The Atmosphere

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The Atmosphere
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Composition of the Atmosphere

• Historical Interest
• Ancient Greeks viewed air as one of the four
  elements.
• 17th Century Air was recognized as a mixture of
  gases
• a confused aggregate of effluviums
• Robert Boyle

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Composition of the Atmosphere

• N2 and O2 major components
• Mixture or compound?

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Composition of the Atmosphere

• Humphrey Davy (1778-1829) thought it was a
  compound
• if it were not, O2 would sink below N2 and O2 gt
  N2 at bottom of the atmosphere)
• (OOPS! Did not appreciate the degree of mixing in
  the atmosphere which destroys gravitational
  separation).

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Air is a mixture!

• Ratio of O2 to N2 does vary (slightly) from place
  to place.
• If it were a compound, the formula would have to
  be N15O4 (unlikely)
• It is possible to separate N2 and O2
• There is no volume change or heat release when N2
  and O2 are mixed.

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Air is a mixture!

• Accurate analysis by Henry Cavendish (1780s)
• also tried to combine N2 and O2 chemically
• N2 O2 ? 2NO (High T)
• but always some unreacted gas left)
• later realized by Rayleigh and Ramsey at the end
  of the 19th Century to be Argon)

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Composition of the Atmosphere

• Rank order of elements in terms of number of
  atoms in major reservoirs
• Often said that composition of sea water and
  atmosphere resembles composition of life
• H and O form H2O

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• first four principal elements of living material
  are contained in the first five major components
  of the atmosphere
• Evolutionary view
• Atmosphere ?? Biosphere ?? Origin of Life

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Residence Times

• The atmosphere is a dynamic resevoir!
• trace components are constantly being produced in
  the atmosphere (gas phase chemistry) or at the
  Earths surface (biological, natural, manmade
  emissions)
• sources ? constant addition of material
• but overall composition of the atmosphere doesnt
  change much over long timescales

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Residence Times

• input is balanced by removal of gases in the
  atmosphere (gas phase chemistry) and at the
  Earths surface (absorption, rain out, biological
  activity)
• sinks ? constant removal of material
• Balance between sources and sinks maintains the
  atmosphere in a steady state (composition appears
  constant)

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Residence times
sinks
sources
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Residence Time

• Residence time, t , characterizes a system only
  when it is well mixed
• Usually a long residence time correlates to being
  well mixed (probability of removal at sink is
  constant)
• average age residence time
• But for slow mixing with source and sinks
  isolated (e.g. particulates injected into the
  stratosphere but can only be removed in lower
  atmosphere) average age can be less than t

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Residence Time

• For source and sink close together (i.e. where
  material can be removed very rapidly) residence
  times can be shorter than the average age

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Residence Time

• Average mixing time of the atmosphere is 80
  days (high mobility)
• e.g. H2O(g) t 10 days not well mixed
• CO2 t 4 years well mixed
• i.e. residence times reflect degree of mixing
• cfcs uniformly distributed (long t)
• global problem
• acid rain (short t) much more regional
• local problem

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Tune in next time for 1.3) Size and pressure
variation 1.4) Radiation and thermal structure