogen

• These new gases would have created a new atmosphere

• Over many more millions of years, the constant production of these gases from the hot Earth interior (outgassing) provided enough water vapour for clouds to form

History of the atmosphere

• After the formation of clouds, rainfall would have occurred forming the rivers, lakes and oceans of the world

• Large amounts of carbon dioxide was dissolved in the oceans and later locked up as carbonate sedimentary rocks such as limestone

• As CO2 dwindled the concentration of chemically inactive nitrogen increased

History of the atmosphere

• Oxygen began increasing in concentration slowly as solar energy split water vapour (H2O) into hydrogen and oxygen - a process called photodissociation

• Lighter hydrogen would have risen and escaped into space whilst the oxygen remained in the atmosphere

• This initial supply of oxygen would have allowed primitive plant life to begin

History of the atmosphere

• Continued plant growth would have enriched the atmosphere with yet more oxygen during photosynthesis

• The atmosphere as we know it today would have taken a great deal of time to develop and we as a species may be altering it more dramatically through pollution and burning of fossil fuels.

History of the atmosphere

GAS PERCENT* GAS & PARTICLES PERCENT** ppmNitrogen 78.08 Water vapour 0 to 4Oxygen 20.95 Carbon dioxide 0.036 365Argon 0.93 Methane 0.00017 1.7Neon 0.0018 Nitrous oxide 0.00003 0.3Helium 0.0005 Ozone 0.000004 0.04Hydrogen 0.00006 Particles 0.000001 0.01-0.15Xenon 0.000009 Chloroflourocarbons 0.00000002 0.0002* = Percent by volume dry air** = Percent by volume From Ahrens C. D, 2000

Composition of the atmosphere

At the surface, there is a balance between Nitrogen/Oxygendestruction and production

Nitrogen is removed from the atmosphere primarily by biologicalprocesses involving soil bacteria. It returns to the atmosphere bythe decay of plant and animal matter

Oxygen is removed from the atmosphere when organic matterdecays - or combined with other substances to make oxides. Itis also removed by animals during respiration (lung exchange).Oxygen is added to the atmosphere by plants during photosynthesis

Cycles

Water vapour is one of the most important atmospheric gases. It’sconcentration will vary geographically and temporally. Values ashigh as 4% are found in the humid tropics, but as low as a fraction of a percent near polar regions

Apart from its contribution to cloud formation and precipitation,water vapour releases large amounts of heat - called latent heatwhen it changes from vapour to liquid

Latent heat is important to the formation of thunderstorms andhurricanes. Water vapour is also an important greenhouse gas asit strongly absorbs and re-emits outgoing longwave radiation

Cycles

Composition of the atmosphere

Composition of the atmosphere

Decreasing wind speed through the friction layerThis layer can be as shallow as 500ft

Any questions?