History of the Atmosphere

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History of the Atmosphere

• Early atmosphere came from volcanic outgassing.
• 95 H2O.
• Also CO2, N2 and S-containing gases.
• Water condensed and formed oceans.
• CO2 and S-gases dissolved into the oceans
• N2 is left behind
• Plants grow in the sea. Photosynthesis. Source
  of O2 to the atmosphere. True?

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Chemisty of the major atmospheric constituents

• Nitrogen, oxygen and carbon dioxide
• Sources and sinks - processes
• Reservoir amounts
• Residence times
• Little atmospheric chemistry here. Most of the
  chemical reactions are occuring in the biosphere,
  soils, and oceans of the earth. The rocks
  (lithosphere) are also important.

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Typical residence times of chemicals in earths
reservoirs

• Atmosphere fast mixing and reactions (seconds
  to years)
• Biosphere (plants) pretty fast reactions
  (seconds to years). Slow mixing.
• Soils slower. Years to decades.
• Oceans even slower. Years to centuries.
• Lithosphere slowest. Millions of years.

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Nitrogen

• Very stable as N2. Much more N is in the
  atmosphere as N2 than in plants, soils and ocean
  reservoirs (Fig 6-3).
• Residence time of N in the atmosphere is very
  long.
• Reactive N environment (NO, NH3) is important to
  ecosystems
• Needed to build enzymes that facilitate
  photosynthesis, proteins in animals and plants
• N deposition fertilizes plants
• N deposition can also be damaging - Acid rain
  (HNO3)

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Nitrogen (continued)

• Creating reactive N is difficult processes are
• fixation by bacteria
• N2 O2 _gt 2NO via lightning or combustion
• Manufacture of agricultural fertilizers
• Fertilizer manufacture and combustion have
  greatly increased the reactive N available to the
  atmosphere and ecosystems. Major anthropogenic
  change to the earth system!
• fertilization of natural ecosystems
• acid rain damage to ecosystems
• atmospheric NO increase O3 pollution, strat
  chemistry
• Atmospheric N2O increase greenhouse gas

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Oxygen

• Only found in abundance on the Earth (Table 6-1).
• O2 is closely linked to the CO2 cycle.
• As we discussed, photosynthesis and burial/
  storage of organic C in sediments creates a net
  source of O2 (and fossil fuel deposits).
• Is O2 continuing to increase over time? No. Why
  not? Dynamic equilibrium requires a sink to
  balance the source. Weathering of C sediments
  and Fe is a sink, and balances burial of organic
  C in sediments.

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Oxygen (continued)

• Residence time of O2 with respect to respiration
  isnt very long, about 3000 years.
• But, total surface reservoir of organic C is
  about 4000 PgC. If burned, this would consume
  how much atmospheric O2?
• about 1.
• Sediment C reserves are large. 30x more than
  would be needed to consume atm O2.
• Residence time of O2 with respect to weathering
  of C sediments and Fe is about 2 million years.

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Carbon Dioxide

• Well bypass the ocean chemistry details at this
  time.
• Linked to O2 by photosynthesis/respiration.
• Small concentrations in Earths atmosphere,
  unlike other planets (Table 6-1).
• Increasing mixing ratio (1.8 ppmv yr-1) due to
  fossil fuel burning and deforestation
• Data ice cores and air samples (Fig 6-6).
• 365 ppmv now, 200-280 ppmv for at least the last
  200,000 years!

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Carbon Dioxide (continued)

• Residence time of CO2 in the atmosphere with
  respect to removal by photosynthesis is what?
• What is the net accumulation rate of CO2 in the
  atmosphere?
• This figure is too large. The land biosphere and
  ocean net exchanges of CO2 are not really zero.
• Ocean Stores 2 Pg C yr-1. Can be estimated
  from ocean chemistry. Rate of deep water
  formation is also very important. See Jacob.
• Land Biosphere Stores 2 Pg C yr-1. Can only
  be determined indirectly at a global scale.
• These two net uptake rates are important! They
  are slowing the rate of CO2 accumulation.

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Atmospheric gases with substantial long-term
trends

• long life times, or residence times many
  years
• Carbon dioxide (CO2)
• Nitrous oxide (N2O)
• Chloroflourocarbons (CFCs)
• Methane (CH4) intermediate life time (months to
  years)
• short residence time months to minutes
• Sulfur dioxide (SO2)
• Free nitrogen (NOx, NH3, HNO3)
• Ozone (O3)

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Summary

• Long residence time gases still have sources
  and sinks, but the source/sink magnitudes are
  small compared to the reservoir sizes.
• Even the permanent and non-reactive
  constituents of the atmosphere are in dynamic
  equilibrium (i.e. have sources and sinks that are
  roughly balanced, so dm/dt 0).
• If the mixing ratio of a gas is changing over
  time (e.g. CO2, O3), then something has upset
  this dynamic equilibrium (e.g. fossil fuel
  burning, increase in stratospheric destruction
  rates).
• The time required for the mixing ratio of an
  atmospheric gas to return to undisturbed
  conditions is related its residence time.
• Remember the most important facts about the
  current state of the earths atmosphere.