The Improved Stratospheric and Mesospheric Sounder (ISAMS) is a multichannel radiometer and forms part of the science payload of the Upper Atmosphere Research Satellite (UARS). ISAMS measures infrared emissions from the Earth's atmosphere in several wavelength bands. Three such bands include emission from nitric oxide, nitrogen dioxide, and dinitrogen pentoxide. In this paper, we briefly discuss how the ISAMS instrument measures NO, NO2, and N2O5. We also present preliminary data from these channels and describe preliminary validation work

Ballard, J.

Kerridge, Brian J.

Knight, R. J.

Morris, P.

Reburn, J.

Remedios, John J.

Stevens, A. D.

Taylor, Fredric W.

English

NASA Technical Reports Server

N95- 11007
MEASUREMENTS OF STRATOSPHERIC NO, NO2 AND N205 BY ISAMS:
PRELIMINARY OBSERVATIONS AND DATA VALIDATION
B J Kerridge, J Ballard, R J Knight and A D Stevens
Rutherford Appleton Laboratory
Chilton, Didcot, Oxon UK
J Reburn, P Morris, J J Remedios and F W Taylor
Atmospheric, Oceanic and Planetary Physics
Oxford University, Oxford UK
ABSTRACT
The Improved Stratospheric and Mesospheric Sounder
(ISAMS) is a multichannel radiometer and forms part of
the science payload of the Upper Atmosphere Research
Satellite (UARS). ISAMS measures infrared emissions from
the Earth's atmosphere in several wavelength bands. Three
such bands include emission from nitric oxide, nitrogen
dioxide and dinitrogen pentoxide. In this paper we briefly
discuss how the ISAMS instrument measures NO, NO2 and
N205, present preliminary data from these channels and
describe preliminary validation work.
1. INTRODUCTION
Catalytic reactions involving NO and NO2 are re-
sponsible for controlling the abundance of Oa throughout
much of the stratosphere. In order to understand the be-
haviour of stratospheric O3, it is essential that the processes
which affect NO and NO2 are understood. A vital contri-
bution to this understanding are global measurements of
the abundance of NO, NO_ and other members of the odd
nitrogen (NO,) family.
Global measurements of stratospheric NO2 at spe-
cific times of day have been made by previous satellite in-
struments, i.e. the Limb Infrared Monitor of the Strato-
sphere on Nimbus 7 (Russell et al., 1984) and the visible
spectrometer on the Solar Mesosphere Explorer (Mount et
al., 1984). SAGE I and II have also accumulated global
NO2 data by near-UV solar occultation measurenmnts from
space (Chu and McCornfick, 1986; Cmmold et al., 1991).
In addition, vertical profih's of NO, NO2 and N20,_ have
been obtained at a restricted set of latitudes from IR solar
occultation measurements by the ATMOS (Russell at. al.,
1988) and Grill_" (Laur¢'nt et al., 1985) spectrometers on
board the space-shuttle . Howew'r, most inforlnation con-
cerning the diurnal variation and vertical profiles of these
inoh,cuh,s has COIII¢"from balloon ir_ situ and remote sans-
ing measurements at northern mid-latitudes (Pommereau,
1982; Roscoe, 1982; Roscoe et al., 1986; Kondo et al., 1988;
Webster et al., 1990 and Abbas et al., 1991).
2. ISAMS MEASUREMENTS
The first height-resolved, global measurements of NO
and N205 in the stratosphere are currently being made
by the Improved Stratospheric and Mesospheric Sounder
(ISAMS), along with the Cryogenic Limb Array Etalon
Spectrometer (CLAES), on the Upper Atmosphere Research
Satellite (UARS). These, together with simultaneous mea-
surements of NO2, HNO3, N20 and other molecules, will
allow the global budget and partitioning of stratospheric
NO_ to be investigated more thoroughly than has been
possible hitherto. The data will also be used to investigate
the effects on the stratosphere of major volcanic eruptions
which occurred during 1991, notably Mt.Pinatubo, specif-
ically by examining the effect of sulphate aerosol upon the
global abundance and partitioning of NO_.
ISAMS measures IR emission from the atmosphere
using ,_ 100cm -1 wide optical filters matched to the funda-
mental vibration-rotation bands of NO, NO2 and N_O5 at
5.3_m, 6.2#m and 8.1#m respectively. Atmospheric emis-
sion from within these filters is modulated at 1 kHz by
100% mechanical chopping and signals synchronously de-
tected at this frequency are referred to as wide-band (WB)
measurements.
For NO and NO2, a gas-correlation technique is used
in addition to the optical filters. Cells containing NO and
NO2 are included in the optical paths of these channels
and their pressures cycled at _35Hz by a piston driven at
resonance. Signals synchronously detected in the 35 Hz
side-babels to chopper frequency are referred to as pres-
sure modulated (PM) measurements. Simultaneous PM
and WB measurements should allow retrievals of NO and
NO2 even in the presence of substantial aerosol and inter-
faring gas emissions (Roscoe et al., 1986).
ISAMS views the atmospheric limb at right angles
to the UARS velocity vector and measurements are made
every two seconds. The tangent-height for each measure-
439
https://ntrs.nasa.gov/search.jsp?R=19950004594 2020-06-16T11:34:17+00:00Z
mentisdeterminedbythepositionofa scanningmirror
whichisprogrammedto sampletangent-heightsbetween
10kinand80kmin 2.5kinsteps.NOandNOasignalsare
recordedat eachmirrorstep;N205signalsarerecorded
lessfrequentlybecausethisfilterismultiplexedwiththose
atotherwavelengthsbyafour-positionfilterwheel.
ValidationofdatafromtheISAMSNOxchannelsis
still atanearlystage,soaccuracieshaveyettobedeter-
mined,however,theprecisionisveryhigh.Forexample,
thesignaltonoiseratioin theNO_PMchannelissufficient
forinversionofindividuallimbscans.Theinstantaneous
fieldofviewofISAMSis2.Skm high by 8km wide at the
limb, and vertical profiles of NO2 mixing ratio are gener-
ated every 32 seconds, or approximately 200kin along the
sub-tangent point track. The intrinsic vertical resolution
of the measurements can be assessed by the sharpness of
the averaging kernels associated with radiance inversion.
Averaging kernels for the NO_ PM channel shown in fig-
ure 1 indicate that good height resolution can be achieved
throughout the stratosphere.
_ . _.-.-,-
..... ./ - _.__ _:_.£- ....
E
40
g
30 . Z>_.U_.
2O
-0.2 0.0 0.2 0.4 0.6 0.8
Averaging Kernels
Figure 1 Averaging kernels for the NO2 PM channel
3. DATA VALIDATION
A programme to validate ISAMS measurements of
NO, NO2 and N2Os is in progress. This includes critical
examination of the radiometric calibration and the resul-
tant calibrated radiances as well as in-depth examination
of instrumental parameters concerning the gas-correlation
cells.
Calibration includes characterising scan-dependent
stray radiation and the radiometric "zero". Accurate de-
termination of radiometric zero in the NO PM channel is
particularly important because radiances from the strato-
sphere are small, and small changes are important scientif-
icaUy. Obtaining a good zero is, however, complicated by
emission from NO in the thermosphere (see Ballard et al.,
this symposium).
For gas-correlation measurements, detailed knowl-
edge of gas cell conditions is also essential, and these are
partially controlled by a silicalite molecular sieve to which
the cell is connected. The mean pressure in each cell is
a few millibars, and can be changed by varying the tem-
perature of the sieve by ground command. The gas cells
were filled some 2 years before UARS was launched and, at
that time, mean gas pressure, piston oscillation frequency
and sieve temperature were carefully characterised. In or-
bit, the mean pressure in a given modulator cell can be
derived from the resonant frequency of the piston and also
from the sieve temperature, by reference to the pre-launch
characterisation.
The composition of the NO and NO_ cells is believed
to have changed somewhat since filling due to chemical re-
actions occurring on the sieves. In the NO cell, total pres-
sure at nominal sieve temperature has fallen since launch
by _10%, which is believed to be due to interconversion of
NO into N20 and N_O3. In the NO_ cell, it is estimated
that _>75% of gas is now NO.
0130
3
0125
2
i
a_
0120
xx\.
0115 i
20 4c 60 80 1o0
4
i , ;
120
OGy_ since launch
Figure 2 Ratio of PM/WB signals in the NO2 channel
when viewing the internal calibration black-body.
More accurate estimates of gas partial pressures in
the two cells will be obtained from (i) measurements of
resonant frequency (ie total pressure) vs sieve temperature
performed recently in orbit and (ii) analysis of signals from
the calibration black-body. The first exploits the unique
relationship bet wean equilibrium pressure of each molecule
440
_-d sieve temperature, which is deter_ned by "binding en-
ergy" and othe_ physical properties. The second exploits
the unique relationship between the partial pressure of ab-
sorbing gas and the ratio of black-body signals in the WB
and PM channels. Comparison of measured and calculated
ratios is currently limited by knowledge of both NO2 spec-
troscopy, particularly self- and NO-broadened linewidths,
and the detailed pressure and temperature cycles. How-
ever, the measured ratio itself serves as an indicator of how
cell composition has changed with time and figure 2 shows
that only small changes have occurred since activation of
ISAMS in orbit.
Day-Night Difference in Zonal Mean Radiance for
NOx Channels
'!' ' ''! ' " " I .... "/ ......I /
• i\'_
40 _ _X. ., .o.s/ .O.,R
i
l
I
\', ) i: / II
i
i •
i __/:1I"
I i ',
I i,I: i [
Z0 l ! II Ii ,
:
\ "
' 11
]01 .... I _ • ,; _ , , • n ....
-io -o.5 o,o 05 1.o
Fraction of Nighttime Rodionce
A
E
v
¢-
C
¢i
C
2
Figure 3 Day-night difference in zonal mean radiance
from the NO, NO2 and N2Os channels and the N20 WB
channel for latitudes between 10°S and 50°S on 4th
November 1991.
Another important aspect of the validation activity
is examination of the flight data for expected atmospheric
signatures. Three such signatures are reported here: (i) the
day-night difference in zonally-averaged radiance; (it) the
step-wise change in NO2 on crossing the terminator at high
latitude and (iii) the steep latitudinal gradient in NO2 at
northern mid-latitude, known as the "Noxon cliff _ (Noxon,
1979).
N2 O5 Day-Night Signature at 20°S
Zor_l M_n 7.8pm RodiQnce
(l_ ,,-,,u .... , .... u .... t ....
a Daytime
• Ni_ttrne
$,o
- %
o
- °
@
°
10
o%"i_;"a;ao.;;' 'o.;a_,,
Radiance (Fraction Of 2gOK BB)
Zonal M_n 8•1pro _iance
60 .... , .... _ .... i .... , ....
O O_ime
= Ni_tirf_
,c
o
,- mO
=O
iO
• a
Radiance (FrGction of 290K BS)
78pm-74pm Day-Night Oiffecectce
"''T'''n''',''',''',''']4
• + •
-O6-O4-02 0.0 0.Z 0._; 0.6
Froctlon Of Nightime Radiance
8.1 prn - "_4pm Day - Night Difference
60 "'v" ,v ,,.
1
N205
÷
4
40" +
_=
-06-0_-O2 0.0 0.Z 0.4 0.6
Froclion of Nightime Radiance
Figure 4 Zonal mean radiance profiles in the 7.8/_m (N_O)
and 8.1pm (N2Os) channels and corresponding day-night
difference profiles referenced to that in the 7.4pm (CH4)
channel.
Signals from the NO_ channels are all expected to
show substantial diurnal variation induced by photochem-
istry, and this is evident in figure 3 where day-night differ-
ences in radiance for all NO_ channels, and also the 7.Spm
(N20) WB channel, are plotted vs tangent-height. The
data are zonal mean averages for latitudes between 10°S
and 50°S for 4th November 1991. The plot indicates that
ISAMS measurements are in qualitative agreement with
theory: NO_ and N_Os nighttime radiances are larger than
daytime values and NO daytime radiances are larger than
nighttime values. Any such differences in the 7.Spm (N20)
441
or7.4#m(CH4)WB channels must be indicative of tem-
perature and/or sampling effects because neither CH4 nor
N20 exhibit any diurnal variation due to photochemistry.
Effects due to temperature and sampling should be very
similar in the 8.1#m (N20_) channel because emissitm is
predominantly from the same constituents and is very close
in wavelength.
The significantly larger day-night difference at 8.1#m
therefore gives confidence that ISAMS is detocting the di-
urnal signature of N_Os in the upper stratosphere.
This conclusion is further supported by figure 4 which
shows, in the left upper and left lower panels respectively,
zonal mean radiance profiles from the 7.8#m (N_O) and
8.1#m (N20_) WB channels. Day-night difference profiles
were generated from these data and then subtracted from
a day-night difference profile from the 7.4#m (CH4) WB
channel. The subtracted profiles are shown in the right
hand panels of figure 4. In the case of (day-night)r_om -
(day-night)r.4om, most values are close to zero, as would be
expected, indicating that temperature and sampling effects
have been largely removed. In the case of (day-night)sa,,,
- (day-night)r.4_,_, values around 40km differ significantly
from zero, indicating a diurnal signature which is unlikely
to be due to temperature or sampling effects.
Radiance profiles from the NO2 PM channel have also
been examined as the tangent-point crosses the terminator
at high, northern latitudes. Figure 5 shows the signal from
NO_ as a function of height, which is seen to increase as
the tangent-point moves from daylight into darkness, in
qualitative accord with theory.
_O
"D
°_
90
8O
70
60
5O
4O
3oi
2O
10
0
0.0066
o<" L \\_'o.os,_os4_=
...... i......... i......... J......... i......... t......... i......H,
70 80 90 I O0 110 120
Solor Zenith Angle
Figure 5 Terminator crossing observed in the NO2 PM
channel
The geographical distribution of retrieved NO_ has
also been examined. Figure 6 shows the distribution of NO2
(relative amounts) for 9/I/92 at 30km altitude. The steep
reduction in NO2 at northern mid-latitudes and longitudes
between 9" and 90*W is strongly suggestive of the so-cMled
"Noxon cliff".
-" ' "." ". ": i ":'""._
"-.
..... .....
; ."; .... !
.... i..... n
Figure 6 Retrieved NO2 (relative amounts) at 30km on
9th January 1992
4. FUTURE WORK
A substantial amount of work still remains before val-
idated nitrogen oxides data will be available from ISAMS.
For NO, this includes provision of a source function for 1--*0
emission throughout the stratosphere, mesosphere and lower
thermosphere and accurate determination of cell composi-
tion. Refinement of the way in which the PM space-view
signal is derived may also be necessary. For NO2, a lot
more work is needed to obtain the best estimate of gas
composition in the modulator cell, including high resolu-
tion spectroscopy of NO2 and measurements of pressure
and temperature cycles in a flight-spare modulator. For
N2Os, despite careful choice of optical filter, other atmo-
spheric constituents, notably CH4, N20 and aerosol, con-
tribute substantial radiance. Reliable N205 retrievals must
therefore await accurate, validated retrievals of these con-
stituents from other ISAMS channels (eg. Lambert et al.,
this symposium).
Once this work has been accomplished, a detailed
intercomparison of retrieved profiles will be undertaken as
part of the UA.RS validation campaign. This will include
measurements by other UARS sensors, the ATLAS mission
and ground-based, aircraft and balloon instruments.
442
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Measurements of stratospheric NO, NO2, and N2O5 by ISAMS: Preliminary observations and data validation

Measurements of stratospheric NO, NO2, and N2O5 by ISAMS: Preliminary observations and data validation

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