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IndianJournalofRadio& SpacePhysics
Vol.22,August1993,pp. 2{)7
••214
5"'5\
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'53
Air
pollution and acid
-- rain problems in the Indian region*
~Ot - \1
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L TIKhemani
(Indian Instituteof TropicalMeteorology,Pune411(08)
Ii 8eptembcc't992yreWed-Z9IM:emhert992;accepted9'March 1993
I~ problemsof air pollution and acid rain in India are reviewed.Gaseouspollutants like 802 and N~ are
high in a few large cities in India, whereas at other locations their concentrations are in the range of
background levels.The concentrations of total suspended particulates in coastal Kerala and forest regions
are within 100 ~ m - 3, but in north-west and central India they are quite high and vary between 200and 550
J.tgm - 3• The concentrations of soil-oriented components (AI, Fe, Mn,ea,_ K and Mg)., are higher in
aerosolsand the concentrations of componentsfrom anthropogenic sources(pb, Ni, Cd, Zn, Cu, Sb, S04 and
N03) are lower than those reported for western countries whichare industrially more developed. There are
more cations (Na, K, Ca and Mg) than anions (S04 and N03) in the aerosols reported from different
environments. These aerosols, whichare potentially basicin nature, influencethe acidificationof rain water.
The mass size distribution of aerosols reveals that coarse particles (natural sources) dominate over the
submicron particles (anthropogenic sources) in India. It is observed that precipitation in India is, by and
large, alkaline. It is of interest to notice that rain water in Chembur, a highly industrialized area in Bombay
regionwhichwas reported acidicfrom 1974to 1980,turned alkalinein 1990.Thi!lcouldbecomepossibled~
to proper pollution control measures taken by industries in the area. Acid rain, wherever it has occurred in
India, is purely a local phenomenon and is restrictedwithin2 km distancein the upwind and downwind of the
industrial complexes.
1 Introduction
If (
15'" \(l ~
t)
There is sufficient evidence available to prove that
large anthropogenic
emissions of sulphur and
nitrogen oxides to the atmosphere have a profound
influence on the chemical climate of Europe and
North America which have shown harmful effects on
aquatic and terrestrial ecosystems of these regions1•2•
But even today very large differences exist in the per
capita emissions. of air pollutants between industrially developed and developing countries. For
example, the per capita emission ofS02 in India is less
than 5% of that in North America3• However, there
are apprehensions that due to increase in population
and large-scale industrialization
in developing
countries like India, per capita emissions will increase
by a factor offour which will bring the level to 30% of
the per capita emissions in North America and
Europe4 - 6.
In view of the above, it is pertinent to examine the
air pollution problems in India and their influence on
acidification of rain water. For this purpose,
published data on gaseous and part~cu1ate pollutants
andprecipitationchemistryhavebeenexamined,and
the results of the study are presented in this paper.
2 Gaseousand particulatepollutants
The principal precursors of acid rain are emissions
of sulphur dioxide (S02) and oxides of nitrogen
(NOx)' In addition, a variety of other emissions also
influence acidity, notably among them are hydrochloric acid, ammonia, volatile organic compounds,
and alkaline dust. These species have both natural
and anthropogenic
sources. Natural emissions
include biogenic emissions from terrestrial, tidal and
neutrient-rich oceanic areas, and non-biogenic
emissions from natural combustion, geothermal
activity, lightning, and airborne soil and water
aerosols. The main anthropogenic
sources are
man-made activities like industries and vehicular
traffic.
As urban areas are the main centres of man-made
activities, the levels ofS02, N02 and total suspended
particulates (TSPs) at these locations will show how
far they are affected by pollution sources. For this
purpose average monthly concentrations of S02,
N02 and TSPs for ten large cities in India, namely,
Ahmedabad, Bombay, Calcutta, Cochin, Delhi,
Hyder~bad, Jaipur, Kanpur, Madras and Nagpur,
are given in Fig. I (Ref. 7). The monthly average
concentrations
are representative of industrial,
commercial,
and
residential areas. The concentra-*This paper was presented at the National Space Science
Symposiumheldduring11-14March1992at PhysicalResearch tions of S02 and N02 are higher in industrial and
Laboratory,Ahmedabad380009.
commercial areas as they are directly affected by the
11.11
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INDIAN J RADIO & SPACE PHYS, AUGUST 1993
TSP
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-
RUidenlial
3 Chemical and physical characteristics of aerosols
Atmospheric aerosols are important in various
fields, such as agriculture, defence applications,
environmental
pollution,
meteorology,
cloud
physics, acid rain, climate change, etc. The
knowledge of the mass size distribution of atmospheric aerosols is of fundamental importance to any
discussion of their measurement, chemistry, physics
and aerosol air quality. We know that atmospheric
aerosols have three modes with respect to their sizes .
First is called the nucleation mode, second the
accumulation mode, and third the coarse mode. The
coarse mode is built of particles greater than I J.lm
radius which are produced by disintegration of sea
spray and/or mineral dust. The nucleation mode (size
< 0.1 J.lmradius) of the particles is generated from
gas-to-particle conversion of precursor gases. The
nucleation mode is not stable and is converted by
coagulation into the accumulation mode (size
between 0.1 and I J.lm radius).
CJ Commercial Area
Are~
AHM
Ten Large Cities in India (1982-85)
Fig. I-Average monthly concentrations of S02, N02 and TSP
for 10 large cities in India (AHM, Ahmedabad; BMB, Bombay;
CAL, Calcutta; CHN, Cochin; DHL, Delhi; HYD, Hyderabad;
JPR, Jaipur; KNP, Kanpur; MDS, Madras; and NGP,
Nagpur).
sources which are close to them. Calcutta is the highly
polluted city and Cochin is the least polluted. Barring
three metropolitan cities, namely, Bombay, Calcutta
and Delhi, the concentrations of S02 and N02 are
quite low in the remaining seven cities. However, the
concentrations of TSPs are, by and large, very high
and vary between 200 and 500 f.lg m-3 except at
Cochin where the concentration is below 150 f.l g m - 3.
Khemani et al.8 have taken observations of trace
gases (S02, NH3, N02 and 03) from 1981 to 1984 at 6
different locations representative of urban industrial,
urban, non-urban, thermal power plant and marine
environments. They concluded that average concentrations of N02 and 03 were in the range of
background values. Except in urban industrial
environment, the concentration of NH3 was in the
range of background value. The concentrations of
SO.2were substantiany higher (by nearly 7 times) in
urban industrial and thermal power plant environments compared to those in other environments
studied. However, the levels are much lower (by
nearly 6 times) than those reported for western
countries. Recent observations on trace gases
referred to above have indicated that the forest areas
in the Nilgiri Biosphere Reserve located in South
India are so far free from the influence ofindustrial
pollution9.
,
1'1
I
"I
I
'I'
., nll'I'" 'II
I~
I
Although investigation on aerosols in India was
initiated in the early sixities, yet the research in this
direction was limited to the physical characteristics of
aerosols till recently. Of late, studies of chemical
composition and mass size distribution of aerosols
have been conducted over the urban regions7.10-18,
and at remote locations and forest areas9,19,20.
4 Chemical composition of aerosols
The average concentrations ofTSPs and their acid
soluble components at different locations are given in
Table I. It is seen from this table that there is a large
variation in the concentrations ofTSPs in India and
they have varied from 39 to 507 J.lgm - 3 • Such large
variations have also been reported for 10 large cities in
India where the levels varied from 200 to 800 J.lgm - 3
(Ref. 12). Soil in north-west and central India is, by
and large, loose and most of the time is not covered by
vegetation. The winds erode this type of soil easily and
raise a lot of dust which contributes to high levels of
particulates in the atmosphere. The areas which are
influenced by dust will record high levels of TSPs.
This inference is also supported by the chemical
composition of aerosols given in Table I. The
concentrations of soil-oriented elements, namely,
AI, Fe and Mn, were much higher compared to those
contributed by anthropogenic sources (Pb, Cd, Zn,
Cu, Ni and Sb). The concentrations of Pb at the
locations referred to in Table I were quite low because
of the locations of sampling sites. The Pb concentration will be high at those places where the traffic
density is ~more. The concentration of Pb was
reported 560 ng m - 3 at King's Circle, Bombay, where
vehicular traffic density is very high. The lowest value
U 1;'Ult 1111111111'
111111,·1111illl,HlIII
Ulhl'
IIIL
,
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---- --
27
0.28
140
21
244
219
5Sb
1Ni
3210
540752
834
39
25I1gm-3
16
699
0.18
0.32
175
76200
10
69
70
25
374
1740
88
181
740
73
37
0.45
0.41
1.21
0.78
1.10
0.35
2.57
0.87
K
720
40
0.21
4.28
3.75
1.30
26
0.28
123
140
1300
17
42753
60
20
2350
1056
3413
600
300
110
18
8240
39
979
19
Ca
507
2.33
2.70
0.44
0.20
0.32
0.39
0.86
0.30
1.98
0.79
2.19
0.23
0.38
0.45
0.55
0.48
16.81
0.46
0.75
0.24
1.00
0.08
0.21
3.38
0.20
0.16
2.20
1.27
0.70
1.03
1.80
1.32
1.01
1.39
45
100
Na
74
1.35 Concentration,
0.40
Mn
Cu
Cd
1940
Fe
Ph
0.47
5.67
180
1961
41203
580
121
33
5320
99
1236
480
122
260
23
12
16
30
U.85
2.28
0.57
0.92
0.97
0.88
0.43
0.40
162
51
TSPs Total
197
7.28
2.43
5.40
490
Ref.
Zn
27
6.90
8.16
1190
511
20
1.60
0.55
116
0.49
0.66
0,85
1.98
1170
325
Ref.
number
54
14
20
21
209
NH.
Mg
0.29
NO)
SO.
ng
m -&3 ACID
KHEMANI:
AIR10.90
POLWTION
Concentration,
RAIN PROBlEMS
I1gm - 3 INNo.
INDIAN REGION
No.
in some
western
countries
in
some
western
countries
Milan (Italy)
t locations in India and
ent locations in India and
(16 ng 1ll- 3) was reported at Matheron, a hill station
located in Maharashtra State, where vehicular traffic
is very low24•
The average concentrations
of water-soluble
components ofTSPs at different locations are given in
Table 2. The concentration of sea salt in the aerosols
was maximum near the coastal regions and decreased
with an increase of distance from the oceans. The
concentration of sulphate was, by and large, in the
range of background values. The concentration of
nitrate in the aerosols was quite high. The high
concentration of nitrate normally indicates the
influence of anthropogenic sources. But, it has been
shown that most of the nitrates in aerosols are
originated from the soil and hence do not playa role in
the acidification of aerosols28, Among the cations,
concentration of calcium seems to be quite high. Very
high values of Ca concentration are reported in
aerosols as well as in rain water throughout India.
Even though the concentrations of remaining cations
(NH4, K and Mg) in aerosols are low, they play an
important role in the neutralization of hydrogen ions
caused by acidic sulphates and nitrates.
It is seen from the above discussion that soil and sea
salt are the major natural sources of aerosols in India
and the concentrations of nitrate and sulphate
210
INDIAN J RADIO
& SPACE
released from industrial sources are quite low. As soil
in India is highly alkaline, alkaline aerosols released
from it have large influence on the pH of rain water in
India.
Khemani28 has surveyed the physical and chemical
compositions of aerosols for a few locations in India
which are representative of urban, non-urban,
marine, and thermal power plant environments. The
conclusions derived from this survey are:
(i) The average levels of TSPs in India are quite
high than the recommended standards in USA and
European countries. In most of the large cities in
Europe and USA, major sources of TSPs in
atmosphere are industrial plants and vehicular
efuissions, whereas in India most of the TSPs are from
dust particles carried by winds from arid zones and
nearby surroundings.
The concentrations
of
soil-oriented elements (AI, Fe, Mn, Ca, K and Mg)
are higher, and concentrations of components from
anthropogenic sources (Pb, Ni, Cd, Zn, Cu, S04 and
N03) are lower in India than those reported
from the western countries which are industrially
more developed. The concentrations
of some
of the above components reported from western
countries are given in Tables I and 2 for comparison21 - 23.25- 27.
(ii) There are more cations (K, Ca and Mg) than
anions (S04 and N03) in the aerosols in India. These
aerosols are potentially basic in nature and hence
influence the acidification of rain water in India.
(iii) The mass size distributions of atmospheric
aerosols have shown bimodal distributions in all the
environments. The lower mode has been observed in
submicron size range (0.4-0.6 /lm diam.) and the
higher mode in the giant size range(5.6/lm diam.).
The particles in submicron mode are formed by
gas-to-particle conversion processes, whereas those
in giant size mode originate mainly from soil. In India,
the giant size mode always dominates the submicron
size mode, whereas in western countries submicron
size mode dominates the giant size mode. This
observation indicates that the natural sources of
aerosols are very strong and anthropogenic sources
of aerosols are weak at present in India.
5 Acid rain
Acid rain is one of the most widely published
environmental issues of the day. Vast areas of the
northern hemisphere including central Europe,
Scandinavia, north-east United States, and Canada
are affected by acid rain. There is a great concern that
acidity of rain can have adverse environmental
impacts.
The free acidity of solution such as rain is
I ,,',
I
PHYS, AUGUST 1993
determined by the concentration of hydrogen (H+)
ions present. It is commonly expressed in terms of pH
scale. Pure water has a pH of7 which is neutral and on
the borderline between acidic and alkaline. Natural,
unpolluted rain water is frequently assumed to be
slightly acidic with pH of 5.65. This is the pH of
distilled water in equilibrium with atmospheric
carbon dioxide, as determined under laboratory
conditions. It has never been etablished that 5.65 is
real pH of unpolluted rain in nature. Nevertheless,
many researchers have accepted this assumption and
refer to rain water having a pH of less than 5.65 as
acidic. Hence, the term acid rain has come to mean
rainfall with pH of less than 5.65.
Contamination in the atmosphere can shift the pH
of rain water either way. Alkaline soil particles
containing carbonates can increase the pH. In
contrast, the presence of acidic particles of suIph uric
or nitric acid would lower the pH. The processes
affecting the acidity of rain water are many and very
complex. They includegas-to-particle
transformations, photochemistry and catalytic chemistry,
aqueous chemistry within cloud drops, precipitation,
regional
and global integrated
atmospheric
transports. The pH of precipitation is an integrated
measure of the relative contributions of all of these
complicated processes29.
Most research efforts dealing with precipitation
acidity have been directed towards understanding the
influence of acid materials or chemical reactions in
the formation of acids in precipitation. As a
result, the role of alkaline substances in determining
precipitation acidity has largely been overlooked.
Precipitation acidity is a function of its contents of
both acids and bases, and any attempt to understand
the processes causing acid precipitation must deal
with the potential acid neutralizing capacity of
alkaline materials. The major cations in precipitation
associated with alkaline compounds (other than
ammonia) are Ca, K, Mg and Na which are mostly
released from crustal sources.
The influence of dust particles in India can be seen
better where concentration ofTSPs has varied from
200 to 800 /lg m - 3. Soil in India is rich with alkaline
components like Ca, K and Mg, and is the major
source for the observed high levels of aerosols in
atmosphere. Therefore, chemical composition of
precipitation in India can indicate the influence of
man-made pollution and natural pollution on the pH
of precipitation.
6 Chemical composition of precipitation
The average concentrations
of major ionic
components along with pH values in precipitation
lo.
KHEMANI: AIR POLUJTION
& ACID
(rain, cloud, fog and snow water) collected at
different locations in India are given in Table 3. The
concentrations of all the ions are higher in fog and
cloud waters than in rain and snow waters. This
observation appears to be logical since fog and cloud
droplets are very concentrafed solutions of soluble
condensation nuclei.
Besides sea salt (CI and Na), S04 and Ca are present
in substantial amounts in rain water. It is known that
concentration of sea salt decreases with an increase in
distance from the sea. The contribution of sea salt in
rain water at coastal as well as inland stations is
mostly from the sea. Sea salt has beeen considered
neutral and its presence does not influence the pH of
rain water. Except at industrial locations (Chembur
and Kalyan), the concentrations of S04 at other
locations are quite low in rain water. Also, the
concentrations of N03 are quite low in rain water
Table 3-Average
Station
concentrations
except at a few locations, namely, Indraprastha
thermal power plant, Delhi, and Muktsar. High
concentration of N03 at Muktsar, a non-urban
location, is little surprising. It was found that N03 in
rain water at M uktsar was originated from soil, since
it showed significant positive correlation with
soil-oriented elements like K, Ca and Mg. The
maximum concentration ofCa in rain water has also
been observed at Muktsar. High concentrations of
Ca in rain water have been reported for coastal as well
as for inland stations in India30• However, the
concentrations of Ca in rain water are substantially
low in north-east (Goraur in Bihar) and South India
(Trivandrum). It seems that in these regions the
influence of dust on TSPs is quite low; concentration
of TSPs in these regions is reported to be about
75 Ilg m-3• Low concentration of Ca and high
concentrations ofS04 and N03 in rain water reported
in mgjlitre of major ionic components
different stations in India
CI
211
RAIN PROBLEMS IN INDIAN REGION
Na
and pH values in precipitation
K
Ca
Mg
collected at
pH
H
.IiCqjlitre
RAINWATER
Coastal
Trivandrum
4.22
1.71
0.98
0.25
2.25
0.27
0.77
0.35
5.30
5.01
Kalyan
3.95
5.20
1.92
0.38
2.36
1.02
1.86
0.47
5.70
2.00
·Chembur36
5.00
20.20
2.10
2.20
UO
3.10
0.68
4.80
15.85
Indraprastha'
2.06
2.10
3.30
0.93
0.98
0.25
2.03
(}.31
5.00
10.00
Koradi
2.35
1.78
1.52
0.39
1.78
0.31
3.13
0.48
6.70
0.18
Urban
Pune
2.57
1.78
0.48
0.10
1.84
0.20
2.07
0.45
6.30
0.50
Delhi
2.54
2.73
2.54
0.53
1.75
1.28
2.95
0.62
6.10
0.79
Non-Urban
Sirur
2.64
1.53
2.64
0.07
1.84
0.95
3.53
0.80
6.70
0.20
Muktsar
2.66
1.59
4.38
0.16
1.84
1.70
5.56
0.63
7.30
0.05
Goraur
0.95
1.85
0.60
0.25
0.59
0.67
1.54
0.22
5.30
5.01
Forest region
Masingudi
0.67
1.85
1.69
0.49
0.44
0.73
1.43
0.23
6.04
0.91
Pune
8.91
1.50
1.80
1.44
5.48
U5
6.90
0.13
12.55
17.53
3.95
6.91
0.12
0.19
0.61
0.07
6.68
0.21
Industrial
Power Plant
CLOUD WATER
0.99
4.5
FOG WATER
Delhi
8.20
4.38
11.65
2.90
Gulmarg
0.64
0.65
0.56
0.30
16.13
SNOW WATER
0.36
212
INDIAN J RADIO
& SPACE
from the western countries are the major causes of
acid rain. The relative contributions of primary
aerosols (Ca, K and Mg) and secondary aerosols (S04
and N03) in rain water will decide the acidity in rain
water. It has been seen that the presence of Ca
neutralizes the H ion concentration and increases the
pH value. However, S04 to some extent increases the
H ion concentration and decreases the pH value31.
N03 in rain does not show any relationship with H ion
concentration since the source of N03 is soil.
However, at Goraur N03 was responsible for the
increase of H ion concentration along with S04.
Among the three crustal elements, K, Mg and Ca,
the last one has maximum acid neutralizing
capacity32. Influence of alkaline particulates from
soil dust onpH of rain water has been reported in the
United States and European countries32 - 35.
PHYS, AUGUST 1993
1976 (Table 3) came down to around 5 mg/litre during
1990 (Ref. 37).
It will be of interest to know the pH of rain water at
other locations in India during various monsoon
periods (1970-90) reported by other investigators.
For this purpose, the average pH values measured
and reported for different stations in India during
various monsoon periods are considered (Refs 31,
38-45). The pH values are high (pH > 7.0) at Srinagar,
Allahabad, Jodhpur, Muktsar, Amritsar, Bikaner,
Jaisalmer, Lucknow and Ahmedabad. The values are
comparatively
low but still in alkaline range
(pH = 6-7) in industrial towns and cities, namely,
Nagpur, Visakhapatnam,
Delhi, Agra, Bhopal,
Calcutta and Chandigarh. At Trivandrum and
Goraur, pH is still lower (pH = 5.3). However, the
rain at Chembur which was acidic during 1974-80
turned to alkaline in 1990.
7 pH of precipitation
The pH values of rain water have been compared
Except at Chembur, a highly industrialized area in with the pH values of soil of different regions ofIndia
Bombay region, the pH of rain, cloud, fog and snow published by ICAR46. The pH values of soil are
waters is alkaline at other locations in India. Also, the reported quite high between 15°N and 35°N, and
pH of rain water at Trivandrum and Goraur (near
substantially low in the longitude belt of8YE to 95°E.
Patna) is near the CO2-equilibrated value. The rain High pH values (pH = 6.5--10.5)of soil are reported in
water at these two places cannot be called acidic Jammu and Kashmir, Punjab, Haryana, Uttar
because not only the concentrations ofS04 and N03
Pradesh, Rajasthan, Madhya Pradesh, Gujarat,
are quite low, but the concentration ofCa is also quite Maharashtra,
Karnataka, Andhra Pradesh, and
low compared to other locations in India. It is some parts of Tamil Nadu. Quite low pH values
reported that pH of rain water in unpolluted areas can (pH = 4.8-5.3) of soil are reported in coastal Kerala
be around 5 due to the presence of naturally produced
and also at Kodaikanal in Tamil Nadu. In Bihar and
organic acids like formic acid and acetic acid29. It is West Bengal, the pH values are between 4.8 and 6.0;
presumed that the lower pH at Trivandrum and pH values are still lower (4.0-5.4) in the east of these
Goraur is not due to pollution sources but may be due two states. It is noticed that the dust load in the
to formic acid and acetic acid which are able to bring atmosphere over north-west parts of India is high
down the pH from 5.65 to around 5.30.
and a significant amount of dust is incorporated into
The first acid rain in India was reported from rainfall either as condensation nuclei or by collision
Chern bur , where average pH of rain was 4.8 in 1976 with falling rain drops. However, the extent to which
(Ref. 36). Recent results of chemical analysis of acid airborne dust influences the precipitation chemistry
rain in Chembur-Trombay area conducted during
of certain geographic regions vaJ;les with its
the monsoon period of 1990 have indicated that the composition and abundance. Neutralizing effect of
rain in this area has become alkaline and the pH varied airborne dust31,37.47-54has been attributed as one of
between 6 and 7 (Ref. 37). Acidic pH (4.8) was the reasons for higher pH values (pH > 5.6). Higher
reported earlier (1976) in the rain water samples
atmospheric temperatures in the tropics, lower
solubility and dissolution rates of CO2 added to rapid
coll~cted at Bhabha Atomic Research Centre
(BNR C) and the trend continued up to 1980 (Ref. 32). processes of cloud formation and precipitation are
The change from acidic to alkaline pH of rain water in also responsible for higher pH values. Greater depth
this area is due to change in fuel from coal to natural
of saturated air during south-west monsoon over the
gas and pollution control measures adopted by Indian subcontinent could be yet another factor for
industries which are reflected from the declining S02 higher pH (Ref. 55).
emission trends reported for Chembur. The average
concentrations ofS02 and N02 during 1971-73 were 8 Conclusions
around 65 and 49 Jlgm - 3which reduced to around 15
The neutralizing effect of airborne dust on rainfall
and 29 Jlg m - 3 respectively in 1990. Also, acidity is now well documented. The alkaline
concentration ofS04 which was around 20 mg/litre in properties of soil particulates have so far sustained
I I" H
I
,
I
KHEMANI: AIR POLUJTION
the alkaline nature of rain water (pH > 5.65) and have
been controlling the spread of acid rain in India. This
has been possible due to low concentrations of acidic
.
.
compo~ents (S?4 and ~03) and high concentratlO~s
of alkalIne particulates m the atmosphere. Also, acld
rain, wherever it has occurred in India, is purely a
local phenomenon and is restricted within 2 km
distance in the upwind andlor downwind of the,
. d
. 1
1
Lo
213
& ACID RAIN PROBLEMS IN INDIAN REGION
f
0
to Sadasivan S, Studies of some important trace constituents in the
mo~oon r?ins ~ their role in atmospheric processes, Ph D
theSIS, Umverslty of Bombay, Bombay, 1977.
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