CF3SF5 - a 'super' greenhouse gas
CF3SF5 - a 'super' greenhouse gas
In Short
• Data from ice reveal CF3SF5 concentration in atmosphere is rapidly rising
• CF3SF5 has an exceptionally long lifetime in the atmosphere, a further cause for concern
Trifluoromethyl sulfur pentafluoride - a byproduct of the electronics industry - has been named a 'super' greenhouse gas by physical chemists. But what evidence do they have that makes this molecule a potential threat to the environment? Richard Tuckett
Over the past seven years the importance of greenhouse gases and global warming has leapt from obscurity to the top of the political agenda in all developed countries, culminating in the Stern report in November 2006 when the economic consequences of unchecked global warming were spelt out. Here we review the science of the greenhouse effect - or, more accurately, radiation trapping - and describe what constitutes a serious greenhouse gas, taking CF3SF5 as a case study.
[pic]
|Trifluoromethyl sulfur pentafluoride |
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|© Created with the free Discovery Studio Visualizer (Accelrys, Inc)|
CF3SF5 in the atmosphere
CF3SF5 is a gas at room temperature with a boiling point of 253 K and an enthalpy of vaporisation of 20 kJ mol-1. The gas was first detected in the Earth's atmosphere in 2000,1 by which time the related SF6 molecule had already been identified as a potential greenhouse gas.2 The source of CF3SF5 is believed to be anthropogenic, and most likely a breakdown product of the dielectric molecule SF6 in high-voltage equipment. The trends in concentration levels of SF6 and CF3SF5 have tracked each other closely over the past 30-40 years (Fig 1(a)), suggesting that CF3SF5 is probably produced in the electronics industry via the recombination of CF3 and SF5 free radicals.
[pic]
|Fig 1 Antarctic ice core data on the concentrations of CF3SF5 and SF6 (a) in the |
|atmosphere; and (b) at depths below the Earth's surface |
Moreover, measurements taken from ice samples in Antarctica show similar variations of concentration of SF6 and CF3SF5 at depths below the Earth's surface (Fig 1(b)). Specifically, the data reveal that the concentration of CF3SF5 has grown from near zero in the late 1960s to ca 0.12 pptv in 1999 (or ca 2.5 × 106 molecules cm-3) with a current growth rate of ca 6 per cent per annum. Stratospheric profiles suggest that the lifetime of CF3SF5 in the atmosphere is between several hundred and a few thousand years. So is CF3SF5 a potentially serious greenhouse gas?
The greenhouse effect
The greenhouse effect describes the trapping of infrared radiation, emitted by the Earth, which is absorbed in the atmosphere by small polyatomic molecules such as CO2, CH4 and H2O. As a result the average temperature of the Earth is raised from 256 K, or -17 ºC, to a hospitable ~290 K. Often called the 'natural' greenhouse effect, this kept the Earth's temperature approximately static for hundreds of years up to the start of the Industrial Revolution, ca 1750.
What we are concerned with today is an 'enhanced' greenhouse effect, caused by increases in concentration of greenhouse gases over the past 250 years. Nobody really doubts the scientific evidence that the concentration of the principal greenhouse gas, CO2, has increased by ca 35 per cent over this period, from ~280 to ~380 parts per million by volume (ppmv), while the average temperature has also increased by ~1 ºC. What has not yet been proven is that there is a cause-and-effect correlation between these two facts. The consensus of world scientists, and certainly physical scientists,3 is that the CO2 concentration and the temperature of the planet are strongly correlated, but there remains a small vociferous minority who believe otherwise.
Although most attention has been given to CO2 (and possibly CH4 and H2O), physical scientists now understand that there are larger polyatomic gases of low concentrations in the atmosphere which can contribute significantly to global warming. These gases, which include SF6 and CF3SF5 ,1,2 absorb infrared radiation strongly in regions where CO2, CH4 and H2O do not absorb.
Properties of greenhouse gases
In qualitative terms, there are two properties that are necessary for a molecule to be an effective greenhouse gas.
• The molecule must absorb infrared radiation strongly in the black-body range of the Earth's emission, ca 5-25 µm, where CO2, CH4 etc do not absorb. In practice, many C-F and C-Cl stretching vibrations around 10 µm contribute strongly. Such transitions are only observed if the vibration causes a change in dipole moment of the molecule. Figure 2 shows the calculated infrared absorption spectrum of CF3SF5. The molecule has 24 vibrational modes, but only six have any significant infrared intensity. The four most intense bands occur in the atmospheric window 8-14 µm or 730-1260 cm-1, where the major greenhouse gases CO2, CH4 and H2O do not absorb. (Note that the vibrations of N2 and O2, comprising 99 per cent of the Earth's atmosphere, are infrared inactive.)
• The molecule must have a long lifetime (at least 10 years) in the atmosphere; it must not be destroyed by photodissociation in the range 200?600 nm, and it must not react with the free radicals prevalent in the atmosphere.
[pic]
|Fig 2 Calculated ir absorption spectrum of CF3SF5 |
Furthermore, a greenhouse gas whose concentration is increasing rapidly owing to human activity will cause special concern. The gas CF3SF5 satisfies these criteria.
Table 1 shows data for four greenhouse gases - CO2 and CH4 together cause ~70 per cent of the total radiation trapping; a chlorofluorocarbon CF2Cl2; and CF3SF5. The 'radiative forcing' is a measure of the strength of the infrared absorption bands over the range 5-25 µm, it is a per molecule microscopic property with units of W m-2 per unit concentration. When multiplied by the change in concentration of pollutant over a defined time, usually 250 years from the start of the Industrial Revolution to the present day, the macroscopic radiative forcing in units of W m-2 is obtained. We can then compare the effect of different pollutant molecules over a specific period. The greenhouse potential (GHP), sometimes called the global warming potential, measures the radiative forcing, Axof pulse emission of a greenhouse gas, x, over a defined time, t, usually 100 years, relative to the time-integrated radiative forcing of a pulse emission of an equal mass of CO2 (see equation (i)).
The GHP is a dimensionless number that tells us how important one unit of mass (eg 1 kg) of a pollutant x is to the greenhouse effect compared with the same unit of mass of CO2. The GHP of CO2 is defined to be unity. Equation (i) can be simplified to give an analytical expression for the GHP of x over a time, t (see equation (ii)).4
The GHP of x reflects values for the microscopic radiative forcing, ao, of greenhouse gases x and CO2; the molecular weights of x and CO2; the lifetime of x in the atmosphere [pic]x; and the constant K CO2 which can be determined for any value of t. K CO2 has units of time, and is related (but not equal) to the lifetime of CO2 in the atmosphere, the values of which range from 50 to 200 years.4
The macroscopic overall contribution of a pollutant to the greenhouse effect involves a complicated convolution of its concentration, lifetime and GHP value. Thus CO2 and CH4 contribute most to the greenhouse effect because of their high atmospheric concentration (note that the microscopic radiative forcing and GHP of both gases are relatively low). By contrast, CF3SF5 has the highest microscopic radiative forcing of any known greenhouse gas (earning it the title 'super' greenhouse gas). (Absolute infrared absorption measurements have shown the microscopic radiative forcing of CF3SF5 to be 0.60 ± 0.03 W m-2 ppbv-1, which is even higher than that of SF6.) The GHP of these two molecules is therefore very high - SF6 is slightly higher because its atmospheric lifetime, 3200 years,4 is about three times greater than that of CF3SF5. However, the contribution of these two molecules to the overall greenhouse effect is still relatively small because their atmospheric concentrations, despite rising rapidly, are still low, at the level of parts per trillion by volume.
Lifetime studies
The reactions that remove CF3SF5 from the atmosphere are important because they contribute to the molecule's lifetime and GHP value. The total removal rate per unit volume per unit time is given by equation (iii), where:
• each of the five terms in the large bracket of equation (iii) is a pseudo-first-order rate constant;
• [x] represents the concentration of species x, in this case [pic]OH, O[pic], cations and electrons;
• the first four terms represent reactions of CF3SF5 with [pic]OH, O[pic], cations and electrons, respectively;
• ki are the corresponding second-order bimolecular rate coefficients.
[pic]
|Fig 3 Mean total kinetic energy released in CF3SF5 + hv forms CF3+ + SF5 = e- for |
|photon energies in the range of 13.3-15.5 eV |
OH radicals and electronically-excited O atoms, O[pic], are the most important oxidising free radicals in the atmosphere. The first term in the large bracket dominates in the troposphere (0 ................
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