# Science of gasses in Earth atmosphere.

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A linear rise in gas concentration does not equate to a linear rise in absorption of radiation. There is only so much energy to be absorbed.

Typically, as gas concentration rises, absorption increases by logarithmic reciprocal.

This kind of conversation is usually about CO2, so I'll just say that the change in absorption between 400 ppm and 800 ppm is vastly less than the change between 200 ppm and 400 ppm. In fact, the greatest rise (by orders of magnitude) happens well before 100 ppm.

CO2 will become a threat as a poison before it becomes a serious threat due to heat.

Add to that that the atmosphere is a stew of many different gases, all with different chemistry and physical characteristics, and you can know one thing: anyone who claims to understand it completely is lying to you. Anyone who claims that his model is definitive is trying to sell you something.

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3 minutes ago, wayne_m said:

A linear rise in gas concentration does not equate to a linear rise in absorption of radiation. There is only so much energy to be absorbed.

Typically, as gas concentration rises, absorption increases by logarithmic reciprocal.

This kind of conversation is usually about CO2, so I'll just say that the change in absorption between 400 ppm and 800 ppm is vastly less than the change between 200 ppm and 400 ppm. In fact, the greatest rise (by orders of magnitude) happens well before 100 ppm.

CO2 will become a threat as a poison before it becomes a serious threat due to heat.

Add to that that the atmosphere is a stew of many different gases, all with different chemistry and physical characteristics, and you can know one thing: anyone who claims to understand it completely is lying to you. Anyone who claims that his model is definitive is trying to sell you something.

Presumably you have some references to scientific fact, figures and measurements to back these statements up ?

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1 hour ago, wayne_m said:

A linear rise in gas concentration does not equate to a linear rise in absorption of radiation. There is only so much energy to be absorbed.

Typically, as gas concentration rises, absorption increases by logarithmic reciprocal.

This kind of conversation is usually about CO2, so I'll just say that the change in absorption between 400 ppm and 800 ppm is vastly less than the change between 200 ppm and 400 ppm. In fact, the greatest rise (by orders of magnitude) happens well before 100 ppm.

CO2 will become a threat as a poison before it becomes a serious threat due to heat.

Add to that that the atmosphere is a stew of many different gases, all with different chemistry and physical characteristics, and you can know one thing: anyone who claims to understand it completely is lying to you. Anyone who claims that his model is definitive is trying to sell you something.

A few things seem not to stack up here:-

- If absorption were to increase "by logarithmic reciprocal", the absorption would get less with increasing concentration (partial pressure). That does not make sense. Can you post the formula you are referring to and provide a source for it?

- According to my admittedly limited understanding of this subject, the CO2 slows down the rate of IR radiation loss to space by absorbing and re-emitting IR radiation as it ascends from the surface of the earth. If you double the partial pressure of CO2 I would think you would double the average number of absorption and re-emission events  an IR photon encounters on its path to space. So the CO2 would trap double the amount of radiation.

- The statement that CO2 will be a poison before it becomes a serious threat due to heat seems to fly in the face of everything that modern climate science is telling us. If you are asserting everybody is wrong and you are right about this, you have an uphill struggle on your hands to convince people. You will need to provide a very sound rationale.

- While there are many gases in the atmosphere, we do know its composition pretty accurately and we know the IR absorption characteristics of all the components present. The challenge is in the complexity of the meteorological processes, e.g. interactions between atmosphere and oceans, clouds formation, icecaps etc, not the composition of the atmosphere. Nobody has ever pretended any of the models is "definitive" but they do all show the same basic trend - and we are seeing the predictions coming true.

This is not about anyone "selling" anything. It is science.

Edited by exchemist
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47 minutes ago, exchemist said:

A few things seem not to stack up here:-

More than a few things methinks.

1) Carbon dioxide is not poisonous, that is carbon monoxide.

2) Anyone who has (like myself) confined spaces training or worked for the Health and Safety Executive will be well aware that carbon dioxide is 'heavier' that air and displaces oxygen in confined spaces such as manhole shafts into whichit can settle.
There concentrations of 10% or more can cause unconsciousness and death by suffocation (not poisoning) if proper precautions are not made.

10% or 100,000 parts per million represents an enormous increase in the present atmospheric level.

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52 minutes ago, studiot said:

More than a few things methinks.

1) Carbon dioxide is not poisonous, that is carbon monoxide.

2) Anyone who has (like myself) confined spaces training or worked for the Health and Safety Executive will be well aware that carbon dioxide is 'heavier' that air and displaces oxygen in confined spaces such as manhole shafts into whichit can settle.
There concentrations of 10% or more can cause unconsciousness and death by suffocation (not poisoning) if proper precautions are not made.

10% or 100,000 parts per million represents an enormous increase in the present atmospheric level.

Yes I ignored that detail. (I too have done those courses, at my time on the refinery......)

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1 hour ago, studiot said:

More than a few things methinks.

1) Carbon dioxide is not poisonous, that is carbon monoxide.

2) Anyone who has (like myself) confined spaces training or worked for the Health and Safety Executive will be well aware that carbon dioxide is 'heavier' that air and displaces oxygen in confined spaces such as manhole shafts into whichit can settle.
There concentrations of 10% or more can cause unconsciousness and death by suffocation (not poisoning) if proper precautions are not made.

10% or 100,000 parts per million represents an enormous increase in the present atmospheric level.

2) - I think CO2 concentrations in confined spaces only stay high if there is a source, ie it starts with high concentrations and air mixing is impeded, by lack of air movement. A A fully enclosed space filled with air will still end up well mixed due to diffusion, not separated. A deep shaft with high CO2 that is open at the top will gradually lose CO2 until it has similar concentrations to the open air.

3 hours ago, wayne_m said:

This kind of conversation is usually about CO2, so I'll just say that the change in absorption between 400 ppm and 800 ppm is vastly less than the change between 200 ppm and 400 ppm. In fact, the greatest rise (by orders of magnitude) happens well before 100 ppm.

CO2 will become a threat as a poison before it becomes a serious threat due to heat.

Add to that that the atmosphere is a stew of many different gases, all with different chemistry and physical characteristics, and you can know one thing: anyone who claims to understand it completely is lying to you. Anyone who claims that his model is definitive is trying to sell you something.

No, not lying. Stop trying to sell the notion that climate scientists must be incompetent or biased, ie wrong; climate science is not about "selling" anything other than the best possible understanding of how our climate system works and responds to change, both natural and human caused. That isn't complete understanding but more than enough to know that the global warming problem is real, very serious and won't be self limiting over the timescales that matter to human civilisation and to remnant natural ecosystems.

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1 hour ago, Ken Fabian said:

2) - I think CO2 concentrations in confined spaces only stay high if there is a source, ie it starts with high concentrations and air mixing is impeded, by lack of air movement. A A fully enclosed space filled with air will still end up well mixed due to diffusion, not separated. A deep shaft with high CO2 that is open at the top will gradually lose CO2 until it has similar concentrations to the open air.

Of course there has to be a source of carbon dioxide, why would anyone think otherwise since its normal concentration in free air is so low ?

But it doesn't have to be a high pressure or continuous source.
Because, as I said, carbon dioxide is heavier than air (has a higher molecular weight) so will tend to accumulate in hollows or ever walled spaces.
It will not necessarily mingle with the free air outside the confined space.

During my training we were made aware of a recent fatality at a nearby chemical plant in an otherwise open area within the plant having side walls/fences, but no roof.
The source was a leak from a CO2 pipe.
A worse disaster occurred in the home counties (in the chalk) where they were laying new deep sewers.
Over the weekend and for perhaps sometme before there had been a lot of rain.
When work recommenced, a man went down the ladder to the bottom of the manhole shaft and collapsed.
When his mates looked down and saw this, two more went down to fetch him.
They too collapsed.
So the last two, instead of calling for professional help, went down to help them.
In all 5 workmen suffocated to death in that manhole shaft.

It is no matter to be treated lightly. Even the definition of a confined space is a legal definition as are the precautions and training necessary to work safely in one.

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7 hours ago, wayne_m said:

A linear rise in gas concentration does not equate to a linear rise in absorption of radiation. There is only so much energy to be absorbed.

Typically, as gas concentration rises, absorption increases by logarithmic reciprocal.

I'm sure others have addressed some of the long established science on CO2 and radiative forcing.  I'm just here to point out that you forgot that water vapor amplifies the GH effect of CO2 and throws off your calculations.  Increased water vapor in the atmosphere amplifies the warming caused by other greenhouse gases. It works like this: As greenhouse gases like carbon dioxide and methane increase, Earth's temperature rises in response. This increases evaporation from both water and land area.

Climatologists have made more predictive models by accounting for this increase in water vapor as a positive feedback mechanism in global warming.  As absorbed energy radiates up from the Earth's surface, water vapor is very good at absorbing and holding that heat in the lower atmosphere.  And the atmosphere can hold a lot of water vapor - it's a very long way to saturation.

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3 hours ago, studiot said:

But it doesn't have to be a high pressure or continuous source.
Because, as I said, carbon dioxide is heavier than air (has a higher molecular weight) so will tend to accumulate in hollows or ever walled spaces.
It will not necessarily mingle with the free air outside the confined space.

Lake Nyos disaster is worth googling, on the dangers of CO2 accumulation on a large scale when there's a sudden belch of a lake.  The cloud hugged the ground and moved down valleys, killing 1700+ people.

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5 hours ago, TheVat said:

Lake Nyos disaster is worth googling, on the dangers of CO2 accumulation on a large scale when there's a sudden belch of a lake.  The cloud hugged the ground and moved down valleys, killing 1700+ people.

I just read about that. The pressure at the 582 foot cold lakebed allowed the water to hold 20L/L of carbon dioxide vs 1L/L near the surface... 20x more.

Edited by StringJunky
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@studiot my point was to clarify for readers that CO2 in mixed air doesn't separate and sink, even under those circumstances. We don't get stratification of the mixed air, we see stratification of pockets/volumes with different CO2 concentrations that have not mixed - yet. Sources will keep it that way but without them the enclosed air will - eventually - homogenize. Or I should say no significant stratification under ordinary Earth gravity; run it through powerful centrifuges and it can become significant.

It is a common misunderstanding (whilst not claiming it of you) that CO2, being more dense, will sink to the bottom - and that the higher atmospheric CO2 concentrations nearer ground level are a result of the CO2 separating rather than the sources of CO2 being at ground level and there being a lag time in mixing. At small scale it mixes by diffusion. At larger scales by bulk air movements, ie wind and turbulence. For example thunderstorms will carry air from ground level to the stratosphere in one go, mixing vigorously as it goes.

Edited by Ken Fabian
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1 hour ago, Ken Fabian said:

@studiot my point was to clarify for readers that CO2 in mixed air doesn't separate and sink, even under those circumstances. We don't get stratification of the mixed air, we see stratification of pockets/volumes with different CO2 concentrations that have not mixed - yet. Sources will keep it that way but without them the enclosed air will - eventually - homogenize. Or I should say no significant stratification under ordinary Earth gravity; run it through powerful centrifuges and it can become significant.

It is a common misunderstanding (whilst not claiming it of you) that CO2, being more dense, will sink to the bottom - and that the higher atmospheric CO2 concentrations nearer ground level are a result of the CO2 separating rather than the sources of CO2 being at ground level and there being a lag time in mixing. At small scale it mixes by diffusion. At larger scales by bulk air movements, ie wind and turbulence. For example thunderstorms will carry air from ground level to the stratosphere in one go, mixing vigorously as it goes.

Well I disagree.

Let us look at it another way.

If the gas was hydrogen or helium or methane what would happen ?

The truth is that mixing will occur if there is sufficient activity in the gas body and then gases will keep remixing so appear to to remain unstratified.

As you correctly point out, there is a lot of activity in the atmousphere.

In perfectly still air there is no mixing.

Equally in a tub of denser gas. with no disturbing flow activity the denser gas will remain in the tub.

This is just the inverse of the lighter gases I mentioned collecting under roofspaces. I have measured this latter effect and designed buildings in danger of this to be safe.

In Australia you have a large amount of spare space and no need to build on former rubbish tips.
In the UK we don't have that luxury, but rubbish tips release methane and some much smaller amount of hydrogen, which are very dangerous gases if allowed to collect in upper spaces.

Edited by studiot
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1 hour ago, studiot said:

In perfectly still air there is no mixing.

I think this is wrong - or rather, there is no perfectly still air (at greater than absolute zero temperature) because of Brownian Motion at the molecule level. Which is what drives Diffusion.

Without a barrier the total volume will end up homogenous -

Edited by Ken Fabian
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6 hours ago, studiot said:

Well I disagree.

Let us look at it another way.

If the gas was hydrogen or helium or methane what would happen ?

The truth is that mixing will occur if there is sufficient activity in the gas body and then gases will keep remixing so appear to to remain unstratified.

As you correctly point out, there is a lot of activity in the atmousphere.

In perfectly still air there is no mixing.

Equally in a tub of denser gas. with no disturbing flow activity the denser gas will remain in the tub.

This is just the inverse of the lighter gases I mentioned collecting under roofspaces. I have measured this latter effect and designed buildings in danger of this to be safe.

In Australia you have a large amount of spare space and no need to build on former rubbish tips.
In the UK we don't have that luxury, but rubbish tips release methane and some much smaller amount of hydrogen, which are very dangerous gases if allowed to collect in upper spaces.

But again this is not spontaneous separation by density. Diffusion alone will mix gases of different density eventually. This should be obvious if you think what happens to a dense gas released into a vacuum chamber. It does not all collect at the bottom. That shows that molecular speeds are sufficient to far outweigh the effect of gravity on individual molecules.

What may cause confusion is that the mean speed of molecules with greater mass is lower, at a given temperature. So their rate of dissipation by diffusion will be lower.

Edited by exchemist
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10 hours ago, Ken Fabian said:

I think this is wrong - or rather, there is no perfectly still air (at greater than absolute zero temperature) because of Brownian Motion at the molecule level. Which is what drives Diffusion.

Without a barrier the total volume will end up homogenous -

4 hours ago, exchemist said:

But again this is not spontaneous separation by density. Diffusion alone will mix gases of different density eventually. This should be obvious if you think what happens to a dense gas released into a vacuum chamber. It does not all collect at the bottom. That shows that molecular speeds are sufficient to far outweigh the effect of gravity on individual molecules.

What may cause confusion is that the mean speed of molecules with greater mass is lower, at a given temperature. So their rate of dissipation by diffusion will be lower.

Thank you both.

So why does do the lighter gases I mentioned not diffuse downwards to form a homogenous gas body inside a large commercial building (tin sheds I call them) ?

I see that you have both avoided this question.

The points I am making are

1) That the Earth's atmousphere is never perfectly still.

2) That 'stratification' fully developed into layering is not happening. However there exists a measurable concentration gradient for any gas in the atmousphere, depending upon the difference of its molecular weight and the average for air. In fact as we rise in the atmousphere there exists a concentration gradient for air itself (or its average).

3) A point I didn't make before, If we look high enough the concentration of very light gases like hydrogen will be changed by processes which change or even break up the molecules.

2)

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32 minutes ago, studiot said:

Thank you both.

So why does do the lighter gases I mentioned not diffuse downwards to form a homogenous gas body inside a large commercial building (tin sheds I call them) ?

I see that you have both avoided this question.

The points I am making are

1) That the Earth's atmousphere is never perfectly still.

2) That 'stratification' fully developed into layering is not happening. However there exists a measurable concentration gradient for any gas in the atmousphere, depending upon the difference of its molecular weight and the average for air. In fact as we rise in the atmousphere there exists a concentration gradient for air itself (or its average).

3) A point I didn't make before, If we look high enough the concentration of very light gases like hydrogen will be changed by processes which change or even break up the molecules.

2)

The lighter gases will diffuse downward. I do not believe they will spontaneously separate at high points, as you suggest. However if a stream of a light gas is introduced, that may rise initially, until diffusion or other processes dissipate it into the general body of air. This will be quite rapid, especially with hydrogen, due to the low molecular weight and consequent high average speed of the molecules.

Have you a source for this idea that atmospheric composition changes with altitude, because of density differences between its component gases? I have not come across this. The compositional variations I am familiar with are due to chemical processes, e.g. ozone formation.  If this indeed does occur, it must be a very small effect, detectable only over altitude differences of many km. Not something you would ever see in a building.

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7 minutes ago, exchemist said:

The lighter gases will diffuse downward. I do not believe they will spontaneously separate at high points, as you suggest. However if a stream of a light gas is introduced, that may rise initially, until diffusion or other processes dissipate it into the general body of air. This will be quite rapid, especially with hydrogen, due to the low molecular weight and consequent high average speed of the molecules.

Have you a source for this idea that atmospheric composition changes with altitude, because of density differences between its component gases? I have not come across this. The compositional variations I am familiar with are due to chemical processes, e.g. ozone formation.  If this indeed does occur, it must be a very small effect, detectable only over altitude differences of many km. Not something you would ever see in a building.

Do the maths.

The diffusion equation describes the response of the particles to a forcing function.

In this case gravity.

It also requires the particles to be (1) non interacting and in the versions referred to to be  (2) identical, and (3) have no significant disturbing function.

What happens when a particle bumps into another, under condition 1 above, that is

a) 22 times a heavy  (hydrogen into carbon dioxide) ?

b) 1.5 times as heavy ? (nitrogen into carbon dioxide) ?

Well momentum and energy is preserved so if m is the mass of the lighter molecule and M the mass of the heavier; u and U the before and after velocities of the heavy molecule; and v=0 and V the before and after velocities of the light molecule then

$Mu = MU + mV \Rightarrow M\left( {u - U} \right) = mV....................1$                                        momentum balance

$Mu{}^2 = M{U^2} + m{V^2} \Rightarrow M\left( {{u^2} - {U^2}} \right) = m{V^2}..........2$           Energy balance

Divide equation  2 by 1

V = (u + U)

So the lighter molecule is greatly accelerated and the heavier one slightly decelerated.

That is the kinetic processes of diffusion tend to speed up the lighter moelcules and slow down the heavier ones.

Remember also that in the atmousphere there are very significant disturbing functions.
So significant that our very climate depends upon really massive amounts of energy being transported from the tropics to the polar regions, after first being high into the air by rotational forces due to the Earths rotation. This energy effect was not realised until the mid 20th century.

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1 hour ago, studiot said:

Do the maths.

The diffusion equation describes the response of the particles to a forcing function.

In this case gravity.

It also requires the particles to be (1) non interacting and in the versions referred to to be  (2) identical, and (3) have no significant disturbing function.

What happens when a particle bumps into another, under condition 1 above, that is

a) 22 times a heavy  (hydrogen into carbon dioxide) ?

b) 1.5 times as heavy ? (nitrogen into carbon dioxide) ?

Well momentum and energy is preserved so if m is the mass of the lighter molecule and M the mass of the heavier; u and U the before and after velocities of the heavy molecule; and v=0 and V the before and after velocities of the light molecule then

Mu=MU+mVM(uU)=mV....................1                                         momentum balance

Mu2=MU2+mV2M(u2U2)=mV2..........2            Energy balance

Divide equation  2 by 1

V = (u + U)

So the lighter molecule is greatly accelerated and the heavier one slightly decelerated.

That is the kinetic processes of diffusion tend to speed up the lighter moelcules and slow down the heavier ones.

Remember also that in the atmousphere there are very significant disturbing functions.
So significant that our very climate depends upon really massive amounts of energy being transported from the tropics to the polar regions, after first being high into the air by rotational forces due to the Earths rotation. This energy effect was not realised until the mid 20th century.

Now it is you that is avoiding my question. What reference can you point to that says the atmosphere stratifies by gravitational separation of its constituent gases.

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15 hours ago, studiot said:

Equally in a tub of denser gas. with no disturbing flow activity the denser gas will remain in the tub.

Side note; maybe related to this discussion? I remembered about gas used in chemical warfare during WWI.

Quote

In addition, because mustard gas was heavier than air or water, it settled in ditches or at the bottom of trenches and puddles and created a persistent environmental hazard for troops, civilians, and animals alike.

Chemical Warfare and Medical Response During World War I, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2376985/ (bold by me)

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When I said CO2 would become poisonous, I meant that in high concentrations, we can't exchange enough out of our bodies, and blood becomes more and more acidic. This is the reason that we feel an urgent need to take a breath, after holding it for an extended period of time.

Even if there is plenty of oxygen in the air, if there is too much CO2, you still die.

The poison is the dose.

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1 hour ago, Ghideon said:

Side note; maybe related to this discussion? I remembered about gas used in chemical warfare during WWI.

Chemical Warfare and Medical Response During World War I, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2376985/ (bold by me)

Tank you for that useful addition. +1

It is also what made these chemical agents so dangerous.

Mustard gas has a molecular weight of 159 and phosphine 34. Compare with carbon dioxide at 44.

51 minutes ago, wayne_m said:

When I said CO2 would become poisonous, I meant that in high concentrations, we can't exchange enough out of our bodies, and blood becomes more and more acidic. This is the reason that we feel an urgent need to take a breath, after holding it for an extended period of time.

Even if there is plenty of oxygen in the air, if there is too much CO2, you still die.

The poison is the dose.

That is my definition of an inhibitor, not a poison.

But nevermind it will kill you all the same.

1 hour ago, exchemist said:

Now it is you that is avoiding my question. What reference can you point to that says the atmosphere stratifies by gravitational separation of its constituent gases.

Well I thought I had provided a pretty comprehensive answer, with several different parts to it.

1) A discussion as to the meaning of the words you guys used, notably stratification and a stement to the effect that there will never be a series of layers, each composed of one gas only, lying one above the other,

2) A discussion about what actually does occur, notably that a concentration gradient develops for all gasses, but the spatial distribution of that gradient varies with the molecular weight of the gas.

3) A discussion of the diffusion equation and the requirements for applying it in a gravitational field.

4) A mathematical outline of the kinetic theory mechanism for this.

In what way was that insufficient ?

You have still to answer the question why do lighter gases such as hydrogen and methane go straight up when released and collect in and under roofspaces ?  Since you now have the maths, why do you need some nerd reference ? Why can you not check it for yourself ?

Why do  the building regulations require roof venting for this collecting gas and why does it continue straight up towards the stratosphere when it exits the vents ?

Edited by studiot
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3 hours ago, studiot said:

Tank you for that useful addition. +1

It is also what made these chemical agents so dangerous.

Mustard gas has a molecular weight of 159 and phosphine 34. Compare with carbon dioxide at 44.

That is my definition of an inhibitor, not a poison.

But nevermind it will kill you all the same.

Well I thought I had provided a pretty comprehensive answer, with several different parts to it.

1) A discussion as to the meaning of the words you guys used, notably stratification and a stement to the effect that there will never be a series of layers, each composed of one gas only, lying one above the other,

2) A discussion about what actually does occur, notably that a concentration gradient develops for all gasses, but the spatial distribution of that gradient varies with the molecular weight of the gas.

3) A discussion of the diffusion equation and the requirements for applying it in a gravitational field.

4) A mathematical outline of the kinetic theory mechanism for this.

In what way was that insufficient ?

You have still to answer the question why do lighter gases such as hydrogen and methane go straight up when released and collect in and under roofspaces ?  Since you now have the maths, why do you need some nerd reference ? Why can you not check it for yourself ?

Why do  the building regulations require roof venting for this collecting gas and why does it continue straight up towards the stratosphere when it exits the vents ?

I've already answered this. If the gas is released as a stream (i.e. unmixed with air), it will do what you say until it diffuses into the surrounding air. Once it has done that it will not do what you say. This was also true of poison gas in WW1. After a while it dissipates. But this will be relatively slow for gases with high molecular weight, due to lower molecular velocity. There is no way that mixed gases will separate appreciably under the influence of gravity. You can do that sort of thing - very partially - in a gas centrifuge, but that involves very high multiples of g.

You have yet to offer any reference supporting your contention that heavy and light components in the atmosphere will spontaneously separate , at least partially, under the influence of gravity.

Edited by exchemist
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52 minutes ago, exchemist said:

I've already answered this. If the gas is released as a stream (i.e. unmixed with air), it will do what you say until it diffuses into the surrounding air. Once it has done that it will not do what you say.

Why go neither the HSE, the Soc of Environmental Health officers or the Association of Building ontrol Officers, nor I believe your bland statements, always qualified by non realistic additons.

How would the gas be released as a stream ?

It is randomly generated in small quantities underground and slowly percolates upwards through the ground. (methane and hydrogen will even get though a 300 deep concrete slab) there is moves quite rapidly upwards through the building air until it concentrates under the roof. No streams of gas are generally involved or the site cleanup has not been properly carried out.

Concentrates is the reverse action from what you are describing.

The gas remains getting more and more concentrated, until it either the supply stops or the gas ignites or it is let out. There is a giant ASDA in one town in Somerset that was built on an old rubbish tip and the disposal method was contolled occasional flare burning. Other methods have been used since on further retail development on the same tip.

Eventually doors or ventilators will open causing natural air currents or there may be forced ventilation. Either way if the supply stops the gas will eventually dissipate by these processes, just as you say. But that could take a very very long time.

The trenches were subject to considerable disturbing agents called winds. It was well known that the gas would 'hang around' much longer on windless days.

Ken's thesis, and you appear to support it, is as far as I can tell, that there diffusion reduces the concentration gradient to zero.

It is a requirement of the diffusion equation that no diffusion occurs unless there is a forcing function.
The pre existence of a concentration gradient constitutes such a forcing function, but slowly subsides to nothing as the concentration gradient is reduced.
So Ken is right in that if the cap is suddenly taken off a concentration gradient and nothing else is acting then the concentration will even out to nothing and then diffusion will cease.

But that is not the case here as there are always other forces acting continuously and intermittently.

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Consider dG = dH - TdS

For a column of atmosphere at uniform density & pressure under a gravitational field, a downwards vertical flow is favoured (supporting the argument of @studiot) since the release of gravitational energy increases total enthalpy sufficiently to counter the reduction in entropy due to reduced occupancy of the higher levels of the column.

So we have established an equilibrium condition with a vertical density/pressure and entropy gradients much as the atmosphere we see around us.

But for further gravitational settling of, say, CO2 to take place, the gravitational potential energy released is now countered not only by the entropy gradient, but also the necessary displacement of an equal volume of lower density gases previously below it generating an adverse temperature gradient and expansion of the lower levels due to both the temperature gradient and the reduced mass of the upper part of the column.

In short, while dH is likely not zero for a perfectly uniform gas mixture (constant mole fractions) it becomes so small that it can support only a tiny mole fraction gradient. I therefore suspect that while @exchemist and @Ken Fabian are not quite 100% accurate in their assertions, in practical terms they are very close to measurable reality. It's certainly an approximation I used throughout my working career without a qualm.

The 'phosgene' counter argument simply reflects the very low rate of diffusion of high molecular weight gases. The thermodynamic equilibrium remains an (approximately) evenly dispersed mixture. It's just that these cases take their time about reaching equilibrium.

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39 minutes ago, studiot said:

Why go neither the HSE, the Soc of Environmental Health officers or the Association of Building ontrol Officers, nor I believe your bland statements, always qualified by non realistic additons.

How would the gas be released as a stream ?

It is randomly generated in small quantities underground and slowly percolates upwards through the ground. (methane and hydrogen will even get though a 300 deep concrete slab) there is moves quite rapidly upwards through the building air until it concentrates under the roof. No streams of gas are generally involved or the site cleanup has not been properly carried out.

Concentrates is the reverse action from what you are describing.

The gas remains getting more and more concentrated, until it either the supply stops or the gas ignites or it is let out. There is a giant ASDA in one town in Somerset that was built on an old rubbish tip and the disposal method was contolled occasional flare burning. Other methods have been used since on further retail development on the same tip.

Eventually doors or ventilators will open causing natural air currents or there may be forced ventilation. Either way if the supply stops the gas will eventually dissipate by these processes, just as you say. But that could take a very very long time.

The trenches were subject to considerable disturbing agents called winds. It was well known that the gas would 'hang around' much longer on windless days.

Ken's thesis, and you appear to support it, is as far as I can tell, that there diffusion reduces the concentration gradient to zero.

It is a requirement of the diffusion equation that no diffusion occurs unless there is a forcing function.
The pre existence of a concentration gradient constitutes such a forcing function, but slowly subsides to nothing as the concentration gradient is reduced.
So Ken is right in that if the cap is suddenly taken off a concentration gradient and nothing else is acting then the concentration will even out to nothing and then diffusion will cease.

But that is not the case here as there are always other forces acting continuously and intermittently.

Well I am open to being corrected on the basis of solid evidence if you have any. If you have had to deal with methane accumulating in buildings you should find it easy to point to a standard, building regulation or paper that says methane or hydrogen will become more concentrated, i.e. with molecules moving against the concentration gradient due to difference in molecular weight.

On the face of it, though, this seems ridiculous to me, though I do acknowledge that at very high artificial g forces, in a gas centrifuge, you can see a bit of partial separation due to this. This scenario will be where @sethoflagos's thermodynamic analysis  has a practical application.

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