The present study concentrates on measurements of NO2 and NO3. NO2 has been measured during twilight period using zenith sky absorption spectrometric technique in the 436 to 448 nm region. NO3 has been measured during night time using direct moon as a source of light in the 655 to 667 nm region. These measurements have been taken at low latitude station, Ahmedabad (23 deg N, 76 deg E), India for the past two years

Chakrabarty, D. K.

Das, S. R.

Lal, M.

Sidhu, J. S.

English

NASA Technical Reports Server

N95- 11065
NEAR SIMULTANEOUS MEASUREMENTS OF NO 2 AND NO 3 OVER TROPICS BY
GROUND-BASED ABSORPTION SPECTROSCOPY
M. Lal, D.K. Chakrabarty, J.S. Sidhu and S.R. Das
Physical Research Laboratory
Ahmedabad - 380 009, India
Abstract
The prcsenl study concentrates on measurements of
NO 2 and N(). N() has been measured during twilight
period using zenith sky absorption spectrometric
technique in the 436 to 44"8 mn region. NO 3 has been
naeasured during night time using direct moon as a
source of light in the 655 to 667 nm region. These
n_east,rements have been taken at low httitude station,
Ahmedabad ( 2._ N, 76"E ), India for the past two
years.
Introduction
NO group of species plays an important role in the
chemical reactions that control the distribution of O
3
ira the stratosphere. The variabilities of NO and NO
2
are knowll to sortie exlenI, those of NO are in
3
considerable darkness. Firsl observation of NO was
3
taken by Noxon et al. ( 1978 ), who used a scanning
spectrometer and moon as a light source above the
Fritz Peak Observatory ( 40"N I in April 1978. The
same spectral feature has been used to study
stratospheric NO by several authors ( Sanders et
al., 1987: Solomon et :ft.. 1989a,b ). However, these
observations are not sufficient to draw a clear
picture of the global NO variability. A number of
observations have been taken on the NO content of the
stratosphere by using ground-based, balloon-borne and
space craft techniques ( Syed and Harrison, 1980,
Mckenzic et al. 1982: Mount et al., 1987: Pommereau et
al., 1988; Johnston et al. 1992). But most of these
observations are of mid- and high latitude zones.
Satellite observations are available for low latitude
zone: but those :are not yet fully validated due to the
lack of enough observations in this region. The
distribution and wtriability of this specms in the
low latitude northern hemisphere, however, are not
properly known. We have, therefore, initiated
measurements of NO and NO over Ahmedabad, 'a low
3
latitude station. The measurements reported here will
contribute significantly to the global data base of
minor neutral con,,tJluents,
Experiment
The details of experimental set-up has beer
described in the companion paper(Lal & Chakrabarty).
Methodology
Using the property that NO has strong absorption
at 662 nm, and NO 2 has strong absorption at 439.5 nm,
NO and NO column abundances have been measured.
2 3
Since the concentration of NO 3 is very low during
daytime, its abundance has been measured during night
time only. The technique of NO measurement has been
described by Noxon (1979), Harrison (1979) and Syed
and Harrison (1980). The analysis of NO 3 has been done
by using matrix inversion technique. For NO
2
measurements, the background spectrum has been
obtained at minimum possible solar zenith angle.
Superficially, twilight and background spectra appear
similar owing to the presence of strong Fraunhofer
structure in the incident sunlight. By taking their
ratio, the Fraunhofer structure is removed. After this
is done, absorption by NO 2 becomes apparent owing to
the much longer path during twilight period. This
ratio spectrum has been used for the calculation of
NO abundance using five point method as given by
2
Harrison (1979).
For NO 3 measurements, direct solar spectrum at 80 °
solar zenith angle has been used as a background
spectrum. All the observations have been taken at
higher lunar zenith angle during night time. Besides
NO 3 absorption, aerosol and Rayleigh scattering, and
absorption due to H O and O are also present. These
have been removed in the manner described by Solomon
et al. (1989). With this residual spectrum, using
least square technique, column abundance of NO had
been calculated.
Results and discussions
Fig. 1 shows an example of the solar spectra taken
during morning and noontime. These spectra contains
Fraunhofer structure and absorption due to atmospheric
species. Morning spectrum has been divided by the noon
time spectrum to remove the Fraunhofer component
683
https://ntrs.nasa.gov/search.jsp?R=19950004652 2020-06-16T11:30:13+00:00Z
1.500
I,-- 1.350
z
.J
b.I 1.200
n,"
Z
>- 1.050
t,-
Z
hi
0.900
Z
0.750
-- 010608.02 JAN '92
.... OI 1412.37
,/
I _ I I I I t t
4390 4420 44.50 4480
WAVELENGTH (El
Figt.0"e 1. Zenith sky spectrum at 0802 hrs and 1237
h_ IST in the month of January.
present in it.
Fig. 2 shows an example of the ratio of the
twilight to noontime spectra. In this spectrum,
Fraunhofer component should have been absent. But, in
some cases the fluctuations in the ratio spectrum
still remain. This is the Ring component. To remove
this Ring contribution, we have taken the running
average of ratio spectrum. The ratio spectrum thus
obtained has the contribution of aerosols and Rayleigh
scattering. To remove this component we have
subtracted the ratio spectrum from a straight line
parallel to the slope of the ratio spectrum. From the
resultant residual spectrum, the slant column density
of NO 2 and 03 have been calculated using the method
given by Harrison (1979). It may be mentioned here
that in this method, only those wavelengths have been
selected which have minimum contribution of Ring
component.To calibrate the instrument, NO 2 cell
spectrum has been scanned using artifical light source
in 436 to 448 nm region, which shows good correlation
with the residual spectrum obtained from the
atmosphere.
RATIO SPECTRUM1.010
F-
ro I.ooo
n,,"
<f
Z
-- 0.990
z
m 0.980
I---
z
0.970 t
436,
I I I
4390 4420 4450
WAVELENGTH (A °)
4480
Figure 2. Ratio of twilight time and noon time
spectra.
15.OE+15
I0.0
o
u 5.0
Z
0.0
0700
SOLAR DATA OCT 1990 TO JAN. 1991
0800
Time in hrs. (IST)
Figure 3. Increase of NO 2 vertical column density
after sunrise.
Fig. 3 shows the vertical column density ot NO
versus time for winter t990-91. An increase in the
value of NO as the sun rises is clearly discernible.
This is due to the formation of N() by the
photodissociation ,_f N205 after sunrise.
Fig. 4 shows the plot of morning and e',ening
vertical column abundance.,, obtained during the peric, d
December 1990 to March 1992. After September 1991. the
values of NO 2 appear to be slightly higher than those
obtained in the earlier months. It may be menti(med
here that Mt.Pinatubo erupted in June 1991 and it
disturbed the aerosol distribution of the earlh's
atmosphere. After volcanic eruption, no alteration v.a,,
made in the instrumental set-up. The interfereno: due
to additional aerosols on the retriewd process of N()
is minimum, as explained by Harrison (1979). We do not
understand why their increase in N(). This could he
due to the increase in value of albedo below aerosols
layer which causes increase in the production rate _f
NO 2 compare to that before eruption period. The
volcanic effect was more pronounced in [he sunset
period data compared to the sunrise peric_d data.
Contrary to our ,.alucs, NO., column density observed hv
aot?
'_E
+ io_
F.
!
,o'_
io14
Figure 4.
the years.
• $UNRIS_
o SUN._ |
MONTHLY AVERI=C,ED NO2 VERTICAL COLUM_
AT AHMEOAOAO , INDIA ( 21"02'N,76")O'E }
o
o
• • •
j I e
•qL
° 1/,
J i t i t i i , , , , i i i , i , , , , , i I , , = i
F_I4AM J J A S 0N D J FM&M J J A $0 NO J FM&M
_91J0 Nan 1_15_
A plot of NO 2 vertical column density over
684
1-15 1
t. loH
0
_ 1.05
E
D I 00
> 095
F-
<[ 0.90
_1
la.I
¢r
>,-
I..-
Z
I,,d
I.--
Z
655(
v/
,j qo.
I I I I I 1 0.83
6570 6590 6610 6630 6650 6670
WAVELENGTH (_)
Figure 5. Direct solar and lunar spectra for large
zenith angle.
similar technique at New Zealand (4. S, 17(FE) show's
decreasing trend after Pmatuho volcanic eruption.
Before June 19!}1. the monthly vertical column
abundance of NO has been found to be in the range of
1-5 xlO 15 cm:. From September 1991 (in the post
eruption period of MI. Pinatubo ) NO 2 values have been
seen to increase, in February 1992, the NO values are
2
I0 le' cm _,
Fig. 5a shows the background direct solar spectrum
and Fig. 5b shows the observed direct lunar spectrum
between 655 and 667 nm. Except H at 656.3 nm, there
is very little solar Fraunhofcr structure in the
0447 hr$ / 0755hr$
= I J I l
6630 6650 6670
WAVELENGTH(_)
Figure 6. Residual absorption spectrum for 655 to
667 nm.
685
wavelength region used, thus the absorption feature
due to NO 3 and H,O can clearly be seen in the raw
spectrum. After taking the ratio of observed to the
backgrotmd spectrunl, ratio spectrum has been obtained.
Since aerosol and Rayleigh scattering is also present
in this wavelength region (i.e., 655 to 667 nm), a
slope in the ratio spectrum has been observed.
Subtracting the ratio spectrum from a straight line
parallel to the slope of the ratio spectrum, residual
absorption spectrum has been observed.
Fig. 6 shows the residual absorption spectrum
obtained in the region 655 to 667 nm region. This
spectrum contains absorption due to NO and water
3
vapour. NO has maximum absorption at 662 nm which can
clearly be seen in the residual spectrum as a
prominent dip. Water vapour has also absorption at
659 nm region, which can be seen as a less pronounced
dip compared to the NO. Besides this absorption
feature, the Bahner-alpha feature is a]so seen in the
residual absorption spectrum at 656.3 nm region. To
verify the absorption feature of NO 3, observations
have also been taken in the 617 to 629 nm region in
which NO 3 has maximum absorption at 623 rim.
Fig. 7 show, s the slant column abundance of NO vs.
3
lunar zenith angle observed on different nights. It
is to be noted that the slant column density of NO
3
decreases as the lunar zenith angle increases. This is
due to the modulation of aureole scattered flux at
higher lunar zenith angle. The higher values of NO
3
observed over Ahmedabad could be due to the
tropospheric contribution from the industrial
production, biomass burning in the tropics and NO
x
species transported from Kuwaity oil fires.
Interestingly, NO 2 values measured simuhaneously are
also higher than the earlier measurements.
Fig. 8 shows the vertical column density of NO
against time observed on different nights. It is to
be seen from this figure that after sunset NO
3
19
10 f I T J
?E to 18
E
2
o
v
c
_ _016
z
1olsl
65
• 1§JAN 1992 (Pre-_unrisel
o 19JAN 1992 IPost-$unset)
x 19JAN 199Z IPre-$unrisel
I 20JAN 1992 (Pre-sunrise) /
", 20JAN 1992- (Posl-sunsel I t
o 21JAN. 1992 (Posl-$unsetl It
/ I
/
oA x A ii
x • ""
A oK i
_x i_ _
LUNAR ZENITH ANGLE (DE6.)
Figure 7. Slant column abundance of NO 3 vs lunar
zenith angle.
]G
IE 32
un
x
g
"o ZO
E
> 8
, o
0
1800
• 18 JAN 1992 [Prll,$unrm_ )
o 19 JAN. 1992 {PoiFsun_,! )
x 19 JA N.1992 ( Pl'l, lsunrl_ I
• 2OJ&N.Ig92 (Pre-su_r*se ]
• ZOJAN.1992 (Post-sunset ]
n 21JAN 1992 (Post-sunset I
r •
o •
A
-- % R_lotJve hum_d,!
ii
z
g.
/g
IO
*1 BB ¢ •
1900 2000 2100 2200 2300 _'400 OtOO 0lO0 0300 0400 O_DO 0600 0700 0800
INDIAN STANDARD TIME (hrs)
Figure 8. Vertical column abundance of NO 3 against time, Relative humidity is shown by continous lmc.
increases. This is in agreement with the restdts of
Platt et al. (1981). Few observations have also been
taken before sunrise, where NO shows a decrease in
column density as one approaches sunrise. Surface
relative humidity during the night has also been
plotted in Fig. 8 which shows a positive correlation
with NO 3 as observed by Platt et al. (19811. Our
measured values of NO lie between 1 and 20 x 1()15
3
-2
cm .
Conclusion
In this paper we have presented results of NO 2 and
NO 3 measurements by a ground-based visible absorption
S_ectroscopy technique. Measurements have been made
at a low latitude northern hemispheric station,
Ahmedabad (23"N, 76°E), India. Several interesting
results have been obtained. The monthly vertical
column abundance of NO has been found to be in the
2
range of 1-5x10 _5 cm 2. An increase in the value of NO 2
was found after Pinatubo volcanic eruption. NO 3
values have also been observed during the nigl:}t time.
Its values varies from 1 to 20 x 10-cmL This
suggest that tropospheric contribution of NO 3 is very
high at our observation site.
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A.R. Ravishankara, 1990 : Absorption
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Near simultaneous measurements of NO2 and NO3 over tropics by ground-based absorption spectroscopy

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