Thursday, February 04, 2010

Passing Gas

About a week ago, an argument broke out between Richard Steckis and Gavin Schmidt about atmospheric oxidation of methane (Start with #comment 12 and search for Steckis). Steckis seems to think that "CO2 is NOT an oxidative byproduct of the atmospheric oxidation of methane" and he has his doubts about water vapor. Gavin loses the thread and wanders off into very detailed and not very to the point stuff. Suffice it to say, much confusion on both sides so Eli thought it would be a good thing to hold a brief review before the test

In looking at complex reaction mechanisms such as the one for methane, diagrams like the one above are great crutches, Eli mostly redrew this one from Ravishankara's 1988 review of methane oxidation which appeared in the Annual Reviews of Physical Chemistry. It's a bit dated but still useful, and anyone interested in a more up to date evaluation of the individual reactions should look at the IUPAC atmospheric chemistry evaluation (open source).

UPDATE: In the comments, Andrew Dessler dates himself by pointing to one of his first papers that summarizes the methane oxidation mechanism in discussing the hydrogen budget (read H2O, H2 and CH4) of the lower stratosphere. It is good and short, but may presume more of the reader than Eli did. As Ravi points out in 1988, the mechanism was pretty well understood by 1971, but the finer details of the kinetics and understanding of the distributions of the various reactants are still being worked on. Today, the emphasis in atmospheric oxidation schemes has shifted to biogenic compounds emitted by those killer trees. Those have complicated spaghetti reaction schemes with large blocks containing the words, "here occurs a miracle".
Bunnies can find methane in blue in the upper right hand corner. The first step is reaction with the hydroxyl radical, HO (in the troposphere) and O(1D) or Cl in the stratosphere (box with dashed lines). O(1D) or singlet oxygen is a low lying excited and reactive electronic state of oxygen which is produced in the photo dissociation of ozone below ~ 306 nm.

The HO, O(1D) or Cl, or Br from photodissociation of halons, reacts with methane, CH4, to produce methyl radicals, CH3. The reaction with HO produces water vapor

CH4 + HO ---> CH3 + H2O

thus, at least one water vapor molecule is immediately produced when methane oxidizes.

Free radicals are molecules with an unpaired electron that react rapidly, cause lonely electrons really want to find a date, and methyl radicals do that, combining quickly with oxygen molecules, O2, to form methyl peroxy radical, CH3O2.

The first step is "rate limiting". It is the slowest step, requiring, on average years. The rate of this reaction = k(T) [CH4][OH]. The square brackets are phys chem speak for the concentration of. The "k(T)" is a rate constant, and is a function of temperature. At 300 K the rate is about 10-14 cm3 s-1. At 1000K, it is two orders of magnitude higher, which is why methane burns faster in a flame than it oxidizes in the atmosphere. The reaction scheme is about the same, and every other step is much faster, the first reaction is still the slowest step, and as we said, it determines the overall speed of the reaction.

The fate of the CH3O2 depends on where in the atmosphere it forms. If it forms in a region where there is lots of NOx (the sum of NO and NO2) then the reactions shown by the red lines become a major part of the scheme. In that case CH3O2 reacts with NO2 to form an adduct CH3O2NO2 . The adduct "stores" CH3O2, but the bond between the CH3O2 and the NO2 moities (a convenient way of designating the molecular units) is weak, and collisions can break it, thus the tight loop.

Lets print the diagram again, so the bunnies don't have to thumb back and forth

In a clean (you ain't running your damn cars, electricity is all generated by wind mills, nuclear power plants and solar units and nature ain't doing its part either) NOx free atmosphere, which is not very many places the CH3O2 can only react with HO2 to form CH3OOH, which Steckis insisted only rained out. Au contraire, that is only one fate. It can react with OH to form formaldehyde, or return to CH3O2 in very roughly equal yields, or it can photodissociate to produce methoxy, CH3O. The reaction with HO yields a molecule of water again

CH3OOH + HO ---> CH3O2 + H2O

Let's return to CH3O2, the peroxide. In a clean atmosphere it can react with itself to form the methoxy, but this is really slow, compared to the channel involving reaction with HO2 which has a much higher concentration.

Reaction rates are proportional to the concentrations of the reactants and a constant, called the rate constant, so if the rate constants are roughly the same, and HO2 is present, as it is in the atmosphere, in a ten times or more higher concentration, the reaction involving HO2 will go ten times faster. The reason that HO is found at lower concentrations is that it is much more reactive that HO2. Among other things it reacts with any hydrocarbon, acting as Mother Nature's atmospheric vacuum cleaner. Eli guesses that it would be good to have a post on the HO2-HO cycle (called the HOx cycle)

Where NO is available, the CH3O2 reacts with it to form methoxy (CH3O). The methoxy reacts with oxygen molecules

CH3O + O2 --> CH2O + HO2

which regenerates HO2. Follow the arrows, and you see if NO is present, the HO2 reacts with the NO to yield HO which reacts with CH4 and around we go again.

CH2O is formaldehyde. Just about every hydrocarbon passed through CH2O at the end of its oxidation mechanism either in the atmosphere or in combustion. There are two photodissociation channels, one to H + HCO, the other to H2 +CO. The details of the second one have created a cottage industry for theoretical and experimental chemical dynamicists for reasons Rabett Run may discuss in the future (Hmm, this may lead to a lot of posts). The reaction with HO is fast. The balance between the three channels depends on wavelengths of light available, but suffice it to say that all three play a role. CO produced in the second channel oxidizes to CO2. The HCO reacts with oxygen molecules to form HO2 and CO, which then oxidizes to CO2. CO2 is the major carbon containing product of methane oxidation in the atmosphere. Not to rub it in, but Richard Steckis was looking at only part of the mechanism.

Have at it.


J. Zimmermann said...

Nicely explained. Just one detail, when already the difference is made between air containing NOx and clean air. With NO below...say...20 ppt HO2-recycling to HO trough O3 + HO2 -> 2 O2 + HO dominates.

EliRabett said...

True, but you gotta have ozone for that and the principle source of ozone in the troposphere is NOx. . .

It gets tangled.

Andrew Dessler said...

At the risk of being accused of Pielke-ing, I'm going to refer to one my first papers. This has (if I do say so myself) a good (and short) discussion of oxidation of CH4 in the stratosphere.

EliRabett said...

At the risk of outing Eli as a charter member of the Pielke Fan Clubbe, Alas, Andrew cannot possible be Pielke-ing himself as the discussion is short.

Moved the link up into the post.

J. Zimmermann said...

True, you need Ozone. It is difficult to not have any even in remote areas in the troposphere ;-).

Marine atmosphere would come to my mind as an example, where ozone takes the part in recycling HO2 to HO. There the loss of CH3O2 almost exclusively goes through reaction with HO2 to methylperoxide. However, even if most of the oxidized methane ends up as methylperoxide in such conditions, methylperoxide will be oxidized in the water and will produce CO2 anyway.
People who run a global model might be able to tell, how big the chunk of methane is, which is rained out as methylperoxide.

David B. Benson said...

My eyes glazed over, but ok...

Gail Zawacki said...

This is all so far over my head, I apologize for even commenting. But for what it's worth, here is a story from Science Daily that also befuddled me. I link to it here because in both cases, I come away with the impression that well - as I put it on my blog, it: leads inexorably to one appalling conclusion: scientific experts - let alone average people - have absolutely no idea what is going on in the atmosphere. There are so many emissions reacting to each other and to radiation that it's anarchy in the skies.

Dr. Rabbet, I would so value your impression of the comments at this story, if you could find the time:

Gail Zawacki said...

oops sorry, I forgot to copy the link to the Science Daily story:

EliRabett said...

David and Gail, Eli had really hoped to make this simple enough that everyone could get something out of it. I understood that many would not understand the entire mechanism.

If I had to boil this down it would be that the slowest step, the reaction with hydroxyl radicals, HO, determines how fast methane is oxidized to CO2 and H2O.

The rest is detail, but interesting to folk who study chemical reactions. The details are important to you, because buried in there are the keys to air pollution modeling which is enabling us to limit damage from smog.

If you read the Science Daily story, the bottom lines is that aerosols can react with and destroy hydroxyl radicals (not a big surprise) This means that there is less HO to react with methane and other hydrocarbons, so methane will remain longer in the atmosphere. Since methane is a much stronger greenhouse gas than CO2, this will increase man made greenhouse warming.

Clearly we need to discuss this over a cup of coffee.

Gail Zawacki said...

Oh, I get it thanks for the clarification! So, in other words, we're FUCKED!

David B. Benson said...

EliRabett --- Thanks, I did learn a little from it, mostly just how much chemistry I never learned and what little I did is largely forgoteen. So to re-iterate the importance of the slowest step was certainly a help.

But I suggest a beer instead.

Hank Roberts said...

Wait, if I just watch the beer, it releases bubbles of CO2.

But if I drink the beer ....


A sobering thought.

How about grain alcohol instead?

Gail Zawacki said...

EliRabett said...

Natasha class.

Gail Zawacki said...

Ahem. I am not clear if this is a stealth attack, or if you actually know that I have been jealously wishful to be named Natasha my whole, entire life, because Gail (until my Italian friend translated it as "a wild tempest in the sea") as so fucking boring which along with much else I blamed on my mother.

anyway I have a really cute golden cocker spaniel - rather she has me - named Natasha.

David B. Benson said...

Hank Roberts --- You can watch while I drink it.

Does that help?

EliRabett said...

Gail, it's an inside joke started with a piece of spam in the comments somewhere that linked to

See Natasha Naked

Since the tread was....contentious some said, that was the highlight. That and the post on fearless leader. . .

Gail Zawacki said...

so what is the the joke????

EliRabett said...

Read the comments (you have to go a distance)

Anonymous said...

Can we plot this to the current atmosphere state?
(Current observed methane, water vapor, co2 etc.)

Is there a model?

Great post, thanks.


Brandon Shollenberger said...

Eli, thanks for this post. Some time back, I had looked for specific information about how methane breaks down in the atmosphere, and I didn't have a lot of luck. Unfortunately, I don't have access to science journals at the moment, so my main resource was Google. I did manage to find the formulas used for the picture in this post, but your post (and papers linked within it) helped me understand things better.

I do have two questions though. The first question is one I had before reading your post, and it is one I haven't found a simple answer for. Approximately what percent of methane in these processes breaks down into carbon dioxide? Since some of it is rained out, it seems you wouldn't get a 1:1 conversion from methane to carbon dioxide (or does it still become CO2 at a later point?).

The second question is one I hadn't considered until reading your post. You discuss how the rate of conversion is primarily limited by the first step, and it is dependent upon concentrations of methane and free radicals. If the concentration of methane increases, but the concentration of free radicals stays constant, will the rate of conversion decrease? If so, that would seem to suggest increasing methane levels might be worse than it would seem.

I'm sorry if these questions seem amateurish, but I don't know much about this field. Any help you could provide would be great!

Anonymous said...

So, what are the reaction rates along the arrows? Without these, it's hard to see how anyone can tell how long a species remains in any of the reservoirs. Residence time tends to be everything in these discussions. So, what's the atmospheric residence time of methane? Where do byproducts go thereafter? No doubt there could potentially be many byproducts, but, just like isotopes of short half lives and little prevalence, they may not matter.

EliRabett said...

Rates are at the IUPAC atmospheric chemistry reaction evaluation site which has been updated many times since. There is also a jpl evaluation run by NASA