METHANE IN THE ATMOSPHERE 145 



Methane is classified as a long-lived pollutant. Initially the atmospheric 

 lifetime of methane was estimated to be about 100 years. More recently the 

 lifetime has been estimated to be only a few years. 



Interest in the CH 4 cycle, particularly in its stratospheric branch, has 

 recently been stimulated by two developments. The first of these was the 

 realization that the oxidation of CH 4 in the stratosphere adds significant 

 amounts of water to the water cycle in the middle and upper stratosphere. It is 

 estimated that about 50% of the water vapor in that region comes from the 

 oxidation of CH 4 . This additional water vapor influences the earth's radiation 

 balance causing a slight increase of the temperature at the earth's surface and a 

 decrease of the stratospheric temperature. 5 The added water vapor is even more 

 important for the stratospheric chemistry because it and the free radicals, OH 

 and HO2 , derived from it interact with the ozone. The net effect is a reduction 

 of the 3 present in the stratosphere. Thus the CH 4 produced in the 

 troposphere influences the stratospheric environment. This has led to the 

 speculation that anthropogenic production of CH 4 , which seems to be 

 increasing, may contribute to changes in the stratospheric environment. 



The second development that has renewed interest in the CH 4 cycle involves 

 the oxidation process of CH 4 in the troposphere. It was theorized recently that 

 the oxidation of CH 4 leads to the formation of CO as an intermediate step. 6 

 This process may very well be the major source of CO. Thus the CH 4 cycle is 

 coupled closely to the atmospheric CO cycle and, incidentally, is also coupled to 

 the atmospheric H 2 cycle. Neither the CO cycle nor the H 2 cycle can be 

 understood quantitatively without understanding the CH 4 cycle. Published 

 measurements over the past 10 years showed that the average tropospheric CH 4 

 concentration is practically constant with time. Apparently the CH 4 cycle is in a 

 steady state, and the balance is solely determined by the sources and sinks of 

 CH 4 . 



SOURCES OF ATMOSPHERIC CH 4 



It is generally agreed that CH 4 is too complicated a molecule to be 

 synthesized within the atmosphere and that it is released mainly at the earth's 

 surface. Even before detailed estimates of the CH 4 sources were available, it was 

 known that most of the atmospheric CH 4 must be produced by anaerobic 

 decomposition of recent organic matter. This information was provided by the 

 measurement of the radiocarbon content in atmospheric CH 4 . Any CH 4 derived 

 from "recent," i.e., recently alive, organic material has a 14 C content very close 

 to that of recent wood. In contrast, CH 4 derived from fossil fuels or volcanic 

 activity contains essentially no 14 C. The fuel deposits are so old that the 

 originally present 1 C has decayed. Several 14 C analyses of atmospheric CH 4 , 

 mostly made by Libby, 7 are available and are summarized in Table 1. Most of 

 these samples were collected before the atmosphere was contaminated with the 



