METHANE IN THE ATMOSPHERE 149 



the troposphere to destroy CH 4 at any significant rate. These reactions were 

 thought to be important only in the stratosphere. This view has changed recently 

 after Levy 1 4 proposed a reaction cycle that maintains the OH concentration at a 

 tropospheric average 6 of about 2.5 X 10 cm" . With such an OH concentration, 

 the destruction of CH 4 in the troposphere becomes very important. Although 

 only the first step in the oxidation of CH 4 is of importance here, the whole 

 sequence is of interest because CO and H 2 appear as intermediate products. Thus 

 the CH 4 cycle is coupled to the CO and H 2 cycle. 



CH 4 + OH->CH 3 + H 2 (1) 



CH 3 + 2 "> CH 3 2 (2) 



CH 3 2 + NO-+CH3O + N0 2 (3) 



CH3O + 2 ^HCOH + H0 2 (4) 



'+ OH^CHO + H 2 

 HCHO < + hi>-»H 2 +CO (5) 



+ h^->CHO + H 



CHO + 2 -> CO + H0 2 (6) 



CO + OH ^ C0 2 + H (7) 



The rate 1 5 of reaction 1 is 9 X 10 _1 5 cm 3 molecule -1 sec -1 at 300°K. From this 

 and the OH concentration, we obtain a tropospheric turnover time of 1.4 years. 

 The corresponding destruction rate is 2.9 X 10 15 g/year (see Table 3). This 

 reaction rate may be an overestimate of about 30% because the rate of reaction 

 1 decreases with temperature 1 5 and the mean temperature in the troposphere is 

 less than 300° K. However, in view of the uncertainties in the tropospheric 

 concentration of OH, such a refinement seems unnecessary at present. 



This reaction scheme works in the stratosphere as well, and the stratosphere 

 also acts as a sink for CH 4 . This was already confirmed experimentally when the 

 first stratospheric CH 4 profiles showed a large decrease in the CH 4 mixing ratio 

 up to 23 km 3 . Figure 1 gives another example of a stratospheric CH 4 profile 

 extending the measured decrease to an altitude of 50 km, where the CH 4 mixing 

 ratio drops to 0.25 ppM. 



The losses of CH 4 to the stratosphere are determined by the rate of 

 transport from the troposphere into the stratosphere. There are two processes to 

 be considered. The first is the Hadley cell circulation that penetrates into the 

 lower tropical stratosphere. Newell estimated that the mean rising motion in 

 the tropics has a velocity of about 0.02 cm/sec. From this he deduced a flux of 

 2.4 X 10 12 g air/sec into the stratosphere. This flux leaves the stratosphere at 



