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Exotic tracers for atmospheric studies
Aaarpkric
&noirownear Vol. 16, No. 6. pp. IM74471.
1982
CWU981’82.061467-05
M3.OOIO
Pqamon
F’ms Ltd.
F’rinted in Great Britain.
EXOTIC TRACERS FOR ATMOSPHERIC
STUDIES
JAMES E. LOVELOCK Brazxos Ltd., St. Giles on the Heath, Launceston, Cornwall, England
and GILBERT J. FERBER Nationa Oceanic and Atmospheric Administmtion Air Resources Laboratories, Silver Spring, MD, U.S.A. (Firsr received 9 July 1981 and in/inalform
21 Seprelnber 1981)
Abstract-Tracer materials can be injected into the atmosphere to study transport and dispersion processes and to validate air polhztion model ~ku~tions. Tracers should be inert, non-toxic and harmless to the environment. Tracrrs for long-range experiments, where dilution is very great, must be measurable at extremely low concentrations, well below the parts per trillion level. Compounds suitable for long-range tracer work are ram and efforts should be made to reservethem for meteorological studies, barring them from commercial uses which would increase atmospheric background concentrations. The use of these exotic tracers, incIuding~~in ~~uor~r~~ and isotopically labelled methanes, should be coordinated within the meteorologic& community to minimize interferences and maximize research benefits.
tracer studies are desired, and many releases would be needed at each location in order to obtain data under a To enjoy life and to live successfully on this planet WC variety of meteorological conditions, the cost of large tracer releases would be prohibitive. As a result, inert need to understand it better. This interest is motivated chemical tracers have been sought which can he readily not only by a proper scientific curiosity about the great detected at concentrations well below the parts per naturai cycles of the atmosphere and oceans but also trillion (lo-I’) level. A number of tracers should be by a growing concern over man’s impact on the available, so that suitable tracers could be chosen for environment. In response to this concern, numerous particular experiments. Further, the use of multiple computer models have been developed in recent years tracers in a single experiment could add silently to to simulate the behaviour of pollutants in the atmosthe information gained, for example, in studies of the phere and calculate long-range dispersion from source regions. However, modelling results often differ, and it effects of varying emission heights. It is apparent that a compound meeting all criteria is generally recognized that tracer experiments are for a long-range atmospheric tracer is a rather unique needed to test the merits of the various models and validate the calculations. The deliberate injection of material. A serious effort should be made to reserve tracers into the atmosphere, in precisely known such tracers, when found, for meteorological studies, amounts, allows direct measurement of the transport barring them from other commercial use if feasible. path and dilution rates aiong the trajectory. Further, the use of these exotic tracers should be To be suitable for atmospheric dispersion studies, a coordinated and controlled within the meteorological tracer material should have a residence time in the community so as to minimize interferences and maxiatmosphere of at least several weeks, several years for mize the research benefits from their use. global experiments. This requires an inert, nonThe historical development of the use of atmosdepositing material. The tracer must be non-toxic and pheric tracers is reviewed by Pasquill(l974) who cites harmless to the environment; it should be inexpensive experiments with smoke, fluorescent powders, sulphur and the cost of sample collection and analysis should hexafluoride and fluorocarbons. However only two classes of substances come near to meeting the exacting also be low. Ideally there should be no sources either natural or man-made, other than its production for requirements for long-range tracers, namely, tracer use. laboratory-made “heavy” methane, distinguished by a Tracers for long-range studies, where measurements rare but stable isotopic composition, and certain are needed out to hundreds or even thousands of km pertiuorocarbons. Recent experiments have demonstrated the feasibility of conducting dispersion from the release point, have additional requirements. experiments over distances of X000km or more using Over these long distances, dilution of the tracer becomes very great, thus requiring that very large an existing perfluorocarbon System (Ferber ec al., amounts be injected into the atmosphere or that the 1981). The development of isotopically labelled methtracer be measurable at extremely low concentrations. anes as tracers has been reported by Cowan et ni. Since there are many potential areas of interest where (1976) and Fowlcr (1979). 1. INTgODtJCTION
1467
14%
J&hiES
E. LOVELOCK and
GILBERT J. FERBER
What has been missing so far from the discussion is information on the properties and potentialities of the many perfluorocarbon compounds from which candidate tracers can be chosen. This lack of information is serious for there is growing industrial use of perfluorocarbons as heat transfer fluids and also in medicine. Already, for some, the global background is sufficient to preclude their use as long-range tracers.
2. PROPERTIES OF PERFLUORO COMPOUNDS Broadly speaking the class of substances from which candidate tracers may be chosen are organic chemicals in which some or all of the hydrogen is replaced by fluorine. In principle there is a universe of fluoro organic compounds which closely resemble in physical properties their corresponding hydrogen analogues. The perfluoro compounds differ principally in: (1) reacting with free electrons; the key property for sensitive detection; (2) having greater stability against thermal and environmental degradation and (3) tending to be either entirely free of toxicity or else highly toxic. Should this last statement seem to exaggerate, there is the exampte of perfluorodecalin, proposed as a blood substitute on account of its ability to carry oxygen, and perfluoroisobutene which is lethal at the parts per mitfion kvet. The preferred analysis method for perfluoro compounds is separation by gas chromatography and detection by electron capture. Air containing the tracer is injected into a fiowing stream of some inert carrier gas such as nitrogen. The tracer material is then separated from the oxygen and other electron attaching substances in the air by a chromatograph column. The isolated tracer, diluted in carrier gas, is then presented to the electron capture detector. ‘In this device gaseous electrons are continuously generated from the carrier gas by irradiation with weak beta radiation from a radioactive foil. The reaction between gaseous electrons and tracer is very rapid and the steady state electron concentration is a sensitive measure of tracer concentration. The tendency of perffuorocarbons to exhibit extremes in their properties with small structural or compositional changes applies also to their reactions with electrons. Figure 1 illustrates the relationship between the number of fluorine atoms in a molecule of perfiuorocarbon and the rate constant for thermaf electron attachment. Note that the scale for the rate of reaction with electrons is logarithmic; there is a range of reactivity of seven orders of magnitude between the least reactive and the most reactive of the perfluorocarbons. Free electron reactivity tends to increase with the number of fluorine atoms per molecule but the relationship is poorty correlated. Moreover there appears to be an upper limit of reactivity with a rate
Fig. 1. The rate constant for electron attachment of perfluoroearbons. Saturated perfiuoro normal alkanes $01. Branched and unsatur+d prfluoro alkanes ( x f Fhoro aromatic compounds ( t ).
constant of 3 x lO-‘cm3 molecule-’ s-l. This upper timit is not confined to the perfluorocarbons but is found with other classes of eIectron attaching compounds. It seems likely that the limit is set by the properties of the gaseous free electron as well as the reacting compounds. It may be significant that the cross section for the reaction at the upper Iimit is close to the dimensions of the De Broghe half wavelength of the thermal (0.025aV) electron. A recognition of this upper limit to electron reactivity is useful in preventing fruitless searches for substances that might be detected even more sensitively. The molecular structural properties which confer reactivity are now being investigated and certain empirical reiation~ps are already apparent. To illustrate this point the perfiuorocarbons in Fig. 1 are divided into three classes: (1) compounds with their carbon atoms linked in a simple chain or single closed ring; (2) compounds which haveat least one carbon atom linked directly to three or more other carbon atoms and (3) compounds which have carbon atoms linked by double or triple bonds or which are part of aromatic ring systems. Increasing molecular weight confers increased reactivity to all three compound classes but the second and third classes are mark&y more reactive at a given molecular weight and clearly the source of potential tracer compounds. In the course af investigation of the effect of structural and compesitionaf changes on the perfluorocarbons it was noticed that the inclusion of oxygen or nitrogen would often greatly diminish the electron reactivity. This observation has some practical importance in su ing the use of those petiuorocarbons as heat transfer fluids which will not, give rise to global backgrounds which interfere with tracer experiments, Compound (A) below is a widely used heat transfer t&id. Fortunately it is a weak electron absorber. The analogous compound (B), where the
Exotic tracers for atmospheric studies
is replaced by carbon, is intensely reactive and is a good candidate tracer.
oxygen
,AqF2
CF, 1
(Bl
i’:
CF, 1
CygjF-C4F9 ctF2;,-,F’ PBTF
PMCP
Both c~m~und A and B are fully fluoridate. The difference in electron reactivity between them is probably attributable to the electron donor properties of the ether oxygen in compound A which partially satisfies the electron affinity of the molecule. Whether or not this explanation is correct. it is a fact that all other perfluorocarbons so far investigated which bear ether oxygen atoms or amine nitrogen atoms are weakly electron attracting. Another desirable property for an ideal tracer substance is that it should not readily attach itself to solid surfaces nor should it be so water soluble as to be scavenged by rain. Few substances other than the perlluorocarbons, light hydrocarbons and the rare gases share these properties. The same molecular properties which confer this convenient indifference to the physical environment also assist in the isolation of the ~~uorocar~ns from the host of other substances, both man-made and natural, present at trace levels in the atmosphere. Thus after concentration by cryotrapping or adsorption, perfluorocarbons can be isolated from all other el~tron-attaching substances simply by heating with hydrogen in the presence of a palladium catalyst. Pet-fluorocarbons can be collected and concentrated by a factor of 1000 with currently available apparatus thereby raising the sensitivity of detection to parts per 10” by volume (Ferber et al., 1981). In principle there appears to be no reason why the perfluorocarbons could not be concentrated lo6 times. It might be possible to detect a concentration of one in lOi* by volume. At such extreme sensitivity, a few kg of tracer would be sufficient for global-scale dispersion experiments. In practice the attainment of these extreme and challenging sensitivities is threatened by the ever increasing atmospheric burden of perfluorocarbons consequent upon their industrial use. Already the background concentration of ~~uorodimethylcyclohexane (PDCH) exceeds one in 1014. More important. trace impurities in this material, commercially used as a heat exchange fluid, threaten to block other molecular “windows” in the spectrum of potential tracers. The efficiency of separation by gas chromatography is sufficient to insure the isolation, even at the lowest levels, of nearly all candidate tracer perfluorocarbons but this is of little avail if the atmospheric background is well above the limit of detection.
3. ATMOSPHERIC
1469 LlFETlMES
OF P~RFL~OROCAR~~
TRACERS
The atmospheric residence times of most potential ~~uoro~r~n tracers is unknown but likely to be very long indeed. CF, has been calculated to have a residence time of greater than lo4 years and is therefore for all practical purposes a permanent resident. The other ~tiuor~rbons may be less stable but to judge from the measured concentration of perfluorodimethylcyclohexane most of the amount released over the past 40 years has remained in the atmosphere. Residence times of at least 100 years seem probable. Shorter lived compounds may be preferable so that experiments would not be limited by increasing background concentrations. The fluorocarbons which bear hydrogen as well as fhtorine and those which have carbon double bonds or aromatic rings are less stable in the environment than are the fully fluorinated compounds. They need not, however, be appreciably less electron-absorbing. The choice of good candidate tracers for these shorter lived classes depends greatly on finding compounds which are neither toxic nor difficult :o separate and analyse. Fluorocarbons with carbon double bonds are very frequently toxic and it would seem wise to avoid them. Fluorocarbons bearing a little hydrogen or simple aromatic fluorocarbons are much less likely to pose a toxic hazard. Per~uorobenzene for example is less toxic than benzene and was considered as an anaesthetic for human use but discarded, not on account of toxicity, but because its mixture with oxygen was explosive on ignition. This compound and its numerous derivatives could form a valuable set of short-lived tracers for both atmospheric and oceanic use. Perfluoromonohydroa&mantane is another intensely electron attracting compound which may have a limited atmospheric lifetime. It gives a single well resolved gas chromatographic peak well away from those of the common industrial fluorocarbons.
4. OTHER ELECTRON ATTACHING COMPOUNDS
SF, was the first efectron attaching tracer to be used (Pasquill, 1974)and were it not for its high background concentration, about 1 ppt, it would be one of the most useful available as well as by far the Ieast expensive of the pertluoro tracers. Compounds in which perfluoro moieties of both carbon and sulphur are present also have heen made and proposed for tracer use. CF,SF, is a gas with properties very similar to those of SF, and easily separated from SF, by gas chromatography. The Telomer compounds SF,(C,F,,),CI also have been proposed and form a series ofexcellent potential tracer compounds with properties very like those of perfluorocarbons. At present these compounds have to be made to order and are therefore very expensive.
So consideration of duorme bearing tracers would ‘be complete iv hich dtd not include the mixed halocarbans. such ~b the common commercial chloro- and bromo-Huorocarbons. .-\lthough a t‘evboitheseare novv so abundant as to pose a potential hazard through the destruction of stratospheric ozone and consequently are unusable as tracers. others like the fire estmguishers CF,BrCI and CFBr hdve background concentrations less than that of SF, and certain b>products of their manufacture. e.g. CF,Br,. have etl’ectively a zero background. The latter compound has been used as a tracer. The present atmospheric background concentrations and comparative costs of pertluorocarbons and other tracers are shown in Table I. Those compounds with an expected atmospheric residence time exceeding 100 years are listed as “permanent” while those with a residence time of less than 50 years are listed as “temporary”. The tracer cost for any experiment depends upon the amount of tracer needed to overcome the existing background concentration at the maximum distance at which measurements are desired.
5.
The communication frequency radiation
coscLcsIo~s of information carried by radio is easily disturbed by the un-
controlled use of broA< -band cmttters such as automotive ignition S)Stsms pi diathermy equipment. In a similar way the chem:er?l information spectrum which includes uniqueand eaji!> resolvable molecular species Ilki:
the pertluorocsrb,~n,.
trolled
emission
1): blurred
ot‘ ?:~~rocarbon
bq the unconmisturss
1Mol.wt.
Atmospheric Background (lo-” by Vol.)
SF, PMCH PDCH PDCB PMCP
146 350 400 300 300
600 2 26 2 2
PD PMD
462
2
PA CF,SF5
512 424 196
1 0.01 1
‘*CD, “CD, C,F, CF,Br,
20 21 186 210
C,F,Br, PHA
260 406
cost Smol-’
Cost Skg-’
Relative Cost per Experiment
“Permanent” II 110 110 IOQO 100
1.6 38 44 300 30
1.0 0.08 1.2 0.6 0.06
60
2s
0.06
100 loo0 loo0
51 420 200
0.05 0.004 0.2
40 630 11 6.3
0.06 0.005 0.1 0.7
16 410
0.2 0.004
“Temporary”
I .Y 0.008: 10 1Oot 10 0.01
2000 30,000 60
30 60
loo0
* SF, PMCH PDCH PDCB PMCP PD PMD PA CF,SF,
Sulfur hexafluoride PerBuoromethyl cyclohexane Pertluorodimethyl cyslohexane Perfluorodimethyl cyclobutane Pertluoromethyl cyclopentane Pertluoro decalin Pefiuoromethyl decalin PerBuoro adamantane Pertluoromethyl sulfur pentafluoride
th;:
In the history of the development of radio frequent) communication and more recently of radio astronomy there has been a continual need to insure. by international cooperation and regulation. that the limited number of frequency channels are shared fairly among the numerous legitimate claimants. Such cooperation has been most effective where the offending radiation is incidental to another use. Witness the almost universal acceptance of suppressors in automotive ignition systems and the limitation of diathermy equipment to certain internationally agreed frequency ranges. The stage is approachmg where the growing number of scientists who use chemical tracer techniques. especially those interested in the natural environment. may need to initiate cooperation or even regulation to limit the release of certain unique and valuable tracer compounds in commerce and industry. Their need is like that of the radio astronomer who gathers exciting and enlightening new knowledge of the Universe. He could not succeed without regulations which reserve certain parts of the spectrum solely for scientific use.
Table 1. Comparative costs of tracer compounds
*Compound
ti)
environment.
“CD, %D, GF6 CF,Br, ClFdBr2 PHA
t Present detection limit; actual background is much lower.
Methane-20 Methane-21 Perfluorobenzenr Dibromodifluoro methane Dibromotetrafluoro ethane Perffuoromonohydro adamantane
1471
Exotic tracers for atmospheric studies
If we assume that a detection limit of one part in 1019 is ultimately possible, then this concentration requires the release of only a few tens of kg to label detectably the entire atmosphere. For this reason alone, the accidental or intentional release of 50 kg of a novel compound with tracer potential could double the cost or diminish the value of subsequent global experiments with the same material. The principal industrial use of perfluorocarbons (PFCs) at present is as coolants, particularly for use with electronic equipment. Their unusual properties render them uniquely valuable in this application also. In particular, they are chemically inert even at high temperatures, have great dielectric strength and efficiently convey heat by convection or by boiling from regions where there is otherwise excessive power dissipation. From this use alone more than 1OOOtof perfluorodimethyl cyclohexane (PDCH) has been released to the atmosphere. The escape to the environment, even of this large quantity, would be less serious if the material were pure. Unfortunately the commercial grade PDCH consists of a mixture which includes also PMCH and PMCP both of which are near ideal as tracers and easily resolved from PDCH. It follows that their use as tracers also is impaired. The escape of impure PDCH to the air is thus the chemical equivalent of a broad band radio frequency emitter like a spark transmitter and its present use is uncontrolled. PFCs bearing ether oxygen are just as efficient as coolants but less objectionable to tracer users since their electron attachment rates are much lower. Even so, the release of thousands of tons into the atmosphere will add significantly to the blurring of the background. Other large scale uses of PFCs have been proposed. One of these is as a temporary replacement for red blood cells on account of their physiological compatibility in the human internal fluid environment and their capacity to dissolve large volumes of oxygen. From this use as a replacement therapy all of the PFC must vent to the atmosphere within a few weeks of injection. Among the PFCs already tried and proposed for this use are the candidate tracers perfluorodecalin and methyl decalin and also perfluoroadamantanes. As with the coolants, those fluorocarbons bearing ether oxygen have been shown to function just as well and would of course be less of a disturbance
to tracer users.
Another potential large scale use for PFCs is in the tagging of explosives to assist in their detection for counter-terrorist and crime prevention purposes. PDCH is one of the candidate materials high on the list of those considered for this use. If it were used in a high proportion of explosives made, the total released to the atmosphere could be > 1kt y- ‘. Eventually the tagging would become counter-productive on account of the background from past use. It would seem wise to investigate the possibility of molecular labels for explosives which have shorter atmospheric life times than PFCs. It might seem that our concern to keep the global background of PFCs and other potential tracers as low as possible is excessive, if not an unwarranted interference with legitimate commercial and other interests. We think otherwise because, unlike radio interference, the cessation of an offending emission is not the end of the matter. So stable are the PFCs that ifall production of PDCH were stopped now, the background in 100 years would almost certainly still be over 1 part in 10”. We propose, mindful of the future needs of the environmental scientist, that some. at least. of the intensely electron attaching PFCs be reserved for tracer use. For the relatively abundant PFCs like PDCH it is too late already to do more than ask that it be used pure and to conserve it by recycling whenever feasible. For those novel compounds with a near zero background and ideal tracer properties, such as perfluoromonohydroadamantane, strict regulation is needed if future generations are not to be denied their unique value in the scientific investigation of the Earth. REFERENCES
Cowan G. A., Ott D. G., Turkevich A., Machta L., Ferber G. J. and Daly N. R. (1976) Heavy methanes as atmospheric tracers. Science 191, 1048-1050. Ferber G. J., Telegadas K., Heffter J. L., Dickson C. R., Dietz R. N. and Krey P. W. (1981) Demonstration ofa lona-ranae atmospheric tracer system using perlIuorocarbons.NOAA Tech. Memo. ERL ARL-101. NOAA Air Resources Laboratories, Silver Spring, MD. Fowler M. M. (1979)The use of heavy methanes as long range atmospheric tracers. Report LA UR-80-1342. Los Alamos National Laboratory, Los Alamos, New Mexico. Pasquill F. (1974) Armospheric DI@~xI. Ellis Horwood, Chichester, U.K.
&noirownear Vol. 16, No. 6. pp. IM74471.
1982
CWU981’82.061467-05
M3.OOIO
Pqamon
F’ms Ltd.
F’rinted in Great Britain.
EXOTIC TRACERS FOR ATMOSPHERIC
STUDIES
JAMES E. LOVELOCK Brazxos Ltd., St. Giles on the Heath, Launceston, Cornwall, England
and GILBERT J. FERBER Nationa Oceanic and Atmospheric Administmtion Air Resources Laboratories, Silver Spring, MD, U.S.A. (Firsr received 9 July 1981 and in/inalform
21 Seprelnber 1981)
Abstract-Tracer materials can be injected into the atmosphere to study transport and dispersion processes and to validate air polhztion model ~ku~tions. Tracers should be inert, non-toxic and harmless to the environment. Tracrrs for long-range experiments, where dilution is very great, must be measurable at extremely low concentrations, well below the parts per trillion level. Compounds suitable for long-range tracer work are ram and efforts should be made to reservethem for meteorological studies, barring them from commercial uses which would increase atmospheric background concentrations. The use of these exotic tracers, incIuding~~in ~~uor~r~~ and isotopically labelled methanes, should be coordinated within the meteorologic& community to minimize interferences and maximize research benefits.
tracer studies are desired, and many releases would be needed at each location in order to obtain data under a To enjoy life and to live successfully on this planet WC variety of meteorological conditions, the cost of large tracer releases would be prohibitive. As a result, inert need to understand it better. This interest is motivated chemical tracers have been sought which can he readily not only by a proper scientific curiosity about the great detected at concentrations well below the parts per naturai cycles of the atmosphere and oceans but also trillion (lo-I’) level. A number of tracers should be by a growing concern over man’s impact on the available, so that suitable tracers could be chosen for environment. In response to this concern, numerous particular experiments. Further, the use of multiple computer models have been developed in recent years tracers in a single experiment could add silently to to simulate the behaviour of pollutants in the atmosthe information gained, for example, in studies of the phere and calculate long-range dispersion from source regions. However, modelling results often differ, and it effects of varying emission heights. It is apparent that a compound meeting all criteria is generally recognized that tracer experiments are for a long-range atmospheric tracer is a rather unique needed to test the merits of the various models and validate the calculations. The deliberate injection of material. A serious effort should be made to reserve tracers into the atmosphere, in precisely known such tracers, when found, for meteorological studies, amounts, allows direct measurement of the transport barring them from other commercial use if feasible. path and dilution rates aiong the trajectory. Further, the use of these exotic tracers should be To be suitable for atmospheric dispersion studies, a coordinated and controlled within the meteorological tracer material should have a residence time in the community so as to minimize interferences and maxiatmosphere of at least several weeks, several years for mize the research benefits from their use. global experiments. This requires an inert, nonThe historical development of the use of atmosdepositing material. The tracer must be non-toxic and pheric tracers is reviewed by Pasquill(l974) who cites harmless to the environment; it should be inexpensive experiments with smoke, fluorescent powders, sulphur and the cost of sample collection and analysis should hexafluoride and fluorocarbons. However only two classes of substances come near to meeting the exacting also be low. Ideally there should be no sources either natural or man-made, other than its production for requirements for long-range tracers, namely, tracer use. laboratory-made “heavy” methane, distinguished by a Tracers for long-range studies, where measurements rare but stable isotopic composition, and certain are needed out to hundreds or even thousands of km pertiuorocarbons. Recent experiments have demonstrated the feasibility of conducting dispersion from the release point, have additional requirements. experiments over distances of X000km or more using Over these long distances, dilution of the tracer becomes very great, thus requiring that very large an existing perfluorocarbon System (Ferber ec al., amounts be injected into the atmosphere or that the 1981). The development of isotopically labelled methtracer be measurable at extremely low concentrations. anes as tracers has been reported by Cowan et ni. Since there are many potential areas of interest where (1976) and Fowlcr (1979). 1. INTgODtJCTION
1467
14%
J&hiES
E. LOVELOCK and
GILBERT J. FERBER
What has been missing so far from the discussion is information on the properties and potentialities of the many perfluorocarbon compounds from which candidate tracers can be chosen. This lack of information is serious for there is growing industrial use of perfluorocarbons as heat transfer fluids and also in medicine. Already, for some, the global background is sufficient to preclude their use as long-range tracers.
2. PROPERTIES OF PERFLUORO COMPOUNDS Broadly speaking the class of substances from which candidate tracers may be chosen are organic chemicals in which some or all of the hydrogen is replaced by fluorine. In principle there is a universe of fluoro organic compounds which closely resemble in physical properties their corresponding hydrogen analogues. The perfluoro compounds differ principally in: (1) reacting with free electrons; the key property for sensitive detection; (2) having greater stability against thermal and environmental degradation and (3) tending to be either entirely free of toxicity or else highly toxic. Should this last statement seem to exaggerate, there is the exampte of perfluorodecalin, proposed as a blood substitute on account of its ability to carry oxygen, and perfluoroisobutene which is lethal at the parts per mitfion kvet. The preferred analysis method for perfluoro compounds is separation by gas chromatography and detection by electron capture. Air containing the tracer is injected into a fiowing stream of some inert carrier gas such as nitrogen. The tracer material is then separated from the oxygen and other electron attaching substances in the air by a chromatograph column. The isolated tracer, diluted in carrier gas, is then presented to the electron capture detector. ‘In this device gaseous electrons are continuously generated from the carrier gas by irradiation with weak beta radiation from a radioactive foil. The reaction between gaseous electrons and tracer is very rapid and the steady state electron concentration is a sensitive measure of tracer concentration. The tendency of perffuorocarbons to exhibit extremes in their properties with small structural or compositional changes applies also to their reactions with electrons. Figure 1 illustrates the relationship between the number of fluorine atoms in a molecule of perfiuorocarbon and the rate constant for thermaf electron attachment. Note that the scale for the rate of reaction with electrons is logarithmic; there is a range of reactivity of seven orders of magnitude between the least reactive and the most reactive of the perfluorocarbons. Free electron reactivity tends to increase with the number of fluorine atoms per molecule but the relationship is poorty correlated. Moreover there appears to be an upper limit of reactivity with a rate
Fig. 1. The rate constant for electron attachment of perfluoroearbons. Saturated perfiuoro normal alkanes $01. Branched and unsatur+d prfluoro alkanes ( x f Fhoro aromatic compounds ( t ).
constant of 3 x lO-‘cm3 molecule-’ s-l. This upper timit is not confined to the perfluorocarbons but is found with other classes of eIectron attaching compounds. It seems likely that the limit is set by the properties of the gaseous free electron as well as the reacting compounds. It may be significant that the cross section for the reaction at the upper Iimit is close to the dimensions of the De Broghe half wavelength of the thermal (0.025aV) electron. A recognition of this upper limit to electron reactivity is useful in preventing fruitless searches for substances that might be detected even more sensitively. The molecular structural properties which confer reactivity are now being investigated and certain empirical reiation~ps are already apparent. To illustrate this point the perfiuorocarbons in Fig. 1 are divided into three classes: (1) compounds with their carbon atoms linked in a simple chain or single closed ring; (2) compounds which haveat least one carbon atom linked directly to three or more other carbon atoms and (3) compounds which have carbon atoms linked by double or triple bonds or which are part of aromatic ring systems. Increasing molecular weight confers increased reactivity to all three compound classes but the second and third classes are mark&y more reactive at a given molecular weight and clearly the source of potential tracer compounds. In the course af investigation of the effect of structural and compesitionaf changes on the perfluorocarbons it was noticed that the inclusion of oxygen or nitrogen would often greatly diminish the electron reactivity. This observation has some practical importance in su ing the use of those petiuorocarbons as heat transfer fluids which will not, give rise to global backgrounds which interfere with tracer experiments, Compound (A) below is a widely used heat transfer t&id. Fortunately it is a weak electron absorber. The analogous compound (B), where the
Exotic tracers for atmospheric studies
is replaced by carbon, is intensely reactive and is a good candidate tracer.
oxygen
,AqF2
CF, 1
(Bl
i’:
CF, 1
CygjF-C4F9 ctF2;,-,F’ PBTF
PMCP
Both c~m~und A and B are fully fluoridate. The difference in electron reactivity between them is probably attributable to the electron donor properties of the ether oxygen in compound A which partially satisfies the electron affinity of the molecule. Whether or not this explanation is correct. it is a fact that all other perfluorocarbons so far investigated which bear ether oxygen atoms or amine nitrogen atoms are weakly electron attracting. Another desirable property for an ideal tracer substance is that it should not readily attach itself to solid surfaces nor should it be so water soluble as to be scavenged by rain. Few substances other than the perlluorocarbons, light hydrocarbons and the rare gases share these properties. The same molecular properties which confer this convenient indifference to the physical environment also assist in the isolation of the ~~uorocar~ns from the host of other substances, both man-made and natural, present at trace levels in the atmosphere. Thus after concentration by cryotrapping or adsorption, perfluorocarbons can be isolated from all other el~tron-attaching substances simply by heating with hydrogen in the presence of a palladium catalyst. Pet-fluorocarbons can be collected and concentrated by a factor of 1000 with currently available apparatus thereby raising the sensitivity of detection to parts per 10” by volume (Ferber et al., 1981). In principle there appears to be no reason why the perfluorocarbons could not be concentrated lo6 times. It might be possible to detect a concentration of one in lOi* by volume. At such extreme sensitivity, a few kg of tracer would be sufficient for global-scale dispersion experiments. In practice the attainment of these extreme and challenging sensitivities is threatened by the ever increasing atmospheric burden of perfluorocarbons consequent upon their industrial use. Already the background concentration of ~~uorodimethylcyclohexane (PDCH) exceeds one in 1014. More important. trace impurities in this material, commercially used as a heat exchange fluid, threaten to block other molecular “windows” in the spectrum of potential tracers. The efficiency of separation by gas chromatography is sufficient to insure the isolation, even at the lowest levels, of nearly all candidate tracer perfluorocarbons but this is of little avail if the atmospheric background is well above the limit of detection.
3. ATMOSPHERIC
1469 LlFETlMES
OF P~RFL~OROCAR~~
TRACERS
The atmospheric residence times of most potential ~~uoro~r~n tracers is unknown but likely to be very long indeed. CF, has been calculated to have a residence time of greater than lo4 years and is therefore for all practical purposes a permanent resident. The other ~tiuor~rbons may be less stable but to judge from the measured concentration of perfluorodimethylcyclohexane most of the amount released over the past 40 years has remained in the atmosphere. Residence times of at least 100 years seem probable. Shorter lived compounds may be preferable so that experiments would not be limited by increasing background concentrations. The fluorocarbons which bear hydrogen as well as fhtorine and those which have carbon double bonds or aromatic rings are less stable in the environment than are the fully fluorinated compounds. They need not, however, be appreciably less electron-absorbing. The choice of good candidate tracers for these shorter lived classes depends greatly on finding compounds which are neither toxic nor difficult :o separate and analyse. Fluorocarbons with carbon double bonds are very frequently toxic and it would seem wise to avoid them. Fluorocarbons bearing a little hydrogen or simple aromatic fluorocarbons are much less likely to pose a toxic hazard. Per~uorobenzene for example is less toxic than benzene and was considered as an anaesthetic for human use but discarded, not on account of toxicity, but because its mixture with oxygen was explosive on ignition. This compound and its numerous derivatives could form a valuable set of short-lived tracers for both atmospheric and oceanic use. Perfluoromonohydroa&mantane is another intensely electron attracting compound which may have a limited atmospheric lifetime. It gives a single well resolved gas chromatographic peak well away from those of the common industrial fluorocarbons.
4. OTHER ELECTRON ATTACHING COMPOUNDS
SF, was the first efectron attaching tracer to be used (Pasquill, 1974)and were it not for its high background concentration, about 1 ppt, it would be one of the most useful available as well as by far the Ieast expensive of the pertluoro tracers. Compounds in which perfluoro moieties of both carbon and sulphur are present also have heen made and proposed for tracer use. CF,SF, is a gas with properties very similar to those of SF, and easily separated from SF, by gas chromatography. The Telomer compounds SF,(C,F,,),CI also have been proposed and form a series ofexcellent potential tracer compounds with properties very like those of perfluorocarbons. At present these compounds have to be made to order and are therefore very expensive.
So consideration of duorme bearing tracers would ‘be complete iv hich dtd not include the mixed halocarbans. such ~b the common commercial chloro- and bromo-Huorocarbons. .-\lthough a t‘evboitheseare novv so abundant as to pose a potential hazard through the destruction of stratospheric ozone and consequently are unusable as tracers. others like the fire estmguishers CF,BrCI and CFBr hdve background concentrations less than that of SF, and certain b>products of their manufacture. e.g. CF,Br,. have etl’ectively a zero background. The latter compound has been used as a tracer. The present atmospheric background concentrations and comparative costs of pertluorocarbons and other tracers are shown in Table I. Those compounds with an expected atmospheric residence time exceeding 100 years are listed as “permanent” while those with a residence time of less than 50 years are listed as “temporary”. The tracer cost for any experiment depends upon the amount of tracer needed to overcome the existing background concentration at the maximum distance at which measurements are desired.
5.
The communication frequency radiation
coscLcsIo~s of information carried by radio is easily disturbed by the un-
controlled use of broA< -band cmttters such as automotive ignition S)Stsms pi diathermy equipment. In a similar way the chem:er?l information spectrum which includes uniqueand eaji!> resolvable molecular species Ilki:
the pertluorocsrb,~n,.
trolled
emission
1): blurred
ot‘ ?:~~rocarbon
bq the unconmisturss
1Mol.wt.
Atmospheric Background (lo-” by Vol.)
SF, PMCH PDCH PDCB PMCP
146 350 400 300 300
600 2 26 2 2
PD PMD
462
2
PA CF,SF5
512 424 196
1 0.01 1
‘*CD, “CD, C,F, CF,Br,
20 21 186 210
C,F,Br, PHA
260 406
cost Smol-’
Cost Skg-’
Relative Cost per Experiment
“Permanent” II 110 110 IOQO 100
1.6 38 44 300 30
1.0 0.08 1.2 0.6 0.06
60
2s
0.06
100 loo0 loo0
51 420 200
0.05 0.004 0.2
40 630 11 6.3
0.06 0.005 0.1 0.7
16 410
0.2 0.004
“Temporary”
I .Y 0.008: 10 1Oot 10 0.01
2000 30,000 60
30 60
loo0
* SF, PMCH PDCH PDCB PMCP PD PMD PA CF,SF,
Sulfur hexafluoride PerBuoromethyl cyclohexane Pertluorodimethyl cyslohexane Perfluorodimethyl cyclobutane Pertluoromethyl cyclopentane Pertluoro decalin Pefiuoromethyl decalin PerBuoro adamantane Pertluoromethyl sulfur pentafluoride
th;:
In the history of the development of radio frequent) communication and more recently of radio astronomy there has been a continual need to insure. by international cooperation and regulation. that the limited number of frequency channels are shared fairly among the numerous legitimate claimants. Such cooperation has been most effective where the offending radiation is incidental to another use. Witness the almost universal acceptance of suppressors in automotive ignition systems and the limitation of diathermy equipment to certain internationally agreed frequency ranges. The stage is approachmg where the growing number of scientists who use chemical tracer techniques. especially those interested in the natural environment. may need to initiate cooperation or even regulation to limit the release of certain unique and valuable tracer compounds in commerce and industry. Their need is like that of the radio astronomer who gathers exciting and enlightening new knowledge of the Universe. He could not succeed without regulations which reserve certain parts of the spectrum solely for scientific use.
Table 1. Comparative costs of tracer compounds
*Compound
ti)
environment.
“CD, %D, GF6 CF,Br, ClFdBr2 PHA
t Present detection limit; actual background is much lower.
Methane-20 Methane-21 Perfluorobenzenr Dibromodifluoro methane Dibromotetrafluoro ethane Perffuoromonohydro adamantane
1471
Exotic tracers for atmospheric studies
If we assume that a detection limit of one part in 1019 is ultimately possible, then this concentration requires the release of only a few tens of kg to label detectably the entire atmosphere. For this reason alone, the accidental or intentional release of 50 kg of a novel compound with tracer potential could double the cost or diminish the value of subsequent global experiments with the same material. The principal industrial use of perfluorocarbons (PFCs) at present is as coolants, particularly for use with electronic equipment. Their unusual properties render them uniquely valuable in this application also. In particular, they are chemically inert even at high temperatures, have great dielectric strength and efficiently convey heat by convection or by boiling from regions where there is otherwise excessive power dissipation. From this use alone more than 1OOOtof perfluorodimethyl cyclohexane (PDCH) has been released to the atmosphere. The escape to the environment, even of this large quantity, would be less serious if the material were pure. Unfortunately the commercial grade PDCH consists of a mixture which includes also PMCH and PMCP both of which are near ideal as tracers and easily resolved from PDCH. It follows that their use as tracers also is impaired. The escape of impure PDCH to the air is thus the chemical equivalent of a broad band radio frequency emitter like a spark transmitter and its present use is uncontrolled. PFCs bearing ether oxygen are just as efficient as coolants but less objectionable to tracer users since their electron attachment rates are much lower. Even so, the release of thousands of tons into the atmosphere will add significantly to the blurring of the background. Other large scale uses of PFCs have been proposed. One of these is as a temporary replacement for red blood cells on account of their physiological compatibility in the human internal fluid environment and their capacity to dissolve large volumes of oxygen. From this use as a replacement therapy all of the PFC must vent to the atmosphere within a few weeks of injection. Among the PFCs already tried and proposed for this use are the candidate tracers perfluorodecalin and methyl decalin and also perfluoroadamantanes. As with the coolants, those fluorocarbons bearing ether oxygen have been shown to function just as well and would of course be less of a disturbance
to tracer users.
Another potential large scale use for PFCs is in the tagging of explosives to assist in their detection for counter-terrorist and crime prevention purposes. PDCH is one of the candidate materials high on the list of those considered for this use. If it were used in a high proportion of explosives made, the total released to the atmosphere could be > 1kt y- ‘. Eventually the tagging would become counter-productive on account of the background from past use. It would seem wise to investigate the possibility of molecular labels for explosives which have shorter atmospheric life times than PFCs. It might seem that our concern to keep the global background of PFCs and other potential tracers as low as possible is excessive, if not an unwarranted interference with legitimate commercial and other interests. We think otherwise because, unlike radio interference, the cessation of an offending emission is not the end of the matter. So stable are the PFCs that ifall production of PDCH were stopped now, the background in 100 years would almost certainly still be over 1 part in 10”. We propose, mindful of the future needs of the environmental scientist, that some. at least. of the intensely electron attaching PFCs be reserved for tracer use. For the relatively abundant PFCs like PDCH it is too late already to do more than ask that it be used pure and to conserve it by recycling whenever feasible. For those novel compounds with a near zero background and ideal tracer properties, such as perfluoromonohydroadamantane, strict regulation is needed if future generations are not to be denied their unique value in the scientific investigation of the Earth. REFERENCES
Cowan G. A., Ott D. G., Turkevich A., Machta L., Ferber G. J. and Daly N. R. (1976) Heavy methanes as atmospheric tracers. Science 191, 1048-1050. Ferber G. J., Telegadas K., Heffter J. L., Dickson C. R., Dietz R. N. and Krey P. W. (1981) Demonstration ofa lona-ranae atmospheric tracer system using perlIuorocarbons.NOAA Tech. Memo. ERL ARL-101. NOAA Air Resources Laboratories, Silver Spring, MD. Fowler M. M. (1979)The use of heavy methanes as long range atmospheric tracers. Report LA UR-80-1342. Los Alamos National Laboratory, Los Alamos, New Mexico. Pasquill F. (1974) Armospheric DI@~xI. Ellis Horwood, Chichester, U.K.