14
$\begingroup$

Why are some gases Greenhouse gases while some are not?

I did search this on the net but didn't get any clearcut/credible answers.

What exactly is the property that is common among Ozone, Water Vapour, CFC and Methane?

$\endgroup$
21
$\begingroup$

A molecular gas is a good infrared absorber if it has several atoms (not just 2, like O2 and N2) or if it is hetero-nuclear (e.g. CO and NO). These type of molecular arrangements allow more infrared energy to be absorbed because there are more vibrational states that are possible. Yes, ammonia fits that description, but it is not long-lived in the atmosphere and it is not widespread. So, even though ammonia is a good infrared absorber, it is not an important greenhouse gas.

Ammonia in the atmosphere is very reactive, and will form to make nitrates and sulfates while in solution (e.g. in cloud droplets). Thus, the atmospheric lifetime of ammonia is short-lived, whereas most important greenhouse gases are long-lived and not reactive. For global warming discussion, you would likely associate ammonia with a net cooling effect since the increased formation of particulate (clouds/haze) increases the albedo of the Earth (which scatters light back to space).

$\endgroup$
  • $\begingroup$ I am not sure of my Chemistry. But why is ammonia in the atmosphere more reactive than carbon di oxide ? Is it due to the structure of the two molecules ? $\endgroup$ – gansub Nov 1 at 9:16
  • $\begingroup$ dgrat is somewhat crude, but correct. Technically, ammonia might be a greenhouse gas, but it is just broken down far to easily to ever be a problem. $\endgroup$ – Gloweye Nov 1 at 14:14
  • 4
    $\begingroup$ This answer is partially correct, but diatomic molecules also absorb infrared light, if they are hetronuclear (for example carbon monoxide CO, nitric oxide NO, hydrogen chloride HCl, etc.). If the electric dipole moment can change due to vibration, the molecule can absorb. Only monoatomics and homonuclear diatomics can be excluded. $\endgroup$ – DavePhD Nov 1 at 15:15
  • 3
    $\begingroup$ @gansub In short, yes. Ammonia is a fairly reactive molecule, while CO2 is not so reactive. The reason why one molecule is more reactive than another molecule is a question, a bit long for the comments here, that could likely find a home on the Chemistry Stack Exchange site $\endgroup$ – fyrepenguin Nov 1 at 15:26
  • 2
    $\begingroup$ @Sristy yes, all triatomics and higher can stretch or bend in a way that changes the electric dipole moment, and all hetronuclear diatomics can also change electric dipole moment by stretching. $\endgroup$ – DavePhD Nov 2 at 16:09
6
$\begingroup$

The common factor among Greenhouse gases is that they absorb and scatter infra-red light.

The Greenhouse effect is caused when energy coming in from the sun is prevented from escaping again. The Sun emits light primarily in the visible spectrum, with some UV and infra-red, most of which is absorbed by the Earth. This causes the Earth to heat up and in turn release it's own black body radiation, which at the temperature of the Earths surface is primarily infra-red. This outgoing radiation is the only way the Earth can cool itself, and the stable temperature at the surface is that at which the outgoing energy of Earths black body radiation exactly matches the incoming energy from the sun which is absorbed (plus a relatively very small amount released from the Earths core).

A Greenhouse gas is one which allows less of this infra-red light back out into space, causing it to be reabsorbed and raising the temperature of the planet. Similarly, any gas which primarily blocks visible or UV light has a cooling effect, since it blocks more incoming energy from the sun than outgoing energy from Earth.

Here is a paper with a very nice diagram showing off the absorption spectrum of several gasses. It's worth noting that water vapor is an even stronger Greenhouse gas than Carbon Dioxide, but since its concentration in the atmosphere is largely affected by temperature it is less concerning as a waste product we produce than it is as a positive feedback mechanism.

Greenhouse gases will have an effect as long as they exist in the atmosphere. Methane is a famous example of a greenhouse gas with a warming potential of about 30 times CO2 over 100 years, but over 20 years has a warming potential 86 times higher than CO2. The large difference over the long time period is because CO2 is very stable, but Methane has a lifetime in the atmosphere of only 12 years.

Ammonia has a lifetime in the atmosphere of only one week. The primary reason for this is the highly reactive nature of Nitrogen-Hydrogen bonds. These bonds tend to be quite weak, and are commonly found in explosives because of how quickly they decompose. I was mistaken, the primary reason is that Ammonia is highly water soluble so quickly comes down in rain, and is also readily absorbed by plants. N-H bonds are not especially weak, but Nitrogen tends to prefer to be in its extremely stable N2 state. The difference in strength between the bonds in these forms is what makes Nitrogen compounds so explosive.

Because Ammonia is so short lived in the atmosphere, it has an effective global warming potential of zero, despite absorbing the right frequencies of infra-red radiation to otherwise be a greenhouse gas.

EDIT: I should also say since you mention CFCs and Ozone in your question: CFCs are not concerning as a Greenhouse gas, they erode the Ozone layer. The Ozone layer itself has little to do with the Greenhouse effect, but instead is involved in blocking UV light which is damaging to living things on the planet (for example, causes skin cancer). Since Ozone blocks UV, it is a gas which cools rather than warms the Earth, though I am not certain how strong the effect is. I imagine not very since I've never seen it mentioned.

$\endgroup$
  • $\begingroup$ Please edit your answer to address the specific question whereas Ammonia is not? $\endgroup$ – Jan Doggen Nov 1 at 9:28
  • $\begingroup$ @Turksarama are you saying the N-H bond is weaker than the C-O bond ? Please provide bond energy estimates as well to support your answer $\endgroup$ – gansub Nov 1 at 11:55
  • 1
    $\begingroup$ Wow did I make a few mistakes in this post. I might have learned more here than @Sristy. I didn't realize how much chemistry I've forgotten. $\endgroup$ – Turksarama Nov 1 at 12:20
  • 1
    $\begingroup$ CFCs are indeed strong greenhouses gases. CFC-11 and CFC-12 have Global Warming Potentials of approximately 6000 and 10000 respectively on a 20 year time horizon. The ozone layer and the greenhouse gas effect are different phenomena, but they do affect each other. For instance, ozone is also a greenhouse gas. $\endgroup$ – jkej Nov 1 at 15:26
  • $\begingroup$ CFCs are important greenhouse gases. Not that important as some tried to suggest (tried removing the blame from CO2) but the the fraction they make is still important for the whole inventory. $\endgroup$ – Vladimir F Nov 2 at 9:14
4
$\begingroup$

A gas is generally considered a greenhouse gas (GHG) if it meets both the following conditions:

  1. GHG molecules absorb and emit electromagnetic radiation in the infrared (IR) range. GHG molecules can do this because they have vibrational states that can store energy when absorbing a photon with an IR energy level (~1-1600 meV), especially those with the subset of IR energy levels (longwave or thermal infrared) most often encountered on Earth (~80-160 meV).
  2. GHG molecules are found in sufficiently large amounts in the Earth's atmosphere to produce significant radiative forcing (which is the preferred way of saying "greenhouse effect"). This can be either because there are large sources of the gas, or the gas is sufficiently long-lived to accumulate in the atmosphere, or both.

Like any classification, this is somewhat arbitrary: any two scientists may disagree on what counts as sufficient absorbing or accumulating for any given atmospheric species. But in practice, the earth-science community tends to agree.

Ammonia (NH3) molecules absorb infrared well, so they meet condition 1, but they are also very reactive, particularly in aqueous solution (e.g., in cloud droplets). Hence they don't survive long enough to accumulate, and thus fail condition 2.

By contrast, ozone (O3), water vapor (H2O), methane (CH4), and CFCs (which are a group of species) are all generally regarded as meeting both these conditions.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.