Transcript Stratospheric Methane - Georgia Institute of Technology

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