Will Josh Silverman be awarded the Service to America Medal for trivial greenhouse gas emissions prevention work?
Department of Energy employee Silverman has been nominated for a high civil service award for reducing sulfur hexafluoride leaks, allegedly worth 1.1 million tons of CO2 emissions, reports the New York Times.
But Princeton physicist Will Happer points out in an e-mail to JunkScience.com (hopefully no error made in translation!):
Here is a little information on the infrared absorption of SF6 (sulfur hexafluride). It has a strong mode centered at about 950 cm-1, which is in the currently open part of the IR window. I would guess that the SF6 mode is the analog of the 667 cm-1 bending mode of CO2. Slogans like “23,900 times more potent than CO2” are designed to confuse the gullible public. The slogan means absolutely nothing for global warming.
If there were no saturation of the CO2 band, the absorption cross sections of SF6 and CO2 would be quite close to each other. The electric dipole moments induced by bending will have similar values in both molecules. So the real (not effective) peak cross section for SF6 has to be within a factor of 10 of the peak cross section of CO2, that is, the cross section can be no larger than sig = 10-17 cm2 for the strong Q branch at the center of both bands.
To put into context the 35,000 lbs of leakage eliminated by the worthy savior of the planet, note that SF6 has 146.07 grams/mole. At 453.5 grams/pound this is M = 1.09 x 105 moles. Since the effective radius of the earth is r = 6371 km the surface area is A= 4*pi*r2 = 5.10 x 1018 cm2 so we have M/A= 2.13 x 10-14 mole/cm2, or with 6.02 x 1023 molecules/mole, a number density of SF6 of N = 1.28 x 1010 molecules/cm2. The probability that a photon at the peak of the SO2 band is absorbed on its path from the earth’s surface to space is sig*N =1.28 x 10-7. This absurdly small contribution to greenhouse warming comes about because the atmosphere contains 1.82 x 1020 moles of molecules, mostly nitrogen and oxygen, so the 35,000 lbs of SF6 amounts to a molar fraction of 5.96 x 10-15. Since the CO2 fraction at 400 ppm is 4 x 10-4, the 35,000 lbs of SF6 amounts to about 1.49 x 10-11 SF6 molecules for every CO2 molecule – 15 parts per trillion! So even if the effective cross section for SF6 is one hundred thousand times larger than that for CO2, it means absolutely nothing when we are talking about controlling a few tens of thousands of pounds of SF6 -– or even many orders of magnitude more.