PHOTOCHEMICAL PROCESSES IN THE UPPER ATMOSPHERE 
AND RESULTANT COMPOSITION 
By SYDNEY CHAPMAN 
Queen’s College, Oxford, England 
INTRODUCTION 
1. Chemistry and the Troposphere. The meteorology 
of the lower atmosphere, the region of weather, is essen- 
tially a physical science, in which chemistry plays 
practically no part; chemists may become good meteor- 
ologists, but meteorologists need not know much chem- 
istry. Their work is concerned with fluid dynamics, 
with heat interchanges by radiation and conduction 
and convection, with mixing and changes of state of 
water vapor—evaporation, condensation, sublimation, 
and precipitation. 
Chemists are indeed interested in the composition of 
air,! but their work has generally been supposed to end 
with the chemical analysis. This shows that air is a 
mixture of several gases, merely coexisting and not 
reacting chemically with each other to any significant 
degree in the lower atmosphere.” This is true for the 
main permanent constituents, nitrogen VN. (78 per cent 
of dry air, by volume) and oxygen O, (21 per cent); 
for the chief variable constituent, water vapor H,0; and 
also for carbon dioxide CO, and the rare gas constitu- 
ents, helium, neon, argon, krypton, xenon, and radon. 
In the lower atmosphere the only changes in the 
composition of dry air are minor and local, such as the 
withdrawal of a small amount of oxygen (and ozone) by 
plants or by oxidation of iron and organic matter; or 
the emission of carbon dioxide by plants, of organic and 
other gases by decaying matter and by factories and 
chimneys, and of helium and radon from radioactive 
matter below ground. Among the few constituents of 
the lower atmosphere that undergo appreciable chemi- 
cal change are ozone and radon; the latter disintegrates 
radioactively, so that its concentration decreases up- 
wards. This is a process of nuclear chemistry, outside 
the realm of ordinary chemistry, which is concerned 
with reactions affecting only the outer structures of 
atoms and molecules. The incidence of cosmic rays 
must also produce nuclear chemical transformations, 
relatively very rare, but worthy of study. However, 
the main fact of tropospheric chemistry is the uni- 
formity of the composition of dry air all over the globe, 
near the ground, as shown by Paneth.) 
Thus ozone is almost the only chemically active con- 
stituent of the tropospheric air, though it may be that 
there are other very rare but chemically active gases 
present, whose changes have not yet been considered. 
Ozone is always present in the air near the ground,* 
1. Consult ‘‘The Composition of Atmospheric Air’ by E. 
Glueckauf, pp. 3-10 in this Compendium. 
2. Consult ‘‘Some Problems of Atmospheric Chemistry”’ by 
H. Cauer, pp. 1126-1136 in this Compendium. 
3. Consult ‘“‘Ozone in the Atmosphere” by F. W. P. Gotz, 
pp. 275-291 in this Compendium. 
262 
though it is reduced in concentration over industrial 
areas and to windward of them by oxidation processes. 
For a long time, however, little attention was paid to 
the chemical problems raised by the continued presence 
in the air of a somewhat unstable gas like ozone. 
2. Atmospheric Chemistry. A more active interest 
in the chemical processes of the atmosphere was aroused 
when it was found that the ozone density is less in the 
air near the ground than in the air well up in the strato- 
sphere—the height of maximum density being 20 to 
25 km, though early estimates were 40 to 50 km. Such 
a distribution differs from that of all the other known 
constituents of the lower atmosphere, which decrease 
upwards in density, maintaining the same relative con- 
centrations at least up to 20-km height, except im the 
cases of radon, whose relative concentration decreases 
upwards, and water vapor, whose distribution is ir- 
regular and variable. 
This peculiar height distribution of ozone may be 
explained by attributing the formation of ozone to the 
dissociation of oxygen molecules into atoms O by sun- 
light; the O atoms then combine with O2 molecules to 
form ozone (O + O: — Os), which is itself dissociated 
by sunlight (O; — O2 + QO). The consequent reactions 
between the three forms of oxygen (O, Os, O3), which 
achieve a slowly changing equilibrium mixture, are 
very complicated, and the relative importance of these 
reactions varies with the height. Above about 80 km 
the ozone concentration sinks to insignificance and the 
atomic oxygen concentration increases to importance, 
and at levels probably of 100 to 120 km O becomes pre- 
dominant over molecular oxygen. Sts presence is indi- 
cated by the emission spectra of the atmosphere, namely 
those of the night sky and of the aurora. Similar spec- 
tral evidence reveals the presence of atomic sodium Na 
and also of atomic nitrogen NV in the upper atmosphere, 
and the terrestrial part of the absorption spectrum of 
sunlight indicates that some oxides of nitrogen also 
exist in the atmosphere. The presence of such chemi- 
cally active gases as atomic oxygen and sodium raises 
very interesting chemical problems, on which there is 
now a growing literature. 
The chemical reactions and their energy mainly origi- 
nate from the absorption of light from the sun, so that 
meteorological chemistry is in the main a branch of 
photochemistry. It includes also some zmpact-chemistry, 
as it may be called, concerned with reactions initiated 
by the entry of fast-moving particles into the atmos- 
phere from outside—a process somewhat analogous to 
the chemistry of reactions inside discharge tubes, in 
which fast-moving ions and electrons produce chemical 
changes in the gas. In the atmosphere the impacts are 
mainly those of the solar particles that cause aurorae 
and magnetic storms. 
