Greenhouse GasesGreenhouse GasesArcher Chapter 4Archer Chapter 4

Garver Garver

GEO 307GEO 307

Greenhouse GasesGreenhouse Gases

Layer model assumes atmosphere acts as a Layer model assumes atmosphere acts as a blackbody in the IR, absorbing and emitting in all blackbody in the IR, absorbing and emitting in all frequencies of light.frequencies of light.

In reality, gases absorb very selectively.In reality, gases absorb very selectively.

Difference has to do with the effect of Difference has to do with the effect of molecular molecular vibrationsvibrations on the electromagnetic field. on the electromagnetic field.

Difference in gasesDifference in gases

• Because gases absorb IR selectivelyBecause gases absorb IR selectively

• Some radiation bands are completely Some radiation bands are completely absorbed.absorbed.

• Others are Others are ‘‘atmospheric windows’atmospheric windows’

• This leads to higher greenhouse forcing per This leads to higher greenhouse forcing per molecule for some gases.molecule for some gases.

Need a Modification to the Layer ModelNeed a Modification to the Layer Model

By looking more closely at gases in atmBy looking more closely at gases in atm ConcentrationConcentration – # of molecules within a – # of molecules within a

volume.volume.– Fewer molecules per volume as gas expands.Fewer molecules per volume as gas expands.

Proportions Proportions (%) – (%) – easiereasier– OO22 21%, N 21%, N22 78%, CO 78%, CO22 0.038% 0.038%– COCO22 380 parts per million (ppm) 380 parts per million (ppm)

Mixing RatioMixing Ratio – CO – CO22 380 parts per million (ppm) 380 parts per million (ppm)– COCO22 is rising at a rate of is rising at a rate of 1.5ppm/yr1.5ppm/yr

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Earth – Range of primary wavelengths being emitted.

Sun – Range of visible wavelengths

1. Nitrous Oxide (N2O)

2. Methane (CH4)

3. Ozone (O3)

4. Water Vapor (H2O)

5. Carbon Dioxide (CO2)

Gases, vibration,lightGases, vibration,light

Most mass of an atom is in its nucleus.Most mass of an atom is in its nucleus.

Electrons - orbitals around the nucleus.Electrons - orbitals around the nucleus.

Nuclei of 2 different atoms repel each other (+ charges).Nuclei of 2 different atoms repel each other (+ charges).

But some electrons of different atoms can fit But some electrons of different atoms can fit

together in their orbitals.together in their orbitals.

Atoms are held together (like a spring) not too close, Atoms are held together (like a spring) not too close,

not too far.not too far.

– Too close they repel each other, too far and there is less Too close they repel each other, too far and there is less

energy gain from sharing the electronsenergy gain from sharing the electrons..

Gases, vibration,lightGases, vibration,light

Chemical bond -Chemical bond - physical process physical process

responsible for the attractive interactions responsible for the attractive interactions

between atoms and molecules, gives between atoms and molecules, gives

stability to chemical compounds.stability to chemical compounds.

Gases, vibration,lightGases, vibration,light

Gases are the simplest molecules.Gases are the simplest molecules.

– Vibrate only in a particular frequency.Vibrate only in a particular frequency.

Vibrations of the major gases in the atm Vibrations of the major gases in the atm

(O(O22, N, N22)) are are invisible to the invisible to the

electromagnetic fieldelectromagnetic field

– Transparent to VIS and IRTransparent to VIS and IR

Gases, vibration,lightGases, vibration,light

O2 & N2:O2 & N2:

These are symmetrical molecules - These are symmetrical molecules - Invisible Invisible

because electric field doesn’t change.because electric field doesn’t change.

– only 2 atoms (that are the same)only 2 atoms (that are the same)

– Not infrared activeNot infrared active

– So, not gh gasesSo, not gh gasesIn general, symmetrical molecules with only 2 atoms In general, symmetrical molecules with only 2 atoms are not are not gh gases.gh gases.

Gases, vibration,lightGases, vibration,light Gas molecules with more then 2 atoms Gas molecules with more then 2 atoms have more than one have more than one

chemical bond.chemical bond.

Water has three normal modes of vibration

All 3 are IR active

Positive Positive chargecharge

negative negative chargecharge

3.0 um

6.0 um

Water - main absorberWater - main absorber 13 million tons of water in the atmosphere 13 million tons of water in the atmosphere Water content the atm varies (0 to 4%)Water content the atm varies (0 to 4%) ~70% of atmospheric absorption of IR~70% of atmospheric absorption of IR Contributes significantly to greenhouse effectContributes significantly to greenhouse effect

– Ensures a warm habitable planetEnsures a warm habitable planet But also, But also, negative feedback effectnegative feedback effect, clouds reflect sunlight., clouds reflect sunlight.

http://en.wikipedia.org/wiki/Electromagnetic_absorption_by_water

Gases, vibration,lightGases, vibration,light

COCO2 2 - is in a straight line with C in the middle - is in a straight line with C in the middle It is a symmetric molecule - both O molecules pull equally.It is a symmetric molecule - both O molecules pull equally.

Most climatically Most climatically importantimportantIR Active

How gh gases interact with IRHow gh gases interact with IR

Earth‘s IR (from clouds, atm, and ground)Earth‘s IR (from clouds, atm, and ground)

How gh gases interact with IRHow gh gases interact with IR

Gases are not good blackbodies! Gases are not good blackbodies! Very choosy about what wavelengths they absorb and emit.Very choosy about what wavelengths they absorb and emit. Blackbody spectra for Earth - T ranges from hot summer day to cold Blackbody spectra for Earth - T ranges from hot summer day to cold

upper atm (model generated data)upper atm (model generated data)

CH4 – 1300 cycle/cm (8 m)

CO2 effects outgoing IR more because there’s more IR energy at 700 cycle/cm (14 m)

Jagged curve – IR light measured by a spectrophotometer looking down

7.0 m50 m 10 m14 m

Optically thick COOptically thick CO22 bend frequency – absorption in bend frequency – absorption in upper atm where T is lower.upper atm where T is lower.

atm. windowatm. window

Comparison of IR

14 um 11 um

Carbon Dioxide Band SaturationCarbon Dioxide Band Saturation

Atm w/no CO2 Atm w/10 ppm CO2

Atm w/100 ppm CO2

Atm w/1000 ppm CO2

Figure 4.5

Band Saturation - Band Saturation - Figure 4.5Figure 4.5

Current COCurrent CO22 is 380 ppm is 380 ppm Figure goes from 0 to 1000 ppmFigure goes from 0 to 1000 ppm

A. atm transparent to IR at 700 (no COA. atm transparent to IR at 700 (no CO22))

B. Add 10 ppm has noticeable change.B. Add 10 ppm has noticeable change.

C. and D. inc. of 100 and 1000 more subtle C. and D. inc. of 100 and 1000 more subtle effect.effect.

That’s what figure 4.6 is showingThat’s what figure 4.6 is showing

You start to get less bang for your buck as you get to higher concentrations.

Assignment #2Assignment #2

16.6 - 14m

Methane & COMethane & CO22 The band saturation for COThe band saturation for CO22 means that it is a less potent gh means that it is a less potent gh

gas than methane.gas than methane. Even though;Even though; Methane absorbs in the tailMethane absorbs in the tail

– CO2 is under the curveCO2 is under the curve– Methane has a much lower concentration in the atm.Methane has a much lower concentration in the atm.– Methane absorption band not saturated.Methane absorption band not saturated.

A molecule of CHA molecule of CH44 is 20x is 20x

More powerful then a moleculeMore powerful then a molecule

of COof CO22

Figure 4.8 - Demo of gh effect Figure 4.8 - Demo of gh effect

A - No COA - No CO22

– 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

Exercise #2Exercise #2

Upward IR Heat Flux

289.602 W/m2

IR Heat Loss (Background)

289.29 W/m2

Ground Temperature

299.7 K

Iout

Exercise #2Exercise #2

Upward IR Heat Flux 320.28 W/m2

IR Heat Loss (Background)

289.29 W/m2

Ground Temperature

299.7 K

Iout

400 ppm Carbon Dioxide

Take Home PointsTake Home Points1.1. Gases absorb/emit IR energy if they vibrate at the Gases absorb/emit IR energy if they vibrate at the

frequency of the IR energy, and if its vibration effects the frequency of the IR energy, and if its vibration effects the electric field.electric field.

O2 and N2 are not gh gasesO2 and N2 are not gh gases All molecules of 3 atoms or more are IR active.All molecules of 3 atoms or more are IR active.

2.2. A gh gas has a strongest impact on the radiative A gh gas has a strongest impact on the radiative balance of Earth if it interacts with energy in the middle balance of Earth if it interacts with energy in the middle of the earth energy spectrum.of the earth energy spectrum.

3.3. Band saturation: a gh gas at high concentrations will be Band saturation: a gh gas at high concentrations will be less effective molecule by molecule than a dilute gas.less effective molecule by molecule than a dilute gas.

4.4. Converting between wavelength, frequency and wave Converting between wavelength, frequency and wave number - www.cactus2000.de/uk/unit/masswav.shtmlnumber - www.cactus2000.de/uk/unit/masswav.shtml

– Keeps Earth ~33 deg C warmer than withoutKeeps Earth ~33 deg C warmer than without– Past century, T inc. by ~0.5 deg CPast century, T inc. by ~0.5 deg C– After 1997 After 1997 Kyoto ProtocolKyoto Protocol world has finally taken world has finally taken

steps in reducing emissions.steps in reducing emissions.

GH GasesGH Gases

Kyoto Treaty (1997)Kyoto Treaty (1997)

Commits industrialized nations to reduce gh gases Commits industrialized nations to reduce gh gases

by ~5.2% below 1990 levels over next decade.by ~5.2% below 1990 levels over next decade.

Agreement needed to be ratified by countries Agreement needed to be ratified by countries

responsible 55% of world's carbon emissions.responsible 55% of world's carbon emissions.

Dealt severe blow in 2001 when Pres. George W. Dealt severe blow in 2001 when Pres. George W.

Bush announced United States would never sign it. Bush announced United States would never sign it.

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.

– CO2 colorless, odorless, most prominent gh gasCO2 colorless, odorless, most prominent gh gas– Recycled through atm by photosynthesisRecycled through atm by photosynthesis– Every year humans add over 30 billion tons of Every year humans add over 30 billion tons of

carbon dioxide to atm.carbon dioxide to atm.

– Mauna Loa (Hawaii) - dataset started by Mauna Loa (Hawaii) - dataset started by Keeling, 1958.Keeling, 1958.

– 316 ppm in 1959 to 360 ppm in 1996316 ppm in 1959 to 360 ppm in 1996

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

– Inc. from developing countries are 3x times Inc. from developing countries are 3x times developed countries. developed countries.

– Middle East 35%, Africa 12%, Eastern Europe Middle East 35%, Africa 12%, Eastern Europe 75% from 1990-1995.75% from 1990-1995.

MethaneMethane Colorless, odorless, flammable.Colorless, odorless, flammable. Formed under anaerobic conditionsFormed under anaerobic conditions

– when plants decaywhen plants decay– bacteria - wetlandsbacteria - wetlands– bacteria - found in cows, sheep, goats, buffalo, bacteria - found in cows, sheep, goats, buffalo,

termites, and camels. termites, and camels. 

Since 1750, methane has doubled, could Since 1750, methane has doubled, could

double again by 2050. double again by 2050. 

MethaneMethane Each year humans add 350-500 million Each year humans add 350-500 million

tons of methane to atmtons of methane to atm livestock, coal mining, drilling for oil and livestock, coal mining, drilling for oil and

natural gas, rice cultivation, and natural gas, rice cultivation, and landfills. landfills.

Rice cultivationRice cultivation - - doubled in the past 45 years.doubled in the past 45 years.– feeds 1/3 of World's pop.  feeds 1/3 of World's pop.  – methanogenesismethanogenesis - bacteria in warm, waterlogged - bacteria in warm, waterlogged

soil releases methane.soil releases methane. 50 -100 million tons/yr.50 -100 million tons/yr. possibly biggest anthropogenic source. possibly biggest anthropogenic source. SolutionsSolutions

– many rice varieties can be grown under much drier many rice varieties can be grown under much drier conditions conditions

– improved varieties of rice, higher yield per acre.improved varieties of rice, higher yield per acre.

The average cow belches ~250 liters of methane/day.The average cow belches ~250 liters of methane/day.

Nitrous oxideNitrous oxide Colorless Colorless Released naturally from oceans and by bacteria in soils.  Released naturally from oceans and by bacteria in soils. 

Risen by more than 15% since 1750. Risen by more than 15% since 1750.    

Nitrogen based fertilizer use has doubled in past 15 years. Nitrogen based fertilizer use has doubled in past 15 years.  – provide nutrients for cropsprovide nutrients for crops– but, breakdown in soil, nitrous oxide released into atm  but, breakdown in soil, nitrous oxide released into atm  – Automobiles Automobiles - released at lower rate than carbon dioxide, - released at lower rate than carbon dioxide,

more carbon in gasoline than nitrogen.more carbon in gasoline than nitrogen.

– Other sources, animal manure, sewage treatmentOther sources, animal manure, sewage treatment

– Also produced naturally from a wide variety of biological Also produced naturally from a wide variety of biological sources in soil and watersources in soil and water microbial action in wet tropical forests. microbial action in wet tropical forests.

FluorocarbonsFluorocarbons        General term for any synthetic organic General term for any synthetic organic

compounds with fluorine and carbon. compounds with fluorine and carbon. – easily converted from liquid to gas. easily converted from liquid to gas. – used in aerosol cans, refrigerators, air used in aerosol cans, refrigerators, air

conditioners.  conditioners. 

1970s - CFCs linked to ozone layer.  1970s - CFCs linked to ozone layer.  – use decreased, banned from production in U. S. use decreased, banned from production in U. S.

Sub for CFCs are hydrofluorocarbons Sub for CFCs are hydrofluorocarbons (HFCs).(HFCs).– doesn’t harm ozone, but is a gh gas.doesn’t harm ozone, but is a gh gas.

Gas molecules with 3 or more atoms are greenhouse gasesGas molecules with 3 or more atoms are greenhouse gases

Water vapor (HWater vapor (H22O), ozone (OO), ozone (O33), carbon dioxide (CO), carbon dioxide (CO22), and ), and

methane (CHmethane (CH44). Also, trace quantities of chloro-fluoro-carbons ). Also, trace quantities of chloro-fluoro-carbons

(CFCs) can have a disproportionately large effect.(CFCs) can have a disproportionately large effect.

The 3 most powerful greenhouse gases are;The 3 most powerful greenhouse gases are;

– Water vapor 36-70%Water vapor 36-70%

– Carbon dioxide 9 - 26%Carbon dioxide 9 - 26%

– Methane 4 – 9%Methane 4 – 9% Misc trace gases:Misc trace gases:

– Ozone, N20, CFCs (HFCs)Ozone, N20, CFCs (HFCs)

NN22 and O and O2 2 major atmospheric constituents not major atmospheric constituents not

greenhouse gases. greenhouse gases. – don’t absorb or emit IR radiationdon’t absorb or emit IR radiation

– no change in these molecules when they vibrate.no change in these molecules when they vibrate.

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Earth – Range of primary wavelengths being emitted.

Sun – Range of visible wavelengths

Main absorbers in atm:

1. Nitrous Oxide (N2O)

2. Methane (CH4)

3. Ozone (O3)

4. Water Vapor (H2O)

5. Carbon Dioxide (CO2)