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Understanding Stratospheric Methane: Sources, Sinks, and Effects

Explore the significance of methane in the stratosphere, its sources, sinks, and role in water vapor production. Learn about the mechanisms of methane destruction, modeling its fate, and the importance of OH concentrations. This insightful study sheds light on the impact of methane on atmospheric chemistry and the greenhouse effect.

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Understanding Stratospheric Methane: Sources, Sinks, and Effects

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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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