copyright © 2010 r. r. dickerson 111 methane lecture aosc 637 atmospheric chemistry russell r....

Copyright © 2010 R. R. Dickerson

111

Methane LectureAOSC 637

Atmospheric ChemistryRussell R. Dickerson

Finlayson-Pitts Chapt. 6 & 14Seinfeld Chapt. 2, 6, 23Wallace & Hobbs Chapt. 5http://www.ipcc.ch/publications_and_data/publications_and_data.htm

OUTLINEImportance

Detection TechniquesSources and Sinks

Global Chemistry & TrendsRemaining 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 m.

• 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

Copyright © 2010 R. R. Dickerson 1111

Gas Chromatography

Detection of trace speciesFlame Ionization DetectionThermal Conductivity DetectionElectron Capture DetectionMass spectroscopy


Flame Ionization DetectorThe 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 cryo-trap. 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.


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.


Gas Chromatograph with a Flame Ionization Detector (GCFID)


GC-FID Chromatogram


Other Gas Chromatograph Detectors

• Thermal Conductivity– Low sensitivity but responsive to nonflammable gases.

Perkin Elmer Autosystem Gas Chromatograph


Electron Capture (EC)Tremendous sensitivity to halogens.

• James Lovelock– ECD– Gaia hypothesis


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.


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.


Quadrupole (TOF) Mass Spectrometer and example with methanol.


SOURCES OF ATMOSPHERIC METHANE

ANIMALS90

LANDFILLS50

GAS60

COAL40RICE

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TERMITES25

WETLANDS180

BIOMASSBURNING20

GLOBAL METHANESOURCES (Tg CH4 yr-1)


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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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Last Updated: Wednesday, 11 January 2006, 23:04 GMT

By Tim Hirsch BBC News environment correspondent

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 RöckmannMethane emissions from terrestrial plants under aerobic conditionsNature, 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 emissions. However, future methane emissions from wetlands are likely to increase in a warmer and wetter climate, and to decrease in a warmer and drier climate.” (IPCC 2007).


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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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