Average Composition of the Troposphere

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• Average Composition of the Troposphere
• Gas Name Formula
  Abundance () Residence time
• (approx)
• Nitrogen N2
  78.08 42,000,000 years
• Oxygen O2
  20.95 5,000 years
• Water H2O
  0 to 4 10 days
• Argon Ar
  0.93 Infinite
• Carbon Dioxide CO2
  0.0360 4 years
• Neon Ne
  0.0018 Infinite
• Helium He
  0.0005 Infinite
• Methane CH4
  0.00017 10 years
• Hydrogen H2
  0.00005 3 years
• Nitrous Oxide N2O
  0.00003 170 years
• Ozone O3
  0.000004 20 days
• variable gases

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• EVOLUTION OF THE ATMOSPHERE
• Earth thought to have formed about 4.5 billion
  years ago
• Atmosphere probably consisted of gases then
  abundant in the solar system -gt hydrogen and
  helium.
• Most of these gases were lost to space
• Over time a secondary atmosphere was formed
• (Current atmosphere doesn't contain much
  hydrogen or helium).
• outgassing from cooling magma
• Volcanoes efflux H20, CO2, SO2, N2, H2, Cl2
• Upon cooling of this prehistoric atmosphere
• Water vapor condensed and precipitated to
  form oceans.
• Some carbon dioxide dissolved in droplets
  also precipitated out.

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• Eventually anaerobic bacteria developed some 3.5
  billion years ago
• Could survive in the absence of oxygen.
• Began the conversion of carbon dioxide to
  oxygen while removing
• Carbon Dioxide which is now stored primarily
  in carbonate rocks.
• Plankton and shellfish continue this process
  more effeciently.
• As oxygen started to become abundant, some of it
  broke down by the
• suns radiation into atomic oxygen and
  eventually formed ozone .
• Ozone absorbed most of harmful ultraviolet
  radiation to make Earth
• suitable for life.
• The atmosphere we know was produced by biological
  processes.

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Geological carbon cycle
In the geological carbon cycle, carbon moves
between rocks and minerals, seawater, and the
atmosphere. Carbon dioxide in the atmosphere
reacts with some minerals to form the mineral
calcium carbonate (limestone). This mineral is
then dissolved by rainwater and carried to the
oceans. Once there, it can precipitate out of the
ocean water, forming layers of sediment on the
sea floor. As the Earths plates move, through
the processes of plate tectonics, these sediments
are subducted underneath the continents. Under
the great heat and pressure far below the Earths
surface, the limestone melts and reacts with
other minerals, releasing carbon dioxide. The
carbon dioxide is then re-emitted into the
atmosphere through volcanic eruptions.
(Illustration by Robert Simmon, NASA GSFC)
Slow time scale - controls atmospheric carbon
dioxide on time scales of hundreds of millions
of years
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Biological/Physical carbon cycle - shorter than
geologic cycle
Land plants 50 years atmosphere 4 years soils
25 years Fossil fuels 650 years oceans 100s
to 1000s years carbonates 150 years
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Addition of O2 to the Atmosphere Today, the
atmosphere is 21 free oxygen. How did oxygen
reach these levels in the atmosphere? Revisit
the oxygen cycle Oxygen Production
o Photochemical dissociation - breakup of
water molecules by ultraviolet
Produced O2 levels approx. 1-2 current levels
At these levels O3 (Ozone) can
form to shield Earth surface from UV o
Photosynthesis - CO2 H2O sunlight organic
compounds O2 - produced by cyanobacteria, and
eventually higher plants - supplied the rest of
O2 to atmosphere. Thus plant populations-gt
Oxygen Consumers o Chemical
Weathering - through oxidation of surface
materials (early consumer) o Animal
Respiration (much later) o Burning
of Fossil Fuels (much, much later) Throughout
the Archean there was little to no free oxygen in
the atmosphere (lt1 of presence levels). What
little was produced by cyanobacteria, was
probably consumed by the weathering process. Once
rocks at the surface were sufficiently oxidized,
more oxygen could remain free in the
atmosphere. During the Proterozoic the amount of
free O2 in the atmosphere rose from 1 - 10 .
Most of this was released by cyanobacteria,
which increase in abundance in the fossil record
2.3 Ga. Present levels of O2 were probably not
achieved until 400 Ma.
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Vertical structure of atmospheric pressure
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Atmospheric temperature vertical structure
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Vertical structure as a function of latitude
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Mars
Venus
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The Ocean composition

• Dissolved salts comprise 3.5 by volume of sea
  water
• Originate from weathered rocks, volcanic, and
  atmosperic sources

Composition of sea water remarkably uniform and
constant over time
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Density of ocean water varies by lt 7
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Variations of density as a function of salinity
and temperature
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Vertical structure of ocean in different latitudes