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CO2 is measured in parts per million (ppm), and recorded as a ten minute average. The urban signal of pCO2 is quite different from the global average, reflecting local events and weather. It is strongly linked to the boundary layer thickness, which is governed both by winds and daily heating and cooling. Note the highest concentrations occur often, but not always, in the early morning.
For reference, as of October, 2009. the global average CO2 concentration was about 386 ppm*.
*Dr. Pieter Tans, NOAA/ESRL (www.esrl.noaa.gov/gmd/ccgg/trends)