In May of 1991 a tunable diode laser heterodyne spectrometer built by the National Physical Laboratory was operated at the International Scientific Station of the Jungfraujoch (46.5 deg N, 8.0 deg E, altitude 3.56 km). Nitric acid spectra in the region of 868 wavenumbers were recorded at sunset and sunrise on two separate days at a resolution of 0.0013 wavenumbers with a signal-to-noise ratio of approximately 130:1. A vertical column amount of HNO3 of 1.61 x 10(exp 16) molecules/sq cm was determined using an atmospheric transmission model developed at the University of Denver. The mean of a number of mid-latitude, northern hemisphere profiles was used as the initial profile for the inversion. A comparison of different initial profiles provides information on the sensitivity of the retrieved column amount of 1.61 x 10(exp 16) molecules/sq cm lies within the range of values published in the World Meteorological Organization Report no. 16 (1986), but is considerably larger than the value of (0.99 - 1.29) x 10(exp 16) reported by Rinsland et al. (1991) for June during the period 1986 to 1990

Fogal, P. F.

Martin, N. A.

Mcelroy, C. T.

Murcray, D. G.

Swann, N. R.

Woods, P. T.

English

NASA Technical Reports Server

N95- 11070
THE DETERMINATION OF HNO COLUMN AMOUNTS FROM TUNABLE DIODE
LASER HETERODYNE S_ECTROMETER SPECTRA TAKEN AT
JUNGFRUAJOCH, SWITZERLAND
P. F. Fogal and D. G. Murcray
University of Denver, Denver, CO
N. A. Martin, N. R. Swann, and P. T. Woods
National Physical Laboratory,
Teddington, UK
C. T. McElroy
Atmospheric Environment Service
Downsview, Ontario, Canada
ABSTRACT
In May of 1991 a Tunable Diode Laser
Heterodyne Spectrometer built by the
National Physical Laboratory was operated
at the International Scientific Station of
the Jungfraujoch (46.5°N, 8.0°E, altitude
3.56 km) . Nitric acid spectra in the
region of 868 wavenumbers were recorded at
sunset and sunrise on two separate days at
a resolution of 0.0013 wavenumbers with a
signal-to-noise ratio of approximately
130:1. A vertical column amount of HNO 3 of16 2 .
1.61 x 10 molecules/cm was determzned
using an atmospheric transmission model
developed at the University of Denver.
The mean of a number of mid-latitude,
northern hemisphere profiles was used as
the initial profile for the inversion. A
comparison of different initial profiles
provides information on the sensitivity of
the retrieved column amounts to the
initial profile. The column amount of
1.61 x 1016 molecules/cm 2 lies within the
range of values published in the World
Meteorological Organization Report #16
[1986], but is ;9nsiderably larger thanthe value of (0. - 1.29) x 10 reported
by Rinsland et al. [1991] for June during
the period 1986 to 1990.
i. INTRODUCTION
Nitric acid is a trace species of
some importance in ozone destruction
mechanisms, particularly those related to
the catalytic destruction of ozone by
chlorine species. It was first measured
in the Earth's atmosphere by Murcray et
al. [1968] using a balloon-borne grating
spectrometer. Subsequently, it has been
measured by various techniques including
Tunable Diode Laser Heterodyne
spectrophotometry (TDLHS). A similar
TDLHS system at the University of Denver
has previously been used to determine
column amounts of ozone [McElroy, et al.,
1990, Fogal et al, 1988], and others have
also reported Laser Heterodyne
Spectrometer (LHS) measurements of
atmospheric constituents [ Frerking and
Muehlner, 1977, Abbas et al., 1978, Hoell
et al., 1979, and Okano, et al., 1989].
Laser heterodyne spectrometer theory has
been discussed at great length by Blaney
[1975], and others, with regards to
optimum performance, and the development
of a signal-to-noise equation.
2. MEASUREMENT DESCRIPTION
The TDLHS constructed by the National
Physical Laboratory (NPL) is configured as
a portable instrument which allows it to
be transported and setup in various
locales such as the International
Scientific Station of the Jungfraujoch.
This particular site was chosen for its
high altitude, its accessibility and also
because of its selection as a world
network site.
A tunable diode laser heterodyne
spectrometer derives its spectra from the
product of a local oscillator field,
produced by the tunable diode laser, and
the signal field which in this case is
infrared solar radiation. Thus the
detected power has the form
• )
For these measurements the TDL temperature
and current were adjusted such that it
lased in an interval centered at 868 cm -I
The intermediate frequency which results
from the oscillatory term generated by the
mixing of the two fields as described
above is filtered by four bandpass filters
as follows: 2-5 MHz, 2-10 MHz, 2-20 MHz,
and 5-50 MHz. These channels result in a
-i
theoretical resolution of 0.00033 cm ,
0.0006 cm -I, 0.0013 cm -I and 0.003 cm -l. A
schematic of the instrument is presented
as Figure I. The TDLHS is provided with a
sun seeker so that sunlight may be
acquired for the measurement.
703
https://ntrs.nasa.gov/search.jsp?R=19950004657 2020-06-16T11:30:20+00:00Z
I `% _ Calibration
Figure i: Instrument schematic for the
NPL TDLHS system. Note that an etalon
could be used in place of the gas cell.
3. DATA ANALYSIS TECHNIQUES
In general, TDLHS spectra require an
external method of frequency calibration
as there is not a frequency scale implicit
In heterodyne detection. For the nitric
acid data presented here, the region
covered by the TDL was determined by the
use of a gas cell containing ammonia. The
extent of the mode was then determined by
counting the fringes from a germanium
etalon. Thus, the TDL was tuned to a
known HNO manifold from approximately
868.0 to 8_B.2 cm -I. The features found in
this manifold were then assigned
frequencies through the use of a
dispersion equation, which is a linear
regression relationship between the point
numbers of the scan and a well-calibrated
interferometer scan of that same manifold,
under similar atmospheric conditions and
zenith angle. Unfortunately, it 'is not
always possible to have the frequency
calibration mechanisms in place during
scans in which data are taken, due to the
noise induced by laser light feeding back
into the primary channel as the laser is
tuned.
Once the spectrum has been assigned a
usable frequency scale, it is compared to
a modelled spectrum. The difference is
calculated, and the model is adjusted
until it agrees with the experimental
spectrum to within some pre-determined
tolerance. The tolerance is nominally set
by the signal-to-noise ratio of the data,
which is calculated as the standard
deviation divided by the mean over a
region in which the signal is constant,
weighted by the theoretical noise
performance as follows
i
The modelling itself is accomplished
through the use of radiation code
developed at the University of Denver,
which has been adapted to run on a PC-type
computer. An atmospheric ray-tracing
calculation is carried out followed by a
llne-by-llne calculation. The line
parameters are taken from the Hitran
database [Rothman et al., 1987] with
corrections made to the HNO 3 parameters [A.
Goldman, private communication] in the
860-880 wave number region. A background
atmosphere calculation containing water
vapor and carbon dioxide as absorbers is
first carried out. This is followed by a
calculation for the nitric acid. The two
are then added, and if needed the nitric
acid calculation is adjusted through
adjustments to its initial profile until
the sum of the two accurately represents
the experimental spectrum to within the
prescribed tolerance. The original
initial profile was calculated using the
average of a number of mid-latitude
northern hemisphere profiles as published
in the World Meteorological Organization
report #16 [1986].
4. RESULTS AND DISCUSSION
The nitric acid manifolds in this
region are not very strong absorptions.
Thus, large solar zenith angles are needed
in order to provide features that can be
easily calibrated and that are large
enough that differences between modelled
and measured radiances can be measured
with confidence. However, this same
scenario makes the calibration of the
spectra more difficult as the features
tend to be broad, leading to some
uncertainty in assigning the frequencies.
On the other hand, at small zenith angles
the weak absorptions are not easily
identified. Thus, we cannot calibrate the
frequencies as well as we would like to,
which results in some error over the
manifold. However, the gross structure
does match up quite well. Furthermore,
there is the possibility that the Hitran
HNO 3 line parameters may require further
adjustments. There is also a background
envelope to deal with, which is due mainly
to the shape of the bandpass function of
the solar grating. The laser mode also
has an envelope, but this can be removed
by subtracting the signal from the laser
704
monitor channel. This procedure also
removes noise directly resulting from
excursions in the laser intensity (ie:
changes on the order of few current
steps). The grating function can be
removedeither by studying the shapeof a
blackbodyscanover the sameregion, or by
an ad hoc approximation to the shape. The
latter wasperformedhere, due to the lack
of an appropriate blackbody scan.
Scans at large zenith angles were
obtained on two different days, the 7th
and 13th of May, 1991. Presentedhere are
four different scans from the 7th of May.The analysis of these scans returned a
columnamountof 1.61 x 1016moleculescm-2.
These four scans are shown in Figure 2. A
similar analysis of data from the 13th
showed the column to be 1.65 x 1016 . The
modelled atmosphere was given 8 layers,
and pressure and temperature profiles were
taken from the nearest radiosonde location
for the two days in question. The
residuals for the comparison of scan
11921126 and its modelled spectrum is
shown in Figure 3.
The initial mixing ratio profile was
moved upwards and downwards in steps of 2
km in order to test the sensitivity of the
modelled spectra to the initial profile.
The results are shown in Figure 4. The
column amount was allowed to vary as the
profile was moved. After the spectra were
scaled to provide the best fit, it was
clear that there were noticeable
differences between the modelled "and
experimental spectra for a change of 4 km.
When the profile was moved upwards 4 km,
the lines in the manifold were much more
clearly resolved than shown in the
experimental spectra. Conversely, when
the profile was moved downwards 4 km we
see that other features are somewhat less
resolved. Moving the profile upwards 2
km, produces modelled spectra that very
clearly resolve the complex indicated by
the arrow. This is not so clearly
resolved in the experimental spectra.
When the profile is moved downwards 2 km,
the modelled spectra provide the best fit
to the experimental spectra. Thus, we
arrive at a representative profile for
these measurements to within 2 km. This
move downwards increased the total column
amount from 1.25 x 1016 to 1.61 x 1016
molecules cm -2
5. CONCLUSIONS
The good agreement between the
modelled spectra and the experimental
spectra indicates the initial profile,
which is an average of many previously
determined northern, mid-latitude
profiles, is a good approximation to the
actual HNO 3 vertical distribution. This is
further substantiated by somewhat
different modelled spectra produced when
I l I I I I I J
J 11758126.pl I slIples TDLCurrent iff Ch2 1-10 ffHz I
1500.00 hie: : : . Screen0 ON ffo4e2 3.00FWHMI
I
c
8 \ 8 o
R Ill L k k t i i\ _ 640
867.9000867,9801 868.0603 868.1404 868.2205868.3000
Figure 2: Four TDLHS scans taken on 7 May
1991. Scan (a) has a zenith angle of
73.37 ° , (b) has a zenith angle of 86.35 ° ,
(c) has a zenith angle of 87.15 ° and (d)
has a zenith angle of 88.73 ° . All scans
have been filtered with a 3 point FWHM
triangular filter.
I0./. I I I I I I I I I 1t11921.di1 i salplesTDLC_rrent Ch3 2-20MHzScreen0 DM Untiltered
-0
[ ' I I 1 I I I _ 640
3
XI
867,9946 868.0473 868,1001 868.1528 868.2055 868.2578
Figure 3: A comparison of scan (c) taken
at 87.15 ° with a spectrum produced by the
University of Denver model. The residuals
are plotted on a scale of -2.5 to I0 % of
full scale.
705
I I I I I r I I I
] 1921Up4.Sl PoinLnulber ElM SYntheLic
!.90Q.00 Screen0 OM Un{il_ered
-I00.00
R ]9211 I I I [ J
867,9000 867.9801 865.0603 868.1404 868.2205 868.3000
Figure 4: A comparison of the spectra
resulting from moving the profiles Upwards
and downwards in 2 km increments. Curve
(a) is the initial profile moved upwards 4
km, (b) up 2 km, (c) initial profile, (d)
the experimental spectrum at a zenith
angle of 87.15 ° , (e) initial profile moved
down 2 km, and (f) moved down 4 km.
that profile is displaced vertically
upwards and downwards. While this
approach is not strictly speaking an
inversion approach, this type of
comparlson between modelled and
experimental spectra, can increase the
acCuracy of column amounts of HNO 3. The
differences found by moving the profile
upwards and downwards demonstrate that
high-resolution spectra provide usable
altitude distribution information on the
scale of 2 km.
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The determination of HNO3 column amounts from tunable diode laser heterodyne spectrometer spectra taken at Jungfruajoch, Switzerland

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