This document summarizes a study of atmospheric dilution indices (ADI) across 20 stations in India. The ADI were calculated twice daily based on mixing heights and wind speeds, with higher values indicating better atmospheric dilution capacity. Spatial analyses found peninsular India generally had higher ADI than northern India. Morning ADI were extremely low nationwide due to low mixing and winds. Afternoon ADI were higher but northern India still showed poor dilution capacity year-round. The study concludes southern regions have more efficient pollution dispersion while northern and central India lack reasonable dilution ability.

1. IJEP 15 ( 10 ) : 734 - 7 3 8 ( Received on A u g u s t 24, 1995 )
Climatological Atmospheric Dilution Indices over India
D.V. V i s w a n a d h a m , S t u t i M i s h r a a n d A.N.V. Satyanarayana
Banaras Hindu University, Department of Geophysics, Faculty of Science, Varanasi - 2 2 1 0 0 5
A t m o s p h e r i c d i l u t i o n indices ( A D l ) given an idea of the capacity of t h e a t m o s p h e r e to disperse
and dilute the p o l l u t a n t s e m i t t e d i n t o it by m y r i a d sources. The higher the index t h e b e t t e r is the
d i l u t i o n . These indices are c o m p u t e d based on the vertical and h o r i z o n t a l e x t e n t of m i x i n g of t h a
atmosphere by s u i t a b l y devising the s t r u c t u r e of the indices. In the present paper A D l have been
c o m p u t e d on a c l i m o t o l o g i c a l bases for 20 stations spread all over India. W i t h the help of isopleth
analysis the spatial d i s t r i b u t i o n of these indices over India has been s t u d i e d f o r 4 m o n t h , nam-
ely January, April, July and October, w h i c h are t y p i c a l of 4 seasons. The analysis has been ca-
rried o u t t w i c e a day at t h e occurance of m i n i m u m a n d m a x i m u m t e m p e r a t u r e . It is observed t h a t
peninsular India shows relatively higher indices in almost all t h e seasons. The early m o r n i n g A D l
are e x t r e m e l y l o w t h r o u g h o u t the c o u n t r y in all the m o n t h s . A m i n i m u m ADl of 4 is d e s i r a b l e f o r
a s u f f i c i e n t d i l u t i o n w h i c h is absent t h r o u g h o u t the c o u n t r y during morning hours.
INTRODUCTION
There is a growing concern and awareness about
the increasing problems caused by air pollution. Se-
veral episodes of air pollution were known to have
occured under adverse atmospheric conditions, such
as stable atmosphere, low mixing, calm winds, te-
mperature inversions, etc. It is known that disper-
sion and dilution of the pollutants emitted into the
atmosphere are solely governed by the prevailing
meteorological conditions. For an effective enviro-
nmental planning it is rather desirable to have an
idea of the dispersion and dilution capacity of the
atmosphere for any given region or locaiitiy. A sui-
table index clearly defining the dispersive and dil-
utive capacity of the atmosphere would be extreme-
ly useful for general public, the industry and the
policy makers.
To this end, a simple but effective structure of indi-
ces has been developed by the authors closely foll-
owing Holzworth ( 1 9 6 4 ) . The indices are develop-
i
ed based on the extent of vertical and horizontal
mixing of the atmosphere. These indices have been
studied on a seasonal basis over the entire country
and the regions of effective and poor dilution have
been delineated.
MATERIAL
The daily upper data for the months of January, Ap-
ril, July and October typical of winter, pre - mon-
soon and post - monsoon seasons, respectively for
a period of 9 year from 1977 to 1 9 8 5 have been
made use of for the present study, hi addition, the
maximum and minimum temperatures, surface win-
ds have been made use of. The stations for which
data have been collected are Patiala, New Delhi,
Jodhpur, Ahemadabad, Nagpur, Gwaliar, Lucknow,
Gorakhpur, Ranchi, Calcutta, Agartala, Bombay,
Goa, Mangalore, Cochin, Trivendrum, Bangalore,
Hyderabad, Madras and Visakhapatnam.
ATMOSPHERIC DILUTION INDICES (ADl)
These indices have been devised in such a way that
low values correspond to poor dilution capacity and
higher values correspond to better dilution. These
indices are numbered from 1 to /, respectively co-
rresponding to very low, low, low medium, medium,
high medium, high, very high. The criteria for these
indices are based on the mixing heights and wind
speeds. Tables 1 to 3 depict the criteria for obtain-
ing ADl.
M E T H O D O L O G Y
The methods of determination of mixing heights has
been presented in several publications and hence
not attempted here ( Vittal Murty et at., 1 9 8 0 ; Vis-
wanadham and Santosh, 1989; Viswanadham and
Pinakapani, 1994). The minimum and maximum mi-
xing heights have been obtained for the period un-
der study. The surface wind speeds nearer to these
timings have been noted. The ADl are first obtained
based on the mixing heights and then based on w i -
7 3 4 INDIAN J. ENVIRONMENTAL PROTECTION, V O L 15, NO. 10, OCTOBER .1995*
<S> 1695 - Kaip&na C o r p o r a t i o n

2. Figure 1. Spatial distribution of AD! over India •
Figure 3. Spatial distribution of ADI over India -
Morning - January
Morning - Aprii
Figure 2. Spatial distribution of ADI over India -
Afternoon - January
nd speeds from tables 1 and 2. These 2 indices
are then combined with the help of table 3 to obt-
ain the final ADI. The ADI are obtained twice a day,
that is at the occurace of minimum and maximum
temperatures since it is at these times that mi-
nimum and maximum mixing heights are obtai-
ned. The mean ADI for morning and afternoon are
finally, made use of for the isopleth analysis. The
INDIAN J. ENVIRONMENTAL PROTECTION, VOL. 15,
© 1995 - Kalpa
Figure 4. Spatial distribution of ADI over India -
Afternoon - April
spatial distribution of these mean ADI for morning
and afternoon are presented for the 4 typical month.
RESULT A N D DISCUSSION
January
Figures 1 and 2 represent spatial distribution of
ADI for morning and afternoon in January. Gener-
NO. 10, OCTOBER 1995 7 3 5
na C o m n r a t l n n

3. fg^tMiiM . „ mse . t i_E
ally the ADl are below 4 everywhere except near pa?ed to that of morning. Once again southern par-
south east coast. Barring southern peninsula the ts have showed extremely low or low. The gradient
rest of the country shows ADl to be very low in the of ADl is from north to south, with the entire pen-
morning. The coastal regions and the inland regio- insular India showing 'high medium'. Quite interes-
ns in southern India show relatively high ADL But tingly the longitudinal variation is insignificant. Ce-
on the whole the entire country shows only low ntral parts of the country and the entire north India
ADl. The afternoon pattern is slightly better com- show low ADl. This being the situation for the max-
7 3 6 INDIAN J. ENVIRONMENTAL PROTECTION, VOL. 15, NO. 10, OCTOBER 1 9 9 5
© 1995 • Kalpana C o r p o r a t i o n
Figure 7. Spatial distribution of ADl over India -
Morning - October
Figure 6. Spatial distribution of AD! over India -
Afternoon - July
Figure 8. Spatial distribution of ADl over India -
Afternoon - October
Figure 5. Spatial distribution of ADl over India-
Morning - July

4. Table 1. ADI based on mixing heights
Indices Mixing heights, m
1
3
< 5 0 0
5 0 0 - 1 0 0 0
5 1 0 0 0 - 1 5 0 0
7 > 1 5 0 0
Table 2. ADI based on wind speeds
Indices Wind speeds, m/sec
1
3
5
7
< 2
2 - 4
4-6
> 6
Table 3. Combined ADI based on mixing heights
and wind speeds
Indices based on
wind speeds
Indices based on mixing hei-
ghts
1 3 5 7
1
3
5
7
1 2 3 4
2 3 4 5
3
4 5 6
4 5 6 7
imum possible index of the day, it certainly is a
cause of concern particularly for the complete cen-
tral and north India.
April
Figures 3 and 4 depict the spatial distribution for
morning and afternoon. The morning situation is
better when compared with that of January, the
only exception being the western parts of central
^ , d i a showing relatively more dilution compared to
it's eastern counterparts. Once again the southern
parts show relatively high ADI but in absolute terms
the maximum ADI are only low medium. The after-
noon picture shows a differential distribution in
that the extreme western parts of north India and
the central parts of peninsular India show the high-
est possible ADI. The entire country shows the ADI
to be generally high. The minimum ADI observed is
medium and, therefore, the entire country can be
said to possess a high dilution capacity.
July
The spatial distribution of ADI for morning and afte-
rnoon is presented in figures 5 and 6. The morning
INDIAN J. ENVIRONMENTAL PROTECTION, VOL. 15, NO. 10. OCTOBER 1995
© 1995 - Kalpana C o r p o r a t i o n
)
distribution shows, as before, relatively high ADi ,
the southern parts and extremely low ADI in the
northern parts barring extreme western tip of north
India. The ADI are between 1 and 3 in the northern
and central parts. The afternoon distribution also
shows more or less a similar pattern as that of mor-
ning. The western parts of north India and the com-
plete southern parts show high or very high ADI.
The north eastern parts and the extreme northern
parts show very low ADI despite monsoon winds.
October
Spatial distribution for morning and afternoon is
presented in figures 7 and 8. Once again the morn-
ing distribution is similar to that of January. Except
the central peninsular India the ADI are extremely
low. The afternoon distribution, however, shows
high to very high in southern peninsula and low in
north India except north western parts.
In all, the morning distribution has depicted rela-
tively low ADI compared to that of afternoon. This
is very natural since both the vertical and horizontal
mixing are low during morning compared to that in
afternoon, when the heat input is high along with
stronger winds. The afternoon values are maximum
possible ADI that one could get. If one examines
the afternoon distribution alone January, July and
October give a very bleak picture for north India.
South India on the otherhand, shows a promise sin-
ce the ADI is generally on the higher side irrespec-
tive of the season, notwithstanding seasonal diffe-
rences in magnitudes of ADI.
The south - north contrast arise essentially due to the
coastal characteristics the former possesses which
cause relatively high mixing during early morning
hours and relatively strong winds due to mesoscale
circulation, such as land and sea breezes. Even du-
ring monsoon season the afternoon ADI could not
go beyond 3 in northern parts. It does not, however,
mean that during the entire monsoon season, nor-
thern parts always show low or very low ADI.
There could always be individual instances when
the ADI could push up to 5 or even more but in
the mean it is low. One possible reason could be
that winds are not that strong whenever the mon-
soon is not active in the northern parts. Monsoon
or no monsoon south India generally records rela-
tively strong winds, more so during monsoon.
The extreme north western parts also show in most
of the cases a reasonable ADI. From the overall stu-
7 3 7

5. dy it appears that generally the extreme southern
parts are quite favourable for better dispersal of po-
llutants followed to some extent by the northwest-
ern parts. The morning situations are extremely ala-
rming since the ADl are well below 4. This number
is not a magic figure, but is certainly considered to
be typical for a reasonably good dispersal of pollu-
tants since an average mixing of more than thousa-
nd meter and an average wind speed of more than
4 m/s, to which this number corresponds to, are es-
sential for a reasonable dilution. The United States
Environmental Protection Agency stipulates a mixi-
ng height of 1500 m and 4 m/s wind speed as the
limits for a good dilution. Either way, the dilution is
very low, during April alone the entire afternoon
ADl are quite reasonable.
The adequacy of the present index is beyond repro-
ach. Some of the earlier indices Sastry and Vital
Murty ( 1979 ) were based on vertical temperature
differences alone. We do mention that the inversio-
ns play a very vital role in restricting dilution of po-
llutants along with stability of the atmosphere. Ex-
plicitly we have considered neither inversions nor
the atmospheric stability. It is implicit, however, si-
nce the determination of mixing height takes care
of the inversions and stability. We, therefore, feel
that the present index is quite reasonable and can
be used universally throughout the country, althou-
gh the limits prescribed in tables 1 and 2 are witho-
ut an experimental basis thereby involving subjec-
tivity. Finer tuning is possible to the limits prescri-
bed but we strongly feel it makes very little differe-
nce in the absolute indices we have obtained.
CONCLUSION
Peninsular India exhibits higher ADl values in con-
trast ;o the relatively low values observed over nor-
thern parts of the country. During monsoon season,
south India which shows strong winds because of
its coastal characteristics shows gives higher values
of ADl in contrast to northern region's ADl values.
Overall study reveals that dispersion of pollutants
is an efficient process over extreme southern regio-
ns. Northern and central parts of India do not app-
ear to possess a reasonable dilution capacity.
ACKNOWLEDGEMENT
The authors are thankful to the India Meteorological
Department for providing data. They are also thank-
ful to Dr K.R. Santosh ana Dr. V.V.S.N. Pinakapani,
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7 t 8 INDIAN J. ENVIRONMENTAL PROTECTION, VOL. 15, NO. 10, OCTOBER 1995
© 1S95 Kelpena C o r p o r a t i o n