How can sulphur hexaflouride both be a greenhouse gas and heavier than air?
December 12, 2010 3:44 PM   Subscribe

How come sulphur hexaflouride is an extremely potent greenhouse gas when it's much heavier than air?

Wikipedia tells me that sulphur hexaflouride is one of the most potent greenhouse gases in existence. However, funny internet videos of people breathing it in and sounding like demons tell me that it's something like six times heavier than air.

But if it's so much heavier than air, doesn't it just stay on the ground when you release it into the atmosphere? I mean, I suppose it could get whipped up by wind and such, but it seems to me that in the long run, it would pretty much stay put and maybe even seep into some loose soil or something. Anyone care to clear this up for me?
posted by gkhan to Science & Nature (6 answers total)
Gases behave statistically; you will notice that the atmosphere is not stratified into layers of N2 O2 CO2 etc based on density either. Some of it spends time high up. Also, it can re-reflect ground radiation at modest height.
posted by a robot made out of meat at 3:52 PM on December 12, 2010

Best answer: The key figure is the scale height. This determines how the partial pressure of a gas drops off as height increases. It's inversely proportional to the mass of the molecule - SF6 has a molar mass of 146 g/mol, while air has one of about 29 g/mol, so the scale height will be about one fifth that of air. A rough scale height for air is about 8km, so for SF6 it's around 1.4km. This means that you have to reach a height of about a kilometre before the amount of SF6 has halved. So there will still be substantial amounts in the atmosphere (relative to the ground) a few miles up.
posted by Electric Dragon at 4:05 PM on December 12, 2010

Best answer: Sulfur hexafluoride is commonly used as a waveguide dielectric in aircraft that need such things (basically anything that emits high energy radio radiation - radars, high power satcom equipment, stuff like that). Aircraft - especially military aircraft that require said radars and antennas - carry the gas up to their operating altitudes where some of it is commonly lost due to leaks and whatnot.
posted by backseatpilot at 4:07 PM on December 12, 2010

What that Wikipedia article is talking about is how a given quantity of SF6 would trap far more heat than an equal quantity of CO2. That doesn't mean there is an equal quantity of it in the atmosphere, or that it makes a non-negligible contribution to global warming. Because it is heavy, it probably doesn't.
posted by Chocolate Pickle at 4:52 PM on December 12, 2010

Because the atmosphere isn't a mixture, it is a solution. (I'm almost sure all gases are soluble with each other, but I'm not sure about that.) So if you had a liquid dye that was heavier than water, you would drop it in and it would sink to the bottom. But over time, it would diffuse to more or less even coverage.

As Electric Dragon says, a differences in mass will cause concentration percentages to change versus altitude, but it's still a solution.

Another thought experiment would be a bunch of helium balloons that are almost exactly weightless in a gymnasium. Some of the balloons are more massive than each other, but still weightless. They would all bounce off of each other like crazy for a really long time until they found an equilibrium. The more massive ones might be closer to the bottom, but they will be dispersed pretty evenly because the chaos of the bouncing has a greater effect than gravity.

Or, think about a feather in the wind. It eventually lands because its weight is greater than what the wind is doing to it. But it stays aloft for a darn long time. Well, SF6 is way, way lighter than that, and it is always getting bounced into by the other molecules in the air. So much so that it pretty much never lands.
posted by gjc at 6:14 PM on December 12, 2010

Um, no, it isn't a solution. In a real solution, such as salt water, the water molecules chemically interact with the salt and cause it to disassociate.

Nothing like that takes place in air, to any significant degree. It is just a mixture.
posted by Chocolate Pickle at 12:54 AM on December 13, 2010

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