PHOTOCHEMICAL PROCESSES IN THE UPPER ATMOSPHERE 
The chemistry of the atmosphere differs from labora- 
tory chemistry because the atmosphere is without any 
material boundary except the ground, which exerts an 
appreciable chemical influence only on the air at the 
lowest levels. Above these levels, the wall-reactions, 
so important in laboratory chemistry of gases, play no 
part in meteorological chemistry. 
The chemistry of the upper atmosphere is a border- 
line subject between the main fields of chemistry and 
meteorology, and naturally involves many aspects and 
conceptions that may be unfamiliar to meteorologists. 
The chief aim of this article is to assist them to read 
past and future papers on the subject by outlining its 
principles and some of its technicalities. The present 
primitive state of the subject is also briefly described, 
and some of its main problems are indicated. In so 
doing it is necessary to pay some attention to at- 
mospheric spectroscopy and ionization. 
SOME GENERAL PRINCIPLES OF GAS 
CHEMISTRY 
3. Atomic and Molecular Particles. Atomic particles 
consist of a positively charged nucleus surrounded by 
(negative) electrons; molecular particles comprise more 
than one such nucleus, with electrons around and be- 
tween them. If the number of electrons is equal to the 
number of positive electronic charges (each of magni- 
tude ¢) in the nucleus or nuclei, the particle is electri- 
cally neutral, and is called a (neutral) atom or molecule 
respectively; otherwise the particle is called an ion, 
atomic or molecular. In general, ions are positive, the 
number of electrons being less than that necessary for 
electrical neutrality; but certain gases, notably (in the 
atmosphere) atomic and molecular oxygen (but not 
nitrogen or the rare gases) form negative ions, taking 
on an extra electron beyond their normal number in the 
neutral state. Negative ions are indicated thus: O", O2. 
Positive ions may be similarly indicated, for example, 
0+, N% or, if doubly ionized, N$*. Otherwise 1, m1, 
m1, are added to the chemical symbol to signify respec- 
tively the neutral state, the first positively ionized 
state, the second, and so on: for example, N21, N21, 
N21 instead of No, Nf, NS*. 
The charge on an atomic nucleus is Ze, where Z is an 
integer, called the atomic number; it determines the 
chemical nature of the atomic particle. For hydrogen 
H, carbon C, nitrogen N, oxygen O, and sodium Na, 
Z is 1, 6, 7, 8, and 11 respectively. 
The unit of atomic mass mp is one-sixteenth of the 
mass of the chief type of oxygen atom, the isotope O01; 
mo = 1823 me, where m. denotes the mass of an elec- 
tron [82]; hence the electrons make a very minor con- 
tribution to the masses of atoms. The mass of an atom 
or molecule is expressed respectively as Amy or Mmp; 
A or M is called the atomic or molecular weight. Atoms 
may have the same Z (and therefore the same chemical 
nature) but different atomic weights A; the different 
forms are called isotopes. They usually differ greatly in 
their relative abundance; for example, in the case of 
oxygen the isotope for which A = 16 far predominates 
over the isotopes for which A (always nearly an in- 
teger number) is approximately 15 or 17, so that the 
263 
average atomic weight of oxygen (including all iso- 
topes) differs very little from 16. The isotopic constitu- 
tion of the chemical elements in the atmosphere forms 
an interesting subject of study, as yet little developed. 
It will not be further mentioned here. 
Chemists call M grams of a substance whose molec- 
ular weight is M a gram-molecule or mole or gram 
molecular weight (and, similarly, A grams of an ele- 
ment whose atomic weight is A, a gram-atom). The 
number NV of molecules (or atoms) in a gram-molecule 
(or gram-atom) is MW grams divided by the mass of one 
molecule, namely Mm, that is, N = 1/m if mo is 
measured in grams; likewise for the number of atoms 
in a gram-atom. This number N is called Loschmidt’s 
(or, less appropriately, Avogadro’s) number: 
1.660 X 10° gram; N = 6.023 X 10. 
4. Energy Levels. Atomic and molecular particles, 
neutral or ionized—hereafter for brevity often referred 
to simply as particles—may exist in more than one 
state, and each state has a definite amount of internal 
energy (as distinguished from the kinetic energy of 
translatory motion). These states form a discrete (not 
continuous) series. 
In an atomic particle the internal energy is deter- 
mined solely by the electronic configuration (including 
the electron spins). In a molecular particle it may also 
include energy of internal vibration and of rotation. 
These latter energies mainly depend on the disposition 
and motion of the nuclei, because of their preponderant 
share of the molecular mass. Hach type of energy has a 
discrete series of values, associated with quantum num- 
bers, electronic, vibrational, and rotational. There is 
consequently a series of electronic energy levels, and in 
the case of molecular particles these levels (for states 
without vibration and rotation) are supplemented by 
many neighbouring levels of higher energy, for states 
in which there is also vibration or rotation or both. 
The state of lowest energy is called the ground state; 
the others are called excited states. The energies of the 
latter are reckoned as differences from the energy of the 
ground state, and are called excitation energies or exct- 
tation potentials. The minimum energy needed to re- 
move an electron altogether from a particle in its 
ground state is called the ionization energy or ionization 
potential; in atmospheric chemistry one mostly considers 
only the first ionization potential, for the removal of 
only one electron from the neutral particle. The energy 
required to detach the excess electron from a negative 
ion such as O~ or O3 is called the attachment energy or 
electron affinity. The energy necessary to divide a molec- 
ular particle (neutral or ionized) into two atomic (or 
molecular) particles is called the dissociation energy or 
dissociation potential. 
In a molecule the distances between its atomic nuclei 
are in general different in different electronic configura- 
tions. A molecule may be raised to a state in which its 
energy, including vibrational energy, is greater than 
that required for dissociation, yet the internuclear dis- 
tances may not be such that the molecule will divide, 
without some rearrangement of the nuclei and electrons. 
If the molecule is such that this redistribution can take 
Mo = 
