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And having dinner a few weeks ago, Niahm (sorry, she only has Twitter account :-) ) asked me about the measurement of carbon dioxide in the atmosphere. My answer was pretty basic. Given that she is a Mechanical Engineer able to code her own drivers, she expected a more complete reply. Therefore, Niahm (and all of you reading this) here you have a much better and complete explanation :-)
http://www.esrl.noaa.gov/gmd/ccgg/about/co2_measurements.html