METHANE IN THE ATMOSPHERE 151 



If we take K z to be 3 X 10 3 cm 2 /sec in the lower stratosphere and use the 

 gradient from Fig. 1, we obtain a global loss of 0.6 X 10 1 g CH 4 /year from 

 upward diffusion of CH 4 . 



Including the stratospheric destruction, the total loss is then 3 X 10 1 5 g/year 

 (Table 3). 



TABLE 3 



SINKS FOR ATMOSPHERIC CH 4 * 



Destruction rate, 



Reaction g/year 



CH 4 +OH->CH 3 +H 2 2.9 xlO 15 



in the troposphere 



Loss to stratosphere in the 0.6 x 10 14 



Hadley circulation 



Loss to stratosphere by 0.6 x 10 1 4 



vertical eddy diffusion 



Total loss rate 3 xlO 15 



*Destruction rates are based on a CH 4 

 mixing ratio of 1.41 ppM. 



ATMOSPHERIC TURNOVER TIME OF CH 4 



We can now compare the production and destruction rates of CH 4 by 

 comparing the respective turnover times in Table 4. Considering the large 

 uncertainties in the estimates of production and destruction, the agreement is 

 not too bad. For further comparison, Table 4 also contains an estimate of the 

 turnover time based on the variance of the observed CH 4 concentration in the 

 free atmosphere. Junge has worked out an empirical rule of thumb that relates 

 the turnover time, r, and the variance expressed as the relative standard 

 deviation of the atmospheric tracer concentration, O: to = 0.14 year. Our 

 measurement of about 400 samples in the free troposphere gave 1.41 ±0.30 

 ppM for the average mixing ratio of CH 4 . The resulting r is 0.7 year. Thus the 



TABLE 4 

 TROPOSPHERIC TURNOVER TIME OF CH 4 



From biological production alone 



(Table 2) 2.6 to 8 years 



From total production (assuming 



addition of 20% 



of dead CH 4 ) 2 to 6 years 



From destruction (Table 3) 1.3 year 



From Junge 's relation, 



or = 0.14 year 0.7 year 



