History of the atmosphere

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History of the atmosphere

• The earths atmosphere has evolved in response to
  geologic and biological changes over the past 4.6
  billion years
• For U238, ?1 9.72E-10
• For U235, ?2 1.55E-10
• k is the current ratio of U238U235
• m is the slope from the isochron

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History of the atmosphere

• Early atmosphere had large amounts of CO2,
  nitrogen, water vapor and methane from volcanic
  outgassing and meteor impacts
• Very hot, all water in vapor form
• Possible cycles of condensing and evaporation
• Impacts lasted until about 3.8 bya.

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Gasses

• Free oxygen would have existed only in very small
  amounts
• No life to produce O2
• Rapidly consumed by reaction with volcanic gasses
• Probably high in nitrogen (N2)
• CO2 was likely high, but the amount is uncertain

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Evolution of modern atmosphere

• About 500 million years after earths formation,
  temperatures cooled and water began to condense
  huge oceans formed and CO2 began to dissolve into
  the water
• Around 3.5 billion years ago bacteria began to
  consume CO2 and produce CH4
• Exact specimens are under debate
• Then about 2.5 billion years ago, oceanic iron
  began to disappear, caused by reactions with
  oxygen produced by photosynthetic cyanobacteria
  (blue-green algae)
• These organisms produced atmospheric oxygen

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Evolution of the atmosphere continued

• Oxygen levels continued to rise, very slowly, for
  about 2 billion years
• Eventually, organisms that could actually use
  oxygen evolved
• 500 million years ago, oxygen levels began to
  stabilize at around todays present value
• Allowed the formation of the stratospheric ozone
  layer

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Composition of the modern atmosphere

• Major atmospheric components
• 78 nitrogen
• 21 oxygen
• 0.9 argon
• Trace gasses
• 0.000001-4 water vapor
• 370 ppm CO2 (and rising)
• 1.6 ppm methane

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Thickness of the atmosphere

• Relative to size of the earth, the atmosphere is
  extremely thin
• 90 of mass below 16 km
• Given that the earths diameter is 12,756 km, the
  atmosphere is about the thickness of the skin on
  an apple

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Vertical structure of the atmosphere

• Gravity holds the atmosphere to the earth
• Consequently, the pressure for any area can be
  defined by the weight (force) of a vertical
  column of air over the area

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Top of the atmosphere
Weight of the entire column of air over the 1 m2
determines the pressure (Pa)
1 m2
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Vertical atmospheric structurePressure
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Pressure decrease I

• PART I
• P ?gh
• P pressure (Pa N m-2 kg m-1 s-2)
• ? density (kg m-3)
• g gravitational acceleration (9.81 m s-2)
• h height of air column (m)

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Pressure decrease II

• PART II
• Expressed in a differential form
• dP - ? g dz (negative indicates decrease with
  height) This is the hydrostatic law
• Or, change in pressure is equal to density times
  acceleration times change in height
• A parcel of air is balanced by the upward and
  downward forces acting upon it

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Pressure decrease III

• The ideal gas law
• P ?RT
• R is the gas constant. For air R is 287.07 J kg-1
  K-1
• T is temperature in Kelvin
• Rearrange to ? P/RT and substitute into the
  hydrostatic law to obtain

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Pressure decrease IV

• Integration of the previous equation gives

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Vertical atmospheric structurePressure
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Pressure decrease summary

• Most of the decrease in pressure occurs at
  relatively low elevations
• The mass of the atmosphere is concentrated at low
  elevations

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Verticalatmospheric structureTemperature
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Reasons for vertical temperature profile

• Troposphere temperature declines due to
  decreases in pressure causing decreases in the
  average kinetic energy
• Conceptually the molecules of air are moving
  around more slowly, causing temperature to
  decline
• This can be predicted mathematically

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Where q is the heat flow in or out of the
parcel cpis the specific heat capacity of air (J
kg-1 K-1) T is temperature (K) P is pressure
(Pa) ? is density (kg m-3)
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We can generally assume that there is no heat in
or out of the parcel
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Remembering the ideal gas law, we can then
substitute for density
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Then divide both sides by T
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Recalling the hydrostatic law dP -? g dz and
then substitute
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We see our old friend density yet again
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After substitution and elimination of units
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And finally rearrange
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Dimensional analysis
The dry adiabatic lapse rate 10K/km. The
environmental lapse rate is about 6K/km due to
the latent heat of condensation
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Temperature profiles

• Increasing temperature in the stratosphere
• Caused by ozone absorption of UV radiation
• Decreasing temperature in the mesosphere
• Caused by decreasing ozone
• In the thermosphere
• Warming caused by absorption of UV radiation by
  O2