Oxides in Global Warming

[Scientist50]

I was wondering why it's the oxides (Nitrogen Oxide, Carbon Dioxide) that are the main causes of global warming? And how does that reason compare to the other gases in terms of not playing a part in global warming?

[John Cuthber]

There's a big effect from oxides because they are common. Also, a lot of other compounds which would be potent greenhouse gases, like methane, are degraded in the air by oxygen. That fate isn't possible for things like CO2.

However there are other compounds with much bigger effects

Sulphur hexafluoride is a notable example - something like 32000 times more effective than CO2

or nitrogen trifluoride with a potency 17000 times that of CO2.

[studiot]

It should be noted that hydrogen oxide has by far the greatest effect simply because its atmospheric concentration is so much higher than other greenhouse gasses.

 

[John Cuthber]

Water vapour isn't just common, but it has a broad absorption so it's less readily saturated too. However as the concentration of it is very variable and temperature dependent it can't be considered in the same way as the other gases.

 

You could also argue that ozone is oxygen oxide* and it's a strong absorber too.

 

* you could argue that, but I wouldn't recommend it.

[Essay]

I was wondering why it's the oxides (Nitrogen Oxide, Carbon Dioxide) that are the main causes of global warming? And how does that reason compare to the other gases in terms of not playing a part in global warming?

 

And, in addition to the great replies above, we should also note:

 

It's because those "oxides" are triatomic, instead of diatomic (as with O2 and N2), that they are better greenhouse gases. How a molecule can vibrate, determines which frequencies of energy it will absorb. The diatomic molecules can't stretch and bend and rotate in a way that "harmonizes" with the IR wavelengths, so they don't absorb IR well. Triatomics, such as O=C=O (CO2) and H-O-H (water), have a shape that lends itself to the asymmetries required for the particular vibrations that absorb certain IR frequencies.

Methane and SF6 also can vibrate in ways that will strongly absorb IR frequencies. I'm not sure how long these molecules will store that energy (vibrate) until they re-radiate the energy, but I recall hearing that CO2 re-emits its IR energy within ten minutes usually--and randomly, in any direction, of course.

If you read up on how to interpret an IR spectrogram, and see which wavelengths are associated with the particular vibrations of certain bonds, then you can predict (in theory) which molecules will be good greenhouse gases.

~

[John Cuthber]

Also, water, as a non- linear triatomic, has more IR active vibrational modes than CO2 which is linear and symetrical.

[Essay]

Also, water, as a non- linear triatomic, has more IR active vibrational modes than CO2 which is linear and symetrical.

 

On average, CO2 is linear; but it will vibrate through non-linear and asymmetrical shapes. I'd expect the double bonds are what make the absorption peak(s) much sharper or narrower, while single bonds absorb over a broader range of wavelengths as their bonds don't resist deformation as much.

[John Cuthber]

The sharpness of the IR absorption bands depends on the rotational levels within the vibration bands, not on whether they are singe or double bonds.

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[Scientist50]

Thanks a lot guys (and not in the sarcastic sense!), especially Essay, you might've just won me a prize in the Year 7 Science Fair Division!