Atmospheric sulfur hexafluoride: Sources, sinks and greenhouse warming

Abstract

Model calculations using estimated reaction rates of sulfur hexafluoride (SF6) with OH and O(1D) indicate that the atmospheric lifetime due to these processes may be very long (25,000 years). An upper limit for the UV cross section would suggest a photolysis lifetime much longer than 1000 years. The possibility of other removal mechanisms are discussed. The estimated lifetimes are consistent with other estimated values based on recent laboratory measurements. There appears to be no known natural source of SF6. An estimate of the current production rate of SF6 is about 5 kt/yr. Based on historical emission rates, we calculated a present‐day atmospheric concentrations for SF6 of about 2.5 parts per trillion by volume (pptv) and compared the results with available atmospheric measurements. It is difficult to estimate the atmospheric lifetime of SF6 based on mass balance of the emission rate and observed abundance. There are large uncertainties concerning what portion of the SF6 is released to the atmosphere. Even if the emission rate were precisely known, it would be difficult to distinguish among lifetimes longer than 100 years since the current abundance of SF6 is due to emission in the past three decades. More information on the measured trends over the past decade and observed vertical and latitudinal distributions of SF6 in the lower stratosphere will help to narrow the uncertainty in the lifetime. Based on laboratory‐measured IR absorption cross section for SF6, we showed that SF6 is about 3 times more effective as a greenhouse gas compared to CFC 11 on a per molecule basis. However, its effect on atmospheric warming will be minimal because of its very small concentration. We estimated the future concentration of SF6 at 2010 to be 8 and 10 pptv based on two projected emission scenarios. The corresponding equilibrium warming of 0.0035°C and 0.0043°C is to be compared with the estimated warming due to CO2 increase of about 0.8°C in the same period.

Quantifying transport into the lowermost stratosphere using simultaneous in-situ measurements of SF<sub>6</sub> and CO<sub>2</sub>

“…The NOAA-ESRL global flask network data shows that the mixing ratio of SF 6 in the troposphere has grown with a nearly constant rate of about 0.2 ppt year −1 since 1996. In the remote and free troposphere, the SF 6 distribution shows no significant variability, but a meridional gradient due to the larger electrical power production in the Northern Hemisphere compared to the Southern Hemisphere (Ko et al, 1993;Maiss and Levin, 1994). Similar to SF 6 , CO 2 increases almost linearly in the atmosphere with an average growth rate of about 1.6 ppmv year −1 over the last decades, due to anthropogenic emissions, mostly fossil fuel burning and deforestation.…”

Oceanographic tracer release experiments using sulphur hexafluoride

“…It is extremely stable in the environment, and other than being an asphyxiant, the pure compound has no known toxic effects. It is produced SF6 is included in the Kyoto Protocol because moleculefor-molecule, it is a powerful greenhouse gas with a long (> 1000 years) lifetime in the atmosphere [Ko et al, 1993;Oram et al, 1998]. However, for any realistic future emission scenario, SF6 will remain insignificant (<1%) as a contributor to the anthropogenic greenhouse effect in the next 50 years [Victor and MacDonald, 1999].…”

Dating young groundwater with sulfur hexafluoride: Natural and anthropogenic sources of sulfur hexafluoride

“…Production and release controls for SF 6 are expected to be set after the protocol is ratified by at least 55 counties; these controls, however, will become applicable only for developed countries. Atmospheric concentrations of SF 6 are expected to continue to increase into the 21st century [Ko et al, 1993]. SF 6 has been extensively used in many studies as a natural …”

“…It is mainly used as an electrical insulator in high-voltage switches and transformers and as a blanket gas in the melting operations of magnesium metal production [Maiss and Brenninkmeijer, 1998]. Industrial production of SF 6 began in 1953 with the introduction of SF6-filled electrical switches, and annual production has increased from nearly zero in 1953 to 85,700 t of SF 6 in 1995 [Ko et al, 1993;Maiss and Brenninkmeijer, 1998]. SF 6 is released to the atmosphere predominantly in the middle latitudes of the Northern Hemisphere and is subsequently incorporated into the hydrosphere.…”