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benefit from looking at its 'founders'
.. but to build the contention on your idea of the difference, without presenting anyone agreeing is a hard thing to do
Doesn't necessarily mean that your ideas is wrong but you should really try to find some supporting evidence before contending what everyone 'mainstream' think is true
Collision diameter (Å)
we are interested in learning and sharing what we know
I'm not sure it is placed in the right forum if it's just a geology/chemistry or physics question
quite close to sounding querulous here
that no scientist can answer your question
Dr. Tim Atkinson, Professor of Environmental Geochemistry .. research interests include earth and environmental sciences, centring around the applications of chemistry and physics to problems in hydrology, hydrogeology, Quaternary geology and palaeoclimate, and geomorphology ..
Dr. Euan Nisbet, Professor of Earth Sciences, Department of Earth Sciences, Royal Holloway, University of London
Note the difference for CO2. Also note the size of He and think of the He-filled party balloon compared with one that you and I might blow up ourselves – which stays up the longest and why?
.. separation on size is possible, when the components to be separated are small enough in kinetic diameter to migrate through the zeolite pores and the components from which they have to be separated have a kinetic diameter that is too large ..
they're foremost Climatologists
.. Secondly, the air is fully enclosed in ice (ie the ice becomes impermeable) only at a depth, typically 60-100 m depending on the site, and the air is sitting in a slowly diffusing column of firn until it is enclosed (typically taking 1-3 decades to reach the enclosure depth) ..
.. the air is sitting in a slowly diffusing column of firn until it is enclosed ..
.. Fig. 3. Basic outline of the multi-scale structure of the EPICA-DML ..
.. Fig. 11. Microstructure-mapping mosaic images of two firn sections from 60 m to 85 m depth ..
as I am not a natural blogger!.
.. The data show that there is a fractionation of some atoms/molecules relative to others in this zone, meaning that .. the bubbles are slightly depleted for some "smaller" molecules compared to the air above .. People have hypothesised that this effect is controlled by a process where smaller molecules are slightly more likely to escape during the final stages of enclosure, but we frankly do not know the exact mechanism ..
.. Depth Where Sealing Occurs - 72m, Age of ice at Sealing Depth – 40years, Mean Age of CO2 in Air at Sealing Depth – 10years, Difference Between Ice Age and Mean Air Age – 30years, Duration of Bubble Close-off Process 8years
.. The accumulation rate at their study site was about .. 1.2 m/y (ice equivalent) .
.. In their figure 2, they (Etheridge et al.) show samples they have measured in the firn air .. and in trapped bubbles at the same depth .. : they conclude that the difference is a random 1.3 ppmv, showing that the enclosure process does not affect the concentration. This is a really direct and elegant measurement which shows that, at least at Law Dome, there is no fractionation of CO2 on enclosure
.. we frankly do not know the exact mechanism ..
.. stop now, and go out to play ..
.. I am surprised that you have had nothing scientific to contribute to the question of how the individual molecules of the different atmospheric gases react within nanoporous media like firn ..
.. Did it occur to you that the fact that no chemical reaction takes place might have some importance there .. Actually I'm an analytical chemist but, as I have pointed out, what you are asking about is physics
Just what is a blog, anyway? Defining this variable form is not easy in the highly opinionated blogosphere ..
.. You've got your answers ..
.. why do paleo-climatologists use collision diameter in preference to kinetic diameter when considering the migration of air molecules through firn and ice? ..
.. I agree that one could imagine constructing a model in which the kinetic diameter is important (and because of it's non-spherical nature, CO2 has a smaller kinetic than collision diameter). However, in such a model, Ar would be more fractionated than O2, whereas Severinghaus et al's data shows it is only one-third as fractionated; and as you are implying, CO2 would be somewhat more fractionated than either Ar or CO2 ..
.. I understand your point that, if the process was just physical permeation through a small gap, then we would expect the kinetic diameter to be relevant. But the data are not consistent with that, so the molecular level process must be somewhat different to that. What Severinghaus and others note is that there seems to be no fractionation for molecules with a collision diameter more than 3.6 Angstroms ..
.. I note that in both papers you have used collision diameter and ignored kinetic diameter, but why?. Have you carried out any research using kinetic diameter instead? If not do you have a feel for what differences this would make? ..
.. it has been answered ..
.. have missed something ..
.. The underlying issue is whether we can believe that the air in bubbles in ice cores is an un-fractionated representation of the atmosphere .. when we crack open a bubble of air, does it contain exactly the relative proportions of different molecules as the air in the atmosphere ..
.. you are more concerned about whether there is a further fractionation at the final stages of enclosure, related to the size of the molecule – do some smaller molecules more easily escape enclosure leading to a fractionation? ..
.. Certainly such a fractionation can exist for some atoms and molecules: it is very strong for neon (Ne), noticeable for O2 and Ar (at the permil to 1 percent level) but the literature says that there is no fractionation for CO2 (or CH4) compared to N2 ..
.. In the present study, however, we are mainly interested at the bottom of the firn air column ..
.. To efficiently separate CO2 (kinetic diameter = 0.33 nm) from CH4 (kinetic diameter = 0.38 nm) or (harder still) N2 (kinetic diameter = 0.364 nm) from methane, it is necessary to accurately control the average pore size between about 0.3 and 0.4 nm and achieve a narrow pore size distribution ..
.. Huber et al. .. say “ .. a critical size of 3.6 Å implies diffusion through channels of about the same dimension .. an outlet from a closing bubble .. changes its dimension steadily from open to closed. .. Diffusion coefficients of gases in ice show a size dependence similar to our findings beside for Ar and O2 .. ”. They should have included CO2 along with those two but didn’t because (I believe) they were focussing on the close-off fractionation of other gases and were misled into ignoring CO2 through using collision not kinetic diameter ..
.. simple model of the bubble close-off fractionation …
.. The model presumes that fractionation is caused by selective permeation of gas through the ice lattice from slightly overpressured bubbles. ..
.. 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 3.6Å 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 ..
.. the ice-core record provides a faithful record of changing atmospheric composition. ..
.. When I discussed this last June with Professor Zbiniew Jaworowski, whose 1992 paper first drew my attention to this issue, he expressed the opinion that “This is a highly specialized field of science. My impression is that it is a terra incognita for glaciologists”. Subsequently I have asked the same question of Professors Richard Alley, Jeffey Severinghaus and Michael Bender without receiving any worthwhile justification for their use of collision diameter. ..