Some Observations of Chloride-Sulfate Relationships in the 
Atmosphere and in Precipitation 
JAMES P. LopGE 
Robert A. Taft Sanitary Engineering Center, U. S. Public Health Service, 
Cincinnati, Ohio 
Abstract—A series of measurements, including the determination of chloride and 
sulfate in particulate matter, were made at a U.S. Coast Guard weather ship midway 
between Honolulu and San Francisco. Even in this extremely isolated location, the 
sulfate concentration frequently exceeded the chloride. In the light of this finding, 
chloride-sulfate relationships were examined for a number of other remote stations; 
published data in the literature have also been studied. The chloride-sulfate regression 
lines were found to vary markedly in slope and intercept. There appears to be a 
definite relationship between chloride and sulfate concentrations; however, the nature 
of the relationship varies markedly with location. An interpretation is offered, based 
upon a three-source model. 
Introduction—Among the salts circulating 
through the atmosphere are sizable quantities of 
chlorides and sulfates. Both species are of im- 
portance in precipitation physics, since they form 
soluble particles which are extremely effective 
as cloud or precipitation nuclei. Both are of in- 
terest in air pollution, since man liberates them, 
or their precursors, in quantity by his activities. 
The levels contributed by nature thus constitute 
a background to the urban levels measured in 
air-pollution studies. On the other hand, the hu- 
man contribution constitutes a sort of ‘noise 
level’ on the natural concentrations, considered 
from a global geochemical viewpoint. 
For several years the U. S. Public Health 
Service has had in progress a small study of 
natural levels of substances which are considered 
pollutants when they are found in urban areas. 
In collaboration with the California Department 
of Public Health, measurements of these natural 
levels were made at a number of remote coastal 
sites [Harrison and Lodge, 1959; Holzworth, 
1959]. Subsequently two series of measurements 
were made during the summers of 1957 and 1958 
at Weather Station November, the weather ship 
located approximately midway between San 
Francisco and Honolulu [Lodge and_ others, 
1959]. When it was discovered that nearly all of 
the particulate samples from this operation con- 
tained more sulfate than chloride, a statistical 
study was made of chloride sulfate relationships, 
from both the coastal sites and from those data 
available in the literature. 
The data—In addition to two sets of measure- 
ments from Station November which were tested 
for homogeneity and pooled, the California study 
made available air data from Southeast Farallon 
Island, Point Piedras Blaneas, San Nicolas Is- 
land, and the top of Mount Hamilton. Also uti- 
lized were the excellent precipitation analyses of 
Larson and Hettick [1956] from central Hlinois, 
and analyses of air or precipitation or both from 
four selected stations of the Swedish network 
[Egner and Eriksson, 1955, and quarterly reports 
by a variety of authors in Tellus}: Kinnvika, 
Spitsbergen; Valentia, on the west coast of Ire- 
land; Edinburgh, Scotland; and Bonn, West 
Germany. 
The statistical analysis was not as complete as 
might be desired, either as to the depth of the 
treatment or as to the number of stations coy- 
ered. It would be desirable to extend the analysis 
to all of the stations of the Swedish network and 
to other historic sets of air and precipitation 
analyses, to see if it would bear out the tenta- 
tive conclusions of this paper. 
The Swedish data are reported in terms of the 
amount of mineral matter brought down by pre- 
cipitation per unit area. However, since total 
precipitation is also recorded, it is possible to 
convert the units given into concentration m 
the rain water. For convenience, the former 
figures will be referred to as ‘rainout’ data, and 
the latter as ‘precipitation’ data. It seems hkely 
that rainout should be a more valid measure of 
the concentration of materials aloft, since it is 
substantially independent of the intensity of pre- 
252 
