Just a heads up, been into Knotlow this morning intending to do the climbing shaft down to the coffin level at the bottom of waterfall chamber. On the ledge at the top of the pitch I found that I was struggling a little to breath and felt a little dizzy, needless to say I didn't go any further. The climb back out was a little trippy though. Once I got back up to Pearl Chamber I was OK again. Take care if you are down there over the next few days and listen to your body if you like me didn't have a gas alarm. I've updated the logbook at the bottom of the climbing shaft.

I wonder if it's something to do with the rapid drop in temperature over the last week? I don't really check air pressure often but it must have been going up and down like a yoyo recently. I don't know that much about Knotlow, but is it possible these rises in CO2 are caused by occasional sumping at the bottom end, cutting off circulation until the water levels drop again?

The level of CO2 in cave can be due to a variety of sources, dilution mechanisms and sinks.  Whilst it is difficult to be confident about a specific cave, one major source of CO2 is due to decomposition of organics in the soil above the cave being picked up in the water percolating through to the soil, then through the limestone and then into the cave.  So a sudden inflush of percolating water could bring a pulse of CO2.  In cave, drafts etc will help dispersion far more effectively than diffusion.  As you suggest, sumping could hinder air flows and hence cause a build up of CO2.  And as you say large atmospheric pressure drops can 'drag' CO2 out at a higher rate than normal.  But the atmospheric pressure at Heathrow airport has not fallen below 1000mBar for over a month so that seems unlikely.  Surface temperature is postulated to cause an impact between winter and summer when a chimney effect could be reversed as the surface air temperature rises below the in cave air temperature.  However, I don't think surface air temperatures have dropped that much. 

You can read a bit more at https://caves.org/pub/journal/PDF/v71/cave-71-01-100.pdf and http://ww.caves.org/pub/journal/PDF/V68/v68n1-Baldini.pdf .  If you want to ponder further, then have a look at the data collected at the BCRC at https://www.cave-science.org.uk/ .

Wow, I'm glad I reading that now, as opposed to 11 pm, as some of it has actually gone in - though it's complicated! I don't need to understand the equations, but the descriptions are pretty clear to a non-specialist. Having dropped into CO2 soup at Nettle Pot once, I certainly have experienced the phenomenon of 'apparent pooling', put down to density, when it's more due to proximity to the CO2 source (and the lack of an oxygen source) and the lack of circulation from evenly-mixed air. Also I guess Nettle's constricted shafts and solid lid don't help with circulation, unlike a 2m-diameter engine shaft with a grille on top. Though isn't the lid solid at Knotlow? Maybe that's part of the problem?

The Knotlow shafts (and Hillocks) have grills fitted to try and alleviate this issue. For some reason the short passage at the top of the waterfall pitch pools bad air - possibly from the constricted passage behind that leads to a choke near the East Level climb down before the Bung. In the past gas detectors have gone beepy beep here and flame safety lamps have gone out, but AFAIK the bottom of the waterfall chamber and the coffin level to 4 ways remains fresh with the water dragging clean air down

I?ve experienced bad air at the bottom of the engine shaft which was worse than the confined area around the ducks in Crimbo pipe. Also Meccano passage can be a bit unpleasant. I think the whole system is prone to it tbh. If in doubt get out. After the experience in the engine shaft it took a few hours to feel better again. Awful.

If you thought the Knotlow air was bad, check this out - a rave in a bunker powered by diesel generators 

https://www.theguardian.com/world/2020/aug/31/oslo-bunker-rave-puts-25-in-hospital-with-carbon-monoxide-poisoning

?We could have ended up with permanent injuries,? a 20-year-old woman who attended the rave told the Norwegian state broadcaster NRK from hospital. ?A doctor called it a collective suicide attempt. It?s totally sick even to think about it.?

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mr conners said:

After the experience in the engine shaft it took a few hours to feel better again. Awful.

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Have a read of https://cscc.org.uk/wiki/doku.php?id=information:co2 but note people do vary quite markedly in their reaction to CO2 so can't be used in place of instruments to guage levels.

I sometimes wonder whether it would be worth doing a systematic study of the ventilation circuits in Knotlow; I suspect the way they interact results in poor ventilation around the engine shaft and Waterfall chamber. There's a circuit from Fourways shaft through Chapeldale Level to the hilltop shaft on the level; there's lots of steam comes out from under the tembos capping it in frosty weather. I think there's also one from the oildrum entrance in Hillocks (which pulls cold air in - icicles form inside), possibly to the climber. However, I'm not so sure there's much circulation around the central shafts.

I also know that prior to the recent rains,  the local farmers were out muck spreading quite a bit. If a significant portion of this has flushed down into the conduits under Flagg then there could be more "organic material" than has been usual of late in the water...