1. Stratospheric Methane Steve Rieck

2. 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

3. CH 4 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. CH 4 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 CH 4 + OH  CH 3 +H 2 O CH 4 + O( 1 D)  CH 3 + 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 H 2 O? I found very little data on the lifetime of methane

10. Method It is necessary to determine the rate constant for oxidation For: –CH 4 + OH  CH 3 + H 2 0 K = 1.85 * 10 -12 * EXP (-1690/T) cm 3 molecule -1 s -1 * Methane lifetimes were measured at 25 and 45 km * IUPAC Kinetic Data

11. 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

12. Results T oxidation = [Reservoir]/[Loss rate] = [CH 4 ]/ (k * [CH 4 ] * [OH]) The lifetime of methane against OH oxidation was found to be about 168 days at 25 km and 26 days at 45 km

13. 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 H 2 O + O( 1 D)  2OH More studies are needed to accurately determine OH concentrations globally