Indian Journal of Radio & Space Physics Vol. 28, December 1999, pp. 291-301

Vertical profile variations of NOz and 0 3 using slant column density observations during twilight period

A L Londhe, C S Bhosale, G S Meena & D B Jadhav

Indian Institute of Tropical Meteor~logy, Pashan, Pune 411008

and

M Gil, 0 Puentedirra & M Yela

Lab. Atmos. Instituto National de Tecnica Aeroespatial, Car Ajalvir KM'l..-Torrejon de Ardoz, 28850, Madrid, Spain

Received 1 June 1999; revised received 27 September 1999

An algorithm developed to derive the vertical profiles of atmospheric species from their slant column density measurements using twilight spectroscopy is discussed. The algorithm has been tested by using the slant column density measurements at polar station, Reykjavik (64°N, 22.6°W) and tropical station, Pune (l8.53°N, 73 .85°E). The vertical profiles of N02 and 0 3 are retrieved by considering slant column densities for ten different solar zenith angles and ten different atmospheric layers of equal thickness. These vertical profiles are used to differentiate the tropospheric and stratospheric contribution of N02 and 0 3. The observations of N02 at polar station and at tropical station showed frequent higher values of tropospheric concentrations due to pollution episodes and are not correlated with stratospheric N02 and 0 3, The correlation between total column density variations of N02 and 0 3 is not observed; however, the stratospheric variations of N02 and 0 3 showed good correlation.

1 Introduction The study of tropospheric ' and stratospheric ozone

(03) has become a topic of local, regional and global concern. The role of the stratospheric 0 3 is of great importance as far as the life on earth is concerned. To understand the 0, chemistry in different environments, regular measurements of 0 3 and its precursor gases such as NOx, CO and Cf4 must be carried out. The discovery of 0 3 hole over Antarctica in particular and the strong heating due to greenhouse gases like 0 " CO2, CH4, etc. in general, witnessed an unprecedented surge of interest in the investigation and monitoring of various trace species.

The vertical profiles of the atmospheric gases are generally derived by balloon-borne instruments, high

"' 2 . resolution lR solar measurements, etc ' . Such type of observations are quite expensive and hence cannot be carried out for longer period of time continuously. However, a new algorithm to derive vertical profiles of N02 and 0 , from slant column measurements is introduced here.

The twilight visible spectroscopy is being used by several groups worldwide to derive the total column density of NO] and 0 3 (Refs 3-6). The slant column of absorbing species measured at twilight hours varies

with solar zenith angle (SZA). Also it depends on the altitude of the absorber. Assuming that the absorber concentration remains constant during twilight hour, the variation in the observed slant column density with increasing solar zenith angle, therefore, contains an information regarding the vertical profile. Inversion technique is proved to be potentially useful both for deducing N02 profiles and converting the twilight slant column density measurements to the vertical column amounts5

,7. The technique provides information on the vertical structure of atmospheric absorbers such as 0 3 or N02 from the surface to about 50 km and is also valuable, particularly, for identifying the influence of pollution on such measurements.

Nearly 90% of 0 3 resid.es in the stratosphere, while only 10% of 0 3 resides in the troposphere. Similarly, the N02 concentration in the stratosphere is found to be maximum during the period when troposphere is unpolluted. But the tropospheric variations of N02 are considerably higher as compared to stratospheric variations. Hence, it is of great importance to separate out the tropospheric and stratospheric concentrations of both N02 "and 0 3. The technique to derive a vertical profile of a gas is described in the following section.

292 INDIAN J RADIO & SPACE PHYS. DECEMBER 1999

An automatic rotating slit scanning spectrometer has been developed at Indian Institute of Tropical Meteorology (IITM), Pune (Fig. I) . This spectrometer is utilized for the observations of zenith sky radiation in the spectral region 400-450 nm. These spectrometer observations (Fig. 2) are analysed and slant column densi ties of N02 are derived for different solar zenith angles.

2 Algorithm to retrieve the vertical profiles of NOz and 0 3

It is possible to retrieve the vertical profile of an absorbing gas from its enhancement factor and a slant column density for different solar zenith angles. The vertical profiles of N02 and 0 3 are retrieved from their slant column densities for different solar zenith angles by adoptin'g the following assumptions.

(i) The vertical guess profiles of N02 and 0 3 are assumed and an iteration method is used to retrieve the vertical profiles of N02 and 0 3•

(ii ) The height of the atmosphere is considered up to 50 km from surface at tropical station and up to 40 km at polar station. This is the maximum height to which most of the gases under study are observed. The atmosphere is di vided into 10 layers of thickness 5 km each at trorical station and \0 layers of 4 km thickness at polar station.

(iii) The absorption cross-sections of N02 and 0 3

do not vary with altitude. A column amount (s;m) observed at solar zenith

angle i is represented as follows (m indicates modelled value).

. . . (1)

where, Ai}=Effecti ve slant path enhancement factor for N02

added to layer j . The enhancement factors considered here are derived by assuming whole gas accumulated In a single layer, i.e. enhancement factors fo r each layer are calculated assuming that the whole gas is residing in a single layer.

Cj=Amount of N02 in layer j Chahine solution methodS is used to derive a

vertical distribution of N02. In the Chahine method, a particular zenith angle is associated with each model layer which is varied in the search of solution. The method converges most rapidly and is least influenced by measurement noise, if the maxima are at distinct angles of the ~tmospheric layers to be solved fo r. This

condition holds good for the N02 enhancement factors for altitudes between 0 and 30 km. The solution process is started by assuming a first guess profile, CrJ

• of N01 for the first iterations. The fitting errors between the observations and the predicted values from the model (S;O -5;,n) are calculated, where st indicates observed column density and Sr indicates model column density. If these errors exceed a pre-set limit, another iteration is made.

Using the basic Chahine method, the (n+l )th estimated concentration profile would be determined by the equation Cr'=ct[S\u/S" klj)] . . . (2) where, k(j) represents the angle at which layer j has the largest enhancement factor.

A weighted matrix Wi} is calculated by the following method.

(i) Th'e maximum element of each row of the matrix A(i,j) is found out and each element of the ' corresponding row is divided by this maximum element.

(ii) The rows and columns of the above modified­matrix are interchanged to form another new matrix.

(iii) Addition of row elements is taken and each element of the same row is divided by its sum.

Thus, performing above three transitions, the weighted matrix Wi} has been obtained and used in Chahine iteration fo; ".1 la.

The weighted Chahine iteration fonnula is used to determine the profiles with

N

P=~:[WjI .(S~(/) /S;(I»] .. . (3) 1='

and CrJ=C/-p ... (4)

where, ON is the number of layers in the model atmosphere and ~I is a set of weighting factors. The factor P is calculated for each iteration and new vertical profile is generated with Eq. (4), until the fitting error is approximately of the same size or the observed and model slant column densities come closer and closer.

The vertical profiles are derived with this method utilizing the data collected at an Iceland station, Reykjavik, during winter season of 1993-94 and at a tropical station, Pune, during March-April 1998. The vertical profile of 0 3 retrieved with the present algorithm is compared with the ozonesonde profile. The error observed between the two is ± 1 0%. The

•

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~94 INDIAN J RADIO & SPACE PHYS, DECEMBER 1999

1.20 Data for zenith angles from. 87° to 92°

0.00 3900 4000 4100 4200 4300 4400 4500

WAVELENGTH , A

Fig. 2-Spectrometer observations for the solar zenith angles from 87° to 92°.

vertical profiles obtained with this algorithm are presented and discussed in detai l.

2.1 Vertical profiles of NOz over Pune

Figure 3(a) represents the slant column densities of N02 for different SZA, while Fig. 3(b) represents the vertical profile of N02 for I April 1998 over the station , Pune . Figure 3(b) shows that the first three layers and last two layers contribute very less amount to the total column density of N02. However, layers four to eight contribute maximum to the total column density of N02. Figure 4 represents the . vertical profiles of N02 obtained on different days during April 1998 and shows decreasing trend of N02

concentrations with time (days) at the station, Pune. The N02 vertical distributions in the polluted and unpolluted troposphere are depicted in Fig. 5[(a) and (b») . Figure 5(a) shows higher concentration of N02 at tropospheric level, while Fig. 5(b) shows less concentrat ion at tropospheric level. The data collected at Iceland stat ion , Reykjavik, have also been analysed and vertica l profiles are denved.

2.2 Vertical profiles of NOz and 0 3 over Iceland (Reykjavik) The slant column densities of N02 and 0 3 derived

from the spectroscopic observations collected at Iceland station, Reykjavik (64°N, 22.6°W), in the form of SESAME-Phasc-I duting winter season of 1993-94 are used to retrieve the 'vertical profiles of N02 and 0 " respectively . The atmosphere up to a height of 40 km from ground Ie:vel is considered for the development of the algorithm. This assumption is based on the actual observations of gases under study made by othe r investigators at different latitudes (Gil, Private Communication). It is found that at higher latitudes, O, and N02 were observed below 40 km only. This atmosphere is divided .into IO layers of thickness 4 km each .

The slant column densities of N02 and 0 3 for solar zenith angles 87.5°-92.0° at an interval of 0 .5° (i.e . IO values) are considered to derive vertical profile. The layerwise air-mass factors of N02 and 0 3 for the same set of solar zenith angles are used in the algorithm. The algorithm is used to derive vertical profile of N02

and 0 3 for the winter period of 1993-94. These

LONDHE et al. : VERTICAL PROFILES OF N02 & 0) USfNG SLANT COLUMN DENSITY

S 1.21E+018 Z

"'" o ;:..1.01£+018 E-< ..... Vl Z ~ 8.10E+017

~ 6.10E+017 o U E-< ~ 4.10E+017

....l Vl

2. 1 OE+O 17

(a)

01 APR.1998

87.00 89.00 91.00 9:5.00 95.00

SOLAR ZENITH ANGLE. deg.

~.OO~--------------------------------------------------,

40.00

] 30.00

~ § f-...... f-~ 20.00

10.00

(b)

01 April 1998

TOTAL N02 = 6.947109E+15

0.00 -tnTTT1ITTTTTnCTTTnTTTrT1ITTTYTT1CTTTnnTTT1ITTTTTT1TrT1fTTTTTT1TTT1rTTTTTT1mTnnTfT1lTTTrrrrj

0 .00 5.00 10.00 15.00 20.00 25.00 JO.OO :55.00 40.00 45.00

N02 CONCENTRATION, X 10 14 per 5 km· em2

Fig. 3-(a) Slant column densities of N02 for differen·t solar zenith angles [E+016 along y·axis represents 101(· and so on) and (b) Vertical profile of N02 at Pune

295

296 INDIAN J R4DI0 & SPACE PHYS, DECEMBER 1999

~.OO,---------------------------------·--------------.

40.00 .

30.00 E

.Jl.

t.J o

~ 20.00

10.00

.,.,. 01 APR 1998 MM& 02 APR 1998 _15 APR 1998 --- 16 APR 1998 ......... 18 APR 1998 ..... 19 APR 1998 ttttf 21 APR 1998 -_. 22 APR 1 gga

0.00 -+,..."..."....,,,--,r-r-'f""T""r-T""rT""rT""T""T"T""rT""rT"T"T"T""TT"TT'TT""T"T-rT-rT ... ...,,,..,,,--,rrl 0.00 10.00 20.00 30.00 40.00 50.00

N~ CONCENTRATION,)( 10'4 per 5 km.cm2

Fig. 4-Venical profiles of N02 for April 1998 at Pune

6O.OQ (a) 50.00 (b)

\ TOTAL NO,; 6.836 X 10" molecules/em' ~6 March 1998

10.00

~~~·~~'~"~"'~4~~"n"'.~.~~.~"~,,,~~~"'~"~\'~~1~~~~~

NO, CONCENTRATION, X 10" per 5 km· em'

TOTAL NO, = 3.91 9 X 10" molecules/em' 17 March 1998

4 ........ ~a.OO"";o.l)O ... 'iJiO""'4.l)O""' • .oo la.1Xl

NO, CONCENTRATION , X 10" per 5 kill ' em'

Fig. 5- Vertical pro fi les of N02 for (a) Polluted troposphere and (b) Unpolluted troposphere

LONDHE et al. : VERTICAL PROFILES OF N0 2 & 0 3 USING SLANT COLUMN DENSITY 297

vertical profiles are utilized to separate out the tropospheric and stratospheric concentrations of N02

and 0 3• For this purpose, out of 10 layers, first three layers are added to represent the tropospheric concentration and remaining seven layers are added to represent the stratospheric concentration. Thus, tropospheric height is taken as 12 km and stratospheric height is taken from 12 to 40 km which is assumed to be reasonable. Figure 6 shows the tropospheric/stratospheric concentrations of N02

derived by using the above algorithm. It is observed from Fig. 6 that the tropospheric concentrations of N02 are higher than the stratospheric concentrations on some days. Thus, if the total column density of N02 is divided into two parts so as to represent the tropospheric and stratospheric contribution ~ important features of troposphere and stratosphere may be studied. The tropospheric pollutipn episodes and contribution of N02 during thunderstorm activity may be studied well with this algorithm9

.

Stratospheric/tropospheric contributions of 0 3

obtained in a similar manner described above are shown in Fig. 7. The amplitudes of variation in stratospheric 0 3 are higher than those of the

tropospheric 0 3 as per expectation. Nearly 90% of 0 3

resides in stratosphere, while only 10% resides in troposphere. Therefore, amplitude of variation in 0 3

at stratospheric level is comparatively more than at tropospheric level (Fig. 7). The trends of stratospheric and tropospheric ozone show inphase relation. However, N02 does not show such behaviour due to the large fluctuations in tropospheric contribution (Fig. 6) .

The stratospheric variations of N02 and 0 3 are shown in Fig. 8. By separating tropospheric/ stratospheric contributions of N02 and 0 ), some interesting results are observed. The stratospheric N02 and 0 3 variations are in phase, although total 0 3

variations and total N02 variations do not show any phase relation. The tropospherio variations of N0 2 are neither related to stratospheric variations of N02 nor to tropospheric/stratos'pheric variations of 0 3.

3 Contour maps of nitrogen dioxide (N02) and ozone (03) at Iceland (Reykjavik) Figure 9 shows the spatio-temporal vtlfiations

(contour maps) of N02 concentrations retrieved from Iceland data. The sunset profiles are used for these

l00~----------------------------~----------------------~

N

E o .... Q) 0.. ... o ...... x Z~

o >00-4

~ U Z o U

N

o Z

80

60

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------ TROPOSPHERIC N02 (Integration of 0-12 krn) I - STRATOSPHERIC N02 (Integration of 12-40 krn)

~ ~

360 380 .wo

~ " " , ' , ' , , , , , , , , , , , , , , ,

DAY NUMBER 420

" " " " :\ I " " " " , , i ~ : ~ ::: ~: t ',' :.: ," '" , ,

Fig. 6-Tropospheri c and stratospheric concentration of N02 [Day number starts from I January. e.g. day No. 360 means Dec 25/26 of a yearl

460

298 INDIAN 1 RADIO & SPACE PHYS, DECEMBER 1999

140~------------------------------------------------~ ------ TROPOSPHERIC 0 3 ( Integration of 0-12 km) -- STRATOSPHERIC 0 3 (Integration of 12-40 km)

120 N

E u ..... v c.. 100 ~ 0

'X

Z 80 0 ..... E--o

~ 50: E--o Z ILl U Z 40': 0 U ---<5 ----

o 460

Fig. 7-Tropospheric and stratospheric concentration of 0 3

lW,-----------------~~------------_, = STRATOSPHi::RIC O •• X 10". = STRATOSPH:RIC No. , X 10'

z o i 120 ,

o z 8 u 60

eJ :x: 0..

140

Of,y NUW8ER

Fig. 8-Stratospheric variations of N02 and 0 3

contour maps. The chemical changes (N021N0 conversion chemistry) during twilight period are taken into account whi le deriving vertical profiles of N02· Day-to-day differences seem to be dominant and expected to be due to dynamical effects and also, possibly. due to photochef1lical effects associated with temperature. Figure 9 shows a peak N02

concentration between 18 and 26 km for day numbers 375-455. However, for day numbers 345-375, the

peak N02 concentration is observed between 12 and 24 km, i.e . during December 1993 and up to first week of January 1994. The peak N02 concentration generally increased from December to January, while it remains constant from January to March (up to the end of available observations) . This indicates the stratospheric NO] correlation with temperature. The tropopause shifts downward during winter and gradually gets lifted to higher altitudes after winter. At an altitude less than 6 km the pollu tion episodes are clearly well marked by thick contour density. The observed tropospheric contribution is much higher than stratospheric contribution in some cases as observed on day nu mbers 347, 421, 430 and 442. These episodes occur at an interval of 5-7 days, as seen in the contours of Fig. 9. The tropospheric episodes seem to be related with the stratospheric concentration of N02 with some delay . Typically, the N02 concentration below 5 km vary between IxlO l4

and sax I 0 14 molecu les cm·2 (Fig. 9). Thi s corresponds to a mixing rati o of 10-500 ppt. Long path absorption measurement ~ at s imilar site indicates that during windy conditions the air is well-mixed wi th .the free troposphere 10.

Contour map for 0 , for Iceland data is shown in Fig. 10. The units are shown in terms of I unit=lxlO 17

E ~ ... U.l

§ f-< -f-<

~

~.

LONDHE el ai. : VERTICAL PROFILES OF N02 & 0 3 USING SLANT COLUMN DENSITY

]8 ]8

]4 ]4

]0 30

26 26

22 22

18 18

14 1-4

10 10

6 6

j45 ]55 365 ]75 385 395 405 415 425 435 445 4552

DAY NUMBER

Fig. 9-Contour map of N02 at Reykjavik (Iceland) during 1993-1994 [I unit=lxlO 14

molecules/cm2/4 km)

E ~ ... ill

~ f:: :t!

38 J8

34 ].<l

"_ ,", ____ ,' ~ \\ -----.....,' "-./.'1 r-- " 3e 30

26

22

18 18

14 1.11

10 10

6 "~\\~""-.J\'v--I'

4552

~45 385 395 405 ~15 425 435 445

DAY NUMBER

Fig. IO-Contour map of 0 3 at Reykjavik (Iceland) during 1993-1994 [I unit=lxlO 17

moIecules/cm2/4 km)

299

300 INDIAN 1 RADIO & SPACE PHYS, DECEMBER 1999

(c) Increase/decrease of the species by chemical change, i.e . chemical reactions change the composition of the species which may modulate the density. (d) Heterogeneous reactions may also be responsible for the changes in the ratio of different layers.

molecules cm'2 per 4 km. It can be seen, from density variations for different layers here, that there is a close correlation between various contours. At stratospheric layers the maxima is around 22 km. The stratospheric maxima are centered around day numbers 365, 387, 410, 427, 432 and 445 . If the stratospheric variations of N02 and 0 3 are compared, then a good correl ation is observed between the maxima of both the species as seen in contour maps.

4 Layer density ratio technique

5 Inter-correlation between layer densities Correlation coefficient between various layers for

N02 and 0 3 are given in Table 1. In thi s technique the ratio of densities observed in

different layers is taken into account which is useful to interpret the density variations in different layers with respect to time.

From thi s ratio technique the following information is expected .

In case of 0 3 the inter-correlation is very good for all the layers . The correlation coefficient is more than 0.998 for all the layers. In case of 0 3, tropospheric layers (1, 2 and 3) and stratospheric layers (4 to 10) are well correlated.

(i) If variations are simultaneous .and of the same order in all layers, they will be eliminated in thi s method .

For N02, a poor inter-correlation is observed between tropospheric layers ( I , 2 and 3) except for layers 2 and 3. The correlation coefficient between layers 2 and 3 is 0.6177. The layers 4, 5 and 6 also show poor correlation with layers 1, 2 and 3. However, layers 7, 8, 9 and 10 show a good correlation with layers 2 to 10 except layer 4 as seen from the values of correlation coefficient in the Table 1. A poor correlation between tropospheric layers may be attributed to large variability caused by tropospheric pollution.

(ii ) The change .in ratio may be due to following processes . . (a) Vertical mixing of the species from di fferent layers, i.e. when dynamics of vertical transport is playing a role in the process . (b) Horizontal transport of the species, i.e. transport due to winds.

Table I--Correlation coefficient for N02 and 0 3

ForNOl

Level I Level 2 Level 3 Level 4 Level 5 Level 6 Level 7

Level I 1.0000 0.298 1 -0.0674 -0.2851 0.0243 0.1023 0.1072 Level 2 0.2981 1.0000 0.6177 0.1 459 -0.0096 0.1614 0.4500 Level 3 -0.0674 0.6177 1.0000 0.5255 -0.1781 -0.1054 0.1383 Level 4 - 0. 285 1 0.1459 0.5255 1.0000 -0.0961 -0.0501 0.0960 LevelS 0.0243 -0.0096 -0.1781 -0.0961 1.0000 0.8053 0.6368 Level 6 0. 1023 0.1614 - 0.1054 -0.0501 0.8053 1.0000 0.8879 Level 7 0.1072 0.4500 0. 1383 0.0960 0.6368 0.8879 1.0000 Level 8 0.11 86 0.6367 0.3380 0.1838 0.4938 0.7396 0.9557 Level 9 0. 1060 0.7458 0.5050 0.2658 0.3954 0.6060 0.8736 Level 10 0.121 0 0.8 153 0.5720 0.2651 0.3099 0.5189 0.8108

ForO)

Level I 1.0000 0.991}9 1.0000 0.9999 0.9997 0.9991 0.9992 Level 2 0.9999 1.0000 0.9999 0.9998 0.9994 0.9987 0.9988 Level 3 1.0000 0.9999 \.0000 1.0000 0.9997 0.9992 0.9992 Level 4 0.9999 0.9998 1.0000 \.0000 0.9999 0.9995 0.9996 levelS 0.9997 0.9994 0.9997 0.9999 \.0000 0.9999 0.9999 Level 6 0.9991 0.9987 0.9992 0.9995 0.9999 1.0000 1.0000 Level 7 0.9992 0.9988 0.9992 0.9996 0.9999 1.0000 1.0000 Level 8 0.9994 0.9990 0.9994 0.9997 1.0000 1.0000 \.0000 Level 9 0.9996 0.9992 0.9996 0.9998 1.0000 0.9999 1.0000

Level 10 0.9996 0.9994 0.9997 0.9999 1.0000 0.9999 0.9999

Level 8 Level 9 Level 10

0. 1186 0.1060 0.1210 0.6367 0.7458 0.8153 0.3380 0.5050 0.5720 0.1838 0.2658 0.2651 0.4938 0.3954 0.3099 0.7396 0.6060 0.5189 0.9557 0.8736 0.8108 1.0000 0.9711 0.9381 0.9711 1.0000 0.9846 0.9381 0.9846 1.0000

0.9994 0.9996 0.9996 0.9990 0.9992 0.9994 0.9994 0.9996 0.9997 0.9997 0.9998 0.9999 \.0000 1.0000 1.0000 1.0000 0.9999 0.9999 1.0000 1.0000 0.9999 1.0000 1.0000 1.0000 1.0000 \.0000 1.0000 1.0000 1.0000 1.0000

. ..,

LONDHE et al. : VERTICAL PROFILES-OF N01 & OJ USING SLANT COLUMN DENSITY 301

6 Results and discussiO .. The vertical profiles of N02 and 0 3 are retrieved

for the Iceland station (Reykjavik). These are utilized to separate out the tropospheric and stratospheric contributions of both tl.l! gases. The highlights of the study are.given as follows:

(i) Tropospheric variations of N02 are more than those of stratospheric ones and are related to tropospheric pollution .

(ii) The stratospheric variations of 0 3 and N02 are in phase, although total column 0 3 variations and total column N02 variations do not show any phase relation.

(iii) The contour maps of N02 and 0 3 columnar densities fur the Iceland station are also drawn . The N02 contour map shows the peak com:entration at 22 km altitude which repr,esents the mean height of layer 20-24 km. However, maximum concentration of N02 is observed between 18 and 26 km on the day numbers 376 and 455 and between 12 and 24 km on the day numbers 345 and 375, i.e. during winter season.

(iv) The contour map of 0 3 shows peak concentration around 22 km. The variations observed in all layers show almost similar trend. Layerwise contributions of N02 total colUJmn density were worked out. The tropospheric layers show larger variations on polluted days. The layer density variations of NO:! are clearly seen in the counter map.

(v) The correlation coefficients between various layers for N02 and 0 3 are calculated. A poor

correlation IS observed between tropospheric layers of N02 (layers I, 2 and 3) except for layers 2 and 3. However, layers 7, 8, 9 and 10 show good correlation with layers 2-10 except layer 4.

Acknowledgements The authors wish to thank the Department of

Science & Technology, New Delhi, for giving the financial support for the development of the Rotating Slit Scanning Spectrometer and the Director, llTM, Pune, for encouragement and help for this work.

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