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Stratospheric Methane

Stratospheric Methane. Steve Rieck. Introduction. CH 4 is emitted from natural and anthropogenic sources Has a long lifetime (8.6 years) Relatively important greenhouse gas Is a source of water vapor in stratosphere. CH 4 Budget (Sources). Natural

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Stratospheric Methane

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  1. Stratospheric Methane Steve Rieck

  2. Introduction • CH4 is emitted from natural and anthropogenic sources • Has a long lifetime (8.6 years) • Relatively important greenhouse gas • Is a source of water vapor in stratosphere

  3. CH4 Budget (Sources) • Natural • Wetlands, termites, ocean 160 (75 – 290) • Anthropogenic • Fossil Fuel related 100 (70 – 120) • Natural gas, coal, petroleum • Biospheric 275 (200 – 350) • Rice paddies, enteric fermentation, landfills, waste Source: IPCC 2001

  4. CH4 Budget (Sinks) • Tropospheric Oxidation by OH 445 (360 – 530) • Stratosphere 40 (30 – 50) • Soil and Microbial activity 30 (15 – 45)

  5. Importance of Stratospheric Methane • Methane oxidation is a large source of Stratospheric water vapor • Water vapor is important due to it’s effects on Ozone chemistry and Greenhouse effect.

  6. Stratospheric Methane Effects on Water Vapor:Mt. Pinatubo 1991

  7. Stratospheric Methane Fate • Methane is destroyed by two mechanisms in the stratosphere • CH4 + OH  CH3 +H2O • CH4 + O(1D)  CH3 + OH • I will focus on the first reaction

  8. Modeling Methane • Once Methane gets to the Stratosphere: • How long does it last? • How much becomes H2O? • I found very little data on the lifetime of methane

  9. Method • It is necessary to determine the rate constant for oxidation • For: • CH4 + OH  CH3 + H20 • K = 1.85 * 10-12 * EXP (-1690/T) cm3 molecule-1 s-1 * • Methane lifetimes were measured at 25 and 45 km • * IUPAC Kinetic Data

  10. Initial conditions • It is necessary to input the following data • Temperature • 25 km: 233 K, 45 km: 258 K • OH concentration • 25 km: 3 ppt, 45 km: 400 ppt • OH concentrations difficult to find • Wennberg et all 1994

  11. Results • Toxidation = [Reservoir]/[Loss rate] = [CH4]/ (k * [CH4] * [OH]) • The lifetime of methane against OH oxidation was found to be about 168 days at 25 km and 26 days at 45 km

  12. Conclusions • The concentration of OH can increase by a large factor with height, making it more efficient at producing water from methane • This is due to more OH being made by reactions like H2O + O(1D)  2OH • More studies are needed to accurately determine OH concentrations globally

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