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Model calculations using estimated reaction rates of sulfur hexafluoride (SF 6 ) with OH and O( 1 D ) 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 SF 6 . An estimate of the current production rate of SF 6 is about 5 kt/yr. Based on historical emission rates, we calculated a present‐day atmospheric concentrations for SF 6 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 SF 6 based on mass balance of the emission rate and observed abundance. There are large uncertainties concerning what portion of the SF 6 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 SF 6 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 SF 6 in the lower stratosphere will help to narrow the uncertainty in the lifetime. Based on laboratory‐measured IR absorption cross section for SF 6 , we showed that SF 6 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 SF 6 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 CO 2 increase of about 0.8°C in the same period.
Ko et al. (Sun,) studied this question.