Atmospheric N2O and sulfur hexafluoride (SF6) are present in lower concentrations than CO2, but the radiative forcing of each is far greater. Nitrous oxide is the third strongest greenhouse gas, while each SF6 molecule is 22,200 times more effective as an infrared absorber than one CO2 molecule and has an atmospheric lifetime of between 500 and 3200 years.
The concentration of both species has grown at a linear rate, N2O at 0.76 ppb/ a (0.25% per year) since 1978 and SF6 at a rate of 0.22 ppt (parts per trillion, 1012, by dry air mole fraction) per year (~5%/a) since 1996. The concentration of 320 ppb N2O in 2005 has added a radiative forcing of around 0.17 W/m2 over the pre-industrial N2O concentration of around 270ppb. Atmospheric N2O is also a major source of stratospheric nitric oxide (NO), a compound that helps to catalytically destroy stratospheric O3. The atmospheric concentration of SF6 has grown due to its use as an electrical insulator for power transmission throughout the world. Its global mean concentration was 5.75 ppt at the end of 2005. While total radiative forcing of SF6 from pre-industrial times to the present is relatively small (0.003 W/m2), its long atmospheric lifetime, high atmospheric growth rate, and high global-warming potential are a concern for the future.
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