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Relations between general and atmospheric physics, particularly concerning atmospheric ions
Aerosol Science, 1971, Vol. 2, pp. 141 to 144. Pergamon Press. Printed in Great Britain.
RELATIONS BETWEEN GENERAL AND ATMOSPHERIC PHYSICS, PARTICULARLY CONCERNING ATMOSPHERIC IONS R. SIKSNA Institutet fOr Hogspanningsforskning, Uppsala, Sweden Abstract-Problems related to the combination and recombination of ions, their mobility and their structure are discussed, to show the necessity of using more critically elaborated ideas taken from general physics for adequate studies of atmospheric ions.
THE TITLE of this presentation may be reformulated as: the relation of the atmospheric physicist to general physics. By the name atmospheric physicist is meant the physicist occupied with physical phenomena in the atmosphere, in our case the physicist occupied with ions, aerosols and radioactivity in the atmosphere. The pioneers in this domain, ELSTER (1854-1920), GEITEL (1855-1923), LENARD (1862-1947), WILSON (1869-1959), LANGEVIN (1872-1946), were pure physicists, since at that time atmospheric physics per se did not exist, and the problems of atmospheric physics were considered like those of general physics. Now the situation is changed and many problems will be not considered so thoroughly, mostly with ad hoc solutions, being satisfied if some formal accordance is reached between the observed phenomena and some general theoretical formulas, without examination of the foundations used for the derivation of these formulas. The situation can be illustrated with some examples concerning the ions. COMBINATION AND RECOMBINATION OF IONS
The problem of charging of aerosol particles by ions is important in atmospheric ionics. Closely connected with this problem are the combination and recombination processes of ions, characterized by the recombination or combination coefficient. As may be shown, comparatively complicated expressions can be derived for the recombination coefficient by using differing assumptions (BRICARD, 1965; NATANSON, 1959; SIKSNA, 1964, 1966; HOPPEL, 1969) with the surprising result that in the outer form the derived expressions appear to be similar. An insight into the relations by deriving corresponding expressions may be obtained by introducing a reciprocal magnitude of the recombination coefficient-recombination time (SIKSNA, 1966)-which is an additive magnitude for different stages involved in the recombination process. An important element in the theoretical consideration of the recombination processes is the mean free path. Unfortunately the meaning of the mean free path is not so clearly defined as is assumed in the elementary gas theory, as shown by a critical consideration of the related matter (SIKSNA, 1964). MOBILITY
Another problem, perhaps even more important in atmospheric ionics, is that of the mobility of the ions. The concept of the mobility of gaseous ions is taken from the consideration of the conductivity in electrolytes. The ions in a gas under the influence of a static, uniform electric field lose energy in elastic collisions with the gas molecules and gain energy from the electric field between collisions. If the energy acquired by the ions between 141
142
R.
SIKSNA
collisions is small compared with their mean thermal energy, their mean drift velocity the direction of the electric field will be proportional to the field strength E Vd
Vd
in
= fLE
The constant of proportionality fL is called the mobility of these ions in the particular gas. Accepting this simple relationship and ignoring the mechanism of the movement of the ions, it is possible to construct different experimental arrangements by which the coefficient fL may be measured. The aspiration condenser is the most commonly used instrument for mobility measurements. Very ingenious devices of this kind have been developed by ERIKSON (1929), MISAKI (1950, 1961), WHIPPLE (1960), HOEGL (1963), DOLEZALEK (1965), BRICARD (1965) and others. A systematic theoretical analysis of the aspiration method of measuring the mobility of atmospheric ions is provided by TAMMET (1967). It may be asked what kind of information we expect to obtain by using these measuring devices for mobility. Some authors have detected discrete mobilities; others, continuous distribution of the mobilities. It seemed that by knowing the mobility it would be possible to say something about the structure of the ions under consideration in other words, to identify the ions. That was one of the driving forces for the development of devices for measuring the mobility of atmospheric ions. However, it is shown that by simultaneous measurements of the drift velocity (from which the mobility is calculated) and by massspectrometric measurements, no correlation between the mobility and the mass (or the structure) of the ions was found. It may seem surprising that no attention is paid by atmospheric physicists occupied with the mobility measurements of atmospheric ions to one essential fact concerning the concept of mobility. A critical survey of this matter is given by KELLER (1968) who pointed out that ion-molecule collisions are absolutely essential when considering the mobility measurements, and species under investigation may be changed by these collisions. In conclusion it can be stated that in spite of highly ingenious experimental devices developed for measuring the mobility of atmospheric ions the expectations of obtaining such information about the nature of the ions were not fulfilled. Problems related to the idea of the mobility of atmospheric ions are still not solved, because they have not been considered from a more general point of view. NATURE OF THE ATMOSPHERIC IONS
Since the beginning of the century when ELSTER and GEITEL (1899a, b), and WILSON (1900) almost simultaneously introduced the term" ions" in considerations of the electric phenomena in atmosphere difficulties have arisen for people investigating these " atmospheric " ions, or, as they were called at that time, "air" ions. The air ions have been considered as something uncertain. Vague models have been proposed without the availability of modern theoretical concepts for explanation of their properties and the efforts made with mobility measurements were but desperate attempts to obtain more information about the nature ofthe atmospheric ions. A turning-point may be noted on October 2, 1967 in Lucerne during the 14th General Assembly of the IUGG when within the scope of the symposia arranged by the Joint Committee on Atmospheric Electricity it was planned to discuss mobility problems. However, the discussion turned out to be a proclamation of a new situation concerning the nature of atmospheric ions (DOLEZALEK, 1968). The basis for the conclusions arrived during this discussion is given in greater detail by MOHNEN (1969) and SIKSNA (1969).
Relations between general and atmospheric physics
143
One of these conclusions was: the dominant atmospheric positive ions are oxonium ions H 3 0+ and its hydrates
It was also shown that hydration of other ions of aeronomic interest takes place in the
atmosphere. With these statements possibilities of a new approach have been opened up for the consideration of the problems relating to atmospheric ions. Very important evidence was obtained by mass-spectrometric measurement both in the laboratory and in the atmosphere. Thus the way lay open for use of more or less plausible structural models of the ions. For example, hydration of the ions may be modelled by using the concept of the hydrogen bond. Since the drastic formulation of Heisenberg's uncertainty principle that it is not possible to determine exactly both the position and the momentum of a particle, physicists have been more hesitant in attempting to build visible models for atoms, molecules and ions with the atomic nuclei and corresponding electrons as the components. On the other hand, chemists have not been so scrupulous and have with success used different models for understanding the structure of the molecules, even the very complicated large molecules of bio-chemical interest. The chemists have also constructed models of simple molecules and molecular ions which can be used by physicists. Without touching the uncertainty principle it is possible to determine the spacing between the atoms building a molecule by using spectroscopical data about the vibrational levels of the molecule or by using the diffraction pictures obtained by the scattering of X-rays or neutrons. In place of the electron coordinates the orbitals giving the probabilities of finding the electron at some points around the atomic nuclei may be used. Orbitals calculated by means of the Schroedinger equation are some orientated directions around the atomic nuclei, and these localized orientated directions may be useful for construction of the models for molecules and ions, especially in the case of anticipated larger structures. For example the hydrates of the ions may be modelled by using the concept of hydrogen bonds. The models may be refined by additional quantum chemistry calculations for which very rich experimental and theoretical material from optical spectroscopy may be used. Hitherto, all the possibilities mentioned for obtaining more information about the ions have not been sufficiently utilized. It might be surprising that so late as 1968 at the Symposium on Laboratory Measurements of Aeronomic Interest held at Toronto, the H30+ ion is mentioned only after a note by one of the participants. On the other hand many related papers have been published during the last three years. However it would be misleading to assume that all the problems are already clarified. It is evident that much work is still necessary. As two problems of importance for the immediate future, the following should be mentioned: (1) Formation of H 30+ ions, and (2) Hydration of the primary atmospheric ions. Many reactions for the formation of H30+ have been proposed, but no definite conclusion can yet be drawn. The same is true concerning hydration. Some experimental and theoretical work is at present, waiting for systematization. If the problems concerning positive atmospheric ions may be regarded as to some extent resolved, those concerning the negative atmospheric ions are less clear. An attempt to systematize the material at our disposal is given by MOHNEN (1970).
144
R. SIKSNA REFERENCES
BRICARD, J. (1965) Action of radioactivity and of pollution upon parameters of atmospheric electricity. In: Problems of Atmospheric and Space Electricity. Proc. Third Int. Con! on Atmospheric and Space Electricity, Montreux, Switzerland, May, 1963, 82-117. BRICARD, J., GIROD, P. and PRADEL, J. (1965) C.R. Acad. Sci., Paris 260,6587. DOLEZALEK, H. and OSTER, A. L. (1965): Spectrometer for atmospheric ions in their uppermost range of mobility. Final Report-Avco-RAD-TR-65-25. DOLEZALEK, H. (1968) Discussion on mobility of atmospheric ions. Int. Ass. of Meteorology and Atmospheric Physics. Report of proceedings. Publication IAMAP No. 14, 238-240. ELSTER, J. and GEITEL, H. (1899a) Terr. Magn. Atmosph. Elect. 4, 213. ELSTER, J. and GElTEL, H. (1899b) Phys. Z. 1,245. ERIKSON, H. A. (1929) Phys. Rev. 34, 635. HOEGL, A. (1963) Z. angew. Phys. 16,252. HOPPEL, W. A. (1969) Pure appl. Geophys. 75, 158. KELLER, G. F. (1968) J. geophys. Res. 73, 3483. MIsAKI, M. (1950) Papers in Meteorology and Geophys. 1, 312. MISAKI, M. (1961) Papers in Meteorology and Geophys. 12, 247-260. MOHNEN, V. (1969) On the nature of tropospheric ions. In: Planetary Electrodynamics. Proc. Fourth Int. Con! on Universal Aspects of Atmospheric Electricity, Tokyo, May, 1968. Vol. 1, pp. 197-206. MOHNEN, V. (1970) J. geophys. Res. 75, 1717. NATANSON, G. L. (1959) Z. Techn. Fiz. 29, 1373. SIKSNA, R. (1964a) Ark. geofys. 4, 473. SIKSNA, R. (I 964b) Pure appl. Geophys. 59, 243. SIKSNA, R. (1966) Tel/us, 18, 619. SIKSNA, R. (1969) Role of the water substance in the structure and by production of ions in the ambient atmospheric air. In: Planetary Electrodynamics. Proc. Fourth Int. Con! Universal Aspects of Atmospheric Electricity, Tokyo, May, 1968. Vol. 1, pp. 207-230. TAMMET, H. F. (1967) Tartu Riikliku Ulikooli Tiomefsed 195, 1. WHIPPLE, E. C. (1960) J. geophys. Res. 65, 3679. WILSON, C. T. R. (1900) Proc. Camb. Phil. Soc. 11, 32. Proc. Symp. on Laboratory Measurements of Aeronomic Interest I.A.G.A. Symposium No.8, September 3-4, 1968, York University, Toronto, Ontario, Canada; Can. J. Chem. 47, 1703 (1969).
RELATIONS BETWEEN GENERAL AND ATMOSPHERIC PHYSICS, PARTICULARLY CONCERNING ATMOSPHERIC IONS R. SIKSNA Institutet fOr Hogspanningsforskning, Uppsala, Sweden Abstract-Problems related to the combination and recombination of ions, their mobility and their structure are discussed, to show the necessity of using more critically elaborated ideas taken from general physics for adequate studies of atmospheric ions.
THE TITLE of this presentation may be reformulated as: the relation of the atmospheric physicist to general physics. By the name atmospheric physicist is meant the physicist occupied with physical phenomena in the atmosphere, in our case the physicist occupied with ions, aerosols and radioactivity in the atmosphere. The pioneers in this domain, ELSTER (1854-1920), GEITEL (1855-1923), LENARD (1862-1947), WILSON (1869-1959), LANGEVIN (1872-1946), were pure physicists, since at that time atmospheric physics per se did not exist, and the problems of atmospheric physics were considered like those of general physics. Now the situation is changed and many problems will be not considered so thoroughly, mostly with ad hoc solutions, being satisfied if some formal accordance is reached between the observed phenomena and some general theoretical formulas, without examination of the foundations used for the derivation of these formulas. The situation can be illustrated with some examples concerning the ions. COMBINATION AND RECOMBINATION OF IONS
The problem of charging of aerosol particles by ions is important in atmospheric ionics. Closely connected with this problem are the combination and recombination processes of ions, characterized by the recombination or combination coefficient. As may be shown, comparatively complicated expressions can be derived for the recombination coefficient by using differing assumptions (BRICARD, 1965; NATANSON, 1959; SIKSNA, 1964, 1966; HOPPEL, 1969) with the surprising result that in the outer form the derived expressions appear to be similar. An insight into the relations by deriving corresponding expressions may be obtained by introducing a reciprocal magnitude of the recombination coefficient-recombination time (SIKSNA, 1966)-which is an additive magnitude for different stages involved in the recombination process. An important element in the theoretical consideration of the recombination processes is the mean free path. Unfortunately the meaning of the mean free path is not so clearly defined as is assumed in the elementary gas theory, as shown by a critical consideration of the related matter (SIKSNA, 1964). MOBILITY
Another problem, perhaps even more important in atmospheric ionics, is that of the mobility of the ions. The concept of the mobility of gaseous ions is taken from the consideration of the conductivity in electrolytes. The ions in a gas under the influence of a static, uniform electric field lose energy in elastic collisions with the gas molecules and gain energy from the electric field between collisions. If the energy acquired by the ions between 141
142
R.
SIKSNA
collisions is small compared with their mean thermal energy, their mean drift velocity the direction of the electric field will be proportional to the field strength E Vd
Vd
in
= fLE
The constant of proportionality fL is called the mobility of these ions in the particular gas. Accepting this simple relationship and ignoring the mechanism of the movement of the ions, it is possible to construct different experimental arrangements by which the coefficient fL may be measured. The aspiration condenser is the most commonly used instrument for mobility measurements. Very ingenious devices of this kind have been developed by ERIKSON (1929), MISAKI (1950, 1961), WHIPPLE (1960), HOEGL (1963), DOLEZALEK (1965), BRICARD (1965) and others. A systematic theoretical analysis of the aspiration method of measuring the mobility of atmospheric ions is provided by TAMMET (1967). It may be asked what kind of information we expect to obtain by using these measuring devices for mobility. Some authors have detected discrete mobilities; others, continuous distribution of the mobilities. It seemed that by knowing the mobility it would be possible to say something about the structure of the ions under consideration in other words, to identify the ions. That was one of the driving forces for the development of devices for measuring the mobility of atmospheric ions. However, it is shown that by simultaneous measurements of the drift velocity (from which the mobility is calculated) and by massspectrometric measurements, no correlation between the mobility and the mass (or the structure) of the ions was found. It may seem surprising that no attention is paid by atmospheric physicists occupied with the mobility measurements of atmospheric ions to one essential fact concerning the concept of mobility. A critical survey of this matter is given by KELLER (1968) who pointed out that ion-molecule collisions are absolutely essential when considering the mobility measurements, and species under investigation may be changed by these collisions. In conclusion it can be stated that in spite of highly ingenious experimental devices developed for measuring the mobility of atmospheric ions the expectations of obtaining such information about the nature of the ions were not fulfilled. Problems related to the idea of the mobility of atmospheric ions are still not solved, because they have not been considered from a more general point of view. NATURE OF THE ATMOSPHERIC IONS
Since the beginning of the century when ELSTER and GEITEL (1899a, b), and WILSON (1900) almost simultaneously introduced the term" ions" in considerations of the electric phenomena in atmosphere difficulties have arisen for people investigating these " atmospheric " ions, or, as they were called at that time, "air" ions. The air ions have been considered as something uncertain. Vague models have been proposed without the availability of modern theoretical concepts for explanation of their properties and the efforts made with mobility measurements were but desperate attempts to obtain more information about the nature ofthe atmospheric ions. A turning-point may be noted on October 2, 1967 in Lucerne during the 14th General Assembly of the IUGG when within the scope of the symposia arranged by the Joint Committee on Atmospheric Electricity it was planned to discuss mobility problems. However, the discussion turned out to be a proclamation of a new situation concerning the nature of atmospheric ions (DOLEZALEK, 1968). The basis for the conclusions arrived during this discussion is given in greater detail by MOHNEN (1969) and SIKSNA (1969).
Relations between general and atmospheric physics
143
One of these conclusions was: the dominant atmospheric positive ions are oxonium ions H 3 0+ and its hydrates
It was also shown that hydration of other ions of aeronomic interest takes place in the
atmosphere. With these statements possibilities of a new approach have been opened up for the consideration of the problems relating to atmospheric ions. Very important evidence was obtained by mass-spectrometric measurement both in the laboratory and in the atmosphere. Thus the way lay open for use of more or less plausible structural models of the ions. For example, hydration of the ions may be modelled by using the concept of the hydrogen bond. Since the drastic formulation of Heisenberg's uncertainty principle that it is not possible to determine exactly both the position and the momentum of a particle, physicists have been more hesitant in attempting to build visible models for atoms, molecules and ions with the atomic nuclei and corresponding electrons as the components. On the other hand, chemists have not been so scrupulous and have with success used different models for understanding the structure of the molecules, even the very complicated large molecules of bio-chemical interest. The chemists have also constructed models of simple molecules and molecular ions which can be used by physicists. Without touching the uncertainty principle it is possible to determine the spacing between the atoms building a molecule by using spectroscopical data about the vibrational levels of the molecule or by using the diffraction pictures obtained by the scattering of X-rays or neutrons. In place of the electron coordinates the orbitals giving the probabilities of finding the electron at some points around the atomic nuclei may be used. Orbitals calculated by means of the Schroedinger equation are some orientated directions around the atomic nuclei, and these localized orientated directions may be useful for construction of the models for molecules and ions, especially in the case of anticipated larger structures. For example the hydrates of the ions may be modelled by using the concept of hydrogen bonds. The models may be refined by additional quantum chemistry calculations for which very rich experimental and theoretical material from optical spectroscopy may be used. Hitherto, all the possibilities mentioned for obtaining more information about the ions have not been sufficiently utilized. It might be surprising that so late as 1968 at the Symposium on Laboratory Measurements of Aeronomic Interest held at Toronto, the H30+ ion is mentioned only after a note by one of the participants. On the other hand many related papers have been published during the last three years. However it would be misleading to assume that all the problems are already clarified. It is evident that much work is still necessary. As two problems of importance for the immediate future, the following should be mentioned: (1) Formation of H 30+ ions, and (2) Hydration of the primary atmospheric ions. Many reactions for the formation of H30+ have been proposed, but no definite conclusion can yet be drawn. The same is true concerning hydration. Some experimental and theoretical work is at present, waiting for systematization. If the problems concerning positive atmospheric ions may be regarded as to some extent resolved, those concerning the negative atmospheric ions are less clear. An attempt to systematize the material at our disposal is given by MOHNEN (1970).
144
R. SIKSNA REFERENCES
BRICARD, J. (1965) Action of radioactivity and of pollution upon parameters of atmospheric electricity. In: Problems of Atmospheric and Space Electricity. Proc. Third Int. Con! on Atmospheric and Space Electricity, Montreux, Switzerland, May, 1963, 82-117. BRICARD, J., GIROD, P. and PRADEL, J. (1965) C.R. Acad. Sci., Paris 260,6587. DOLEZALEK, H. and OSTER, A. L. (1965): Spectrometer for atmospheric ions in their uppermost range of mobility. Final Report-Avco-RAD-TR-65-25. DOLEZALEK, H. (1968) Discussion on mobility of atmospheric ions. Int. Ass. of Meteorology and Atmospheric Physics. Report of proceedings. Publication IAMAP No. 14, 238-240. ELSTER, J. and GEITEL, H. (1899a) Terr. Magn. Atmosph. Elect. 4, 213. ELSTER, J. and GElTEL, H. (1899b) Phys. Z. 1,245. ERIKSON, H. A. (1929) Phys. Rev. 34, 635. HOEGL, A. (1963) Z. angew. Phys. 16,252. HOPPEL, W. A. (1969) Pure appl. Geophys. 75, 158. KELLER, G. F. (1968) J. geophys. Res. 73, 3483. MIsAKI, M. (1950) Papers in Meteorology and Geophys. 1, 312. MISAKI, M. (1961) Papers in Meteorology and Geophys. 12, 247-260. MOHNEN, V. (1969) On the nature of tropospheric ions. In: Planetary Electrodynamics. Proc. Fourth Int. Con! on Universal Aspects of Atmospheric Electricity, Tokyo, May, 1968. Vol. 1, pp. 197-206. MOHNEN, V. (1970) J. geophys. Res. 75, 1717. NATANSON, G. L. (1959) Z. Techn. Fiz. 29, 1373. SIKSNA, R. (1964a) Ark. geofys. 4, 473. SIKSNA, R. (I 964b) Pure appl. Geophys. 59, 243. SIKSNA, R. (1966) Tel/us, 18, 619. SIKSNA, R. (1969) Role of the water substance in the structure and by production of ions in the ambient atmospheric air. In: Planetary Electrodynamics. Proc. Fourth Int. Con! Universal Aspects of Atmospheric Electricity, Tokyo, May, 1968. Vol. 1, pp. 207-230. TAMMET, H. F. (1967) Tartu Riikliku Ulikooli Tiomefsed 195, 1. WHIPPLE, E. C. (1960) J. geophys. Res. 65, 3679. WILSON, C. T. R. (1900) Proc. Camb. Phil. Soc. 11, 32. Proc. Symp. on Laboratory Measurements of Aeronomic Interest I.A.G.A. Symposium No.8, September 3-4, 1968, York University, Toronto, Ontario, Canada; Can. J. Chem. 47, 1703 (1969).