Very strong evidence has been found that the formation of
uncharged condensation nuclei in filtered Edinburgh air, by
UV radiation of wavelength greater than 2900A, was due to a
photochemical reaction involving sulphur dioxide. The
intensity of the UV light used, weighted according to
absorption by sulphur dioxide, was calculated to be approximately
equal to the similarly weighted intensity of the
direct and indirect radiation from the sun, at the zenith in
a clear sky.
It seems that these UV nuclei are only formed when
water vapour is present in the air, on irradiation. The
nuclei may be produced from sulphuric acid molecules formed
by water vapour and sulphur trioxide. The sulphur trioxide
would result from the catylised photochemical reaction of
sulphur dioxide and molecular oxygen or from the reaction of
sulphur dioxide with atomic oxygen, formed by the photochemical
dissociation of nitrogen dioxide and ozone. Alternatively the
nuclei may be formed from ammonium sulphate or ammonium hydrogen
sulphate, which would be produced by a reaction of sulphuric
acid molecules produced as above, and ammonia. Another possible
reaction leading to UV nucleus formation, is the photochemical
reaction of sulphur dioxide with a mixture of nitrogen oxides
and olefin hydrocarbons.
The nuclei are probably formed, due to a chemical super-saturation of the air with the material of the nuclei.
IMPORTANCE OF UV NUCLEI IN THE ATMOSPHERE:
It is probable that UV nuclei similar to those observed
in the author's experiments, are produced in the open atmosphere,
when both the pollution level of the air, and the intensity of
solar radiation, weighted according to absorption by sulphur
dioxide, are sufficiently high. It must be remembered, however,
that the concentration of UV nuclei produced in untreated
atmospheric air would be smaller than the concentration produced
in filtered atmospheric air. This would be so, because the
nucleogenic material, photochemically produced, would partly
condense on nuclei already existing in untreated atmospheric
air, rather than form new nuclei (see pages 84 -87).
The layer of relatively large sulphate particles found at
a height of 20 km. in the stratosphere, and described by
Junge et al(86,87) and Byers(88), almost certainly owe their
formation to a photochemical reaction involving sulphur dioxide.
It is also possible that the particles, which act as nuclei for
the ice crystals believed by some workers such as Paton(89) to
form noctilucent clouds, may have a photochemical origin.
Photochemical "smog" or haze formation is a serious problem
in numerous cities, such as Los Angeles. When such a "smog" or
haze is formed, the haze particles or nuclei act as centres of
physical or chemical condensation for various pollutants.
Since large concentrations of various pollutants are then present
in the nuclei, reactions can occur in the nucleus or on the
nucleus surface, which would not take place in the absence of
haze. Photochemical haze is injurious to health, causing
irritation of the eyes and of the respiratory tract. It should
be realised that for proper control of air pollution, it is not
sufficient to control emission into the atmosphere of visible
smoke only, but that the emission of trace gases and vapours,
particularly sulphur dioxide, should also be controlled.
POSSIBLE USE OF CONDENSATION NUCLEI TECHNIQUES, FOR THE CHEMICAL
ANALYSIS OF TRACE GASES
The formation of UV nuclei in filtered atmospheric air
was found to increase with the sulphur dioxide concentration.
It seems therefore, that a method could be developed to measure
the concentration of sulphur dioxide in the atmosphere or in a
gas mixture, by forming nuclei from sulphur dioxide and its
reaction partner using UV radiation, under carefully controlled
conditions. The concentration of nuclei produced would be
measured by means of a suitable nucleus counter. Similar
methods could be used for the chemical analysis of other trace
gases such as ammonia, hydrogen sulphide, and hydrocarbons, which
are known to take part in photochemical reactions leading to UV
nucleus formation. However, it is stressed that since there is
very little information available, on the production of UV
nuclei by irradiation of various gases and vapours, that there is
need for much more research on this problem, before the above
method will be of practical use.
SUGGESTIONS FOR FUTURE RESEARCH:
It is extremely important that great care should be taken
in future experiments on the production of nuclei by UV irradiation
to use experimental conditions similar to the conditions
obtaining in the atmosphere, if it is desired to use the results of such experiments to find out what may happen in the open atmosphere.
Thus the UV radiation used, should not contain
light of wavelengths lower than the wavelengths of sunlight in
the lower atmosphere. The intensity of the UV light used
should be as uniform as possible and of the same order of magnitude
as the intensity of sunlight in the lower atmosphere.
The concentrations of trace gases and vapours used in such
experiments should be of the same order of magnitude as the
concentrations found in the atmosphere. Great care should
be taken that no materials are used in the apparatus, especially
in the irradiation chamber, which might form nuclei on irradiation.
It should be extremely interesting to investigate in detail
possible nucleus forming photochemical reactions between sulphur
dioxide and the various reaction partners discussed in chapter 27,
particularly the reactions of sulphur dioxide with molecular oxygen,
atomic oxygen, ammonia, and a mixture of nitrogen oxides and olefin
hydrocarbons. It is stressed again, that these investigations
should be carried out under conditions similar to those obtaining
in the atmosphere.
It is intended to continue the research reported in this work,
along these lines.