PII: 0004-6981(71)90144-2


Atmospheric En~~ro~e~r Per@mon Press 1971. Vol. 5, pp. 413-422. Printed in Great Britain. 

DIURNAL VARIATIONS OF AEROSOL TRACE ELEMENT 
CONCENTRATIONS AS DETERMINED BY 

NONDESTRUCTIVE NEUTRON ACTIVATION ANALYSIS 

K. A. RAHX, R. DAMS,* J. A. ROBBIX and J. W. WINCHESTERS 

Dept. of Xfeteorology and Oceanography and Great Lakes Research Division, University of 
Michigan, Ann Arbor, Michigan 48104, U.S.A. 

(First received 27 April 1970 and in find form 13 August 1970) 

Abstract-Diurnal concentration variations of 20 trace elements in surface air have been studied 
over a 34-h period in a rural area. Twenty-five millimetre polystyrene air filter samples were 
taken each 90 min and analyzed by nondestructive neutron activation analysis using Ge(Li) 
gamma-spectrometry and computer assisted data handling. Basic diurnal patterns are related 
to meterological variables but variations from element to element are ascribed to differences in 
particle size distributions as determined by an Andersen Cascade Impactor. The behavior and 
particle size distribution of some elements suggested local sources. 

INTRODUCTION 

A STUDY of the diurnal concentration variations of atmospheric trace elements has 
recently been begun, as part of a larger study of their general atmospheric behavior. 
Previous investigations of daily patterns of pollution concentrations have been both 
theoretical (HEWSON, 1960; U.S. DEPARTMENT OF HEALTH, EDUCATION AND WELF~~RE, 
1965) and empirical (COMMINS~ 1967; MUNN, 1959; SU;MMERS, 1966; WEISMAIU’, 1969; 
U.S. PUBLIC HEALTH SERVICE, 1968) where in the latter cases soihng index, smoke, 
sulfur dioxide, nitrogen oxides and total particulate have served as indicators. To our 
knowledge, however, daily variations of individual trace elements have not yet been 
reported. Previous studies have all been in urban areas, whereas a rural setting was 
chosen for this investigation in an attempt to minimize the effects of source processes 
and urban micrometeorology relative to the mesometeorology. 

This study was made possible by the recent development by the authors of a 
technique for nondestructive neutron activation analysis of aerosols collected on a 
high purity filter (DAMS, 1970) in which up to 33 elements may be determined from 24 
hour samples, many of which are recognized as being of pollution origin. Diurnal 
studies require sample lengths as short as possible, and the extreme sensitivity of neutron 
activation allows the determination of at least 15 elements in samples collected for as 
short a time as 90 min. In an effort to better correlate time variation patterns from 
element to element, particle size distributions were measured concurrently. 

SAMPLING 

Sequential 90 min filter samples were taken throughout the period, using a poly- 
styrene material (DAMS, 1971) which combines good filtering performance with 
reasonably high purity. A high flow rate per unit surface area was achieved through 
the combination of a high vacuum pump (Gelman twin cylinder) and 25 mm dia. 
filter holders. Such a system produces flow rates through the poIystyrene of 12 

*Present address: Institute for Nuclear Sciences, University of Ghent, Ghent, Belgium. 
tPresent address: Dept. of Oceanography, Florida State University, Tallahassee, Florida 32306, 

U.S.A. 

413 



414 K. A. RAHN, R. DAMS, J. A. ROBBISS and J. W. WISCHESTER 

04 06 08 IO I.2 14 16 I8 20 22 24 02 04 06 08 IO I2 14 16 18 203’ 

HOUR (EST) AUG. 21.1969 AUG. 22, 1969 

FIG. 1. Relative humidity, temperature and dewpoint during sampling. 

I L2 
-w 

6- 

E: 

: 

g 
F 

06 08 IO 12 14 16 I8 20 22 24 02 04 06 06 IO 12 14 16 I8 20 
HOURKST) AUG. 21, 1969 AUG. 22, 1969 

FIG. 2. Wind speed and direction during sampling. 

I-min-‘-cm-‘, as opposed to the figure of 4.5 I-min-‘-cm-’ obtained with a high 
volume sampler (20 x 2.5 cm titer and low vacuum pump). Each sample consisted 
of aerosols from approximately 4 m3 of air. 

Particle size distribution of the elements were determined by use of an Andersen 
Cascade Impactor, which separates aerosols into 7 fractions ranging from radii of 
roughly 8 pm down to 0.1 pm. High purity polyethylene sheets were used as impaction 
surfaces and analyzed in the same manner as the titers. A backup polystyrene filter 
(25 mm dia.) was used to catch the smaller particles. Because of the low flow rate of 







Diurnal VariationS of Aerosol Trace Element Concentrarions 415 

this instrument (28 l-min”“) only one size distribution sample was taken throughout 
the experiment. All samples were taken in a ventilated instrument shelter 1.5 m above 
ground, over short grass. 

Sampling was performed in a rural area 5 km west of Niles, LMichigan and 15 km 
north of South Bend, Indiana. It is approximately 45 km east of Lake Michigan and 
100 km northeast of the heavily industrialized Northwest Indiana area. Samples were 
taken during 34 h on 21 and 22 August, 1969, when the entire north central and 
northeastern U.S. was under the influence of a broad Canadian high pressure area. 
Winds during this period varied from calm at night to 6.5 m s-r during the after- 
noon, and the extreme stability of the air mass prevented any clouds from forming. 
Light ground fog was observed during the early morning hours of the second day. 
FIGLT~~E~ I and 2 give some of the meteorological conditions recorded at ESSA WBAS 
South Bend, Indiana. FKXJF~ES 3 and 4 show surface weather conditions for 7 AM EST 
on both sampling days. 

ANALYSXS 

The technique as described earlier (Dx~s, 1970) was applied. The complete filter 
(25 mm dia.} or the complete polyethylene sheet was irradiated twice with slow 
neutrons in the Ford Nuclear Reactor at the University of Michigan, first for 5 min, 
later for 4 h. Each sample was counted 4 times, at 3 and I5 min after the first irradia- 
tion, and 20 h and 20 days after the second irradiation. Counting was performed with 
a 30 cm3 Ge(Li) high resolution gamma ray detector coupled to a 4096 channel pulse 
height analyzer. A digital computer program was used to integrate photopeaks of the 
sample, compare them with standard spectra, subtract blank values, calculate con- 
centrations of the elements in air and standard deviations on the obtained results. 

IO00 

Gross Particulate - Niies,Mic~.(Rural~ August 1969 

%3451145 1445 1745 2045 2345 0245 0545 0645 1145 1445 1745 
TIME 

FIG. 5, Concentration variations of 6 elements during sampling. 



K. A. RAHS, R. DAMS, J. A, Roesr;is and J. W. WISCKESTER 



Diurnal Variations of Aerosol Trace Element Concentrations 417 

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Diurnal Variations of Aerosol Trace Element Concentrations 119 

RESULTS 

The results are summarized in TABLE 1. They reveal that 15 elements could be 
determined in nearly al1 of the samples, namely Al, V, Br, Na, K, Mn, Ti, Sm, ELI, 
La, SC, Zn, Fe, Co, and Cr. A half dozen others (Sb, As, Ga, Mg, Cu, and Ce) could 
only be determined in some samples. Due to their incompleteness the results for the 
latter 6 elements are not very useful and are not given in TABLE 1. The behavior 
of a number of representative elements is shown in FIG. 5. Particle size distributions 

ANDERSEN iMPACTOR STAGE 

FIG. 6. Particle size distribution of 4 elements as measured with an Andersen Impactor. 

001~ ! I 1 
F 7 6 5 4 3 2 i 

ANDERSEN tMPACTOR ST&GE 

FIG. 7. Particle size distribution of 5 elements as measured with an Andersen Impactor. 



420 K. .I\. RAHS, R. D&q J. A. ROBBISS and J. W. WIXCHESTER 

of 18 elements are summarized in TABLE 2 and 9 of these are plotted in FIGS. 6 and 7. 

Standard deviations are given in parentheses. If high count rates were obtained these 

standard deviations may be as low as 10 per cent. Because of the short sampling times 

the concentrations of several elements were at or near the limit of detecrion, and stan- 
dard deviations of these results are much higher. On some stages of the Andersen 

impactor only an upper limit could be set for the concentration of several elements. 

DISCUSSION 

It is obvious that very large variations occur in the concentrations of several 

elements during a 24 h period. The behavior of aluminum, shown in FIG. 5, is repre- 
sentative of a number of other elements such as Ti, Mg, SC and the rare earths, Sm, 
Eu, and La. Concentration variations by a factor of up to 10 occur within a few hours. 
A number of other elements such as Na, K, Fe, Co, and Cr shows less prominent 
variations: on the order of 2.5 rather than 10. The behavior of 1Mn seems to be in 

between these two extreme groups. 
How can this consistent behavior pattern for a large number of elements be under- 

stood? Smoke, SO,, NO, N02, CO, total oxidants, total hydrocarbons and visibility 
measurements have also shown consistent diurnal patterns, but with average variations 

of at most a factor of three (COBI~MINS, 1967; Mm, 1959; S~IMERS, 1966; WEISX-\X, 
1969; U.S. PUBLIC HEALTH SERVICE, 1968). Being urban measurements, these maria- 
tions uere mostly related to variations in local source processes and city ventilation. 
In the present case, one deals with a rural area, and measures primarily distant 
sources, and diurnal meteorological variations should thus be more important. The 

pattern found is indeed consistent with the predicted variations of ground-level con- 
centrations from elevated sources (U.S. DEPART~IENT OF HEALTH, EDUCATIOS. ASD 

WELF~ZRE, 1965). 
With the previous considerations in mind the following tentative explanation is 

offered. Particulate pollutants released at stack height during the fair weather nocturnal 
temperature inversion conditions tend to remain at stack height until after sunrise. 

Daylight hours bring ground heatin g and generation of turbulent motions nhich 
build rapidly upwards until the pollutant layer is reached some one to three hours 

after sunrise. Eddy transport of these pollutants to the surface (Hewson fumigation 
type I) causes the steep morning peak. Continued increase of the maximum mixing 

level until the midafternoon further dilutes concentrations, after which the lolvering 
mixing level and increasing thermal stability initiates the gradual concentration 
increase of late afternoon and early evening. 

The concentration levels of evening, being effectively cut off from elet-ated sources 
by the temperature inversion, might be expected to decrease slightly during the night. 

Instead, they are in most cases observed to drop rapidly during the early morning 
hours, often reaching the lowest levels of the sampling period. Our hypothesis is that 
local ground fog at the sampling site was the primary agent responsible for this 
decrease, due to fog droplet nucleation, probably predominantly by the giant particle 
component of the aerosol population, followed by enhanced sedimentation and/or 
impaction of the enlarged droplets. Though fog was not recorded at the more urban 
South Bend Airport, the temperature there decreased to within 2 degrees of the 
dewpoint, and the relative humidity reached a maximum of 93 per cent at the 4 a.m. 
and 8 a.m. observations (FIG. 1). 



Diurnal Variations of Aerosol Trace Element Concentrations 321 

Confirmation of this idea comes from the observation that in general those elements 
showing the largest concentration variations share two characteristics, namely rela- 
tively deep morning minima and masses concentrated on larger particles. This 
relation can be verified from the plots of Al and V, and is also noted for Ti, Mg, SC, 
La, ELI, and Sm. On the other hand, elements like Na, K, Cr, Co, and Mn, showing 
smaller diurnal variations, are more equally distributed over the 0.1-10 pm size 
range. 

We then visualize the fog formation as involving the larger and more soluble 
aerosols (i.e. those ordinarily collected on the first stages of the impactor), rapidly 
growing to a quasi-equilibrium radius of some 5-10 pm. Because the droplet concen- 
tration in typical fogs (1~5~ cmm3) (DISGLE, 1970) is large relative to the giant 
aerosol number density in continental aerosols (1 cmv3) (JUNGE, 1963) but small 
relative to the large particle component of the same (1000 cms3), it is possible that a 
majority of the giant particles and a minority of the large particles served as nucleating 
agents. Removal of these droplets would preferentially decrease coucentrations of 
those elements primarily in giant aerosols. 

Certain of the observed elements show diurnal patterns suggestive of local sources. 
Br is the clearest example, the probable source being automotive exhaust from the 
road some 50 m distant. Distinct traffic maxima are observed about 6-7 a.m., 3-5 
p.m., and 11-12 p.m., very nearly the times of Br maxima. Further evidence comes 
from the measured particle size distribution of Br, where 65 per cent of its mass is 
found on the backup filter (TABLE 2). This is in agreement with the very small, con- 
densation aerosol nature of auto exhausts, and indicates the fresh nature of most of 
the Br (LOUCKS, 1970). 

The diurnal variation of Zn is not very consistent with the other elements, which 
may be partially due to the low quality of the analytical results for this element. It 
may however also be correlated with its predominant distribution on small particles, 
probably due to formation via a condensation process. 

The steep morning peak makes the V pattern somewhat different from those of the 
other elements. The most important source of V is known to be fuel oils, but because 
of the August date this does not seem to provide a sufficient explanation. Heating 
of commercial establishments may be related to the steep morning peak, however. 
In addition, a considerable fraction (30 per cent) of this element is found on the very 
small particles, pointing toward a condensation formation process and recent age 
for its Aitken component. 

CONCLUSION 

The obtained results demonstrate that nondestructive neutron activation analysis 
can very favorably be applied to the study of diurnal variations of trace elements in 
the atmosphere. The 15 elements which can be measured after sampling times as short 
as 90 min in a rural area show distinct variations in diurnal behavior and should be 
indicators enough for the behavior of most other trace elements. It seems that the 
application of this technique in studies of simultaneously measured total concentration 
variations and particle size distribution of airborne particulates should lead to signifi- 
cant advances in the understanding of source processes and identi~cation of dilution 
and removal mechanisms. The observed size distribution patterns remote from the 
source may not only reflect differing source processes but may also result in a tendency 



422 K. A. RGW, R. DX.IS, J. A. ROBBINS and J. W. WLXHESTER 

toward different atmospheric behavior patterns. This specific investigation was 
however only of an exploratory nature and further experiments under different 
meteorological conditions are under way in order to expand upon the tentative 
conclusions reported here. 

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COMMC;~ B. T. and WALTER R. E. (1967) Atmospheric Environment 1.49. 
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DAMS R., RAHX K. A. and WC‘ICHESTER J. W. (1971) Environ. Sci. Tech&. submitted. 
DNGLE A. N. (1970) Dept. of Met. and Ocean., Univ. of Mich., private communication. 
HEWSON E. W. (1960) Meteorological measuring techniques and methods in air pollution studies. 

In: Zncfustriu/ Hygiene ard Toxicology, Vol. 3, edited by L. SIL~IAN, Inter-science, New York. 
JLTNGE C. E. (1963) Air Chemistry and Radioactivity. Academic Press, New York. 
LOUCKS R. H. and WNCHE~TER J. W. (1970) J. geophys. Res. 75 (12), 231 I. 
IMUNN R. E. and KATZ M. (1959) In?. J. Air. Polfut. 2(l), 51. 
SIRS P. W. (1966) J. Air Polhrt. Control ASS. 16(4) 33. 
U.S. DEPARTMENT OF HEALTH, EDUCATION and WELFARE (1965) Meteorological Aspects of Air 

PolIution. Course conducted by Air Pollution Training Program, Cincinnati, Ohio. 
U.S. PUBLX HEALTH SERVICE, (1968) Air Quality Data from the National Air Sampling Networks and 

Contributing State and Local Networks, 1966, Durham, N.C. 
WEISM.G~ B., MATHESON D. M. and HIRT M. (1969) Atmospheric Environment 3, 1.