AEROBIOLOGY 
cates the presence of eddy masses which still retain the 
properties derived from their sources. However, the 
longer the elapsed time since such an eddy mass left its 
source, the more completely will its population become 
equalized with that of its surroundings because of 
diffusion and small-scale turbulence; hence, for any 
class of microorganisms, the ratio referred to above 
should diminish with distance from the source. 
The writer has previously analyzed data obtained by 
Rittenberg [26] on the distribution of molds’ over the 
ocean from shore to a distance of 400 miles westward 
from the Southern California coast. The mold samples 
represent collections made on shipboard at oceano- 
graphic stations at approximately equal distances along 
courses normal to the coastline. The region off the coast 
of California presents unusually stable conditions 
throughout practically all months, therefore it is not 
expected that the mixing processes are as effective here 
as in some other areas. Nevertheless, the data ac- 
cumulated by Rittenberg show the reasoning in the 
preceding paragraphs to be valid. His collections give 
an average mold count per hour of 155.6 for the region 
from 0-100 miles off the coast and a count of 29.0 for 
the region from 100-400 miles off the coast. If the mean 
of the differences in count between individual collecting 
stations is taken as a measure of the variability, a value 
of 250.1 molds per hour is found for the coastal region 
and 30.8 molds for the region beyond the 100-mile 
zone. These figures are interesting in that they give 
ratios between the mean differences in count and mean 
counts of 1.61 for the first region and 1.06 for the 
second. However, the actual mixing processes are prob- 
ably best represented if the ratio of the variance to the 
mean of the counts is taken. In this case, the ratio is 
402.0 in the first region and 32.7 in the second. As 
suggested, if the mixing were accomplished solely by 
pure diffusion, the ratios would approach 1.00 in both 
areas. Approximately the same number of counts were 
obtained in each of the areas under consideration. 
One may speculate as to the origin of the molds. 
The prevailing winds in the Southern California area 
are from the sea, and thus the air masses usually 
present over the coastal region are essentially marine 
in character and may have passed over several thousand 
miles of sea surface without contact with land. Since 
the mold collections were made by Rittenberg at inter- 
vals of varying length and on several cruises, it may be 
assumed that a large fraction of the counts were made 
within truly marine air masses. If the molds are pri- 
marily of land origin, they must have originated either 
over the continental land masses to the east or over the 
far removed land areas bordering the North Pacific 
Ocean on the west or north. The fact that Rittenberg 
notes a decrease in the mold counts from shore seaward 
appears to rule out the latter possibility which in- 
dicates that these organisms actually originated from 
7. Rittenberg assumes that all the molds are of land origin 
regardless of whether they develop on sea-water media or tap- 
water media, although it is admitted that at least a portion 
of the molds collected may have had a temporary sojourn in 
the sea. 
1109 
the land surface of the United States but were trans- 
ported westward across the prevailing wind stream 
through lateral mixing. Whether the effect is the result 
of a more or less continuous mixing process or is 
brought about by sporadic incursions of continental 
air masses over the ocean is a matter for further 
speculation. 
Considerations such as these suggest the use of bac- 
terial counts in the identification of air masses and in 
the study of large-scale mixing processes in the at- 
mosphere. The identification of the origins of species of 
organisms found in the atmosphere should give infor- 
mation concerning the origin of the air masses, while the 
study of the ratios (at the surface and aloft) between 
organisms representing two or more source areas should 
give additional information concerning the nature and 
effectiveness of lateral and vertical mixing. Few of the 
available aerobiological data, however, are complete 
enough to allow this type of analysis. 
Many observers have reported great irregularities in 
concentrations of organisms collected at different levels 
in the atmosphere, often finding heavier concentrations 
at higher altitudes than at lower levels. In some cases 
the investigators have reported encountering clouds of 
spores or bacteria several thousand feet above the 
surface of the earth, seemingly borne along by the wind 
as more or less discrete, sharply bounded units. How- 
ever, it should be pointed out that such collections 
have been obtained through the use of aircraft, and it 
is quite probable that in many cases the variations in 
samples merely represent the passage of the aircraft 
from and into convective (rising) columns of air which 
would be expected to have greater populations of or- 
ganisms than the surrounding (and perhaps descending) 
portions of the atmosphere. Wellington [84] has sug- 
gested the use of the helicopter for the purpose of 
“selective sampling’’—a technique which could be used 
to obtain more meaningful data on the vertical dis- 
tribution of organisms in the atmosphere. 
Several investigators have noted that increased 
counts of microorganisms are obtained in airplane 
collections upon entering clouds and have ascribed the 
condition to the fact that the cloud is a more favorable 
environment for the organisms than is clear air, al- 
though Durham [7] ascribes the increase to the loss of 
electrostatic charge and the consequent dislodging of 
particles which adhere to the plane. It does not seem 
to have occurred to the investigators that the vertical 
cloud boundary probably also represents the vertical 
boundary of a rising air column which should contain 
a heavier concentration of organisms than the sur- 
rounding clear (nonconvective) area. 
The discussion so far is intended to indicate the im- 
portance of the horizontal and vertical mixing in dis- 
tributing organisms throughout the atmosphere. The 
height to which the organisms may be transported 
depends upon the degree of stability (or instability) 
near the surface and upon the height of the turbulent or 
convective layer of air. The presence of a stable layer 
at the surface will prevent or retard the introduction of 
surface organisms into the upper atmosphere but will, 
