Methane Lecture
AOSC 637
Atmospheric Chemistry
Russell R. Dickerson
Finlayson-Pitts Chapt. 6 & 14
Seinfeld Chapt. 2, 6, 23
Wallace & Hobbs Chapt. 5
http://www.ipcc.ch/publications_and_data/publications_and_data.htm
OUTLINE
Importance
Detection Techniques
Sources and Sinks
Global Chemistry & Trends
Remaining Challenges
Bibliography
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Methane
Importance
• Greenhouse gas with 25 times the warming potential of CO2. Absorption bands
at 3.5 & 7.5 mm.
• Primary air pollutant, but produced primarily by biogenic processes in anaerobic
environments such as swamps, rice paddies, and the guts of ruminants.
Biogenic but also anthropogenic.
• Major conversion of OH to HO2
Thompson et al. (1989); Shindell et al. (2009)
• Source of CO, H2, H2CO and source/sink of O3 depending on NOx
• Source of water vapor sink or Cl in stratosphere.
• Nontoxic
• Sources hard to pin down.
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IPCC, 2007
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In the remote atmosphere there is often insufficient NOx to drive this
reaction to two O3; the process reduces OH. Globally, Thompson et
al. (1989) predict that increased CH4 increases H2O2 and the ratio of
HO2 to OH. A longer lifetime for CH4 and O3 contributes to global
warming, e.g., Shindell et al., (2009); EPA (2010)
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Chemistry
Methane oxidation in a clean environment:
(1) O3 + h  O2 + O(1D)
(2)
O(1D) + H2O  2OH
(3) OH + CH4  H2O + CH3
(4) CH3 + O2 + M  H3CO2 + M†
(5) HO2 + H3CO2  O2 + HOOCH3
(6) HOOCH3  dry dep (insoluble)
----------------------------------------(3+4)
2O3  3O2
NET
Note photolysis of HOOCH3 is almost a do-nothing reaction.
HOOCH3 + hv  H3CO + OH
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Chemistry, continued
Methane oxidation in a dirty (polluted) environment:
OH + CH4  CH3 + H2O
CH3 + O2 + M  CH3O2 + M†
CH3O2 + NO  NO2 + CH3O
CH3O + O2  HO2 + CH2O
HO2 + NO  NO2 + OH
NO2 + h  NO + O
O + O2 + M  O3 + M
------------------------------------------------(3'-7') CH4 + 2 O2  H2O + 2O3 + CH2O NET
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Detection Methods
• GC-FID
• FTIR
• Tunable Diode Laser Spectroscopy
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Gas Chromatography
Detection of trace species
Flame Ionization Detection
Thermal Conductivity Detection
Electron Capture Detection
Mass spectroscopy
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Flame Ionization Detector
The sample containing hydrocarbons is
mixed with fuel (H2 and O2) and burned
between two electrodes. The cations
go to the cathode and the anions to the
anode, and the current is proportional
to the mass of hydrocarbon.
To detect methane specifically, the
other VOC’s are first captured in a cryotrap. Sometimes the remainder is
detected as total non-methane
hydrocarbons (NMHC’s). To detect
specific VOC’s the individual
compounds must first be separated on
a column.
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Characteristics of FID
• Great sensitivity (picograms, 10-12 g)
• Broad linear dynamic range, 106
• Most HC’s, such as alkanes and alkenes, detected
with similar sensitivity; concentration
proportional to peak area.
• Poor sensitivity to oxygenates such as aldehydes.
• Flammable gases expendable.
• Separations are black magic.
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Gas Chromatograph with a Flame
Ionization Detector (GCFID)
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GC-FID Chromatogram
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Other Gas Chromatograph Detectors
• Thermal Conductivity
– Low sensitivity but responsive to nonflammable gases.
Perkin Elmer
Autosystem
Gas
Chromatograph
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Electron Capture (EC)
Tremendous sensitivity to halogens.
• James Lovelock
– ECD
– Gaia hypothesis
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Electron Capture Detector (ECD)
A beta (e-) emitter such as 63Ni ionizes the carrier gas, usually N2. Fast beta
particles collide with the carrier gas producing free, slow-moving electrons that
generate a steady base-line current. When the GC effluent contains organic
molecules with electronegative functional groups, such as halogens, phosphorous
and nitro groups (inc. N2O), they capture electrons and reduce the current. The
reduction in electron flow is proportional to the quantity of electrophilic sample
components.
Electron Capture Detectors, developed by James Lovelock in 1957, are up to 1000
times more sensitive than Flame Ionization Detectors and were the first detectors
able to measure components at parts-per-billion (ppb) and parts-per-trillion (ppt)
levels. Found DDT is penguins and showed that CFC’s are ubiquitous.
Lovelock, J.E. 1958. A sensitive detector for gas chromatography. Journal of
Chromatography, l, 35-46.
Disadvantage – only sensitive to halogens and N-compounds.
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Example GC-ECD Chromatogram
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GC/MS
A mass spectrometer
creates charged particles (ions) from molecules. It then analyzes those ions to
provide information about the molecular weight of the compound and its chemical
structure. There are many types of mass spectrometers and sample introduction
techniques which allow a wide range of analyses. Mass spectrometry is powerful
and widely used method of identifying and detecting VOC’s
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Mass Spectroscopy separates ions by
their mass to charge ratio: M/z.
MS instruments consist of three parts: an
ion source, to convert gas-phase sample
molecules into ions, a mass analyzer,
which sorts the ions by their masses by
applying electromagnetic fields, and an
ion detector. The technique has both
qualitative and quantitative uses. These
include identifying unknown compounds,
determining the isotopic composition of
elements in a molecule, and determining
the structure of a compound by observing
its fragmentation.
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Quadrupole (TOF) Mass Spectrometer and example with
methanol.
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SOURCES OF ATMOSPHERIC METHANE
WETLANDS
180
BIOMASS
BURNING ANIMALS
90
20
LANDFILLS
50
GLOBAL METHANE
SOURCES (Tg CH4 yr-1)
GAS
60
TERMITES
25
RICE
85
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COAL
40
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Anaerobic conditions in
the waterlogged soils of
rice paddies can host
methanogenic bacteria.
These are believed to
generate 50-100 Tg
CH4/yr.
Methane hydrates can
exist in permafrost or
Arctic oceans. As the
Earth warms these
release methane to het
atmosphere.
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Remaining Challenges related to CH4 in the atmosphere
How accurate are the emissions?
Bakerblocker et al. (1977) estimated 300 Tg/yr from wetlands.
Zimmerman et al. Science, 1982. Termites 150 Tg.yr??
Does chlorine consume much methane?
CH4 + Cl → CH3 + HCl
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By Tim Hirsch
BBC News environment
correspondent
Last Updated: Wednesday, 11 January 2006, 23:04 GMT
Plants revealed as methane source
Forests may add to methane levels,
scientists say Scientists in
Germany have discovered that
ordinary plants produce
significant amounts of methane, a
powerful greenhouse gas which
helps trap the sun's energy in the
atmosphere. (despite aerobic
conditions! RRD)
http://news.bbc.co.uk/2/hi/science/nature/4604332.stm
Frank Keppler, John T. G. Hamilton, Marc Brass and Thomas Röckmann
Methane emissions from terrestrial plants under aerobic conditions
Nature, January 12, 2006
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In terms of total amount of production worldwide, the scientists' first guesses are
between 60 and 240 million tonnes of methane per year. That means that about
10 to 30 percent of present annual methane production comes from plants.
Problem with scaling from lab to world. Upper limit 125 Tg.
S Houweling et al, Geophysical Research Letters, 2006, 33, DOI:
1029/2006GL026162.
“The results of a single publication stating that terrestrial plants emit methane
has sparked a discussion in several scientific journals, but an independent test
has not yet been performed. Here it is shown, with the use of the stable isotope
13C and a laser-based measuring technique, that there is no evidence for
substantial aerobic methane emission by terrestrial plants, maximally 0.3% (0.4
ng g−1 h−1) of the previously published values. Data presented here indicate that
the contribution of terrestrial plants to global methane emission is very small at
best.” Dueck, T. et al. New Phytol. 175, 29-35 (2007).
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Ellen Nisbet, an evolutionary biologist at the University of South Australia in
Adelaide, previously reported that plants do not have the biochemical
pathways needed to generate methane. "I'm pretty sure from our studies that
[plants] aren't making methane themselves," she says. "This paper is really
showing that methane is moving around the plants, that it's being transported
up and out." Nisbet, R. E. R. et al. Proc. R. Soc. B 276, 1347-1354 (2009).
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Uncertainty:
Ambient measurements: [CH4] = 1,774 ± 1.8 ppb (0.1%)
Sinks: OH ± 103 Tg/yr (20%)
Soil ± 15 Tg/yr (50%)
Stratosphere ± 8 Tg/yr (20%)
Chlorine 20Tg/yr???
Overall ± 15% uncertainty in sink strength
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Take Home Messages
•
Methane is an important tropospheric trace gas with adverse effects on climate and the
oxidizing capacity of the atmosphere.
•
The uncertainty in the emissions is larger than can be explained by measurement
uncertainty.
•
A warmer, wetter climate will lead to faster methane release from soils and methane
hydrates.
•
Recent evidence indicates that chlorine atoms may be a substantial sink for CH 4. (Thornton
et al, 2010; von Glasow, 2010).
•
“Observed increases in atmospheric methane concentration, compared with pre-industrial
estimates, are directly linked to human activity, including agriculture, energy production,
waste management and biomass burning. Constraints from methyl chloroform observations
show that there have been no significant trends in hydroxyl radical (OH) concentrations,
and hence in methane removal rates, over the past few decades (see Chapter 2). The
recent slowdown in the growth rate of atmospheric methane since about 1993 is thus likely
due to the atmosphere approaching an equilibrium during a period of near-constant total
Copyright
© 2010from
R. R.
emissions. However, future methane
emissions
wetlands are likely to increase in a 34
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warmer and wetter climate, and to decrease
in a warmer and drier climate.” (IPCC 2007).
Bibliography
Bakerblocker, A., T. M. Donahue, and K. H. Mancy (1977), Methane Flux from Wetlands Areas, Tellus, 29,
245-250.
Cicerone, R. J. (1983), Methane in the atmosphere, paper presented at Twelfth International Conf. on the
Unity of the Sciences, Chicago, Illinois, Nov. 24-27, 1983.
Cicerone, R. J. and R. S. Oremland (1988), Biogeochemical aspects of atmospheric methane, Global.
Biogeochem. Cycles, 2, 299-327.
Dueck, T. and A. van der Werf (2008), Are plants precursors for methane?, New Phytologist, 178, 693-695.
Ehhalt, D. H. (1974), The atmospheric cycle of methane, Tellus, 26, 58-70.
Houweling, S., T. Rockmann, I. Aben, F. Keppler, M. Krol, J. F. Meirink, E. J. Dlugokencky, and C.
Frankenberg (2006), Atmospheric constraints on global emissions of methane from plants, Geophysical
Research Letters, 33.
Isaksen, I. S. A., C. Granier, G. Myhre, T. K. Berntsen, S. B. Dalsoren, M. Gauss, Z. Klimont, R. Benestad, P.
Bousquet, W. Collins, T. Cox, V. Eyring, D. Fowler, S. Fuzzi, P. Jockel, P. Laj, U. Lohmann, M. Maione,
P. Monks, A. S. H. Prevot, F. Raes, A. Richter, B. Rognerud, M. Schulz, D. Shindell, D. S. Stevenson, T.
Storelvmo, W. C. Wang, M. van Weele, M. Wild, and D. Wuebbles (2009), Atmospheric composition
change: Climate-Chemistry interactions, Atmospheric Environment, 43, 5138-5192.
Keppler, F., J. T. G. Hamilton, M. Brass, and T. Rockmann (2006), Methane emissions from terrestrial plants
under aerobic conditions, Nature, 439, 187-191.
Shindell, D. T., G. Faluvegi, D. M. Koch, G. A. Schmidt, N. Unger, and S. E. Bauer (2009), Improved
Attribution of Climate Forcing to Emissions, Science, 326, 716-718.
Thornton, J. A., J. P. Kercher, T. P. Riedel, N. L. Wagner, J. Cozic, J. S. Holloway, W. P. Dube, G. M. Wolfe,
P. K. Quinn, A. M. Middlebrook, B. Alexander, and S. S. Brown (2010), A large atomic chlorine source
inferred from mid-continental reactive nitrogen chemistry, Nature, 464, 271-274.
von Glasow, R. (2010), ATMOSPHERIC CHEMISTRY Wider role for airborne chlorine, Nature, 464, 168-169.
Zimmerman, P. R. (1982), A potentially large source of atmospheric methane, carbon dioxide, and molecular
hydrogen, Science, 218, 563-565.
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