THE COMPOSITION OF ATMOSPHERIC AIR 
By E. GLUECKAUF 
Atomic Energy Research Establishment, Harwell, England 
For many reasons it is desirable to have a complete 
knowledge of the composition of the atmosphere as 
regards both the molecular species present and their 
absolute quantities. This applies not only to the main 
components, but also to rare species of polyatomic 
molecules whose importance in the radiation balance 
is often quite out of proportion to their actual quantity. 
Any observed variation in the composition of air—with 
time, with geographical location, with height, with the 
seasons, or with meteorological conditions—seriously 
affects our conception of the processes in the atmos- 
phere. It follows from this that the present article must 
lay its emphasis on facts in order to see where our 
knowledge is still inadequate. 
SURVEYS OF VARIATIONS IN THE 
COMPOSITION OF THE 
ATMOSPHERE 
Oxygen (O:). The first ‘international’’ investigation 
of the O2 content with adequate equipment was carried 
out as early as 1852 by Regnault [18] (see Table I). 
He concluded that atmospheric air presents only small 
variations. 
TasLe I. Survey or Oxycen ContTEeNT 
(After Regnault [18]) 
Location Average Oz (per cent) 
Wontpellier esses ochecei ccs ce aseeeenos 20.95 + 0.00 
AVOID. 0 Cdk Oe eS CRE eCeac SR 20.94 + 0.01 
Nearaa AT Cd yiap-t suis Ayre. oeinyorse Siesevered As Sloe eves 20.95 
13a hha, Se eee eee 20.96 + 0.01 
Mediterranean* 20.94 + 0.01 
Atlantic........... 20.94 + 0.01 
Hast Indian Ocean t 
Arctic Ocean........ 20.91 + 0.01 
TP BIT Bo 0.5.6 cS RRO IEE Ee eae 20.96 + 0.01 
ENIGHEYGOR Dae ban ome eee ee eee 20.944 
* Two samples from Algiers have been omitted. 
t Greatly varying, mostly low. 
Sixty years later, in a survey involving many hun- 
dreds of precision analyses, Benedict [c. 16]! proved 
conclusively that during a period of about two years, 
which involved a great variety of weather conditions, 
no material variations occurred in the O2 content of 
air at the Nutrition Laboratory of the Carnegie Insti- 
tution of Washington. A statistical analysis of Bene- 
dict’s data gives a standard deviation of +0.006 per 
cent, while the repeated analysis of a bottled air sample 
gave a standard deviation of only +0.0025 per cent. 
Tt is likely that the higher standard deviation of the 
1. A “c”’ is used in front of reference numbers to indicate 
that detailed reference is given in the paper cited. 
former originated from minute variations in the sam- 
pling procedure. 
Similar precision was obtained by Krogh [c. 16] 
whose analyses from October 4, 1917, to January 25, 
1918, showed a standard deviation of --0.005 per cent 
for Oo, 0.0025 per cent for CO2, and +0.0020 per 
cent for O2 plus CO2, all in uncontaminated air. The 
absolute values for O2 and CO, in dry air obtained by 
Krogh after careful calibrations are shown in Table II. 
This last figure—or the corresponding figure of 79.0215 
per cent for the content of “atmospheric nitrogen” 
(i.e., No + rare gases)—is considered by Krogh to be 
a geophysical constant which does not vary more than 
indicated by the standard deviation observed (--0.0020 
per cent). 
TaBLE II. OxyGEN AND CARBON D1oxIDE VALUES IN Dry AIR 
(After Krogh [c. 16]) 
Content (per cent) 
Constituent 
Experiment I Experiment II 
COS ee eee or atysrsieto tesa 0.0305 0.0300 
ORR atic chin da heme meeR ae ea 20.9480 20.9485 
OFC OTe etre sane re 20.9785 20.9785 
The absolute value of the O2 content of air obtained 
by Benedict, taking into account a small correction 
resulting from the formation of CO from the pyrogal- 
late, was later computed by Benedict and by Haldane 
as 20.952 per cent and by Krogh [c. 16] as 20.954 per 
cent. 
While Benedict’s experiments prove conclusively that 
only trifling changes occur in air observations at one 
locality, it is more difficult to show that there is uni- 
formity over all the earth’s surface. For such a survey 
it is necessary to bottle air samples, and changes of one 
or two parts in ten thousand can easily occur during 
transit of the air sample. Table III gives a summary of 
Benedict’s analyses of air of various origins. It is not 
Taste III. Survey or OxyGEn ConTENTS 
(After Benedict [c. 16]) 
O02 
sea 6 No. of 
Origin of air cana (eee 
Boston Ming! f <a ct cle sie a cicduerets s eseversis 212 20.952 
Ocean air (Montreal to Liverpool)....... 7 20.950 
Ocean air (Genoa to Boston)............. 36 20.946 
PikesuReakierrn, scwcd: tisccaptomntere at adits ss 5 20.945 
* The data for August 14, 1911 which showed abnormally 
low values have been omitted. 
likely that significance can be attached to these small 
variations. 
