JOURNALOF GEOPHYSICALRESEARCH,VOL. 93, NO. D7, PAGES 8378-8382,
INSOLUBLE
BACKGROUND
AEROSOL
PARTICLES
SIZE
Michael
IN
ANTARCTIC
DISTRIBUTION
1988
ICE:
AND DIATOM
Ram and Robert I.
JULY 20,
CONCENTRATION
Gayley
Department of Physics and Astronomy
State University
of New York at Buffalo
Jean-Robert
Laboratoire
de Glaciologie
St.
et de G•ophysique de l'Environnement
Martin
d'Heres,
Abstract.
We have measured insoluble
particle
size distributions
covering the radius range 0.05
- 1.31 •m for
C, Antarctica.
six sections
Two of the
of ice core from Dome
sections
are from the
Holocene, two are from the last glacial
maximum
(LGM), and another two are from the period that
preceded it.
Our measurements lead us to the
conclusion that the southern hemisphere insoluble
background aerosol size distribution,
in the range
of our measurements, has not changed significantly
over the 26,000-year
period that we studied.
We
also compared the concentration
of diatoms in a
sample of Holocene ice with that in two samples of
LGM ice and found that the concentration
of
diatoms whose largest dimension was equal to or
greater than 10 •m was 20 times larger during the
LGM, the same as the ratio we measured for the
concentration
of insoluble
particles.
We
interpret
this to mean that the higher dust levels
were mainly due to an increase in wind strength
rather than to increased continental
aridity.
Greenland and Antarctic
ice sheets, it brings down
with it particles
that were incorporated
in the
snow as nucleation
seeds (rainout)
and also those
that were swept out by falling
snow (washout).
Thus particles
that come down with snow should be
representative
of those present in the atmosphere
at the time of snow deposition,
and it is
reasonable
to
assume
that
the
dust
By definition
'
n • n_ + •o
n_ is
.M + L
the total insolubleparticle concentration
in the
distribution
of
the
scattering/absorbing
particles
[Study of Man's
Impact on Climate (SMIG), 1971].
Insoluble
particles
are a relatively
small
fraction
of the total aerosol.
Many, however,
such as iron-oxide-bearing
minerals and soot, are
strongly absorbing and therefore
play a role that
is larger than their numbers would seem to imply
[Junge, 1977a].
In addition,
as has been observed
range of our measurements.
We have chosen to use these percentages
to
describe our size distributions,
since they are
well defined regardless of the shape of the
distribution
curves and do not depend, for
example, on whether or not the curves exhibit
a
distinct
maximum within the range of our
measurements.
In addition,
we have found this
description
to be very useful
the measured distributions.
that the •eciston
to break
get approximately equal radii
significant
limits
nuclei
consists of insoluble
clay (silicate)
out
into these three radius ranges was made during the
analysis period, after the data had already been
taken.
Consequently, because of our original
choice of measured radii,
it was only possible to
by Kumai [1977] in CampCentury, Greenland, snow,
of ice-forming
in characterizing
We wish to point
up our distributions
a very
proportion
insoluble
concentration
and size distribution
of particles
in polar ice sheets reflect
past insoluble
atmospheric dust levels and size distributions.
In fact,
as we discuss in the next section,
there
are good theoretical
and experimental
arguments to
support such a conjecture.
Our measurements cover the optically
interesting
radius range 0.05 - 1.31 •m. To give
a simple indication
of size distribution,
we
divide the measured radius range into three
subranges:
"small" (0.05 - 0.13 •m), "medium"
(0.13 - 0.38 •m), and "large" (0.38 - 1.31 •m).
We denote the number of particles
per gram of
respectively.
Past climate change and climate forecasting
are
important subjects for both scientific
and
practical
reasons.
One factor that is thought to
be important in climate behavior is the reflection
and absorption of solar radiation
by particles
in
the atmosphere, which depends on the composition,
and size
France
meltwaterin eachsubrange
by nS, nM, andnL,
Introduction
concentration,
Petit
of the three
In an earlier
ratios
for the
intervals.
study, Gayley and Ram [1985]
minerals. This seems
to showthe importance
of
measured
valuesof 60, 30, and10%for nS/n, nM/n,
hence their
importance for snowfall.
The polar ice sheets contain a record of past
insoluble
particles
which can give clues to past
insoluble
atmospheric
dust levels
and particle
These values
insolubleparticlesin inducing
ice nucleation
and and
nL/n, respectively,
for a 200-year-old
section
of ice core from Cr•te in central
Greenland.
size
distributions.
Copyright
As
snow
falls
on
same
values
represent
were
yearly
measured
for
averages,
each
of
and the
3
consecutive years.
They conjectured that these
values also represented
the size distribution
of
insoluble
particles
in the northern hemisphere
background aerosol at the time of snow deposition.
For a 2,500-year-old
sample of south pole ice
core, Gayley and Ram found corresponding values of
74, 22, and 4%. They interpreted
these
percentages
to be representative
of the southern
the
1988 by the American Geophysical Union.
Paper number 8D0258.
0148-0227/88/008D-0258505.00
8378
Ram et
al.:
Particles
in Antarctic
Ice
- Sizes
and Diatoms
83?9
and upper troposphere is made up of the
aerosol originating
from the continents.
This is important to realize
for all studies
of the big ice sheets in Greenland and
Antarctica
which are mostly higher than 2 km
and also to a large degree of the glaciers
in
mid-latitudes."
On the theoretical
side, Junge [1977b] has
argued that the incorporation
of condensation
nuclei in falling
snow is the dominant mechanism
for transfer
of aerosol particles
to the ice
sheets.
Junge presents a curve (curve 1 in his
Figure 6) giving the ratio of aerosol
concentration
air
in
Examination
Fig. 1. A map of Antarctica
showing the location
of Dome C and the south pole.
hemisphere insoluble background aerosol at the
time of snow deposition.
(Note that because of a
misprint,
the heading of the fourth column in
Table 1 of Gayley and Ram [1985] reads n x 10,
gr-1.
It shouldread n x 10-•, gr-1).
In this paper we report on our measurements of
the size distribution
of insoluble
particles
recovered by filtration
from six sections of ice
core from Dome C, Antarctica
(74ø39'S,
124ø10'E,
3,240 m above sea level (asl),
Figure 1), and
relate
these
insoluble
size
distributions
to
those
of
the
Wealso report on our measurements
of the
concentration of diatoms in DomeG ice and compare
the values measuredin two samplesof the last
glacial maximum
(LGM)ice (approximately 16,000
and 22,000 years B.P.), with those in a 10,000year-old sampleof Holoceneice. These diatoms
are of potential interest, since their
concentration can provide an indication of wind
and sourcestrength in the past. In addition, if
the diatomspecies and the location of their
sources
can be determined,
something about past air
Relation
of
Insoluble
one will
It
learn
circulation
Particles
Sheets to the Insoluble
patterns.
in
Polar
Ice
Background Aerosol
has been proposed [Junge, 1977b] that
3 km over
the
oceans
and
above
5 km over
above
the
continents
there exists a "global background
aerosol" which varies little
with time and place
within one hemisphere.
One might guess that because of the high
elevation
and
remoteness
of
the
Greenland
and
Antarctic
ice sheets, it would be possible
to
detect the insoluble background aerosol particles
in snow, firn,
and ice from the ice sheets.
This
seems particularly
reasonable for Antarctica
[Thompson, 1977], which is very distant from any
large exposed land masses but, as our previous
work [Gayley and Ram, 1985] seems to indicate,
it
also seems to be true for Greenland.
According to
Junge [1977b],
"the main part
of
of the aerosols
in the middle
to
aerosol
of aerosol
this
curve
concentration
particle
shows
in
radius.
that
the
ratio
is
almost constant throughout the range of our
measurements.
This implies that the aerosol
size
distribution
to
in
air
and
in
rain
are
the
same
a
very good degree of approximation
in this size
range.
This argument is based on considerations
of liquid precipitation.
Junge argues, however,
that as a first
approximation,
the conclusions
should also apply to solid precipitation.
In an effort
to test Junge's theory, Pourchet
et al. [1983] made measurements of long-lived
Sactivity
in south pole snow and compared the
results with corresponding measurements in south
pole air.
Unfortunately,
their results are not
conclusive.
It is, nevertheless,
generally
assumed that the concentration
of impurities
in
polar snow is directly
proportional
to the
concentration
in the atmosphere [Lorius et al.,
1984].
If we compare curves showing the background
aerosol
size
[Jaenicke,
background aerosol.
rain
as a function
distribution
1980]
measured
in
air
(which are dominated by the
soluble particles) with the insoluble particle
size distributions that we have reported
previously for Greenlandand Antarctic ice [Ram
and Gayley, 1983; Gayley and Ram, 1985], we find
very strong similarities (both insofar as the
general shape of the curves and the location of
the maximum
are concerned). This also leads us to
believe that the size distributions that we
measurein polar ice may, indeed, be those of the
insoluble backgroundaerosol. Oneof our goals is
to put this conjecture on a firm footing by
studying the size distribution
of insoluble
particles
at several locations
in Antarctica
and
Greenland.
If the conjecture
is correct,
we do
not expect to measure any significant
changes in
particle
size distribution
for ice of the same age
at different
locations
in Antarctica,
and we would
expect a similar
result
for Greenland.
We do,
however, expect different
size distributions
for
Antarctica
and Greenland,
since there is little
atmospheric mixing between the two hemispheres and
the amount of exposed continental
area is much
larger in the northern hemisphere.
One
of
the
remarkable
results
of
our
work
is
that the insoluble
particle
size distribution
in
the range of our measurements does not seem to
change with time over a span of many thousand
years for Antarctica.
This result still
needs to
be
understood.
Measurements
We have
insoluble
of
measured
particles
Particle
Size
the
distribution
size
Distribution
of
recovered from six samples
8380
Ram et
TABLE 1.
al.:
Particles
Slope Parameter,
in Antarctic
Concentration,
Particles
in
Dome
Ice
- Sizes
and Diatoms
and Size Distribution
C and
South
Pole
of Insoluble
Ice
Specimen
Depth,
Age, nxl.0•
n.•/n, xln•O
/n xln•O
/n Parameter
Slope
m
Years
gr 5 xlOO
B.P.
1
2
3
4
5
6
SP
Dome
180
361
531
665
793
859
212
c
5,000
10,000
16,000
22,000
28,000
31,000
2,500
4.6
1.5
30
27
10
13
0.8
C
68
73
72
70
73
78
74
25
22
19
25
22
17
22
7
5
9
5
5
5
4
-1.41
-1.75
-1.20
-1.62
-1.32
-1.61
-1.89
72
22
6
-1.46
average
Dome C samples are labeled 1-6.
The variability
in the percentages
corresponding
to one standard deviation
in the measurements are 3, 3, and 1 for
the small, medium, and large particles,
respectively.
SP indicates
south pole.
Depthmeasuredfrom 1978 true surface.
The standard
deviation
for
the slope parameter
is 0.15.
from the DomeC ice core. According to published
data for this core [Lorius et al"..•, 1979] the
samples we studied span a period from 5,000 31,000 years B.P.
As can be seen from Table 1 and
Figure 2, two of the samples correspond to the
Using a Zeiss microparticle
size analyzer,
measured the smaller particles
from X5000
Holocene, two to the LGM, and two to the period
that immediately preceded it.
Samples ranged in
length from 5 to 22 cm and corresponded
approximately
to 2 - 7 years of precipitation.
The amount of water actually
filtered
varied from
We take
3 to
89 mL, with
the
smallest
amounts
we
photographs after an additional
X3 enlargement.
This gives the radius of the circle having the
same area as the projected
area of the particle.
this
radius
to
be
the
"effective
radius"
of the particle.
The same was done for the larger
particles,
using X1000 photographs.
The average
number of particles
measured per specimen was 850.
Control samples showed that contamination
was
negligible.
The
size
distributions
measured
for
the
six
corresponding
to the "dirtier"
LGM ice.
In all
cases the amount of water used was determined by
the requirement that recovered particles
stand out
distinctly
on the filter
with little
overlap.
samples are shown in Figure 3. We have denoted by
N the number of particles
per gram of meltwater
with radii larger than some radius r^ and by r the
melt.
Each ice sample was cleaned by rinsing with
doubly distilled,
filtered,
and deionized Milli-Q
water and then melted and filtered
through 0.04-•m
curves are similar in their general features and
show log-linear
behavior for large values of r.
For small-particle
radii,
three of the curves
pore radius Nuclepore filters.
The filters
were
gold coated, and randomly selected areas were
photographed in a scanning electron microscope.
exhibit
Particleswererecovered
by filtration fromthe
effectiveparticle radiusin micrometers.
All six
of 0.1
a clear
•m.
distinctive
maximum at an approximate
Two of the
curves
maxima
seem
but
to
show
flatten
out.
maximum is not always easy to recognize,
occurs
MICROPARTICLE
MASS-9
CONCENTRATION
(10gcj
-•) -54•
500
I000
•80
to
the
lower
limit
of
range.
The curves are reminiscent
distribution
curves [Junge, 1963]
%0
-50
our
The
since it
measurement
of the Junge
for tropospheric
aerosols.
oO
200•
•0
5
-
,
,, '
close
radius
do not
,.
Fig. 2. Microparticle
mass concentration,
oxygen
isotope ratio,
and approximate age versus depth
for the Dome C ice core, from Royer et al. [1983].
The alternate
dates given in parentheses are
discussed by Lorius et al. [1979].
For values of r larger than 0.15
were approximated by straight
lines
plot)
and fitted
to the distribution
•m, the curves
(on a log-log
function
c
dN/d(log
r)
• ar
This is common practice
in atmospheric physics
[Junge, 1963; Ram and Gayley, 1983].
The measured
values of the slope parameter c are given in Table
1.
As can be seen from Table 1, the slopes for
all curves are equal within
the measured
uncertainty.
all
In Table 1 we give the measured values of n for
six Dome C samples.
The Table also gives the
ratios n_/n
n•./n in percentageform.
Inaddition'
•g and
1alsW
liststhecorresponding
values for our 2,500-year-old
[Gayley and Ram, 1985].
south pole sample
Ram e• al.'
Particles
in Antarctic
Ice - Sizes and Diatoms
8381
7
4
!0
0.1
0.3
1.0
Effective radius r (•m)
I
I
0.1
0.3
I
1.0
Effective radius r (/•rn)
Fig. 3. Microparticle
size distributions
for our six Dome C samples and for a
south pole (SP) sample that was reported previously
[Gayley and Ram, 1985] and
which is reproduced here for comparison.
The samples are described in Table 1.
The "effective
radius"
r is determined
from the particle
area measured on
scanning electron microscope photographs, and dN is the number of particles
per
gram of ice in a given small range of log r.
Data points (circles)
are shown for
samples 2 and 6 to indicate
the typical
scatter of our results.
Note that
although these curves differ
somewhat from each other in their slope at large
radius and in the location
of their maximum, the percentages of small, medium,
and large particles
do not vary significantly
from sample to sample (see Table
1).
Aswesee fromTable1, the valuesof ns/n,
•m filter.
Sincethe number
of particles
qn•u/n,
n•./n
for
the
Dome
C
samples
areall
recovered
by
thisprocedure
very
large,
we
ite and
similar
and
do
notsix
differ
significantly
from
limited our
search
to diatomswas
that
had
at least
the values measured for our south pole ice sample.
We have argued [Gayley and Ram, 1985] that these
percentages should be representative
of the
insoluble background aerosol in the southern
one linear dimension greater than or equal to 10•m.
In one sample, representing 371 g of ice from
the Holocene, we found four diatoms, which
hemisphere. The fact that the percentages for the
south pole and Dome C are so similar lends
corresponds to 0.01 diatoms g-•.
Two samples from
the LGM, totaling
114 g of ice, yielded 24
credence to such an hypothesis. This is also
supported by the similarity of the slope c for all
measured samples (Table 1).
The constancy of the percentages for DomeC ice
diatoms, or, equivalently, 0.2 diatoms g-• of ice.
These results are not of high accuracy because of
the small number of diatoms involved, but it is
clear that the concentration of diatoms in the LGM
seems to indicate that the insoluble background
aerosol size distribution
did not change
significantly
over the 26,000-year period covered
by our measurements, even though the density of
particles
(as reflected by n) changed
ice studied is larger than that in the Holocene
sample by a factor of approximately 20. We are
now in the process of identifying
the diatom
species, and the results will be reported
elsewhere.
considerably.
Discussion
Measurement of Diatom Density
The concentration
of diatoms in Antarctic
Our work on south pole and DomeC ice seems to
ice
is small, and relatively large quantities of water
have to be filtered
to recover a significant
number The procedure for sample preparation and
ß was
filtering
similarto theoneusedin
recovering microparticles.
One of the samples was
filtered
through a Nuclepore filter
with 5-•m pore
diameter and the others were filtered
through a 1-
imply that the southern hemisphere insoluble
background aerosol size distribution
in the size
range of our measurementshas not changed
significantly
from LGMto Holocene periods.
In
fact, none of the measured values of n_ •o, and
•'
from
n•.reported
in Table1 differssignificantly
t•e average values of 72, 22, and 6% for Dome C.
This remarkable result indicates to us that the
size distribution,
in the range of our
8382
Ram et
al.:
Particles
in Antarctic
Ice
- Sizes
measurements, averaged over several years, may
Nicolis,
actually
1984.
be a property
particles
of background aerosol
independent of climate.
In this
connection
it
pp. 23-45, Elsevier Science, New York,
Gayley, R. I.,
is interesting
to
polar
and Diatoms
and M. Ram, Atmospheric dust in
ice and the background aerosol,
•.
speculate that the soluble particle size
distribution may also have remained unchanged over
the past 30,000 years. This, of course, is still
very hypothetical,
since the sources and evolution
Geophys. Res., 90, 12,921-12,925,
1985.
Jaenicke, R., Atmospheric aerosols and global
climate, •. Aerosol Sci., 11, 577-588, 1980.
Junge, C. E., Air Chemistry and Radioactivity,
of soluble and insoluble
particles
are so
different.
Unfortunately,
it is not possible to
test this conjecture by direct measurements on ice
cores, since soluble particles
dissolve in ice,
and the question has to be investigated
with
theoretical
models.
If the conjecture
can be
Academic, San Diego, Calif.,
1963.
Junge, C. E., The importance of mineral dust as an
atmospheric constituent,
in Saharan Dust, SCOPE
vol. 14, edited by C. Morales, pp. 49-60, John
Wiley, New York, 1977a.
Junge, C. E., Processes responsible for the trace
content in precipitation,
International
Symposium on Isotopes and Impurities
in Snow
shown to be true,
it will
allow us to infer
the
past size distribution
of the important soluble
aerosol component by measurements in the present
atmosphere.
We have
found
that
the
concentration
of
and Ice,
diatoms
in two samples of LGM ice from Dome C is 20 times
larger than that in a sample of Holocene ice from
the
as
same location.
the
one
In
we measured
fact,
for
the
the
ratio
is
the
concentration
same
of
insoluble
particles.
This seems to indicate
that
the mechanisms that enhanced transport
of crustal
material
to Antarctica
during the LGM were the
same as those that lead to much higher
concentrations
of diatoms.
Since it is unlikely
that
the
sources
of
diatoms
and
insoluble
particles
would have changed in the same way, we
suggest that the larger particle
concentrations
in
the LGM are mainly due to increased wind strength.
It was found by de Angelis et al. [1984] that
(crustal
particleconcentration)LG
M
= 17
(crustal particle concentration)Holocen
e
IAHS Publ.
118,
63-77,
118,
Lorius,
341-350,
1977.
C., L. Merlivat,
Pourchet,
Jouzel,
and M.
A 30,000 year isotope
climatic
from Antarctic
ice,
J.
Nature,
280,
aerosol
content
from
East
Antarctic
samples and past wind strength,
391-394,
sabbatical
Part
of the work on diatoms
when one of us (R.I.G.)
leave
in
France.
was on
R.I.G.
would
thank C. Lorius of the Laboratoire
et G•ophysique de l'Environnement,
and
their
M.
Lefevre
of
the
hospitality.
Creseveur
and
U.
Universit•
de
He would also
Ezat
for
their
like
to
de Glaciologie
CNRS, Grenoble,
Paris
like
XII
for
core
293,
Pourchet, M., F. Pinglot,
and C. Lorius,
Some
meteorological
applications
of radioactive
fallout
measurements
in Antarctic
snows,
•.
Geophys. Res., 88, 6013-6020, 1983.
Ram, M., and R. I. Gayley, Insoluble microparticle
size distributions
in Greenland ice, •. Phys.
Study of Man's Impact on Climate,
Acknowledgments.
ice
Nature,
1981.
dominant factor.
was started
1979.
core studies,
Ann. Glaciol.,
• 88-94,
1984.
Petit,
J. R., M. Briat,
and A. Royer, Ice age
Royer, A., M.
30,000 year
properties
Antarctica
paleoclimate
Change, •,
This suggests that de Angelis et
record
644-648,
Lorius, C., D. Raynaud, J. R. Petit,
J. Jouzel,
and L. Merlivat,
Late-glacial
maximum-Holocene
atmospheric and ice-thickness
changes from ice-
They attributed
the large value of this ratio to
increased continental
aridity
and stronger winds
during the LGM. Using a factor of 5 for the
influence
of increased aridity
and assuming that
the particle
flux is proportional
to the third
power of the wind velocity,
they estimated an
upper limit
of a factor of 1.4 for the increase in
wind speed during the LGM. Our results seem to
indicate,
however, that wind velocity
was the
al. [1984] may have underestimated
the wind speed
change and that it could have been as much as a
factor of 2.6 greater during the LGM.
1977b.
Kumai, M., Electron microscope analysis of
aerosols in snow and deep ice cores from
Greenland, International
Symposium on Isotopes
and Impurities
in Snow and Ice, IAHS Publ.,
Chem.,
87,
4120-4121,
1983.
de Angelis, and J. R. Petit,
A
record of physical and optical
of microparticles
from an east
ice core and implications
for
reconstruction
models, Clim.
381-412, 1983.
Inadvertent
Climate
Modification,
Man's Impact on Climate,
Mass.,
SMIC,
Report
MIT Press,
on
Cambridge,
1971.
Thompson, L. G., Variations
in microparticle
concentration,
size distribution
and elemental
composition found in Camp Century, Greenland,
and Byrd Station,
Antarctica,
deep ice cores,
International
Symposium on Isotopes and
Impurities
in Snow and Ice, IAHS Publ.
118,
351-364,
1977.
to thank M.
assistance.
R. I. Gayley and M. Ram, Department of Physics
and Astronomy, State University
of New York at
Buffalo,
References
de Angelis, M., J. Jouzel, C. Lorius, R. Merlivat,
J. R. Petit,
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(Received September 10, 1987;
revised March 24, 1988;
accepted March 24, 1988.)