METHANE IN THE ATMOSPHERE 



DIETER H. EHHALT 



National Center for Atmospheric Research,* Boulder, Colorado 



ABSTRACT 



The atmospheric cycle of CH 4 is described. Most of the CH 4 released to the atmosphere is 

 of recent biologic origin. Carbon-14 analyses show that, at most, 20% of the atmospheric 

 CH 4 is released by fossil sources. The estimated total annual production of CH 4 is between 

 6.5 xlO 14 and 19 x 10 1 4 g/year. The destruction of CH 4 takes place mainly in the 

 troposphere, most probably through the reaction CH 4 + OH -* CH 3 + H 2 0. Only about 10% 

 of the CH 4 is destroyed in the stratosphere. The total destruction rate is estimated to be 

 3 x 10 15 g/year. Apparently the CH 4 cycle contributes about 1% to the atmospheric carbon 

 cycle. The average volume mixing ratio in the troposphere is 1.41 ppM, which corresponds 

 to a total amount of 4 x 10 1 s g of CH 4 present in the atmosphere. Thus the turnover times 

 of atmospheric CH 4 range from 1 to 6 years. The vertical profiles of CH 4 in the troposphere 

 show no large systematic gradients. In the stratosphere the CH 4 mixing ratio decreases with 

 altitude, reaching 0.25 ppM at 50 km. Finally, the isotopic composition of atmospheric CH 4 

 and its sources are discussed briefly. 



The presence of methane in the earth's atmosphere was discovered by Migeotte. 1 

 It was identified by its infrared absorption band in the solar spectrum. The early 

 optical measurements indicated a uniform distribution of CH 4 throughout the 

 atmosphere, with an average volume concentration of about 2 ppM. Subsequent 

 measurements 2 lowered this value to 1.5 ppM and then to 1.4 ppM. Our own 

 measurements using gas chromatography give an average of 1.41 ppM for the 

 troposphere and indicate a decrease in concentration in the stratosphere. Our 

 measurements also show that the CH 4 concentration fluctuates greatly even in 

 tropospheric profiles. 4 



*The National Center for Atmospheric Research is sponsored by the National Science 

 Foundation. 



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