IONS 
for believing that in many of these determinations the 
precision was not especially high, and in some cases it 
seems probable that considerable error resulted from 
the collection of lower-mobility ions by the ion counter 
(resulting in too low a value for the mobility). The 
latter was probable because at one time it was custom- 
ary to apply to the ion counter a rather high potential, 
frequently as much as several times that required for 
small-ion saturation. A list of a few mobility values in 
air, from measurements where it is known that precau- 
tions were taken to avoid various possible errors, is 
given in Table I. The several values are in quite good 
TasiLe [. Somme Mopiniry VALuEs ror Smauu Ions 
Melati in cm 
Bieter. Station Period Say NOL Gear 
k+ k— 
[44] | Carnegie, Cruise VII 1928-1929 | 1.30 | 1.39 
[39] | College, Alaska 1932-1933 | 1.45 | 1.75 
[14] | Canberra, Austraha 1934 1.29 | 1.40 
[29] | Glencree, Ireland 1937 1.28 | 1.40 
agreement, especially in view of the very large differ- 
ence in meteorological conditions at the various stations 
and the possibility that such conditions might affect 
the mobility of the atmospheric ion. 
Since the mobility of an ion is affected by impurities 
in the gas, it seems likely that atmospheric ions may be 
affected by impurities in the air. A sudden change in 
mobility, believed to be due to such causes, was found 
by Parkinson [35] at Huancayo, Peru (altitude 3300 m, 
mean pressure 518 mm). From carefully controlled 
mobility determinations and from simultaneous meas- 
urements of air conductivity and ion concentrations 
which were corrected for the lower-mobility ions caught 
by the ion counter, 1t was found that a sudden drop in 
mobility took place each morning at about seven o’clock 
local time. This drop occurred at the precise time when 
there was a large influx of molecular impurities into the 
atmosphere. The average values of k+ and k— for the 
7-hr period before the influx of impurities were 2.40 
and 3.46, respectively. The mean values for the 7-hr 
period immediately following the influx were 2.00 and 
2.35, respectively. The change in negative-ion mobility 
was nearly twice as great as the change in positive-ion 
mobility. Thus it appears that the molecular impurities 
at Huancayo associate more readily with the negative 
than with the positive ion. 
Impurities in the atmosphere tend to be graded as to 
size, so that ions of a mobility lower than that of the 
small ion are often, although not always, found within 
rather narrow mobility ranges. Pollock [37] in Sydney, 
Australia, observed a group (intermediate ion) with a 
mobility between 0.1 and 0.02 cm. A similar mobility 
group has been found in other localities [14, 16, 47]. 
In some localities, on the other hand, there appears to 
be no such group [52, 56]. A lower-mobility group 
(around 10-4 em) first detected by Langevin [21] has 
since been observed in a number of localities, and the 
ions in this group are now called the “Langevin,” or 
IN THE ATMOSPHERE 
121 
large, ions of the atmosphere. Ions of still lower mobility 
have been observed in some localities [17]. 
Pollock found that the mobility of the termediate 
ion was a function of the vapor pressure of the atmos- 
phere, diminishing from around 0.1 to about 0.02 cm 
when the vapor pressure increased to about 17 mm, 
whereupon the ion was suddenly transformed into the 
large ion. Both Hogg [14] and Wait [47] examined this 
matter and neither found any tendency for the inter- 
mediate ion to be transformed into the large ion at any 
vapor pressure. Wait, however, found that the mobility 
of the intermediate ion was a function of the vapor 
pressure. At a pressure corresponding to 0 mm the 
mobility was about 0.5 em while at 30 mm pressure it 
was only 0.05 em. 
Yunker [56] found a continuous mobility distribution 
in California. He presented a distribution in which the 
number of ions per mobility imterval increases with 
increasing mobility. His results on artificially ionized 
air indicate that the concentration of ions of mobility 
down to 7 X 10~* em varies inversely with the nuclei 
concentration, which shows that the ions formed by 
charging of condensation nuclei have a still lower mo- 
bility. 
Nature of Slow Ions 
It is usually considered that a large ion is a charged 
condensation nucleus of the type discovered by Aitken 
[1]. This ion is usually singly charged, but multiply 
charged ions can exist [5, 15]. In general, nuclei appear 
to consist of some hygroscopic core around which a 
stable agglomeration of water molecules can form. 
Landsberg [20] has presented a detailed discussion of 
what is known of their properties. Sulphuric acid (a 
common product of combustion) and oxides of nitrogen 
probably play important parts in the formation of 
nuclei. Wright [55] discusses the matter of salt spray 
forming the core material for a nucleus which appears to 
be somewhat larger and is important in the atmosphere 
over the oceans and at some coastal eae but less 
important in most inland localities. 
Pollock [37], in his announcement of the existence of 
intermediate ions, considered them to consist of a 
“rioid nucleus enveloped by a dense atmosphere of 
water vapour.” Since the intermediate ion appears to 
exist only in certain localities, its nature probably 
varies greatly. If, as found by Wait [47], its mobility 
varies with water-vapor pressure in accordance with 
Blane’s law, then the size of the ion is pr obably con- 
stant. This argues against a hygroscopic ion. Hogg [16], 
working in ondan! detected intermediate ions the sizes 
of which were multiples of an aggregate of some 2000 
molecules. He assumed them to be composed of sul- 
phurie acid. 
Rate of Ionization 
The term zonization as used here refers to the produc- 
tion of ions, and not to ion concentration (as is some- 
times the case). Small ions may be produced in air by a 
variety of methods, such as by chemical and mechanical 
means, by ultraviolet light of sufficiently short wave 
