Stratospheric Methane

Steve Rieck

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

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

CH4 Budget (Sinks)

• Tropospheric Oxidation by OH 445 (360 – 530)

• Stratosphere 40 (30 – 50)

• Soil and Microbial activity 30 (15 – 45)

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.

Stratospheric Methane Effects on Water Vapor:Mt. Pinatubo 1991

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

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

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

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

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

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