140 CADLE 



CONCENTRATIONS 



Numerous studies have been made of CO concentrations in the ambient 

 troposphere, and the results have been discussed in the reviews mentioned in the 

 preceding paragraphs. ' '* The average concentration is about 0.1 to 0.2 ppMv, 

 but there is a large variation about this mean. For example, Robinson and 

 Robbins 2 obtained values at Inge Lehmann station in Greenland varying from 

 about 0.1 to 0.7 ppMv. Measurements by Seiler and Junge 4 in the vicinity of the 

 tropopause showed that a rapid decrease in CO concentration with increasing 

 altitude occurs in the lower stratosphere. 



In polluted atmospheres, such as city smog, the concentrations are of course 

 very much higher and extremely variable. A typical concentration would be 40 

 ppMv. 



RESIDENCE TIMES 



Residence times for an atmospheric species can be defined in at least two 

 ways. One is the mean or average lifetime that can be obtained by dividing the 

 total amount in the atmosphere by the rate of addition to the atmosphere. 

 Another definition involves the so-called tau value [thus: t(CO)] , which is the 

 atmospheric concentration divided by the removal rate. For many processes this 

 is the time required to decrease the concentration to 1/e of the original value. 



Junge, Seiler, and Warneck 5 suggest a value of 1 year for the average 

 residence time and that there must be differences between the tropospheric 

 lifetimes of 14 CO and 12 CO. Weinstock and Niki 1 6 estimated a residence time, 

 defined as the reciprocal of the first-order rate constant for CO removal, of 0.1 

 year. They assumed that the rate constants for 12 CO and 14 CO are identical. 



CO-REMOVAL PROCESSES 



Our knowledge of the processes for the removal of CO from the atmosphere 

 is at least as uncertain as our knowledge of the sources and the residence time. 

 Pressman and Warneck 1 7 suggest that chemical reactions in the stratosphere 

 contribute significantly, but only partially, to the overall removal of CO from 

 the atmosphere. They suggest that the most important removal process is 



OH + CO -> C0 2 + H (4) 



Another reaction that must remove some CO in the stratosphere is 



0( 3 P) + CO + M -> C0 2 + M (5) 



