METHANE IN THE ATMOSPHERE 153 



the eastern Pacific about 200 km offshore from Santa Barbara, Calif., during 

 1966 and 1967 (Ref. 18). Each profile represents an average of two to four 

 individual profiles. If we consider first the Scottsbluff profiles, it appears as if 

 there were a systematic trend in the CH 4 concentration with a maximum in 

 September and October and a decrease during the following months, reaching a 

 minimum in May, June, and July. However, if we look at the standard deviations 

 that are indicated for the summer profile over Scottsbluff, it is obvious that the 

 differences between the seasonal profiles cannot be considered significant at this 

 stage. Further years of observation are required to establish a seasonal variation 

 beyond doubt. In addition, over the Pacific the highest CH 4 concentrations are 

 observed in the spring profiles. The periods during which the fall and winter 

 profiles over Scottsbluff and the Pacific were obtained overlap, and these 

 profiles do show good agreement. The spring and summer profiles were collected 

 in 1966 and 1967, respectively, and the difference between Scottsbluff and 

 Pacific profiles in these cases could include yearly changes in the CH 4 

 concentration as well as local differences. 



The lack of conspicuous seasonal variation in the observed atmospheric CH 4 

 concentration can have several causes. One is that there are a number of sources, 

 possibly with their seasonal variations out of phase so that only small and short 

 oscillations occur. Another is that the seasonal variation of CH 4 destruction is 

 similar to the seasonal variation of production. In fact, water vapor, ozone, and 

 the total solar-radiation flux that determine the concentration of the OH radical 

 have maximum values in spring or summer. Thus the CH 4 destruction rate 

 should be larger during the warmer seasons, as is the CH 4 production rate. The 

 two seasonal variations tend to cancel each other and leave little imprint on the 

 atmospheric CH 4 concentration. 



We were also interested in seeing if the average tropospheric profiles would 

 show systematic gradients, as is the case in the stratosphere. There appears to be 

 at least one. All the Scottsbluff profiles do show a decrease of the CH 4 

 concentration close to the surface, which seems to indicate that there is a sink of 

 CH 4 close to the surface. Since it is well known that soil microorganisms 19 

 consume CH 4 , the uptake of CH 4 by soil was considered a possible sink. 

 However, preliminary experiments in our laboratory have so far failed to 

 demonstrate that soil microorganisms can utilize the small atmospheric CH 4 

 concentrations. The annual average profiles do not show significant gradients 

 except those very close to the surface. Because of the higher concentrations in 

 spring, however, the annual average CH 4 concentration over the Pacific is higher 

 than over Scottsbluff. 



THE ISOTOPIC COMPOSITION OF CH 4 



Another possible source of information is the isotopic composition of CH 4 . 

 We have already seen that the 14 C content proved quite useful in distinguishing 



