[Pete Ridley (OP)]

Hi Professor Wolff, thanks for responding so promptly to my Email of 20th with your detailed explanation. This is just the sort of response that I have been trying to elicit for over a year now. You have given me plenty to think about and I was in the middle of replying to your E-mail of yesterday when I spotted that this thread had been unlocked (thanks moderators) and you had posted here. Your explanation here is very similar to what you E-mailed, apart from the background information that you have provided for those who aren’t so familiar with the issue. I’m sure they’ll appreciate it.

I have several points that I’d like to discuss and will do so one post at a time until you get tired of responding. I had thought of simply exchanging E-mails but I’m sure that there are others who are interested in this, judging by the views this thread is getting.

You say

.. 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) ..

so maybe I’m misunderstanding the ice sheet structure.

Here’s a diagram that might help  [ Invalid Attachment ]

I was under the impression that from the top down there is:
- a very small depth (much less than 1m?) of snow, which is affected by surface activity such as wind and others, below which there is
- a significant depth (tens of metres) of increasingly dense firn (see Figs. 2 & 3 of the Goodwin reference below) in which all air components diffuse relatively freely (is this also known as the stagnant zone?), below which there is
- a relatively short depth (10m?) in which the pores and channels interconnecting air pockets gradually reduce to nanometric size, where free diffusion no longer takes place and size-dependent fractionation occurs (is this known as the transition zone?) below which
- all air pockets are sealed to molecular movement other than through the crystal lattice via the breaking of hydrogen bonds.

Consequently, I find your

.. the air is sitting in a slowly diffusing column of firn until it is enclosed ..

surprising, because I would expect that the air components having the largest molecular size (e.g. CH4, N2, O2, Ar) would be “enclosed” before the smaller molecules like CO2, Ne and He, which will continue to leave the air pockets and move down the pressure gradient towards the surface.

Can we concentrate on that first. Are you saying that this does not happen?

For anyone who is interested, one description of the typical ice sheet structure is given in “Snow-accumulation variability from seasonal surface observations and firn-core stratigraphy, eastern Wilkes Land, Antarctica” by Ian D. Goodwin in 1991 (http://www.igsoc.org/journal/37/127/igs_journal_vol37_issue127_pg383-387.pdf). Another paper “Polar ice structure and the integrity of ice-core paleoclimate records” by Faria et al. in 2009 (http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6VBC-4XT2CSW-1&_user=10&_coverDate=01%2F31%2F2010&_rdoc=1&_fmt=high&_orig=gateway&_origin=gateway&_sort=d&_docanchor=&view=c&_searchStrId=1733120326&_rerunOrigin=google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=a1695e258504733d23512b27bd34e36c&searchtype=a) provides a helpful diagram in

.. Fig. 3. Basic outline of the multi-scale structure of the EPICA-DML ..

plus some excellent images, mostly of deep ice but

.. Fig. 11. Microstructure-mapping mosaic images of two firn sections from 60 m to 85 m depth ..

relates to this discussion.
There are also some excellent SEM images of ice structure in “Snow crystal imaging using scanning electron microscopy: III. Glacier ice, snow and biota” by Rango et al. (http://itia.ntua.gr/hsj/45/hysj_45_03_0357.pdf).

Best regards, Pete Ridley

[ericwolff]

Pete,
I will respond (though not endlessly, as I am not a natural blogger!).

Yes, your descriptions is generally fine, and not in contradiction with the one-sentence summary I gave. But just to summarise in my own words:
1. At the top of the firn (the name for the whole column of material that is between loose snow and solid, impermeable ice) there is a convective zone in which the air exchanges relatively freely with the atmosphere.  At most sites today, this is very shallow (less than a couple of metres).
2. Most of the firn column is what is called the "diffusive zone".  It is in this zone that gravitational fractionation can occur.
3. Then there are a few metres in which diffusion with the overlying column appears not to take place, but which still has some permeability (this is the non-diffusive zone).  At any depth in this zone, it is likely that some bubbles are closed off and some are not (this is the most obvious way in which the enclosure is "gradual", and the main reason why gas profiles in ice cores have an inherent limit in their time resolution).
4. Finally one reaches solid ice where all the bubbles are enclosed.

Again maybe for the general reader, I should say how we know this, and what we don't know.  While people probably know that we can sample the air bubbles by crushing a core to release the air, they probably don't know that we can also sample the firn.  This is done by drilling a hole, and lowering a pumping device, that has a bladder above the sampling entries, to the bottom of the hole.  By inflating the bladder, one can make sure that the device pumps air only laterally from the firn surrounding the depth the inlets are at.  With such a device it is easy to know when one has reached zone 4: it becomes impossible to pump any air (but obviously this is easily verified when the core is drilled as the air bubbles are intact).  Zone 3 is characterised by the fact that the 15N14N/14N14N ratio, which increases slightly (by a few tenths of a part per thousand) with depth in the diffusive zone (zone 2 in my description), stops increasing.

The question in this thread is entirely about what happens in zone 3.  The answer is of course that we don't fully know what mechanism is taking place because we cannot reasonably observe or reproduce the very slow enclosure process in the lab.  All that scientists can do is observe the results of the process, and hypothesise about the cause.  The data show that there is a fractionation of some atoms/molecules relative to others in this zone, meaning that (using nitrogen as the standardiser) the bubbles are slightly depleted for some "smaller" molecules compared to the air above.  One should not exaggerate this effect, which is of order 0.3% for oxygen.  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.

I cannot emphasise enough though that the exploration of the mechanism (in the papers I cited yesterday) is only a way of explaining the data, which clearly show that CO2 is NOT fractionated during enclosure, at least within the analytical resolution.  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.  If this leads someone to a better description of the mechanism, that would be great: but the data are the data.

[Geezer]

as I am not a natural blogger!.

You could have fooled me! Many thanks for the post.

[Bored chemist]

OK, To summarise the thread .
Pete asked " My question in a nut shell to the scientists here is “why do paleo-climatologists use collision diameter in preference to kinetic diameter when considering the migration of air molecules through firn and ice”? "
And the answer is

"the data, which clearly show that CO2 is NOT fractionated during enclosure, at least within the analytical resolution.  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.  If this leads someone to a better description of the mechanism, that would be great: but the data are the data."


We can stop now, and go out to play.

[Pete Ridley (OP)]

Hi Professor Wolff, I think that Geezer’s comment will reflect what many of the silent viewers of this thread will be thinking. I’ll be most surprised if others do not start jumping in with comments. Thanks for another excellent, helpful response which, as far as the main question in this thread is concerned, seems to be covered in Para. 3

.. 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 ..

In other words, this type of fractionation is evidenced by the data and could be due to size-dependent fractionation as I have described it but this has not been properly researched and we just do not know. My understanding of the work of Severinghaus and Huber, which is the basis for many other papers on the subject and specifically addresses this type of fractionation, is that they rely upon models which use the wrong measure of molecular size, collision rather than the appropriate measure, kinetic diameter.

Let me leave that for a moment to ask about ice sheet structure. Am I correct in thinking that what you refer to in 2. of your summary as the “diffusion zone” is also called by others the “stagnant zone” or is this what you call in 3. the “non-diffusive zone” or is this zone also referred to as the close-off zone? (pause for breath).

Also, I think that it would be sensible to resolve any uncertainties arising from your first comment here before tackling those in your second. Your structure summary point 3. leads nicely into something that came to mind after I read your E-mailed response of 26th.

In your E-mail you sent me the paper “Natural and anthropogenic changes in atmospheric CO2 over the last 1000 years from air in Antarctic ice and firn” (http://www.agu.org/journals/ABS/1996/95JD03410.shtml) by Etheridge et al. (one of the papers that Dr. Nisbet had also sent me). Then (and on the thread here on 27th) you make reference to Fig. 2 on Page 4120. I’m unable to provide a link to the full paper so for anyone who is interested and can’t get a copy of the paper I’ll try to describe what Fig. 2 shows. Alternatively, the CO2 (and CH4) “data and descriptions” are available free from NOAA (http://www.ncdc.noaa.gov/paleo/icecore/antarctica/law/law_data.html).

Fig. 2 is a graph of CO2 on the Y-axis (origin 310-360ppm) and depth in the ice sheet (origin 0-120m). The plotted points start at 10m, 353ppm with an almost linear fall to 70m, 343ppm. a gradient of 0.17ppm/m. Around 70m there is a turning point to a steeper fall from 70m, 343ppm to 115m, 310ppm,  a gradient of about 0.73ppm/m.

Related to Fig. 2 is Table 2 on Page 4119, which says

.. 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 inferences that I draw from Table 2 are:
- ice accumulation rate = 72m/40years = 1.8m’year, but this conflicts with the statement in “Interactive comment on “Post-coring entrapment of modern air in polar ice cores collected near the firn-ice transition: evidence from CFC-12 measurements in Antarctic firn air and shallow ice cores” by M. Aydin et al. 30 March 2010” (http://www.atmos-chem-phys-discuss.net/10/C1119/2010/acpd-10-C1119-2010-print.pdf) that

.. The accumulation rate at their study site was about .. 1.2 m/y (ice equivalent) .

(any suggestions?)
- the close-off process is very gradual, with fractionation of the smaller molecules taking place over several years (around 10), which I understand to be the period in Fig. 2 during which the CO2/depth gradient of 0.17ppm/m in the firn (the diffusion zone?) is gradually changing (in the non-diffusive zone?) to a gradient of 0.73ppm/m (in the “solid” ice – I use parentheses because of the persistent movement of molecules within that zone due perhaps to H-bond breaking and fractures, another aspect of the fractionation process to be considered later).

The first thing that struck me as odd when I first saw Fig. 2 in 2008 was the discontinuity occurring around 70m. At that time I had come across a related graph (http://maps.grida.no/library/files/historical-trends-in-carbon-dioxide-concentrations-and-temperature-on-a-geological-and-recent-time-scale.jpg) on the site of Norwegian GRID-Arendal, a collaborating centre of the United Nations Environment Programme (UNEP). More recently I found the US EPA’s page “Atmospheric Concentrations of Greenhouse Gases” which presents a graph superimposing data from 6 Antarctic ice cores, spliced with measurements from Mauna Loa and sundry other SH and NH locations (http://cfpub.epa.gov/eroe/index.cfm?fuseaction=detail.viewInd&lv=list.listByAlpha&r=231323&subtop=342). That too displays a puzzling discontinuity for CO2 concentration, but in that case relating to the difference between the “normal” periodic swings between ice ages and interglacials).

Rather than jump to the conclusion that the apparent increase in mean global atmospheric CO2 was a consequence of the industrial revolution and our increasing emissions of CO2 into the atmosphere (not least from using fossil fuels) I considered it worthwhile trying to identify another reason for that discontinuity seen in ice core data from numerous different locations. I was reminded of the concerns expressed by Professor Zbinew Jaworowski about fractionation in the ice (see my opening post on this thread) and that led me to research this issue of size-dependent fractionation in the deep firn.

The inferences that I drew from Fig. 2 of the Etheridge paper were:
- CO2 experiences significant size-dependent fractionation down the pressure gradient towards the surface at depths around the nominal close-off depth of 72m where the larger gas molecules (N2, O2, Ar, CH4, Kr, Xe) are trapped in the air bubbles,
- above the nominal close-off zone CO2 is no longer experiencing size-dependent fractionation but is being mixed by diffusion with younger air within the firn.
 
You said that

.. 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

. I need to look more closely at that and revert. Meanwhile I propose to E-mail the scientists I have been exchanging E-mails with for the past year to see if any of them wish to get involved here. After all, opinions from other specialists, especially some who have the necessary expertise in the movement of different molecules in sub-nano-porous media (such as from those involved in gas purification for the energy industry, e.g. removal of of impurities from mine gas to bring it up to grid standard) could help to remove some of the uncertainties arising from the fact that

.. we frankly do not know the exact mechanism ..

.

If anyone viewing this thread who has such expertise or knows specialists who have it then I’d appreciate them inviting their involvement.

BTW, for anyone who was/is interested in taking a look at those links to ice sheet structure that I provided at the end of my comment yesterday @ 22:39:34, Dr. Al Rango, the main author of “Snow crystal imaging using scanning electron microscopy: III. Glacier ice, snow and biota” (http://itia.ntua.gr/hsj/45/hysj_45_03_0357.pdf) has expressed an interest in joining the discussion. Dr. Rango has kindly offered to try to get his hands on more of those excellent images and the data used as the basis for that paper. I again recommend a look at those images.

Best regards, Pete Ridley
[attachment=post_tmp_24442_0][/attachment][attachment=post_tmp_24442_0][/attachment]

[Pete Ridley (OP)]

Hi Bored chemist, although you may have lost interest in discussing this size-dependent fractionation issue and want to

.. stop now, and go out to play ..

you did raise a relevant question on the “What does Iain Stewart's "CO2 experiment" Demonstrate” thread in your comment of 22nd April @ 17:35:55. Rather than your question being followed up there, which is on a separate topic altogether, maybe it should be addressed here or, better still, as a new question related to the validity of the attempts to reconstruct past atmospheric CO2 content from air in ice sheets.

In response to my

.. 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 ..

you said

.. 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

.

Perhaps, as an analytical chemist, you can help clear up other significant uncertainties regarding those other air fractionation processes that take place right from the beginning of the process of ice-sheet development. After all, the validity of those attempts to reconstruct past atmosheric CO2 content from air “trapped” in ice is rather fundamental to the argument presented by the IPCC and its supporters.

Let’s start with the very first part of the ice sheet development process, the falling snow. In my list of scientists I mentioned Professor Hartmut Frank, Chair of Environmental Chemistry & Ecotoxicology, University of Bayreuth. Professor Frank wrote a forward to Jaworowski’s 1994 paper “Ancient Atmosphere – Validity of Ice Records” (http://www.springerlink.com/content/284n23943h8g687p/fulltext.pdf?page=1). Professor Frank sent me a slide from one of his presentations last year to graduates at Technical University in Gdansk. [ Invalid Attachment ]

He described it  as “ .. a simplified illustration of the major processes which are leading to changes of gas concentrations in the secondary bubbles (including and especially of carbon dioxide) .. ”. I don’t know how to embed a pdf page into this comment so will have to describe it (if anyone would like a copy then send me a “Personal Message” and I’ll E-mail it).

The slide provides a diagram showing falling snow beneath which is the section of a fully developed ice sheet. It discusses the manner in which atmospheric air collected within the voids of forming snow has already been depleted in CO2 before it even hits the top of the ice sheet. Alongside it says “Assuming a specific weight of 1 dm3 snow as 0.1 kg/L, it consists of 10 Vol-% of snow-ice and 90 Vol-% of air. Thus, a dm3 snow may contain a total carbon dioxide content of which 4 mg is adsorbed and 0.54 mg comes from the air between the snow flakes –explaining the high values found in ice cores by the gas extraction over long time in the molten state”.

It then goes on to summarise the snow/firn/ice compaction stages after “Deposition:
- Compaction to firn, air bubble closure (when?),
- Chlathrate formation (CO2~ 5 atm) [CO2•5 H2O] preferred diffusion of CO2 into the ice matrix
- Chlathrate formation (N2, O2at ~ 20 atm)
- Carbonic acid formation (formulae shown) N2, O2 non-reactive: selective depletion of CO2
- Primary bubbles disappear.
- Upon drilling and horizontal storage of ice cores, expansion and back diffusion of N2, O2 and CO2 (and slow decomposition of carbonic acid) into secondary bubbles occurs, at different rates”.

My question to you at this stage is whether or not that initial adsorption of CO2 in the snow is a chemical or a physical process (http://www.tutorvista.com/content/chemistry/chemistry-iv/surface-chemistry/absorption-types.php). My suspicion is that it is Chemisorption but as I’m not a chemist I’d appreciate your opinion. Following on from that is another question about adsorption in the deep firn, but that can be followed up later. Let’s work our way down the ice sheet looking at the different processes that Professor Zbiniew Jaworowski has been expressing concern about since 1992 and return to deep firn adsorption of CO2 and clathrate formation at a later stage.

I can ask Professor Frank and Professor Jaworowski and other scientists (see my comment of 13th April @ 21:58:19 on this thread) if they are interested in helping on those other processes.

Best regards, Pete Ridley
 [ Invalid Attachment ]

[Pete Ridley (OP)]

I see that I managed to attach that pdf without realising I’d done it. What a great blog this is.

[imatfaal]

Pete - it's a forum not a blog. 

A blog tends to be a regular personal piece of writing with potential for others to comment on it.  a forum has a different structure - it is a public discussion arena, a modern agora, where people can meet and debate diverse topics.  It is structured around the threads, which are back and forth conversations between two or more participants.  Any member (or on some fora anyone at all) can start a question or participate in a thread  - whereas on a blog there is an owner who writes the blog and others who comment.  To stop the naked science forum from descending into a free for all, in common with most fora, there are rules of posting and moderators who gently or not so gently enforce those rules.  I have mentioned this to perhaps help explain why previous posters and mods were talking about editorialising;  in a blog one is expected to be editorial and proprietorial - in a forum it is presumed that debate is the key. 

http://en.wikipedia.org/wiki/Internet_forum
http://en.wikipedia.org/wiki/Blog

[Pete Ridley (OP)]

Hi Imatfaal, I’m at risk of upsetting the moderator of this blog by responding to your off-question comment, but I’ll risk it with a “quickie”.

Just what is a blog, anyway? Defining this variable form is not easy in the highly opinionated blogosphere ..

(http://www.ojr.org/ojr/stories/050929/) – but at least I’ve learned what an agora is.

Now, what have you to say about collision v kinetic diameter in the close-off zone of deep firn?

Best regards, Pete Ridley

[yor_on]

Well, I say we better put a stop to this Peter

You've got your answers, now you want more, and if Eric answers those, you will find other things to question That's perfectly correct in a 'blog' I presume, or in a private correspondence, but not here. We've delivered what I think is above and beyond the 'call of duty' for our head-bofins managing TNS, just for you Peter, and notably without letting your comments about our incompetence, not being 'firn experts', getting to us.

Why not take some of those courses that are available instead, somewhat slower maybe but then you will know what you're talking about. Or maybe you consider yourself to 'know'? Anyway, it's never wrong to study, if one is burning as much as you for a subject. Alternatively, if you consider yourself knowing already, create that final experiment that will put your question to rest, once and for all. As I read Eric that should be quite a challenge to create? And as i see it that's what it comes down to, your question needs a experiment to answer it. As for how to create it I can't say though? It has to to over quite a long time period, and you will need to prove that you've considered all parameters that may have a influence, and isolated those that you're interested of. You're welcome back with some practical results. This discussion though, seems already have gone to on too long to me.

Until then Peter.

[Pete Ridley (OP)]

Hi Yor_on, I do not agree with your

.. You've got your answers ..

Here is my opening question again

.. why do paleo-climatologists use collision diameter in preference to kinetic diameter when considering the migration of air molecules through firn and ice? ..

Only one person, Professor Eric Wolff, has tried to respond properly to that question (see 27the April @ 14:53:30 and 28th April @ 08:45:37). He is not one of The Naked Scientists but works with the British Antarctic Survey and is recognised as a specialist in the area relating to part of my question. He has shown that he fully understands my question but even he has not answered it. Others here may think that he has but by reading his penultimate paragraph in his response of 27the April @ 14:53:30 carefully enough and having done whatever background reading is needed to understand it and my original question, it will be realised that the specific question remains unanswered. Proferssor Wolff focussed on CO2 but my question is not specifically about CO2. It refers to “air molecules”, which covers not only CO2 but N2, O2, Ar, CH4, He, Ne, etc.

I expect that Professor Wolff would quite readily accept what I have just said. More exchanges are needed before the question is answered and that is one reason why, after receiving Professor Wolff’s response, I E-mailed specialists like Professor Jeff Severinghaus to take a look at this thread. Professor Severinghaus and his associate Dr. Chris Huber have written papers on this specific issue of size-dependent fractionation of “air molecules” but used collision diameter not kinetic diameter and did not specifically investigate CO2. The reason appears to be because their model led them to conclude that molecules having a diameter larger than 0.36nm are not affected by the size-dependent fractionation process. CO2 has a collision diameter of 0.39nm but a kinetic diameter of 0.33nm.

If they had used the kinetic diameter measure of molecular size then I hypothesise that their conclusion s about CO2 would have been different. As I have said already, expert practitioners in gas purification through nanoporous media do not use collision diameter in their analyses, they use kinetic diameter. That is what gave rise to my still unanswered question.

Professor Wolff acknowledged

.. 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 think that last should be O2). I expect that gas purification specialists had been involved and the research done by Severinghaus and Huber kinetic diameter would have been used in the model.

As Professor Wolff said in his second response on 28th April @ 08:45:37 

.. 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 ..

Following several E-mail exchanges with Professor Severinghaus I asked on 29th December about the structure of the models that he had used, including

.. 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? ..

For some reason I didn’t get a response.

The validity of the attempts to reconstruct past atmospheric composition from air recovered from ice is fundamental to the CACC hypothesis. Many questions about this have been raised repeatedly since at least 1992 by others such as Professor Zbiniew Jaworowski and Professor Hartmut Frank. The question that I posted here initially is just the first of a long string of such questions.

Contrary to what has been suggested here, that first question of mine remains unanswered, hence, in my opinion this very popular “Hot Topic” thread, which continues to attract significant views, should remain open.

Best regards, Pete Ridley

[yor_on]

That's okay with me Peter. Email as many as you like, and keep on searching for those answers. But the question has been answered as I see it. The next step if you want to prove your point is without doubt a experiment, no words in the world can replace that.

May I most respectfully suggest you to use mails if you won't, instead of TNS.

[Bored chemist]

Pete,
you seem to have missed something
"If they had used the kinetic diameter measure of molecular size then I hypothesise that their conclusion s about CO2 would have been different. "
Yes, specifically their conclusions would have been wrong.
The results they calculated tally with the data.
Any different result would not agree with the real world.
Scientific results that don't agree with reality are called "wrong".

Also, where you say "Contrary to what has been suggested here, that first question of mine remains unanswered" you are mistaken, it has been answered. You just don't like the answer.
They use the numbers to verify the model.
They observe that "the data, which clearly show that CO2 is NOT fractionated during enclosure, at least within the analytical resolution. "

[Pete Ridley (OP)]

Hi Yor_on, I consider that The Naked Scientists blog/forum is an excellent place to have this issue discussed. The viewing figures tell me that this thread is attracting a lot of attention but maybe you interpret those figures in a different way to what I do. I repeat, my question has not yet been answered so let’s wait for further help from those who have specialist knowledge in the subject. I believe that the only person to contribute to this thread so far who fits that category is Professor Wolff. Hopefully he will continue to give his valued assistance and others with such expertise will join in.

Hi Bored chemist, from your contributions so far am I being fair if I suggest that you do not have the necessary expertise in the subject to state that

.. it has been answered ..

You are mistaken if you  have the impression that I

.. have missed something ..

I certainly don’t miss anything that Professor Wolff says and have every intention of following up on all of it. In the mean time I hope that Professor Severinghaus or Dr. Huber will join in because it was they who modelled the process so should be able to give a  satisfactory answer to my original question. It is possible that Professor Wolff will return to it too and others may join in.

Best regards, Pete Ridley

[yor_on]

Well Peter, I'll give it the benefit of a doubt But if Eric feels he answered your original question, then that's that, as far as I'm concerned. And then we will move on.

[Bored chemist]

And, once again, for those who still don't get it.
Q
"why do paleo-climatologists use collision diameter in preference to kinetic diameter when considering the migration of air molecules through firn and ice"
A
because it gives the right answer.

[Pete Ridley (OP)]

In response to my original question Professor Wolff has provide two excellent overviews of the fundamental issue of how valid are the attempts to reconstruct past atmospheric composition from air recovered from ice sheets. I thank him for making the time to put together those submissions and hope eventually to cover all questions about the validity of those reconstructions. It is sensible to tackle in a structured manner all of the different complex processes that distort the composition of the original atmospheric air, To this end I have asked the founder of this blog/forum, Chris Smith, if it is possible to expand the thread question to read “Another Hockey Stick Illusion?” – “Do glaciers tell a true atmospheric CO2 story?”. That then allows us to follow the relevant processes from the top of the ice sheet down to the depths of the ice beneath.

I first would like to focus on the matter covered in my original question, which is

.. why do paleo-climatologists use collision diameter in preference to kinetic diameter when considering the migration of air molecules through firn and ice? ..

I shouldn’t have to keep repeating this but there are several people who have commented here who seem not to have read it properly. Professor Wolff went to a great deal of trouble to explain his understanding of it in his very first comment here (27th April @ 14:53:30) and I can’t improve on that.

Once a convincing answer has been given to my original question I would like to return to those other important points made by Professor Wolff that do not directly address my original question but are relevant to “Do glaciers tell a true atmospheric CO2 story?”.

In that comment Professor Wolff said

.. 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 ..

Note that reference to “air”, , not N2, not O2, not Ar, not CO2, etc. etc. etc. but “air” – all of it.

He then went on to say

.. 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? ..

Note this time that Professor Wolff focuses on “the final stages of enclosure”, i.e. the deep firn, which is precisely where my original question is targeted at.

Professor Wolff then said

.. 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 ..

The issue in question should by that stage have been reasonably clear to anyone following this thread.

Let’s look now at what “the literature says” about size-dependent fractionation in deep firn as it approaches close-off, using three papers that Professor Wolff made reference to. First let’s see what is said in the 1996 paper “Natural and anthropogenic changes in atmospheric CO2 over the last 1000 years from air in Antarctic ice and firn” by Etheridge et el. As I pointed out almost a year ago elsewhere, that paper (and as far as I could find then, no other that Etheridge was involved in) made any reference to size-dependent fractionation of air in the deep firn, the only references to fractionation being to the gravitational type. I propose to return to this paper at the appropriate point in the wider discussion about “Do glaciers tell a true atmospheric CO2 story?”.

The papers that are much more relevant to my original question are the two from 2006 that Professor Wolff makes reference to. First let’s look at what “Evidence for molecular size dependent gas fractionation in firn air derived from noble gases, oxygen, and nitrogen measurements” by Huber, Severinghaus, et al. (http://icebubbles.ucsd.edu/Publications/Huber_closeoff_EPSL2006.pdf) says about this. I make the assumption that all eight co-authors can reasonably be called paleoclimatologists. The next point to clear up is whether or not their paper is about the migration of air molecules through firn and ice. The title of the paper certainly suggests so, the paper discusses the molecules of all of the different atmospheric air components and their diffusion within the ice sheet and most importantly

.. In the present study, however, we are mainly interested at the bottom of the firn air column ..

This is precisely the area on which I am focussing, as Professor Wolff acknowledged when saying in his first comment here

.. 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? ..

At the bottom of the firn air column, as close-off is approached, pores on the surface of the closing air bubbles and the channels that link air pockets must at some stage approach molecular size, i.e. the firn becomes a nanoporous medium. Gas purification experts use nanoporous media (e.g. zeolites) in industrial applications and numerous papers and patents can be found that describe the process. All of these refer to molecular kinetic diameter, not collision diameter, e.g. see the 2002 paper “Application of natural zeolites in the purification and separation of gases” by Ackley et al. (http://www.zeolitanatural.com/docs/gasseparation2.pdf) and  “Sol-Gel Processing of Inorganic Membranes for Natural Gas Purification” (http://www.netl.doe.gov/kmd/cds/disk28/NG8-3.PDF) which says

.. 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 ..

So, I repeat my original question

.. why do paleo-climatologists use collision diameter in preference to kinetic diameter when considering the migration of air molecules through firn and ice? ..

As I discussed over a month ago on the Climate Conversation Group’s “Fallen snow” thread (which I provided a link to in my first post on this thread)

.. 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 ..

I won’t say much here at this stage about the paper “Fractionation of gases in polar ice during bubble close-off: New constraints from firn air Ne, Kr and Xe observations” by Severinghaus & Battle (http://icebubbles.ucsd.edu/Publications/closeoff_EPSL.pdf). In Section 1.2 and 3.1 of the paper they provide a good description of the process that I am focussing on and they discuss the results of their

.. simple model of the bubble close-off fractionation …

They go on to say

.. The model presumes that fractionation is caused by selective permeation of gas through the ice lattice from slightly overpressured bubbles. ..

I’m puzzled by that reference to “ice lattice” because there is another process that is thought to take place after close-off. This is a process related to the one that I am focussing on at present and I would like to consider this one later, however, it is noticeable that they again refer to the 0.36nm molecular size. They say

.. 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 ..

Once again, they ignore kinetic diameter in favour of collision diameter (see Table 1).

If anyone is interested, more comments of mine on this paper can be found on the Climate Conversation blog threads:
- “It’s not warming, you nitwit — it’s cooling” (http://www.climateconversation.wordshine.co.nz/2011/03/its-not-warming-you-nitwit-its-cooling/) in my comment of 21st March @ 11:26 and
- “Fallen snow” (http://www.climateconversation.wordshine.co.nz/2011/03/fallen-snow/) in my comment of 23rd March @ 11:17.

Professor Wolff, are you at liberty to attach a copy of your 2011 paper “Greenhouse gases in the Earth system: a palaeoclimate perspective” so that subsequently we can look at the evidence provided therein supporting your conviction that

.. the ice-core record provides a faithful record of changing atmospheric composition. ..

(http://www.ncbi.nlm.nih.gov/pubmed/21502180)? That paper may provide the answer to the question used by Professor Jaworowski et al. as the title to their 1992 paper “Do glaciers tell a true atmospheric CO2 story?” (http://www.co2web.info/stoten92.pdf).

I referred to that paper in my comment of 13th April @ 21:58:19) and as I said then

.. 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. ..

Best regards, Pete Ridley

[ericwolff]

Pete and other readers,

My first post (353863) was intended to be a very clear and direct answer to your original question; additionally I have since contacted Professors Alley and Severinghaus: although I have not seen their correspondence with you, they both tell me that they felt they had already answered you, in similar terms, in private e-mails in the past.  Given this, I am surprised that you are still claiming your question has not been answered.  I will therefore make one closing attempt to be absolutely clear.  Your specific question was:

"why do paleo-climatologists use collision diameter in preference to kinetic diameter when considering the migration of air molecules through firn and ice?".
The problem that we are all having is that this is a false or loaded question: the implication elsewhere in your posts is that the idea that CO2 is unfractionated compared to N2 comes from authors making this assumption.  In fact it is quite the other way round: the empirical evidence that CO2 is not fractionated on enclosure (as well as the observation that Ar is less fractionated than O2) is what led these authors to hypothesise that collision diameter was the controlling variable.  So the specific answer is that they use collision diameter because this is what allows them to rationalise the data they observe.

I think that none of us has a definite molecular-level understanding of the physical process occurring at closeoff, and it would be great if someone can do the experiments in the lab to understand that better.  But it won't alter the empirical facts.

Incidentally it may be worth mentioning that, in their expts, Severinghaus and Huber used trace gases (such as Ar and O2) that are understood to have been invariant in concentration over recent decades, precisely so they could look at the diffusion and enclosure processes free from any assumptions about temporal change in concentration. 

You raised several other points in different posts, and I simply don't have time to answer them all.  Your questions about kinetic diameter imply you have got quite deep into the literature, so I am surprised that you are confused about the kink in firn air profiles, as this is clearly explained in several of the papers.  It is the result of moving from the diffusive zone (in which the air is ageing according to diffusive mixing with the air above) into the non-diffusive zone, where the air is ageing with the ice, ie much faster ageing. Just to give a schematic example:
Imagine a site at which the snow accumulation rate is 1 m ice equivalent per year, and the non-diffusive zone starts at 70 m snow depth (which because the firn is much less dense near the surface, might be 50 m in ice equivalent).  This is similar to the situation at Law Dome, but I have chosen to use a hypothetical site with round numbers for simplicity.  At 70 m, the ice is 50 years old, but the air is about 10 years old.  At 80 m, the ice is 60 years old, and the air is 20 years old.  Thus the CO2, would change from about 385 ppmv at the surface (2010 concentration, South Pole) to 365 ppmv at 70 m, then 350 ppmv at 80 m.  This would of course show a sharp kink if plotted against depth.

Finally, I am surprised you are citing Jaworowski as an expert.  His monographs about ice core CO2 have been comprehensively refuted by those who actually work on the topic, most notably some 15 years ago by Professor Hans Oeschger.  I myself looked into his work a decade ago, but found it so full of misunderstandings (if I am generous) that I did not bother to formulate any specific refutation, but such material is easily available on the web.  The saddest part is that Jaworowski cites as issues aspects of ice core analysis that many of the pioneers of the field were very aware of, and spent decades carefully studying and overcoming.  It is therefore galling to see their efforts ignored.

I will be happy to answer other well-formulated questions about ice cores, but I don't think I have any more to add to this thread.

[yor_on]

Thanks Eric. And, you're most welcome back whenever you find the time and inclination. It's been a, ah, interesting journey, with a lot of twists if I may say so, but myself I think your posts have been oasis's of clearness.

I will close this thread now, as I think your post says it all.