ATMOSPHERIC SULFUR-HEXAFLUORIDE - SOURCES, SINKS AND GREENHOUSE WARMING

Model calculations using estimated reaction rates of sulfur hexafluori de (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 discu ssed. The estimated lifetimes are consistent with other estimated valu es based on recent laboratory measurements. There appears to be no kno wn natural source of SF6. An estimate of the current production rate o f SF6 is about 5 kt/yr. Based on historical emission rates, we calcula ted a present-day atmospheric concentrations for SF6 of about 2.5 part s per trillion by volume (pptv) and compared the results with availabl e atmospheric measurements. It is difficult to estimate the atmospheri c lifetime of SF6 based on mass balance of the emission rate and obser ved abundance. There are large uncertainties concerning what portion o f the SF6 is released to the atmosphere. Even if the emission rate wer e precisely known, it would be difficult to distinguish among lifetime s longer than 100 years since the current abundance of SF6 is due to e mission in the past three decades. More information on the measured tr ends over the past decade and observed vertical and latitudinal distri butions of SF6 in the lower stratosphere will help to narrow the uncer tainty in the lifetime. Based on laboratory-measured IR absorption cro ss 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. Howev er, its effect on atmospheric warming will be minimal because of its v ery 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-degrees-C and 0.0043- degrees-C is to be compared with the estimated warming due to CO2 incr ease of about 0.8-degrees-C in the same period.