# Why does nitrous oxide have 300 times the global warming potential of CO2?

Both nitrous oxide and carbon dioxide have roughly the same atmospheric lifetime (nitrous oxide slightly longer), so I thought that it would have to do with the infrared absorbance associated with each of them. So I looked at their spectrum.

Perhaps this is misleading because it is my understanding that not much infrared light at around the 2300 $\pu{cm^{-1}}$ region exists. But even so, that means that the two peaks that matter are $\ce{CO2}$'s 700 peak, and $\ce{N2O}$'s 1300 peak. It is my understanding that the blackbody radiation of earth peaks at around 700, and though sizable, is much less at 1300. So, I don't see why nitrous oxide would have a greater global warming potential than carbon dioxide.

• On a molecule to molecule basis besides any basic transition intensity effect perhaps it is due to the fact that $\ce{CO2}$ belongs to the $D_{\infty h}$ point group and has zero nuclear spin which means that odd J rotational levels are absent. – porphyrin Apr 20 '17 at 7:07
• I'm not sure whether global warming potential takes this into account, but $\ce{N2O}$ may oxidize/decay into other compounds which have far greater GWP, so "by proxy" $\ce{N2O}$ could end up having a significant impact. – Nicolau Saker Neto Apr 20 '17 at 9:21
• From Wikipedia "Even if a gas absorbs radiation efficiently at a certain wavelength, this may not affect its GWP (Global Warming Potential) much if the atmosphere already absorbs most radiation at that wavelength. A gas has the most effect if it absorbs in a "window" of wavelengths where the atmosphere is fairly transparent." $\ce{N2O}$ happens to lie within one of these "windows" where nothing else is absorbing much. – airhuff Apr 20 '17 at 17:20
• @NicolauSakerNeto , methane is a good example of what you are talking about. It is a strong greenhouse gas with a short atmospheric lifetime (~12yr) whose primary sink is oxidation to other greenhouse gases, $\ce{CO2}$ and water. – airhuff Apr 20 '17 at 17:25
• @NicolauSakerNeto It is a good point to raise, but not applicable to the GWP of N2O as a discrete molecule. As said, it is important to raise, as N2O is involved in ozone depletion. It's not as effective at O3 depletion as CFCs, but the increasing concentrations of N2O (coupled with decreases in CFCs) make it a major player. – R Ramsay Jun 12 '18 at 15:04

There are two really important properties of N2O that make it such an important greenhouse gas, both of which you have raised in your question but need some clarification.

1. Infrared absorption - it's good to think in terms of gaps here. There's a lot of overlap between CO2, CH4 and (a frequently ignored GHG, but very important) water vapour in the wavelengths which they absorb. At the 4-5 micron and 7-8 micron range, N2O is extremely efficient at absorbing infrared radiation. These are important regions, precisely because there is no overlap with other GHGs in these regions. N2O is absorbing infrared very efficiently, and is doing so without "competition".

2. Lifetime - you noted that the lifetime of N2O and CO2 are comparable, which is sort of true. N2O, as a stable, inert molecule that is well mixed in the atmosphere, has a very long lifetime of 120 years compared to some other GHGs (CH4, for example is 8 years, based on the fact that its removed by the hydroxyl OH radical). It's possible to determine this fairly accurately as the only sinks for N2O are its stratospheric photo-dissociation and reaction with O(1D) radical. CO2 by comparison has a lifetime that varies anywhere between 5 and 200 years. The comparable part comes from the fact that for calculations for global warming potential, CO2 is given an "averaged" lifetime.

With N2O, therefore, we have a very efficient infrared absorber at two "windows" at which there is no overlap with other GHGs, and a long lifetime. The Global Warming Potential is the time integrated radiative forcing for a 1 kg pulse emission of the compound, where the upper limit of the integration is time horizon (usually set at 100 yr), relative to the same quantity of the reference compound CO2. It becomes clear from this definition why a stable, inert molecule with a long lifetime and which is very efficient at absorbing at particular infrared wavelengths would have a greater global warming potential than CO2 (shorter lifetime, absorbing in "windows" shared by other compounds) on a kg to kg comparison.