Re: How can the scientists measure the concentration of CFC in the atmosphere ?

Atmospheric concentrations of CFCs can be known in a number of ways.  
First,if we know the industrial production of CFCs, if we know the transport 
of air from the troposphere to the stratosphere, we can calculated steady 
state concentrations of CFCs in the troposphere.  We can do this because we 
can assume that chemical loss of CFCs in the tropophere (via oxidation with 
OH) is minimal and that CFCs have a very low solubility in the oceans.  

We can also directly measure CFCs by "grab sampling."  That is, get an 
evacuated pyrex bulb and take it into the clean troposphere and open it.  
This will suck in the air in that region.  Then purify that air in the 
laboratory by removing CO2, O2, N2, by crogenic distillation.  Then you can 
either run the sample through a gas chromatograph or a mass spectrometer to 
view its contents.