Maybe a simple question, but if sulfur hexafluoride is 6 times heavier than air then how does it get into the high atmosphere. All be it in low quantities now, but there is a large stock of the stuff in potentially vulnerable and prevalent infrastructure such as substations of the electricity grid. If there was a much larger leak of the chemical than is now observed would we expect it to reach the upper atmosphere to become a dangerous greenhouse gas at that position in the atmosphere... as i understand potent ghg's induce greater warming when high in the atmosphere. Anyone know more about this? its mode of transport there, what activities are getting it up there? Ive done some research on existing and past growing concentrations but wonder if its Global warming potential is dangerous unless it can actually accumulate significantly high in the atmosphere?
1 Answer
Carbon dioxide is heavier than air but is found in nearly equal proportions throughout the well-mixed portion of the atmosphere. Xenon is much heavier than air, almost as heavy as sulfur hexaflouride, but it too is found in nearly equal proportions throughout the well-mixed portion of the atmosphere.
Turbulence is what makes the well-mixed portion of the atmosphere be well-mixed. It is only in the uppermost layers of the atmosphere, where the air is very thin, that turbulence fails to keep things well-mixed. The turbopause, which marks the border between the well-mixed lower portions of the atmosphere and heterogenous upper portions of the atmosphere, is about 100 km above the surface. The stratosphere is only 10 to 50 km above the surface, so it is well within the well-mixed portion of the atmosphere.
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$\begingroup$ (So if I read ideatank's question right, he suggests there is data to show some $SF_6$ in the upper atmosphere [if not, perhaps the question should be altered])... are you suggesting the process that got it there is that it mixes up to the turbopause, and small amounts get above the mixed realm and settle just above that? $\endgroup$ Feb 27, 2021 at 7:50
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$\begingroup$ I should have stated just atmosphere instead of atmosphere, i really just meant off the ground as i thought SF6 would constantly fall to the ground and stay there. But the data does indeed state that it is measurements of the troposphere so im assuming the SF6 is at altitude. Many thanks $\endgroup$ Feb 27, 2021 at 10:23
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1$\begingroup$ @ideatank17 There are chlorofluorocarbons (CFCs) that have molar masses similar to that of SF6, and those CFCs that contain bromine can have an even greater molar mass. All CFCs have a molar mass that is greater than the mean molar mass of the atmosphere. Despite these high molar masses, those CFCs manage to make their way to the stratosphere. One way to look at it: gases have a near infinite potential to dissolve other gases. $\endgroup$ Feb 27, 2021 at 16:02
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$\begingroup$ Excellent point, thankyou. Id been doing a fair bit of research on SF6 but wasn't sure how it contributes to the greenhouse effect if it was so heavy and whether or how it could get into the high atmosphere. It falls into the same category as other chemicals with high GWPs such that they are currently leaking from existing infrastructure. It is a great concern since such infrastructure like electric substations are at risk from damage from extreme weather events and sea level rise that i wonder how high on the agenda we should place this risk in order to consider installing alternatives. Thnx $\endgroup$ Feb 28, 2021 at 19:02