Modification of air standard composition by diffusive and surface processes

We present experimental evidence of modification of O 2 /N 2 , Ar/N 2 , 29 N 2 / 28 N 2 , 34 O 2 / 32 O 2 ratios and CO 2 mole fraction in dry air standards resulting from gas handling. Correlated variations among high precision measurements of multiple molecular pair ratios show diffusive fractionation to be the main modifying process. This mechanism can account for much of the previously unexplained CO 2 instability commonly observed in air standards used by the atmospheric CO 2 measurement community. Identification of the effects of diffusive fractionation helps to isolate effects of other processes such as surface adsorption, which is also implicated as a cause of CO 2 modification in some applications. These findings have direct implications for the preparation and maintenance of O 2 /N 2 /Ar and CO 2 calibration standards and are relevant to other atmospheric measurement activities that involve gas handling.