Modification of air standard composition by diffusive and surface processes

Abstract

[1] 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.

Long-term continuous atmospheric CO<sub>2</sub> measurements at Baring Head, New Zealand

“…When working gases have been found to be drifting in CO 2 mole fraction, we also assign their values based on a linear trend over the drifting period. Cylinder drift is typically less than 0.2 ppm over the life of the cylinder as found elsewhere (Kitzis, 2009;Langenfelds et al, 2005), but there have been several cases of cylinders that drifted by as much as 2 ppm.…”

Gas adsorption and desorption effects on cylinders and their importance for long-term gas records

“…Similar arguments can be used to exclude ordinary diffusion on the low pressure side, though with slightly lower confidence since the diffusion length is only half of the decanting volume. Furthermore, the diffusion fractionation should decrease with increasing gas flow, just opposite to what has been observed in Langenfelds et al (2005), thermal diffusion induced by the Joule-Thomson cooling effect might only play a role for the low-flow decanting experiment, as shown below. Therefore, we follow the adsorption theory.…”

“…These instabilities can either be viewed as temporal drifts of gas composition or as pressure-dependent composition changes along the lifetime of the cylinder gas. These drifts have been attributed to numerous diffusional fractionation processes such as ordinary diffusion (depending on molecular mass and molecular size), thermal diffusion, or effusion (Bender et al, 1994;Keeling et al, 1998Keeling et al, , 2007Langenfelds et al, 2005), or were related to surface interaction alterations (Yokohata et al, 1985). These latter processes, i.e.…”

In-situ measurements of oxygen, carbon monoxide and greenhouse gases from Ochsenkopf tall tower in Germany

“…To measure PBL concentrations, the sampling point needs to be above the surface layer (about 100 m) in order to minimise the influence of the very nearfield (Gloor et al, 2001), that is, the area within a radius of the order of 10 km. Furthermore, high frequency measurements are required for optimal signal to noise ratio and to resolve diurnal and synoptic variability (Law et al, 2002). Such measurements can be achieved by using tall towers as sampling platforms with in-situ analysers.…”