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Background

Among all the discussion of 'greenhouse gas removal' (GGR) to aid climate change mitigation, almost all is concerned with removal of carbon dioxide. There are one or two mentions of chemical techniques for removing methane and nitrous oxide. I've found nothing on F-gas removal, despite destruction of HFCs being a source of carbon offsets and recent media focus on SF6 having a GWP100 of 23,500 and an atmospheric lifetime of 3,200 years; others include tetrafluoromethane (lifetime 50,000 years), dichlorodifluoromethane and chloropentafluoroethane. Presumably F-gas removal is never mentioned because concentrations even for long-lived species are unworkably low (less than one part in 109), the warming effects are far smaller than that of CO₂, it's not useful cover for the non-renewable energy industry, and production can be addressed through the Montreal Protocol and Kigali amendment (which covers HFCs but not PFCs or SF6). I am aware that F-gases represent less than 3% of GHG emissions and the main focus of mitigation has to be fossil fuels.

Question

What I would like to know is whether there is any chemical or physical method to remove F-gases from the atmosphere. How could it be done? Or is it definitively impossible?

Pointers to relevant publications would be particularly appreciated.

Initial thought: distillation

Could fractional distillation be useful? In production of liquid nitrogen, any solid impurity will be a greenhouse gas, mostly water. Apparently there's a technique to separate substances in this state. Xenon is isolated through fractional distillation of air, but occurs at around 87 parts in 109, still 104 times more than SF₆. Lackner's nitrous oxide comment linked to above mentions 'Sherwood's Rule' - 'separation costs tend to scale linearly with dilution', making this seem impossible.

Or is there any other relevant technique or chemical reaction or way of trapping fluorinated molecules at scale? Could resonance electron capture be in any way relevant?

Digression about dilution

Obviously liquefaction of the entire atmosphere might be a tad inconvenient and expensive. However, some F-gases are much denser than air, so might they pool in low, still air in drains and caves near production and waste recovery facilities? Edit: probably not - there's little sign of a CO₂ gradient in caves, so is there any research on distribution of denser substances like SF6 which can be detected in very low concentrations (measurable by electron capture detector, infra-red spectrophotometry, flame ionisation or mass spec)? How long do they take to diffuse away?

Lackner also mentions passive absorption of carbon dioxide using natural wind. Is there any scope for passive concentration using gravity? A useful answer here on diffusion suggests for the heavier F-gases mentioned, you'd get a 65% ratio of concentration of still air per km. So that implies you'd need a really deep mineshaft to collect heavier gases; about 6 km deep, Maybe tapering from a wide collector in open air down through filters could eventually achieve such a 20x concentration for SF6, and then be used in some separation process.

Sub-question: economics

Supposing a 20x concentration were located and used as the source by the compressed gas industry and it could also use a carbon price around $500/tCO2e - is there any way recovery, recycling or destruction of F-gases could be economic? Back-of-the-envelope suggests that the price of xenon (about 15 USD/L) is comparable to the carbon price of a similar sample of SF6 at the current GHG price (80 USD/tCO₂e on EU ETS), but of course the concentrations of the latter are much lower.

Sub-question and follow-on

The focus of this question remains the environmental context as above. I'm not a professional in this field and don't even know which of the 'sub-questions' I've added are relevant and what aspects have been missed. I've posted a separate question, free of environmental context, focused on SF6 at How would you remove sulfur hexafluoride from a gas sample?.

Cedric Knight
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    No, once mixed into the atmosphere, HFC's stay mixed and do not pool in low places (though those with low vapor pressure at Antarctic temps might accumulate there). The issue is that the gases are man-made, but widely dispersed as refrigerants, aerosol propellants, etc. Though production of some are banned, there are viloators. – DrMoishe Pippik Aug 12 '22 at 15:53
  • Can you also search the concentration of these fluorinated compounds in the lower atmosphere? It must be in picograms/m$^3$...Imagine finding a tip of a broken needle in a haystack. – AChem Aug 12 '22 at 15:54
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    @AChem I know concentrations are measured. I don't know if it's using an electron capture detector or some other means, but presumably feasible to locate higher concentrations - indeed SF6 is used as a tracer. Those I mention range from 5-500 ppt, so nanograms per m³. – Cedric Knight Aug 12 '22 at 16:10
  • @DrMoishePippik Thanks. It does look like most HFCs are well-mixed. I was initially thinking of SF6 and the old idea that CO₂ pools in caves; there is a period when it is not well-mixed as in the Lake Nyos disaster.

    Xenon is commercially extracted from air, but is at concentrations about 100 times higher - wondered if possible for F-gases. It's not only violators releasing CFCs, but that many HFCs are not completely banned.

     – Cedric Knight Aug 12 '22 at 16:35
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    Bulk gases denser than air can stay in depressions for a long time. In the end, in thermal equillibrium, all gase mix perfectly with each other, unless you are at very low pressure and/or very high $g$ fields. And even then they never separate, but only develop a shall concentration gradient. – Karl Aug 12 '22 at 21:50
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    In the Lake Nyla case, the CO2 was dissolved in the cold lake water at the bottom until it was disturbed. Then it came out of solution quickly like a shaken soda… – Jon Custer Aug 13 '22 at 00:24
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    If anything concerns the whole atmosphere, the main question must be about the method scalability, mainly in economy, resources, technology and environment aspects. Many methods for purging air of trace components, that are easy and efficient in the lab scale, are not applicable in atmospheric scale. // Environmental problems are very seldom as simple to be addressable in the scope of a single Q/A. – Poutnik Aug 16 '22 at 11:29

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Short answer? No.

All gases are prefectly mixed in the atmosphere, except if you can catch them directly an the source. That's just thermodynamics.

It's already an imo crazy idea trying to remove CO2 from the ambient air. Of course it can be done, but the amount of energy needed is huge, even before you liquify or compress the gas and store it. Anything with orders of magnitude lower concentration is just hopeless. We do not currently live on "planet free energy". ;)

If you really go and separate CO2 from air (on a really large scale, so it'd actually make a dent in the atmospheric carbon dioxide concentration), you will also capture most of the other gases you're thinking about. Except the methane, because it has a significantly lower boiling point, 80 K below CO2.

I don't know the actual numbers, but my gut feeling is CO2 makes up 80% of the global warming problem, and 90% of the rest is, unfortunately, methane. (If you want to research the numbers, please add them here.)

So once we have excess solar or fusion energy , we can start capturing CO2, and will get some small amount of SF6 and friends with it, improving the efficiency of the climate-saving effort by a few percent. They however need special disposal, so they'd probably be more of a nuissance here.

Karl
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    (+1) Nice answer. If I was going to capture carbon dioxide from the atmosphere, I would grow lots of fast growing trees, harvest them, process just enough to get logs, and stash the logs in old salt mines and the like. And re-plant one for one, at least. That captures the carbon and you have wood if you need it later. – Ed V Aug 12 '22 at 22:32
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    @EdV That's my favourite, too. You can make money by selling as construction wood. Turn the rest into charcoal, stash that (or sell for soil improvement), and sell the energy. Re-forest the sahara! You need large scale seawater desalination, but that's cheap in hot regions. – Karl Aug 12 '22 at 23:08
  • CO₂ removal etc is addressed in the links in the question. As percentages of the problem, it depends on timescale: see IPCC AR6 WG1 figs SPM.2 and TS.20 and Annex III - F-gases look tiny after 100 years, but I'm interested anyway. BECCS (burying tree carbon) uses huge amounts of land. – Cedric Knight Aug 12 '22 at 23:47
  • 'Just thermodynamics' - statistically separation is unlikely, but not if there is something (gravity, electrical field, etc) producing a gradient. Water vapour is not well-mixed and very heterogeneous You could consider this topic like remediating heavy metal contamination. – Cedric Knight Aug 12 '22 at 23:52
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    @CedricKnight Atmospheric water freezes, melts, evaporates, and condenses all the time. It has a huge turnover by itself. None of the trace gases you are thinking about does. They are not even well water-soluble, which would make a difference. Sorry, but all I can say is "forget it". ;) – Karl Aug 13 '22 at 00:04
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    And timescale? I'm not concerned with 22nd century problems. They'll have to clean up, just like we have to for our grandparents. I know that sounds harsh, but come on. You only live once. – Karl Aug 13 '22 at 00:14
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    "statistically separation is unlikely": no, it cannot happen. The gravitational or electrical fields you mention do no exists on earth. You're orders of magnitude off. – Karl Aug 13 '22 at 00:18
  • By "statistically separation is unlikely", I meant the second law of thermodynamics is a question of probability. You could spontaneously get all the nitrogen on one side of a room and all the oxygen on the other. Unlikely is a British understatement :). Just trying to elicit some possibilities, involving energy input or not, phase changes or not. I corrected 'm' to 'km' in my question, but hope have orders of magnitude for gravity correct now. – Cedric Knight Aug 14 '22 at 13:29
  • On 'timescale' I just meant that there's a big difference between GWP10 and GWP100 as shown in that IPCC TS.20 figure. Methane emitted now has almost negligible impact in the 22nd century, but many F-gases will still be around. Some people are looking even further ahead: https://doi.org/10.1007/s10584-020-02785-4 – Cedric Knight Aug 14 '22 at 13:35
  • Thanks for answers so far, and I do get the general direction this is going. 'If you want to research the numbers, please add them here.' Of 2020 GHG emissions, it looks like F-gases are 2.6%, compared to N2O at 4.9% and deforestation at 10%, with the bulk of course being coal, oil and gas. https://wedocs.unep.org/xmlui/bitstream/handle/20.500.11822/34426/EGR20.pdf p5 – Cedric Knight Aug 14 '22 at 13:36