CHEMICAL COMPOSITION OF THE ATMOSPHERE AT AN ALTITUDE OF 70 KM

For the solution of a great variety of problems in geophysics, radiophysics, meteorology, aeronautics, and many other branches of science and technology, it is extremely important to know the chemical composition of the atmosphere at different altitudes. Up to the present time, direct chemical analysis of the composition of the air has been successfully carried out only up to 29 km. This analysis was performed by taking air samples at various altitudes during stratostat flights (up to 22 km) and, in a few cases, pilot balloons (between 25 and 29 km). Careful measurements of the oxygen and helium content in these samples showed that up to an altitude of approximately 20 km the composition of the air remains unchanged, while beginning with this altitude the percentage content of oxygen decreases somewhat and the percentage content of helium increases somewhat; this could be interpreted as an indication of the onset, from these altitudes, of diffusive separation of gases in accordance with the barometric formula. However, the observed changes in the composition of the air were very small, and the number and accuracy of the measurements did not provide a basis for confident conclusions. In particular, the observed deviations could be attributed to irregularities in the atmosphere, such as weather[^1].

At the same time, indirect methods based on the study of the spectra of aurorae and the glow of the night sky, the laws of twilight luminescence, and other phenomena convincingly indicated that, up to the greatest altitudes accessible to investigation, the Earth’s atmosphere remains basically oxygen–nitrogen in composition, and that hydrogen is present in it in at most negligible quantities[^2]. Theoretical consideration of the processes of diffusive separation of a gas mixture at different altitudes, together with evidence for the presence at great altitudes of powerful air currents, obtained from observations of the drift of noctilucent clouds, meteor trails, and other formations, led to the conclusion that, at least up to altitudes on the order of 100 km, the atmosphere must be completely mixed[^3].

Thus, although there was confidence in the uniformity of the composition of the air up to very great altitudes, the question of the degree of this uniformity remained open until the possibility appeared of carrying out direct chemical analysis.

In the cited work[^4], the results are presented of direct chemical analysis of air samples taken at altitudes of about 70 km, which apparently fully confirm the invariability of the composition of the air up to these altitudes. The samples were obtained during research flights of V-2 rockets into the stratosphere, carried out in recent years at the White Sands Proving Ground in New Mexico (33° N lat.).

The technique for obtaining the samples is described as follows. The rocket was equipped with evacuated steel cylinders, which, by means of thin-walled annealed copper tubes, were connected with chambers located beneath the rocket skin. The chambers were situated in the tail section of the rocket and, through openings in the rocket side, were blown through by a stream of the surrounding air. At a predetermined moment a machine automatically opened a valve closing the connecting tube, and air from the chamber gained access to the cylinder. After several seconds the cylinder was sealed again by simultaneous heating and flattening of the connecting copper tube. The samples were taken during the free ascent of the rocket. Some of the cylinders returned to the ground intact, having only small dents, and the pressure of the air enclosed in them proved to be in agreement with expectations for the case of proper operation of the mechanisms. The air from the cylinders was then pumped into glass vessels of 50 cm³ volume under a pressure of about 4 cm of mercury, which corresponded ...

approximately \(2.5\ \mathrm{cm}^3\) of gas under normal conditions. Some of these vessels were obtained by the authors for analysis; each analysis required \(0.2\)—\(0.5\ \mathrm{cm}^3\) of air. The samples were obtained during three flights carried out at different times. The article gives the results of analyses of five samples taken during two flights at approximately the same altitude, 70 km.

As the experiments showed, air samples analyzed not immediately after collection, but after some time had elapsed (which inevitably occurs when samples are taken by the method described), prove to be poorer in oxygen than the air introduced into the flask. (This can be explained by oxidation processes taking place in the flask.) Therefore, as in the preceding works of Paneth and Glückauf,\(^{1,5}\) the judgment about the composition of the atmosphere was based not on the relatively inaccurate measurement of the percentage content of oxygen and nitrogen, but on the measurement of the relative concentration of the noble gases. In the present case the content of helium, neon, and argon was measured relative to the mixture nitrogen plus argon. If the atmosphere were unmixed, and diffusive separation of the gases in accordance with the barometric formula were observed, then the first two relative concentrations should increase with altitude, while the third should decrease; moreover, for a given sample these three ratios should be related to one another in a quite definite way.

The authors indicate that the analyses for helium and neon were carried out by means of a procedure that is a further improvement of that used in the preceding studies of Paneth and Glückauf.\(^{1,5}\) The determination of the argon content essentially amounted to its purification and subsequent measurement of pressure.

The first stage of all the analyses was the removal of oxygen. The authors found that in the samples studied, oxygen was not present in any noticeable amounts (less than 0.1%). They explain this by saying that it had been consumed in the oxidation processes accompanying the sealing of the copper tube. The hydrogen content also proved to be less than 0.1%. However, as the authors note, this result cannot be referred to the outside air at the corresponding altitude, for hydrogen may have been absorbed by the walls of the metal and glass vessels. The results of measurements of the concentrations of helium, neon, and argon, expressed as percentages of the content of the mixture nitrogen plus argon, are presented in the table.

Relative concentrations of helium, neon, and argon, expressed as percentages of the content of the mixture nitrogen plus argon

The values of the relative concentrations for an altitude of 70 km are averages from five different samples. It is clear from the table that the relative concentrations of argon and neon remain within the limits of measurement accuracy unchanged in comparison with the air of the near-surface layers. The relative concentration of helium appears to decrease somewhat (by approximately 4%). But, as the authors point out, this decrease should not be given serious significance because of possible inaccuracies of the determination, as well as the unreliability of the error estimate.

The authors point out one more source of errors that makes their results not entirely reliable. According to aerodynamic calculations, it is possible that during the 80 seconds needed for the rocket to reach the altitude at which the samples were taken, the ground-level air that filled the rocket before its ascent will not be completely replaced by stratospheric air. In other words, there is a possibility that, when the sample is taken, together with the air surrounding the rocket at the given altitude, some quantity of ground-level air (or air from lower altitudes) carried along by the rocket will enter the cylinder. This makes further investigations in this area necessary. However, as the authors rightly note, even the assumption that 50% of the air entering the sample consisted of ground-level air cannot shake the principal result—namely, that the chemical composition of the air at an altitude of 70 km is practically identical with its composition at the earth’s surface. As was already indicated above, this result is in full agreement with the entire complex of ideas about the structure of the upper layers of the atmosphere and the processes taking place in them.

T. Rozenberg

REFERENCES

1. E. Glueckauf and F. A. Paneth, Proc. Roy. Soc. A 185, 89 (1945); see also E. O. Khalbart, UFN 34, 481 (1948).
2. See, for example, I. A. Khvostikov, The Glow of the Night Sky, Publishing House of the Academy of Sciences of the USSR, 1948.
3. H. B. Maris, Terr. Mag. 33, 233 (1928); 34, 45 (1929); E. H. Gowan, Proc. Roy. Soc. A 120, 655 (1928); A 128, 531 (1930); see also E. O. Khalbart, UFN 34, 481 (1948).
4. K. E. Chackett, F. A. Paneth and E. J. Wilson, Nature 164, 128 (1949).
5. F. Glueckauf, Proc. Roy. Soc. A 185, 38 (1945).