PHOTOCHEMICAL PROCESSES IN THE UPPER ATMOSPHERE 
light has a part which is of terrestrial origin and includes 
bands due to Ns, Oo, O3, COs, and H,0. Except for 
O3, the major part of all these gases lies in the lowest 
and densest part of the atmosphere, but the absorption 
in the ultraviolet region by Ne, Oo, and O3 occurs at 
higher levels. 
The amount of ozone present varies with the latitude, 
season and weather, but is of the order 3 atmo-milli- 
metres. (It is convenient to express the amount of the 
rarer gases of the atmosphere by the thickness of the 
layers they would form if all of each such gas were sepa- 
rated out from each vertical column of atmosphere, 
and collected at ground level at normal temperature 
(OC) and pressure (760 mm of mercury); the amount 
may then be named as being in atmo-centimetres or 
atmo-millimetres.) 
There is also good evidence for the presence of about 
1 atmo-em of nitrous oxide (N20); it seems likely that 
it is mainly in the lower atmosphere. This gas is very 
transparent for radiation of wave lengths greater than 
2000 A, and must therefore be very stable photochemi- 
cally. Study of its ultraviolet spectrum indicates an 
ionization potential of 12.66 ev, but its dissociation 
potential and products are still uncertain. Other oxides 
of nitrogen may also be present; it appears to be pos- 
sible to set upper limits of 0.1 atmo-mm for the amounts 
of NO; and NO;; there is some evidence for the pres- 
ence of less than 1 atmo-mm of N20;. If formaldehyde 
(CH,0) is present (presumably in the troposphere and 
lower stratosphere), its amount must be less than 1 
atmo-mm [26]. 
The sun’s absorption spectrum gives information 
concerning atmospheric composition by day; similar 
information about the atmosphere at night could be 
obtained from the spectrum of moonlight or starlight, 
but the practical difficulties are much greater than with 
sunlight, owing to the much lower intensity of such 
light. 
19. Emission-Spectral Evidence Regarding Upper At- 
mospheric Composition. The spectrum of the night sky 
shows prominently the green and red lines of atomic 
oxygen and the yellow line of atomic sodium (§ 12). 
In the infrared there are strong hydroxyl (OH) bands 
[83] (probably including bands at 10,400 A originally 
ascribed to N4 by a process depending on recombination 
of N atoms) and also Kaplan-Meinel bands of O2 (1.7 
ev). These are the parts of the spectrum as yet most cer- 
tainly identified. There is also a continuous background 
throughout the visible region, fading in the ultraviolet 
at \ 3900; somewhat lost in the blue and violet part of 
this continuum (A > 3900) there are very many lines 
and bands, and there are others, very distinct, in the 
ultraviolet (A < 3900). Of the latter, the strongest have 
been ascribed to the Herzberg bands of O» (excitation 
energy 4.7 ev), and others, weaker, as the Schumann- 
Runge bands of O; (6.2 ev). In the visible region many 
_ bands are ascribed to the Vegard-Kaplan bands of N» 
(>7.0 ev). Barbier has ascribed some of the visible 
bands to CO. Not all these ascriptions are certain. 
The OH emission may be due to the reaction 
H + 0; — OH' + Os, (+76 kilocalories), 
271 
the H atoms being produced by dissociation of water 
vapor by sunlight. 
The excitation energies indicated for the oxygen green 
and red lines respectively are 4.2 ev and 2.0 ev, and 2.1 
ev for the sodium yellow (or D) line. 
It is likely that the spectrum is mainly what is called a 
recombination spectrum, the energy being provided by 
the recombination of atomic O to form O2, and (less 
probably) of atomic nitrogen to form N2 . One argument 
for the latter hypothesis is that the recombination of 
oxygen provides no more than 5.09 ev, whereas that of 
nitrogen can provide at least 7.38 and possibly 9.76 ev. 
(The dissociation energy of Ne is still doubtful.) The 
energy provided by the recombination of ions and elec- 
trons will also contribute to the night-sky spectrum 
to a smaller degree. 
At twilight (dawn and sunset), part of the emission 
spectrum is enhanced: chiefly the red oxygen line and 
the sodium yellow line. At these times there also appears 
emission of light in the band spectrum of ionized 
nitrogen (N3), in the region 3914 A, correlated with the 
enhanced emission of the red oxygen line. This betokens 
higher energy absorption from sunlight in the high 
atmosphere then irradiated, and consequent emission, 
some of which is visible from places on the ground where 
the sun has already set or not yet risen. 
The auroral spectrum shows chiefly the green and red 
lines of atomic oxygen, bands of Ny and N3; it seems 
also to indicate the presence of atomic nitrogen, hy- 
drogen, helium, and perhaps Het . The energy of excita- 
tion is much greater than for the night-sky spectrum, 
and can reasonably be attributed to impacts caused by 
fast-moving particles coming from the sun and entering 
the atmosphere from above; they will “knock on” 
many atmospheric particles, which will thus be second- 
arily responsible for the impacts causing most of the 
observed excitation effects. Auroral emission caused by 
weak corpuscular streams impinging on the upper at- 
mosphere may at times be mingled with the true 
night-sky recombination spectrum, even when there is 
no obvious visual sign of the presence of an aurora. 
Recently it has been found that the atomic hydrogen 
lines in the auroral spectrum, when their light is received 
at a small inclination to the auroral rays, may show 
much broadening and large Doppler displacements to- 
wards the ultraviolet. This indicates that the emitting 
H atoms are travelling downwards through the at- 
mosphere with speeds up to some thousands of kilo- 
metres per second. 
20. The Heights of the Absorbing and Emitting 
Layers. By observing the intensity of absorption or 
emission of any particular kind along paths of different 
inclination to the vertical, or (by day) of different zenith 
distances of the sun, it is possible to estimate the level 
of absorption or emission, and in some cases to infer 
also the height-distribution of the atmospheric con- 
stituent concerned. In this way it has been possible to 
determine the height-distribution of atmospheric ozone,” 
5. Consult ‘Ozone in the Atmosphere” by F. W. P. Gotz, 
pp. 275-291 in this Compendium. 
