Radiative Forcing between 1750 and 2005

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Radiative Forcing between 1750 and 2005
IPCC, FAQ 2.1, Fig. 2
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Nitrous Oxide (N2O)
Fourth most important greenhouse gas (soon
to be third)
Natural Sources
Emitted from soils and the ocean
Anthropogenic Sources
Cultivation of soils Production and use of
fertilizers Burning of Fossil Fuels Biomass
burning
263 ppb pre-industrial
Sinks
Photolysis in the stratosphere (90) or reaction
with O(1D)
Some uptake by soils (very small)
Lifetime 120 yrs GWP(100 yr) 296
Also a major source of ozone depleting NOx in
the stratosphere (from the photolysis)
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Sources
TgN/yr
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From 2002 Ozone Assessment Report
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N2O from combustion
Vehicles
Biomass
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N2O from Soil Effects of Fertilization
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Halocarbons (i.e. halogen-containing hydrocarbons)

• 100 man-made
• very long lifetimes
• large GWPs

Main Classes
CFCs Chlorocarbons HCFCs HFCs Halons
IPCC, AR4
Exception is CH3Br and CH3Cl
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Chlorofluorocarbons (CFCs)
Developed by Thomas Midgley at GM in 1928 as a
replacement for NH3 as a refrigerant
Example CFC-12 is CF2Cl2
Eventually used as refrigerants, insulation
blowing agents, solvents for cleaning electronics
and metals
Positives
Negatives
Generally non-toxic Non-reactive Low (variable)
boiling point
Destroy the ozone layer
Side-note Midgley also developed tetraethyl
lead additive for gasoline!
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Hydrochlorofluorocarbons (HCFCs) Phase-out
Initial replacements for CFCslower ozone
depletion potential b/c they have fewer chlorine
atoms
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Hydrofluorocarbons (HFCs)
NOT controlled under Montreal Protocol (they
dont contain Cl or Br)
Used as replacements for CFCs and HCFCs
SF6
Extremely potent GHG Very long lifetime
used as an insulator for circuit breakers, switch
gear, and other electrical equipment
Not emitted on purposefugitive emissions are
primary source
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Stratospheric Ozone
Small negative forcing (-0.05 W/m2)
Forcing impact related directly to emissions and
current abundance of CFCs, HCFCs, etc.
UNEP Assessment Report
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Tropospheric Ozone
Ozone Production
NO2 hv ? NO O O O2 M ? O3 M
Ozone Destruction
O3 NO ? NO2 O2 O3 hv ? O(1D) O2 O(1D)
H2O ? OH
CO OH ? CO2 H H O2 M ? HO2 M HO2 NO ?
NO2 OH
Carbon Monoxide
OH is recycled
Methane and other hydrocarbons
CH4 OH ? CH3 H2O CH3 O2 M ? CH3O2 CH3O2
NO ? CH3O NO2 CH3O O2 ? HCHO HO2
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Tropospheric Ozone
For pre-industrial run use zero anthropogenic
emissions of NOx and VOCs and reduce biomass
burning by 90
There were still soil NOx sources and biogenic
VOC sources so O3 was not zero in pre-industrial
times
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Tropospheric Ozone
Recent trends are hard to determinesome
countries are actively working to clean up O3
precursor emissions while others are not
Because O3 depends on chemistry natural
emissions anthro. emissions it will likely
change as the climate change
Oltmans et al. Atmos. Environ. (2006)
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The Hydroxyl Radical
Not a GHG itself, but the primary sink for many
GHGs
Long term trends unclear
Future projections indicate OH could increase or
decrease
Really hard to measure.
OH often inferred from methyl chloroform
concentrations (which is being phased out under
the Montreal Protocol)