Greenhouse Gases Archer Chapter 4

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Greenhouse GasesArcher Chapter 4

• Garver
• GEO 307

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Greenhouse Gases

• Layer model assumes atmosphere acts as a
  blackbody in the IR, absorbing and emitting in
  all frequencies of light.
• In reality, gases absorb very selectively.
• Difference has to do with the effect of molecular
  vibrations on the electromagnetic field.

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Difference in gases

• Because gases absorb IR selectively
• Some radiation bands are completely absorbed.
• Others are atmospheric windows
• This leads to higher greenhouse forcing per
  molecule for some gases.

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Need a Modification to the Layer Model

• By looking more closely at gases in atm
• Concentration of molecules within a volume.
• Fewer molecules per volume as gas expands.
• Proportions () easier
• O2 21, N2 78, CO2 0.038
• CO2 380 parts per million (ppm)
• Mixing Ratio CO2 380 parts per million (ppm)
• CO2 is rising at a rate of 1.5ppm/yr

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1. Nitrous Oxide (N2O) 2. Methane (CH4) 3.
Ozone (O3) 4. Water Vapor (H2O) 5. Carbon
Dioxide (CO2)
Sun Range of visible wavelengths
Earth Range of primary wavelengths being
emitted.
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Gases, vibration,light (page 30)

• Most mass of an atom is in its nucleus.
• Electrons - orbitals around the nucleus.
• Nuclei of 2 different atoms repel each other (
  charges).
• But some electrons of different atoms can fit
  together in their orbitals.
• Atoms are held together (like a spring) not too
  close, not too far.
• Too close they repel each other, too far and
  there is less energy gain from sharing the
  electrons.

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Gases, vibration,light

• Chemical bond - physical process responsible for
  the attractive interactions between atoms and
  molecules, gives stability to chemical compounds.

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Gases, vibration,light

• Gases are the simplest molecules.
• Vibrate only in a particular frequency.
• Vibrations of the major gases in the atm (O2, N2)
  are invisible to the electromagnetic field
• Transparent to VIS and IR

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Gases, vibration,light

• O2 N2
• These are symmetrical molecules - Invisible
  because electric field doesnt change.
• only 2 atoms (that are the same)
• Not infrared active
• So, not gh gases
• In general, symmetrical molecules with only 2
  atoms are not gh gases.

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Gases, vibration,light

• Gas molecules with more then 2 atoms have more
  than one chemical bond.

• Water has three normal modes of vibration
• All 3 are IR active

Positive charge
6.0 um
negative charge
3.0 um
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Water - main absorber

• 13 million tons of water in the atmosphere
• Water content the atm varies (0 to 4)
• 70 of atmospheric absorption of IR
• Contributes significantly to greenhouse effect
• Ensures a warm habitable planet
• But also, negative feedback effect, clouds
  reflect sunlight.
• http//en.wikipedia.org/wiki/Electromagnetic_absor
  ption_by_water

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Gases, vibration,light

• CO2 - is in a straight line with C in the middle
• It is a symmetric molecule - both O molecules
  pull equally.

Most climatically important
IR Active
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How gh gases interact with IR

• Earths IR (from clouds, atm, and ground)

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How gh gases interact with IR (page 32)

• Gases are not good blackbodies!
• Very choosy about what wavelengths they absorb
  and emit.
• Blackbody spectra for Earth - T ranges from hot
  summer day to cold upper atm (model generated
  data)

CH4 1300 cycle/cm (8 mm) CO2 effects outgoing
IR more because theres more IR energy at 700
cycle/cm (14 mm)
14 mm
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Comparison of IR

• Optically thick CO2 bend frequency absorption
  in upper atm where T is lower.
• atm. window

14 um
11 um
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Carbon Dioxide Band Saturation
Figure 4.5
Atm w/no CO2
Atm w/10 ppm CO2
Atm w/100 ppm CO2
Atm w/1000 ppm CO2
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Band Saturation - Figure 4.5

• Current CO2 is 400 ppm
• Figure goes from 0 to 1000 ppm
• A. atm transparent to IR at 700 (no CO2)
• B. Add 10 ppm has noticeable change.
• C. and D. inc. of 100 and 1000 more subtle effect.

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Thats what figure 4.6 is showing
You start to get less bang for your buck as you
get to higher concentrations.
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Assignment 2
16.6 - 14mm
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Methane CO2

• The band saturation for CO2 means that it is a
  less potent gh gas than methane.
• Even though
• Methane absorbs in the tail
• CO2 is under the curve
• Methane has a much lower concentration in the
  atm.
• Methane absorption band not saturated.
• A molecule of CH4 is 20x
• More powerful then a molecule
• of CO2

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Figure 4.8 - Demo of gh effect

• A - No CO2
• Earth in energy balance
• Ground T 270 K
• B - Add 1000 ppm
• Dec. outgoing energy flux
• See CO2 trough
• C - Ground and atm respond by
• warming up 8.5 deg K
• New output spectrum has risen
• System rebalances

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Exercise 2

Upward IR Heat Flux 289.602 W/m2
IR Heat Loss (Background) 289.29 W/m2
Ground Temperature 299.7 K
Iout
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Exercise 2
400 ppm Carbon Dioxide

Upward IR Heat Flux 320.28 W/m2
IR Heat Loss (Background) 289.29 W/m2
Ground Temperature 299.7 K
Iout
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Take Home Points

• Gases absorb/emit IR energy if they vibrate at
  the frequency of the IR energy, and if its
  vibration effects the electric field.
• O2 and N2 are not gh gases
• All molecules of 3 atoms or more are IR active.
• A gh gas has a strongest impact on the radiative
  balance of Earth if it interacts with energy in
  the middle of the earth energy spectrum.
• Band saturation a gh gas at high concentrations
  will be less effective molecule by molecule than
  a dilute gas.
• Converting between wavelength, frequency and wave
  number - www.cactus2000.de/uk/unit/masswav.shtml

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GH Gases

• Keeps Earth 33 deg C warmer than without
• Past century, T inc. by 0.5 deg C
• After 1997 Kyoto Protocol world has finally taken
  steps in reducing emissions.

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Kyoto Treaty (1997)

• Commits industrialized nations to reduce gh gases
  by 5.2 below 1990 levels over next decade.
• Agreement needed to be ratified by countries
  responsible 55 of world's carbon emissions.
• Dealt severe blow in 2001 when Pres. George W.
  Bush announced United States would never sign it.
  

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• Reasons for not ratifying Treaty were expressed
  by U.S. Senate and Clinton Administration in
  1997, and Bush Administration in 2001.
• In 1997, U.S. Senate voted 95-0 that Treaty
  would not enter Senate floor for ratification.
• Said ratification would result in harm to
  American economy and workforce.
• Loose restrictions imposed on India and China
  would render treaty ineffective in the long run
  for goal of lowering total carbon emissions.

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• CO2 colorless, odorless, most prominent gh gas
• Recycled through atm by photosynthesis
• Every year humans add over 30 billion tons of
  carbon dioxide to atm.

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• Mauna Loa (Hawaii) - dataset started by Keeling,
  1958.
• 316 ppm in 1959 to 360 ppm in 1996

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• Ice core samples - also show inc. in carbon
  dioxide levels.   

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• Inc. from developing countries are 3x times
  developed countries.
• Middle East 35, Africa 12, Eastern Europe 75
  from 1990-1995.

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Methane

• Colorless, odorless, flammable.
• Formed under anaerobic conditions
• when plants decay
• bacteria - wetlands
• bacteria - found in cows, sheep, goats, buffalo,
  termites, and camels. 
• Since 1750, methane has doubled, could double
  again by 2050. 

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Methane

• Each year humans add 350-500 million tons of
  methane to atm
• livestock, coal mining, drilling for oil and
  natural gas, rice cultivation, and landfills.

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• Rice cultivation - doubled in the past 45 years.
• feeds 1/3 of World's pop. 
• methanogenesis - bacteria in warm, waterlogged
  soil releases methane.
• 50 -100 million tons/yr.
• possibly biggest anthropogenic source.
• Solutions
• many rice varieties can be grown under much drier
  conditions
• improved varieties of rice, higher yield per acre.

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The average cow belches 250 liters of
methane/day.
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• Nitrous oxide
• Colorless
• Released naturally from oceans and by bacteria in
  soils. 
• Risen by more than 15 since 1750.  

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• Nitrogen based fertilizer use has doubled in past
  15 years. 
• provide nutrients for crops
• but, breakdown in soil, nitrous oxide released
  into atm 
• Automobiles - released at lower rate than carbon
  dioxide, more carbon in gasoline than nitrogen.
• Other sources, animal manure, sewage treatment
• Also produced naturally from a wide variety of
  biological sources in soil and water
• microbial action in wet tropical forests.

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• Fluorocarbons   
• General term for any synthetic organic compounds
  with fluorine and carbon. 
• easily converted from liquid to gas. 
• used in aerosol cans, refrigerators, air
  conditioners. 
• 1970s - CFCs linked to ozone layer. 
• use decreased, banned from production in U. S.
• Sub for CFCs are hydrofluorocarbons (HFCs).
• doesnt harm ozone, but is a gh gas.

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Gas molecules with 3 or more atoms are greenhouse
gasesWater vapor (H2O), ozone (O3), carbon
dioxide (CO2), and methane (CH4). Also, trace
quantities of chloro-fluoro-carbons (CFCs) can
have a disproportionately large effect.
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• The 3 most powerful greenhouse gases are
• Water vapor 36-70
• Carbon dioxide 9 - 26
• Methane 4 9
• Misc trace gases
• Ozone, N20, CFCs (HFCs)
• N2 and O2 major atmospheric constituents not
  greenhouse gases.
• dont absorb or emit IR radiation
• no change in these molecules when they vibrate.

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Main absorbers in atm 1. Nitrous Oxide (N2O) 2.
Methane (CH4) 3. Ozone (O3) 4. Water Vapor
(H2O) 5. Carbon Dioxide (CO2)
Sun Range of visible wavelengths
Earth Range of primary wavelengths being
emitted.
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