Atmospheric Environment VoL 24A, No. 5, pp. 1143 1151, 1990.
0004 6981/90 $3.00+0.00
Pergamon Press pie
Printed in Great Britain.
THE CONTRIBUTION OF SULFATE AND DESERT AEROSOLS
TO THE ACIDIFICATION OF CLOUDS AND RAIN IN ISRAEL
ZEV LEVIN, COLIN PRICE a n d ELIEZER GANOR
Department of Geophysics and Planetary Sciences, Reymond and Beverly Sackler Faculty of Exact
Sciences, Tel Aviv University, Ramat Aviv, 69978, Israel
(First received 5 April t989 and in final form 17 October 1989)
Abstract--Measurements of the sulfatecontent in individual particles were carried out on top of a mountain
in Northern Israel in order to determine the role played by acidic sulfate particles in changing the pH of
cloud drops. The investigation was carried out during the passage of convective clouds over the station.
Since these clouds developedin cold fronts originating from differentair masses it was possible to determine
the role of sulfateas wellas desert particles in affectingthe chemistryof cloud and raindrops. It was observed
that the cloud droplets were frequently very acidic with the pH as low as 2.5. As the cloud droplets grew by
condensation they became less ackiicdue to dilution. The presenceof desert particlesdid not affect the pH of
the cloud droplets since they remained as interstitial particles. However, once rain started the pH of the
raindrops was found to be as high as 8.2. On these occasions the drops contained large amounts of dust
indicating that the process of scavengingeither by the cloud drops or by the fallingand melting graupels is
very efficient.Sulfate concentrations in air containing large amounts of dust were found to be much higher
than on days lacking dust aerosols. The source of these sulfate particles has not yet been identified.
Key word index: Sulfate particles, acid rain, cloud chemistry, desert aerosols.
INTRODUCTION
Measurements of aerosol particles in desert and urban
areas in Israel by Levin and Lindberg (1979) and
Ganor and Mamane (1982) have shown that the
aerosols can be characterized as consisting mostly of
desert and marine particles with some additional ones
of anthropogenic origin. The relative fraction of each
component is mostly determined by the origin of the
air mass and its history. Air masses moving from the
northwest carry a larger fraction of marine and anthropogenic aerosols and a relatively small fraction of
desert particles. Air masses coming from the southwest
and from the west on the other hand, carry a larger
fraction of desert and marine aerosols with smaller
fractions of anthropogenic particles. It has been observed that many of the desert particles contain sulfur
(gypsum) or are covered with sulfate (Mamane et al.,
1980). As will be discussed later these particles may
become effective as condensation or freezing nuclei.
Measurements of the acidity of rain in Israel (Loewengart, 1964; Yaalon and Ganor, 1968; Mamane,
1987) have revealed values of pH ranging from 4.1 to
8.7, depending on the trajectories of the air masses that
make up the rain system.
These variations in pH may be attributed to the
relative contribution of the various aerosols scavenged
by the cloud and rain drops. However, thus far all the
studies in Israel have concentrated on ground measurements from which definite conclusions about the
chemical processes in the clouds can only be tentatively stated. Also in many other studies (e.g. Parungo
et al., 1987; Pueschel et al., 1986; Hegg and Hobbs,
1981; Hegg et al., 1984; Castillo and Juisto, 1984) at
other locations around the world conclusions have
been drawn based on analysis of bulk samples of cloud
water. In these experiments the effects of desert particles on the pH of the cloud water were generally
ignored since their concentrations were small and
since most of these particles were insoluble. In the
Eastern Mediterranean region many of the desert
particles contain calcium carbonate. Even though
these particles are not efficient cloud condensation
nuclei, once collected by cloud or rain drops, they can
be partly dissolved and modify the acidity.
The purpose of this study was to use a method of
single aerosol particle analysis to investigate the role
of sulfate and desert particles in changing the pH of
the cloud and of the rain as they are scavenged by
cloud and rain drops.
Most rain storms in Israel are formed in winter
continental convective clouds associated with cold
fronts or post frontal systems. In these clouds rain is
produced only via the ice formation process (Gagin,
1975). The cloud base is often around 1000 m above
sea level with the 0°C isotherm around 2000 m. Since
natural Ice Nuclei (IN) are found in reasonable concentrations at levels where the temperatures are lower
than about - 16°C it is expected that ice will form in
large concentrations only at altitudes greater than 3.5
km. Since most chemical reactions affecting cloud
chemistry take place by dissolving aerosol particles in
drops and by absorption of gas through drops' surfaces, it should occur between the cloud base and
about 3.5 km above. Some chemical reactions may
also take place in the ice phase but at slower rates and
1143
1144
ZEV LEVINet al.
mostly through the collection of supercooled cloud
droplets. These droplets may be acidic due to some
early collection of acidic aerosols or due to oxidation
of SO2 in them.
It has been shown (Jensen and Charlson, 1984;
Sievering et al., 1984; Daum et al., 1987) that the
chemistry of cloud droplets is mostly affected by the
process of nucleation scavenging of aerosols to form
either water droplets or ice crystals. The process of
S O 2 oxidation to form SO 2- in drops has been shown
to be relatively slow, and strongly dependent on the
presence of catalysts. Beilke and Gravenhorst (1978)
have also shown that in clouds the oxidation of SO2 is
of secondary importance in the formation of sulfate,
and that the scavenging of SO 2- particles is the most
important process affecting cloud acidity. In this light
we have concentrated our efforts on determining
aerosol content and composition below cloud base
and in cloud droplets just above the cloud base.
Measurements were carried out on the the top of
Mt. Meron (l120m ASL) in Northern Israel. The
measurements permitted us to determine the characteristics of individual aerosols entering the cloud base,
the cloud droplet size distribution, their chemical
composition and pH, the composition and concentration of the interstitial particles, and on some occasions
the composition and pH of the rain.
DETERMINATION OF SULFUR MASS IN I N D I V I D U A L
AEROSOL PARTICLES
The method used to determine the SO~- content of
individual cloud droplets and aerosol particles was
adapted, with some modifications from methods used
by others in the past (Bigg et al., 1974; Lodge, 1986).
The method involves initially sampling aerosols onto
carbon coated transmission electron microscope
(TEM) grids. Droplets impacting the surface produce
a crater which can be correlated with the original drop
size using results from laboratory calibration. In these
calibration experiments drops of known sizes are
generated by an ultrasonic drop generator and allowed to impact carbon coated grids placed in a
cascade impactor. For drops of diameters smaller than
15 #m, a fairly constant value of 0.9 is obtained for the
ratio of drop to crater size. Dry aerosols which impact
the surface do not produce any crater and so can be
easily identified as dry particles. Figure 1 represents an
example of a sample collected on a carbon coating and
which contains both dry interstitial aerosol particles
(marked P) and cloud droplets (marked D).
Once the size of the droplets and dry particles are
known, the samples are coated with a thin layer of
BaCIz, and kept for 2 h in a humidity chamber maintained at 75% relative humidity (r.h.). If SO42- is
present in the aerosols, it reacts with the coating to
form BaSO4 which appears as a reaction halo around
each aerosol (Fig. 2).
By comparing the aerosols with and without halos
one can determine the percentage of aerosols which
contain hygroscopic SO~- (Mamane and de Pena,
1978).
Once the number concentration of sulfate containing aerosols in the sample is known, the next step is to
calculate the exact mass of S (and hence SO 2-) in each
droplet. This can be done by using scanning electron
microscopy (SEM) with an energy dispersive X-ray
analyzer (EDS) and with the aid of calibration curves
developed by Pardess et al. (1989). In their calibration
experiments particles of known sizes and compositions [(NH4)2SO , and Na2SO4) ] were allowed to
impact C coated grids. The X-ray counts over a period
of 100 s in the S energy window (2300 ev) were correlated to the known S mass of the laboratory generated
particles using different accelerating voltages of the
electron beam in the microscope. The element Co was
used as reference in order to normalize the calibration.
The normalized counts for S were calculated using the
following equation:
S-B
NC = - -
C
xA
(1)
where S = S counts for 100 s (2300 ev)
B = background counts for 100 s (2500 ev)
C = Co standard counts for 100 s (6920 ev)
A = analyzed area (/zm2).
Once the normalized counts are known, the S mass
can easily be calculated using the calibration curves
(see Fig. 3d) from Pardess et al. (1989).
In this way the size and S content of each droplet or
particle can be determined. A summary of the whole
process is shown in Fig. 3 for two individual drops. In
Fig. 3a the drops collected on C coated grids can easily
be measured. Drop A and B are 4.3 and 11/zm in
diameter, respectively. The presence of SO~- is verifiled both by the BaC12 post-coating and reaction halo
in 3b, as well as by the X-ray analyzer output in 3c.
Finally, the S mass is calculated using the normalized
counts from the X-ray analyzer, and the calibration
curves shown in 3d. The masses of the S in the drops
are shown by the arrows.
INSTRUMENTATION
Size spectra of aerosol particles between 0.3 and
20/~m diameter were measured using an optical aerosol spectrometer (CSSP-100) connected to a microcomputer. The spectrometer had been previously calibrated using standard latex particles of various sizes.
In addition, a four stage cascade impactor (Casella)
was used to correlate between size spectra and chemical composition. On each stage of the impactor we
placed four electron microscope grids coated with
formvar and heavy carbon. Following each sample
collection the grids were removed to a dry box, to
protect the samples from the humidity outside. Manu-
Acidification of clouds and rain in Israel
1145
Fig. 1. Photomicrograph (TEM) of droplets (D) and dry particles (P)
sampled on stage 4 of the cascade impactor. The arrows show the impact
craters of the droplets.
Fig. 2. Photomicrograph (TEM) of aerosols after coating
with BaCI2 and 2 h in a humidity chamber of 75% r.h. The
halos around the particles represent the presence of SO]in the aerosols.
facturer calibration curves were used to estimate size
selection on each stage. However, since size measurements were performed using the electron microscope,
the calibration curves were used only as first indicators. One possible error in evaluating drop size
based on crater size in the impactor could be their
evaporation in flight through the impactor before
being collected on the stage. This evaporation can
occur due to the increase in pressure that the air
experiences as it passes through the narrow passages
of the instrument. Our estimate of this error based on
changes in air velocity suggests that evaporation can
amount to no more than 5% of the volume and
therefore to less than 2% in the estimated drop
diameter.
A pH meter with a microelectrode was used to
determine the variation in the acidity of the rain. This
was done by collecting about 2~4 cm 3 of rain water in
small test tubes and recording its pH immediately.
Acidity in cloud droplets was recorded by sampling
cloud droplets through the cascade impactor. In this
case pH paper was placed on each stage of the
impactor. Drops impacting the paper left a mark of
their pH by changing its color. The lowest pH found in
each cloud sample was recorded this way because once
a drop with a low pH impacted the paper no other
drop with higher pH could revert its color.
Standard meteorological parameters such as wind
speed and direction, temperature and humidity were
recorded every 10 min during each experiment.
RESULTSAND DISCUSSION
Table 1 lists the average meteorological parameters
measured before, during and after the passage of the
1146
ZEV LEVINet al.
(c)
S
A
Si
IF -II
\
(d)
Mg At
IE - 1 2
IE-13
s
~
o IOKV
Si
Ca
u)
/
o 15 KV
A 25 KV
tE-~
IE-4
Etements
I
IE-3
I
IE-2
I
I
I
IE-I
I
I0
NormaLized counts
Fig. 3. (a) Two drops collectedon carbon coated EM grids.(b) The same two drops after BaCI2 postcoating. (c) The X-ray
analysis showing the presence of S. (d) The calibration curves used to convert X-ray normalized counts to S mass. The
arrows represent the mass orS in drops A and B. The points on the three curves are those used by Pardess et al. (1989) in
their calibration.
cloud over the station on the 3 days of 3-5 of March
1987 and during our measurements on 25 January
1988. Using wind direction measurements and meteorological synoptic maps, provided by the Israeli
Meteorological Service, we carried out back trajectory
calculations to determine the origin of the air masses
on each day (see Fig. 4). Our analysis revealed that on
3 March 1987 the air mass originated from the north,
possibly from Turkey and even Eastern Europe. On 4
March 1987, the wind shifted and brought air from the
Mediterranean Sea. On the third day the wind shifted
again and brought air from North Africa. The wind
shift during this period was a result of the passage of
two rapidly moving cold fronts.
During our field experiment on 25 January 1988 the
air originated from the Balkans and Southern Europe
and made a long path over the Mediterranean Sea
before passing over our station.
Samples were collected before the cloud passed over
the mountain station. These measurements were later
compared with those carried out during the presence
of the cloud and after its dissipation. The objective was
to determine the chemical composition of the particles
and whether they were dry or wet. Depending on the
Acidification of clouds and rain in Israel
1147
Table 1. Meteorological data collected before, in and after cloud passage on 3 March
1987, 4 March 1987, 5 March 1987 and 25 January 1988
Before
cloud
Date
Wind velocity (ms- 1)
Wind direction (°)
Dry temperature (°C)
Relative
humidity (%)
3 March 1987
In
After
cloud
cloud
2
360
6.1
99
Before
cloud
Date
Wind velocity (ms-t)
Wind direction (°)
Dry temperature (°C)
Relative humidity (%)
2
350
6.0
100
5 March 1987
In
After
cloud
cloud
7
225
7.2
95
7
260
5.6
97
40"
Mediterroneon
I~
Seo
Mt.
Meron
30 °
3o°
0.5
360
6.9
91
40"
Fig. 4. Trajectories of air masses reaching the mountain station on the days of the experiments. A,B,C,D,
represent, respectively, the air trajectories arriving at
Mt. Meron on 3 March 1987, 4 March 1987, 5 March
1987 and 25 January 1988.
source of the aerosols, it was found that 5-20% of the
aerosols >0.3/~m were insoluble and remained as
interstitial dry aerosols in the cloud. Between 80 and
95% of the aerosols were activated to form small
droplets at cloud base. However, from the number size
distributions (Fig. 5) we observed that only drops
> 3 #m in diameter continued to grow as cloud drops
in cloud base. Smaller droplets with sizes < 3 # m
remained as unactivated haze particles within cloud
base (they did not pass the critical supersaturation). In
other words, our method of collecting and analyzing
aerosols allowed us to separate dry aerosols, haze
droplets and cloud drops. F r o m the point of view of
the cloud these small haze droplets could also be
considered as being interstitial particles which could
9.5
240
5.6
94
Before
cloud
4 March 1987
In
After
cloud
cloud
3
180
5.5
92
2.5
200
5.6
97
5
190
11.6
85
25 January 1988
Before
In
After
cloud
cloud
cloud
5.2
250
5.2
97
2.6
250
5.0
100
3.6
270
5.0
98
be scavenged at a later time by larger cloud drops or
by falling rain drops.
Measurements were carried out during two field
experiments. During these days rain often fell over the
station. The air masses producing the rain originated
from different directions resulting in a variety of
different aerosol compositions.
Figure 5 represents examples of the size spectra of
particles that were measured using the optical aerosol
spectrometer before the cloud engulfed the station, at
the base of the cloud during its presence over the
station and after the cloud dissipated. These examples
were taken on the 3rd, 4th and 5th of March 1987
during the passage of a front with an air mass moving
from the west. As can be seen the size spectra before
the cloud arrived and after the cloud dissipated are
similar and so are the number concentrations. The size
spectra at cloud base show that the cloud droplets are
those that are larger than 3/~m, namely those that
managed to pass the critical supersaturation and
become activated. Smaller particles remained as haze
droplets or as dry particles. The number concentration of cloud drops was between 500 and 1500 cm 3.
These concentrations include the cloud as well as the
haze droplets. The increase in the number concentration in the cloud as compared to the number prior to
the cloud arrival is mostly due to the growth by
condensation of the aerosols smaller than the lower
threshold of the optical spectrometer and which could
not have been counted in the measurements performed prior to the cloud's arrival. Once the r.h.
increased, these small particles grew and were counted
by the instrument.
The optical counter cannot differentiate between
wet and dry particles or between particles of different
chemical compositions or morphology. F o r this we
had to resort to the cascade impactor and to single
1148
ZEV LEVINet al.
1~0,0
1E4
.....
Before
Cloud
E
~ \
--
In Cloud
:ii,'~,...~
E
o
...... Particles 4 ~
100.0
( N - 7 1 2 cm~ )
~
--Totol
E
( N = 3 4 5 c m -= )
Droplets 96~
E
- - - A f t e r Cloud
(.=352 ~ 1
/
L)
10.0
leo~
CI
~
Z
"13
E3
-o
1.0
Z
10
"I:3
0.1
1.0
0.1
1
1.0
0.1
I O.O
DIAMETER
10.0
100.0
DIAMFER (.m)
,
100,0
(/zm)
1~.0
-
b
--Total
1E4
E
..... Before Cloud
(N=113 cm -a )
E
--
In Cloud
(N= 1575 cm "= )
---
After CIoud
1000
'~': ~
E
u
~
v
..... P a r t i c l e s 2 2 ~
lOO.O
3t
10.0
"''"
- -Droplets
78~
E
O
I
v
100
a
1.0-
a
Lle~
10
'
Z
1o
i
z
I3
~:~,
'.
O.I
0.1
1.0
:l
.. .. .. .. .. .. ..
1
0.1
ii
1.0
. . . . ~. . . . . . ,i
DIAMETER
i
I ::::::,
10.0
100.0
(/zm)
1E4
c
..... Before
Cloud
100.0
(.m)
Fig. 6. Aerosol size distributions measured by the
cascade impactor on the 4 March 1987. (a) Represents cloud droplets and interstitial particle
distributions at cloud base. (b) Haze droplets and
dry particle distributions after cloud passage.
( N - 1 O0 c m -= )
E
--In
1000.
~
~
..
(3
cloud
(N=IO6g cm -=)
•.. \
E
- - - A f t e r cloud
~
( N = 5 2 c m -~ )
::tl:
r'~
-O
DIAMETER
10.0
10'
~ ~ _ ~l'i.
Z
'O
1 ,
0.1
:
:
:
1.0
DIAMETER
10.0
: : :::::l
100.0
(/.zm)
Fig. 5. Aerosol size distributions from the optical
aerosol counter for (a) 3 March 1987, (b) 4 March
1987 and (c) 5 March 1987. Each distribution was
calculated by sampling aerosols over a time interval
of approximately 15 min. The corresponding relative humidities are presented in Table I.
particle analysis. Even though this method has its
drawbacks such as the low collection efficiency of
particles, the relatively low size resolution and the
time consuming analysis of each sample, it is still a
powerful method for determining particle composi-
tion as a function of their size. It is also the only way
known to us that distinguishes between dry and wet
particles (Ganor and Pueschel, 1988). Figure 6 represents examples of the size spectra measured on 4
March 1987 using the EM photomicrographs for
identification of dry particles and drops. Figure 6a
presents in-cloud spectra while Fig. 6b represents the
spectra after the cloud dissipated. In the cloud 96% of
the particles were in the form of droplets (not necessarily cloud drops). About 4% were unaffected by the
moisture and remained dry interstitial particles. After
the cloud dissipated a much higher fraction of dry
particles (22%) appeared, even though many small
droplets could still be found. It should be pointed out
that the total number of particles and droplets measured by the cascade impactor was much smaller than
that recorded by the optical counter. This was due to
the lower collection efficiency of the impactor and the
higher size cutoff in the smallest size range (about
0.4 pm on the fourth stage). In our analysis, for lack of
better data for comparison we assumed that the ratio
of the concentrations of the dry and wet particles in
the atmosphere were similar to those measured by the
Acidification of clouds and rain in Israel
l°° I
II
D= 1.05x 106M°'~
E
10~
L
<
r-~
m
: : : :::::lg
11r-1 ,
1E-14
:
1E-13
: : ::::::
: : i iiiii
1E-12
1E-11
SULFUR MASS ( g r a m s )
11111
b
D= 7.76x 10SM°'52
E
rY
LLJ
L
<
123
1E-1
1E-14,
:
: ::::::',
1E-13
:
: ::::::',
1E-12
:
: ::::::,
1E-11
SULFUR MASS ( g r a m s )
c
D= 1.95x 10~I °':~
~
lO
<
n
t(-14
1E-13
1!r-12
1E-11
SULFUR MASS (grams)
Fig. 7. Relationships between diameter of droplets and the mass of sulfur in them on the (a)
3 March 1987, (b) 4 March 1987 and (c) 5 March
1987.
cascade impactor. In other words, a rough comparison between the results of the optical counter and
the cascade impactor could still be made.
Using our methods of sampling and analysis we
found that for air masses originating from Europe,
approximately 95% of the aerosols (>0.3/~m) contained hygroscopic nuclei. Most of these were observed to contain sulfate while about 26% contained
~E(A) Z4:s-K
1149
both NaCI and sulfate together. On the other hand, air
masses from North Africa often carried less hygroscopic SO 2- (about 80%), and more dry insoluble
particles of desert minerals.
Using data collected just prior to cloud appearance,
when the humidity approached saturation, we obtained a direct relationship between sulfate concentration and haze droplet size. This was done using the fact
that for aerosols of known composition and size,
Kohler theory (Pruppacher and Klett, 1980) predicts
the exact size of the haze droplet at saturation. Using
the samples collected on the grids the sulfur masses
were obtained using the method described before.
Using this method we also obtained for each experiment a relation between haze droplet diameter (from
the crater) and sulfur mass (Fig. 7). One should
emphasize that only particles coated with some water
were used for this analysis, namely, only hygroscopic
sulfate. Since most pure sulfate in the atmosphere is
usually found in the small ( < 1/lm) particle range, the
present observation suggests that some of the large
particles which we collected and which contained
sulfate, contained other soluble materials as well.
Indeed, many of the particles we collected contained
both NaC1 and sulfate together.
The results of Fig. 7, namely the equations representing the best fit lines for each case, were combined with
the appropriate size distribution to obtain either the
number concentration as a function of sulfur mass or
sulfur mass distribution as a function of diameter.
Using the best fit lines and the size spectra of the
haze particles and assuming that all the sulfur detected
was in the form of sulfate, we obtained values of sulfate
concentration in air between 6.7 #gm -3 and 8.2 pg
m - 3. These are in agreement with values measured at
other locations in Israel (Luria and Meagher, 1986).
Even though these sulfate concentrations are similar they do not tell the whole story. The fact is that the
type of particles that contained sulfate was considerably different depending on the air mass. On days in
which the air mass came from Europe the sulfate
particles were normally small and tended to be unattached to other particles. Air masses that originated
in the west or south west were characterized as having
sulfate attached to marine and mineral dust particles,
respectively. These values of sulfate in the air correspond to 56-172 mgcm 3 of haze water. When the
same analysis was carried out in samples of cloud
droplets taken at cloud base we found that sulfate
concentrations in the water decreased due to the
diluting effect of growth by condensation. The corresponding values were 38.4-99.8 mg cm- 3. Table 2 summarizes the values of the sulfate concentrations in air,
in haze (below cloud base) and in cloud droplets
during three different experiments.
Others in the field of cloud chemistry have measured SOl concentrations in cloud droplets of the
order of mg { ~ (Pruppacher and Klett, 1980), while
we calculated concentrations of mgml-1 (Table 2).
The reason for this discrepancy of three orders of
1150
ZEV LEVINet al.
Table 2. Concentrations of SO~- in the air (A) and in
droplets (C), below and within cloud base as a function of the
air mass origin
Origin of air mass
Below cloud
base
Within cloud
base
Table 3. Ion concentrations (a), mineral dust analysis (b),
and clay minerals (c), in rain sample collected at Mt. Meron
on 5 March 1987
(a)
SO4
NE Europe
Southern Europe
and N. Africa
N. Africa
A=7.1 #gm -3
C=56.1 mgcm -3
A=8.2 #gm -3
C=172mgcm -3
A =6.7 #gm -3
C=33.3 mgcm -3
NO 3
Cl
pH
2.9 mgd -1
103 mgd -1
7.4
Ions
121 mgd -1
C=38.4 mgcm -3
C=99.8mgcm -3
---
(b)
Mineral
dust
Quartz Calcite Dolomite Feldspar Halite
51.3%
19%
20.4%
5.6%
3.7%
(c)
6
Clay
minerals
5,
4
"113_
32
IE-I
1
10
DROP DIAMETER (/J,rn)
Fig. 8. pH of cloud droplets sampled at
cloud base on 25 January 1988.
magnitude is that most other measurements were
performed using bulk analysis methods. These methods include averages over the whole cloud drop size
spectrum, of which the mean drop diameter can be
20-30#m (Hegg and Hobbs, 1981). This implies that
the mean volume of the droplets in the above size
range is 103 times larger than the mean volume for
droplets of 3 #m diameter used in our analysis. If the
volume of liquid water is 103 times larger for bulk
measurements, then it is obvious that, provided SO2
oxidation is small, the sulfate concentration will be 103
smaller when compared with the concentration in
small cloud droplets.
The same trend was observed when we measured
the pH of the cloud droplets. Using the method for
measuring pH of cloud droplets as discussed before,
we observed changes in the pH as a function of the
drop size. The results of samples collected on 25
January 1988 are shown in Fig. 8.
The decrease in acidity with the increase in drop size
is once again a result of the droplet growth by
condensation, resulting in the dilution of the droplets.
Since the cloud droplets < 1 0 # m grow mainly by
condensation on CCN, we conclude that the pH of
small cloud droplets < 10#m is mainly influenced by
the process of heterogeneous nucleation at cloud base.
On the other hand, the pH of the raindrops is
probably affected by processes such as the coalescence
Montomorilonite
Illite
Kaolinite
0%
33%
67%
of drops of various pH values and by impaction
scavenging of dry insoluble and partly soluble particles. It is reasonable to expect that the pH of the rain
can become more acidic or more alkaline depending
on whether the raindrops collect more acidic or
alkaline particles, respectively. On 5 March 1987, an
air mass originating from North Africa containing
large amounts of desert dust aerosols reached the
station. It was found that the pH of the rain vacillated
over 2 h between 6.2 and 8.2 with an average pH of 7.3.
The raindrops were opaque yellow with large amounts
of dust in them. Rain samples were taken every 10 min
and were later analyzed using an ion chromatograph.
Table 3 shows (a) the ion concentrations and mineral
composition for a rain sample with pH of 7.4, (b) the
relative concentrations of the different non-clay minerals (the relative concentrations are given in percent
by weight), and (c) the relative concentration of the
various clay minerals. We see that very high concentrations of sulfate ions were observed. In fact this
concentration is about 10 times higher than those
reported by Mamane et al. (1987) for sulfate concentrations in storms of similar type. The difference stems
from the fact that our samples were taken over a short
time interval, while Mamane's data refer to averages
over the whole storm. Naturally, the large variations
in pH over the lifetime of the storm imply that the
sulfate concentrations vary as well. Therefore, averages over the whole storm give lower sulfate ion
concentrations. It should be pointed out that measurements in the Northern part of the Negev desert
reported by Nativ et al. (1985) show similar results to
ours. Even though their data also refer to averages
over longer times, the rain in the desert is often light-due to evaporation. This would tend to enhance the
dust concentrations in the drops. Together with the
high sulfate concentration the chlorine ion concentration was high as well (probably NaCI). In contrast the
nitrate ion concentration was relatively low.
On 25 January 1988, (see Fig. 4) the air mass
originated from Southern Europe, and the minimum
Acidification of clouds and rain in Israel
Table 4. Ion concentrations (mgf-1) and pH
of rain samples collected at Mt. Meron on 25
January 1988
Sample
SO4
NO 3
CI
pH
1
2
3
14.35
19.55
18.62
0.98
2.65
2.61
35.9
39.2
46.5
5.0
5.0
5.0
p H m e a s u r e d was 5.0. Table 4 shows t h a t on this day
the sulfate ion c o n c e n t r a t i o n was m u c h lower t h a n on
the dusty day. Since very little dust was present in the
a t m o s p h e r e no neutralizing particles were collected by
the drops a n d the p H r e m a i n e d low throughout.
We propose t h a t the m a i n difference between the
p H o n the two days stems from the differences between
the a t m o s p h e r i c loading of desert dust aerosols. We
believe that generally the cloud droplets are initially
very acidic due to nucleation on acidic cloud condensation nuclei, mostly sulfate containing particles. As
these droplet grow by c o n d e n s a t i o n their p H increases
due to dilution. W h e n the droplets reach sizes of
20-30 #m, the processes of coalescence a n d i m p a c t i o n
scavenging become d o m i n a n t . W h e n desert dust aerosols, which are mostly hydrophobic, are present in
the a t m o s p h e r e in high concentrations, they can enter
the drops via a variety of scavenging processes. Any
acidic SO 2- present in the drops will be neutralized by
minerals such as C a C O 3 to form CaSO4, and the
excess c a r b o n a t e s will dissociate in solution causing
the r a i n d r o p s to become more alkaline.
The source of the very high c o n c e n t r a t i o n s of sulfate
in rain d r o p s t h a t fall out of clouds which follow dust
storms has still not been clearly identified.
Acknowledgements--We would like to thank the National
Council for Research and Development, Israel, and the
BMFT (through the G. S. F., M/inchen, F.R.G.) for supporting this research.
Thanks are also due to Mr Doron Pardess for his assistance in the measurements and the analysis of the data.
We also thank Mr M. Devorachek and Mr F. Skandrany
for their assistance with the use of the electron microscope.
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