Methane Decay in atmosphere 4

[Roger.Bryenton]

Hello Engineers, Help Needed on Methane Decay ... Thx
I am researching climate change, and with Covid and a few years, seems foggier that usual. However, from what I can determine, methane, CH4 has a half-life of about 7 years. That means it decays at about 10% per year. The concentration in the atmosphere is slowly increasing. If in year one, the rate of decline is 10%, then it would take about 10% to maintain the roughly constant concentration. Now, what about the CH4 that is there from the year before? It will decline about 9% in its second year.

If I sum the total of declines over, say 21 years, its down to 25% so the overall decline is 75%, correct?


Here's another description. In a dynamic atmospheric model of methane decay to CO2, methane's half life of 7 years means it decays into CO2 at 10%, declining, per year. Thus in order to maintain to roughly steady amount of 1920 parts per billion, in year 1 it will decline 10%. In year 2 it declines about another 9%, year 3 about 8% etc. This assumes it is not constantly refreshed, to maintain the constant concentration.

How much is required each year to stay constant? My mind is a bit foggy after several bouts with Covid, however the answer is germain to our planet's health. 10% new methane a year is a huge flow, and methane's warming effect during year 1 is about 120 times as bad as CO2.

THUS, the first year's decay at 10% is a huge amount. What about the dynamics of the continuing decay from the "2 year old" methane? Doesn't it also have to be replaced in order to maintain a constant methane concentration? Similarly with the 3 year old methane, etc? If it were a "stock" or fixed volume, it would decay to 50% over 7 years.

BUT it holds constant concentration, ie it is a "flow", dynamic, being constantly refreshed while simultaneously decaying at 10% per year. One part of the brain says it is a 10% "refreshment flow",annually while another part of the brain says "account for all the decay, constantly, which approximates, over 14 years, to be 75% decline, which would me an unimaginable massive flow of new methane annually.

What % of the methane concentration is needed to maintain a steady concentration, please? IF it's only 10% per year, what about the decay in year 2, 3 etc? How is that accounted for. Thank you VERY much. Roger

 

[dik]

Don't know if this helps...

"Methane has important implications for climate change, particularly in the near term.

Two key characteristics determine the impact of different greenhouse gases on the climate: the length of time they remain in the atmosphere and their ability to absorb energy. Methane has a much shorter atmospheric lifetime than CO2 (around 12 years compared with centuries for CO2), but it is a much more potent greenhouse gas, absorbing much more energy while it exists in the atmosphere.

There are various ways to combine these factors to estimate the effect on global warming; the most common is the global warming potential (GWP). This can be used to express a tonne of a greenhouse-gas emitted in CO2 equivalent terms, in order to provide a single measure of total greenhouse-gas emissions (in CO2-eq).

The Intergovernmental Panel on Climate Change (IPCC) has indicated a GWP for methane between 84-87 when considering its impact over a 20-year timeframe (GWP20) and between 28-36 when considering its impact over a 100-year timeframe (GWP100). This means that one tonne of methane can considered to be equivalent to 28 to 36 tonnes of CO2 if looking at its impact over 100 years.

In addition to its climate impacts, methane also affects air quality because it is an ingredient in the formation of ground level (tropospheric) ozone, a dangerous air pollutant."

https://www.iea.org/reports/methane-tracker-2021/methane-and-climate-change

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Roger.Bryenton]

Thx dik .... I am actually trying to determine what the flow is to replace CH4 decaying, in order to keep an almost constant concentration in the atmosphere. I believe it is 10% per year, which is a massive amount! By comparison CO2 takes only a small fraction of that due to it's long lifetime. Thank you ...
Any suggestions as to who else on the forum might be able to confirm my numbers, please? Thank you, Roger

 

[dik]

No I don't...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[shvet]

This is a chemical engineering forum. Chemical engineers communicate here, not environmental ones or, more appropriately, climate scientists. Why do you believe chemical engineers know how the atmosphere of this planet works?

 

[dik]

It is a chemical question that an environmental site may not be able to answer. "Chemical Engineering-Other Topics" seems reasonable.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[GBTorpenhow]

Half life of 7 years means about 9.5% decay per year. The amount of methane emitted per year to maintain constant concentration would be 9.5% of the current amount of methane in the atmosphere.

In year 2 it declines about another 9%, year 3 about 8% etc

No, the 9.5% per year rate doesn't change depending how long the methane has been in the atmosphere. 9.5% of any methane currently present, whether it was emitted this year or last year or ten years ago, will decay in a given year.

 

[shvet]

It is a chemical question

Seriously? Where in a chemical engineering handbook can I find a general info about a mechanism of methane degradation in the atmosphere of this planet? Perry's handbook?

Half life of 7 years means about 9.5% decay per year.

Gentlemen, why 9.5%? Not 0.95% or 95%? Why 9.5%? At which chemical handbook can I verify this data?

 

[GBTorpenhow]

The topic of exponential decay can be found in various grade school level math textbooks.

 

[shvet]

No-no-no. No math. No decay.
Methane degradation rate and mechanism - see first post.

 

[georgeverghese]

The chemical kinetics for methane decomposition in the atmosphere may be 1st order decay - check. Or it may even be some fractional order ? A given half life doesnt indicate what the underlying reaction order is.

If the planet's atmosphere can be compared to a well mixed stirred tank reactor, atmospheric concentration with continuous feed of fresh methane will follow a given time profile which is characteristic of CSTR operation, depending on the flow of fresh feed. I suspect temperature will have a significant effect on kinetics also (summer, winter, tropics or temperates etc). I'm rusty on this topic, but I think this is where answers are. If you dont hava a textbook on chemical engg reaction kinetics, this is also covered in Perry Chem Engg Handbook in the chapter on reactor kinetics.
The major assumption here is that of a well mixed reactor volume. In real life, local cells may predominate. Making this simplified assumption will however yield some answers, which is better than nothing.
Maybe some others reading this can help.

 

[TiCl4]

Shvet,

Seriously? Where in a chemical engineering handbook can I find a general info about a mechanism of methane degradation in the atmosphere of this planet? Perry's handbook?

Perry's Engineering Handbook (7th Edition), Section 7: Reaction Kinetics

I don't see how this ISN'T a chemical question. This is simply a reaction of CH4 + 2O2 -> CO2 + 2H20

This equation will have a particular rate constant, activation energy, etc, that are typical with chemical equations. Alternative reaction mechanisms may be present(reaction with atmospheric O3, for example), and should be factored into the overall "decay" rate. Chemical Engineering addresses reaction kinetics that deal with all of this.

This analysis is certainly in the wheelhouse of a chemical engineer. If you are saying that because the particular dataset for this reaction is not in the "chemical engineering handbook", then you are gatekeeping, pure and simple. Chemical engineers will obtain that dataset from the same source as climatologists. Neither "side" has a monopoly on performing reaction kinetic calcultions.

 

[Roger.Bryenton]

Thx GBTorpenhow ... I treated it as a "stock" to start with, allowing it to decay, without replacement, then cumulatively over 14 years it was down to 25%. (Dreaded Covid: lungs and brain, just "ain't the same"....). Then I got confused as I mixed the "stock" with a flow ...

I also got about 10%/ yr replacement so really appreciate you folks here.
Into the atmosphere, that's really a lot of methane, compared to CO2 which decays very slowly, that replacement is really high.
cheers and thanks to all .... you are a great service to the well-being of people, so don't get old, please. We need your wisdom and guidance.
cheers, Roger

 

[georgeverghese]

Wiki says methane decomposition in the atmosphere is by reaction with free OH- radicals, but doesnt say much more. So mechanistic reaction order is >1. Even then, in the presence of a large amount of OH- radicals, if this were to be the case, a second order reaction or any order >1 will approach that of order 1 (i.e. pseudo first order).

 

[hacksaw]

The density of CH4 os drastically smaller than CO2, and quickly moves into the upper atmosphere, where solar ionization is much more intense. As a consequence it is converted to CO2 and sinks closer to the earths surface.

 

[shvet]

@TiCl4, georgeverghese, hacksaw, dik and others
Aah, I have got it now. This is a joke, this is a way to troll newcomers to the forum. The Earth's atmosphere is a continuous ideal strirring tank, where 2 gases reacts by an oversimplified reaction of the 1st order that can be found in an engineering handbook.

@Roger
Let me join the party too.
You are able to obtain a comprehensive and reliable data of megascale natural processes just looking through a proper section in Perry's Handbook or similar and performing calculations. Any level that one might need - ammonia clouds formation on Titan or sulfuric acid rainfalls on Venus.

 

[pierreick]

Hi Shvet,
What about the impact of climate change on permafrost with the release of methane?
myth or reality?
I'm sure data are available in your country.
Pierre

 

[TiCl4]

Shvet,

Your sarcasm is noted. Try to stay professional. You are confusing chemistry with modeling. The chemistry can be determined experimentally to get relatively accurate rate constants, etc.

However, the application of that chemistry to real-life (like out atmosphere) presents, as you snarkily noted, numerous difficulties. I personally do not think atmospheric modeling can be detailed to such a degree as to accurately predict true levels of methane degradation. Additionally, the magnitude of Earth's volume presents sampling difficulties due to non-homogeneity. Thus, empirical models that rely on sampling data rather than first-principles are also likely prone to error.

I simply noted that the question of oxidation of CH4 to CO2 and H20 IS a chemistry question. The application of that chemistry to our atmosphere is a different story entirely.

Edit: Clarity of 2nd paragraph.

 

[dik]

I thought the original question was a real question that I did not have an answer for but was able to find a little info that might help.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[shvet]

@all
My idea is still clear. Chemical engineering has nothing to do with planet scale processes. The topicstarter has no chance to find here a clue except a case he/she has been tasked by a midschool teacher of chemistry. If so then why just not to ask a Copilot/ChatGPT to write a report instead of him/her?

I propose focusing on the topicstarter and his/her task. Nobody of us here, the forummembers of this CheEng forum, are able to help the topicstarter. If this is true then what are all of us discussing here other than my flawless sparkling wit?

PS
Rhetorical - Some of you have told this was a sarcasm. But was that a sarcasm or a sorrow actually?