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"We" do NOT measure CO2 - resurrected thread.
Fractionation of gases in polar ice during bubble close-off: New constraints from firn air Ne, Kr and Xe observations

Severinghaus, Jeffrey P.; Battle, Mark O.

Earth and Planetary Science Letters, Volume 244, Issue 1-2, p. 474-500.

Gas ratios in air withdrawn from polar firn (snowpack) show systematic enrichments of Ne/N2, O2/N2 and Ar/N2, in the firn ice transition region where bubbles are closing off. Air from the bubbles in polar ice is correspondingly depleted in these ratios, after accounting for gravitational effects. Gas in the bubbles becomes fractionated during the process of bubble close-off and fractionation may continue as ice cores are stored prior to analysis. We present results from firn air studies at South Pole and Siple Dome, Antarctica, which add Ne, Kr and Xe measurements to the suite of observations. Ne, O2 and Ar appear to be preferentially excluded from the shrinking and occluding bubbles, and these gases therefore accumulate in the residual firn air, creating a progressive enrichment with time (and depth) in firn air. Early sealing of gases by thin horizontal impermeable layers into a non-diffusive zone or “lock-in zone” greatly enhances this enrichment. A simple model of the bubble close-off fractionation and lock-in zone enrichment fits the data adequately. The model presumes that fractionation is caused by selective permeation of gas through the ice lattice from slightly overpressured bubbles. The effect appears to be size-dependent, because Ne, O2 and Ar have smaller effective molecular diameters than N2, and fractionation increases strongly with decreasing size. Ne is fractionated 34 ± 2 times more than O2 in South Pole firn air and reaches an enrichment of 90‰ in the deepest sample. The large atoms Kr and Xe do not appear to be fractionated by this process, despite the large size difference between the two gases, suggesting a threshold atomic diameter of ˜ 3.6 Å above which the probability becomes very small that the gas will escape from the bubble. These findings have implications for ice core and firn air studies that use gas ratios to infer paleotemperature, chronology and past atmospheric composition.
DOI: 10.1016/j.epsl.2006.01.032

Notice that they don't mention CO2?

Read the following to put a context on the molecular size of CO2:

Quote:A sublimation technique for high-precision δ13C on CO2 and CO2 mixing ratio from air trapped in deep ice cores

Jochen Schmitt

2.1.7 Processes during bubble close-off.
Contrary to the three processes described above, where diffusion takes place in the gas phase, the following phenomenon deals with diffusion through the ice lattice. Elemental ratios in air pumped out of polar firn layers near the firn-ice transition show systematic enrichments of Ne/N2, O2/N2 and Ar/N2 (Huber et al., 2006; Severinghaus and Battle, 2006). In turn, air extracted from already closed-off bubbles is relatively depleted. This unexpected depletion in O2/N2 ratios was recognized early, when O2/N2 ratios were measured with the aim to gain information about the strength of the photosynthetic oxygen production (Bender et al., 1994 and references therein). The O2/N2 depletion was found to be systematic and cyclical and although the underlying mechanism remained obscure, the fractionation effect was exploited for dating issues (Bender, 2002). Only recently, the effect of gas fractionation during bubble close-off gained more attention and can now be described as a special diffusion process through the ice lattice (Huber et al., 2006; Severinghaus and Battle, 2006). In contrast to gas phase diffusion, here the mass of the species is not a relevant parameter, but rather its diameter.

Large atoms, like Kr and Xe, are not affected by this process, however, the small atoms He and Ne are subject to considerable losses. Below a critical diameter of 3.6 Å, gas molecules and atoms migrate through the lattice and are partially lost to the firn air and are therefore relatively depleted within the bubble. The dependency of the measured loss with diameter is strongly nonlinear (Severinghaus and Battle, 2006).

Analogue to the preferential gas loss during the bubble close-off, a similar diffusion process through the ice matrix is present during the storage of ice cores after drilling, which causes further off-sets for the gas composition, e.g. for N2/O2 (Bender, 2002). CO2 has a molecular diameter of 3.94 Å, thus, should lie on the safe side of this size criterion. The values for the diameters are collision diameters, derived form viscosity experiments. Note that besides collision diameters, kinetic sieving diameters are used in the field of studying diffusion and permeation through porous membranes. Here, the effective diameter of CO2 (3.3 Å) is smaller than for N2 (3.6 Å) and CO2 preferentially passes inorganic membranes (Yeom et al., 2000 and references therein).

To derive effective diameters for a given process, both the dimension of the gas of interest, but also for the second collision partner, in our case the ice lattice, have to be considered. The data basis for effective diameters is sparse, therefore, the conclusion that CO2 is not affected by this process has yet to be proven. Firn air measurements near the close-off zone did not indicate any effect on the CO2 concentration. However, in contrast to other gases like O2, N2, and the noble gases, for which a relative enrichment against a fairly stable background concentration can be detected with relative ease, CO2 is more uncertain due to the anthropogenic rise. Furthermore, the CO2 concentration profile with depth is often an input parameter to tune the model, thus, CO2 is not a free parameter, which limits the interpretation.

Jochen Schmitt has assumed the apparent rise in CO2 to be due to anthropogenic activity, possibly to appease those assessing his work. Whilst there may be some rise in atmospheric CO2 levels over the last century, there is no evidence that it is due to burning hydrocarbons.

Kinetic diameters, rather than collision diameters, are almost exclusively used in disciplines outside of glaciology for diffusion and permeation work. Permeable membranes are used in many industries for separating gases and there is plenty of literature covering these areas, both in the academic and commercial (patents) worlds.

There are also well known mechanisms whereby solvent on the surface of membranes enhances the passage of those molecules that dissolve in said solvent. Thin layers of ice with its quasi-liquid surface and firn with its grain boundaries might be regarded as such a system. CO2 is highly soluble in cold water, much more so than N2 and O2 and thus is more likely to be fractionated by this process. Another thing to note is that as soon as the bubbles in ice form they become over-pressured. Again, this generally enhances fractionation processes.

CO2 is also capable of distorting the ice lattice and thus can become caged as in clathrates. It does this far more readily than O2 and N2. The relative depths ate which they occur is testament to this.

In addition to all these phenomena, it is essential to realise that the cavities in the snow become smaller as the snow sinters into firn and then ice. Thus as the density increases the air is squeezed out from the base and therefore there must always be a general upward migration of some of the semi-trapped air. This process must cause a certain amount of air motion and mixing which will to some extent counter any gravitational effects. It will also mean that a fractionation gradient is produced because the more rapidly fractionating species will escape first leaving behind the slower ones.

Together these mechanisms mean that gases trapped in ice are not and cannot be representative of the atmosphere in any absolute sense. However, there is the possibility that some climatic signal is present but it will, in most cases, require large corrections and a great deal of care in interpretation.
"Correlation is NOT Causation"
I have been looking for the attached thread for a bit now,
so I'll put it here for reference.

Hint - "enjoy" the later "ironman, and Dim wit" comments..
Sinkin’ the Source
the Air Vent blog
Jeff Id on November 11, 2009
The whole aim of practical politics is to keep the populace alarmed
(and hence clamorous to be led to safety)
by menacing it with an endless series of hobgoblins, all of them imaginary.

H. L. Mencken.  

The hobgoblins have to be imaginary so that
"they" can offer their solutions, not THE solutions.
I have just remembered I meant to put some reference links re CO2 and measurements.

Historic variations in CO2 measurements.
Posted by Jeff Id on March 6, 2010
Guest post by Tony Brown

Carbon cycle modelling and
the residence time of natural and
anthropogenic atmospheric CO2:
on the construction of the
"Greenhouse Effect Global Warming" dogma.

Tom V. Segalstad

by Jeffrey A. Glassman, PhD
Revised 11/16/09.

Gavin Schmidt on the Acquittal of CO2

by Jeffrey A. Glassman, PhD
Revised 3/18/10.

CO2: "WHY ME?"

by Jeffrey A. Glassman, PhD
Revised 3/14/10.

The whole aim of practical politics is to keep the populace alarmed
(and hence clamorous to be led to safety)
by menacing it with an endless series of hobgoblins, all of them imaginary.

H. L. Mencken.  

The hobgoblins have to be imaginary so that
"they" can offer their solutions, not THE solutions.

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