Waters, J. W.

English

NASA Technical Reports Server

1-71531 !
MICROWAVE, MM, AND SUB-MM MEASUREMENT CAPABILITY
Position Paper for Workshop on Early Detection of Stratospheric Changes
Boulder, CO 5-7 March 1986
J.W. Waters, Jet Propulsion Laboratory
The microwave, millimeter, and sub-millimeter spectral range covers
wavelengths between about 10 mm and 0.1 mm and contains spectral lines
due to rotational transitions of many molecules of interest to upper
atmospheric research. These spectral lines can be measured, either in
thermal emission or in absorption, to remotely determine the abundances
of certain molecules. Temperature and pressure can also be determined.
Measurements in this spectral region have been performed from ground,
aircraft, balloon, and are now being developed for Earth orbit. Most
measurements have been made of thermal emission due to the sensitivity
of the technique and the desire to obtain data over all portions of the
diurnal cycle.
At wavelengths longer than about 1 mm measurements are generally
performed using coherent microwave (superheterodyne) techniques. At
wavelengths shorter than about 1mm measurements are generally performed
using incoherent (filter, interferometric) techniques, although coherent
techniques are rapidly being pushed to the shorter wavelengths. Coherent
techniques have the advantage of greater sensitivity and spectral
resolution than incoherent techniques; this spectral resolution can be
used to infer vertical profiles from the measured line shape of
pressure-broadened lines. Incoherent techniques have the advantage of
more easily covering a wider spectral range than coherent techniques
and, because they now operate at shorter wavelengths, can measure the
important species of OH and atomic 0 which do not have measurable lines
at longer wavelengths. However, the sensitivity to other radicals such
as CIO, H02, and N02 appears to be about lOx worse for incoherent
techniques than for coherent techniques even though the incoherent
techniques operate in a region of larger line strengths!
The spectroscopy in this spectral region is relatively well-known and
apectroscopic uncertainties appear not to be a limitation in either
planning or interpreting measurements. Line strengths in this region can
be reliably calculated from the molecular dipole moment which is
measured very accurately from Stark or Zeeman effects.
Figure 1 shows the calculated stratospheric spectrum for balloon
measurements of a layer around 30 km using expected stratospheric
abundances. This figure, which covers a wavelength range of only 1.5mm
to 1.0mm, Illustrates the many lines which are available for
measurement; it also illustrates the care which must be taken to avoid
Interference from unwanted lines. The vertical scale in the figure is
the logarithm of the optical depth (for weak lines the optical depth is
https://ntrs.nasa.gov/search.jsp?R=19910073976 2020-03-17T14:51:58+00:00Z
proportional to the strength of the emission). The heavy horizontal line
represents rms noise for a liquid nitrogen cooled radiometer with a few
minutes integration; anticipated technological developments, e.g.
SIS-based radiometers, could improve sensitivity lOx. Many of the lines
in Figure 1 can be measured from ground; Figure 2 compares vertical
resolution of balloon-based and ground-based measurements.
Our program at JPL has involved measurements with a balloon-borne
microwave limb sounder which is tuned to lines of CLO, 03, and H202 near
205 GHz and is now operational. A similar instrument is now being
developed for the Upper Atmosphere Research Satellite, which has
additional measurements of H20 and pressure. W. Wilson at JPL and
collaborators have recently performed ground-based measurements of
mesospheric H20 using a borrowed radiometer . which is no longer
available; there Is interest in continuing these measurements, but an
Instrument needs to be constructed. Wilson and D.Muhlemann at Caltech
now have a developmental model of a 233 GHz radiometer for a Mars
mission; tentative plans are to test that instrument with ground-based
measurements of stratospheric and mesospheric 03 and mesospheric CO.
H.Pickett and collaborators at JPL are also developing an incoherent
balloon instrument operating at O.lmm wavelength for measurements of OH
and possibly other species.
Table 1 Indicates the general capability of measurements which could be
performed from balloon at millimeter wavelengths where heterodyne
radiometers can be constructed using existing technology.
Table 1. Some millimeter-wavelength spectral lines of stratospheric
molecules and approximate measurement sensitivities for balloon-based
measurements. The lines given here are expected to be substantially free
from other interfering lines based on the JPL catalog. The approximate
sensitivity is the volume mixing ratio required to produce an emission
brightness temperature of approximately 0.05 Kelvin for a balloon-based
measurement of a 3 km thick layer at 30 km height; this signal
corresponds to the rms noise for a liquid-nitrogen cooled radiometer
(system noise temperature of 1500 Kelvin) with square-wave switching,
spectral resolution of 30 MHz (approximate width of the lines at 30 km),
and Integration time of 2 minutes. Many of these lines can also be
measured from the ground.
Molecule
03
03(16-17-16)
03(16-16-17)
03(16-18-16)
03(16-16-18)
H20
HDO
H202
HO 2
N20 .
NO
N02
HN03
CL-35-0
CL-37-0
HOCL-35
HOCL-37
BR-79-0
BR-81-0
CO
HCN
H2CO
S02
OCS
Line
Frequency
(GHz)
206.13
283.42
266.92
278.45
277.93
183.31
255.05
204.57
265.77
276.33
250.44
265.56
269.2
204.35
237.42
227.60
297.01
320.48
395.65
230.54
265.89
225.70
221.97
267.53
Approximate
Sensitivity
(vmr)
3 E-09
2 E-10
6 E-10
3 E-10
4 E-10
2 E-10
8 E-ll
1 E-10
3 E-ll
2 E-10
1 E-09
3 E-09
2 E-ll
2 E-ll
1 E-ll
5 E-ll
3 E-ll
3 E-12
2 E-12
6 E-10
5 E-13
8 E-12
4 E-ll
2 E-ll
Note
2,3
3
3
3
Notes: 1. This line is given because of the insensitivity of its
emission strength to stratospheric temperature variations.
Other 03 lines exist which are ten times stronger.
2. Not observable from ground, but the 22.23 GHz H20 line can
be used from ground to measure mesospheric H20 with a
sensitivity of approximately 5 E-07 volume mixing ratio.
3. An additional mesospheric contribution must be accounted for
4. CLO line at 278.63 GHz is approximately 3x stronger.
408 J. W. WATERS ei at.
2
0
-2
H20,02 (16-16. 16-18, V,)
03 (16-16-16, 18-16-16, 17-16-16, 16-17-16, V2, V3, 2V2)
E 2
o.
LU
D
-2
m
5
o
o"
HNOi
2
0
-2
-4
-6
OTHERS
H037Ct
N,0
200 250
FREQUENCY (GHz)
300
Fig. I. Atmospheric spectral lines in the 200-300 GHz range. The vertical scale is the logarithm of the
calculated optical depth for a limb sounding path through the middle stratosphere. The calculations are
for a temperature of 225 K and the species abundances in Table fe..The heavy horizontal line is the
approximate noise level for emission measurements using a sute-of-ihe-an radiometer with a few minutes
integration and spectral resolution to resolve the stratospheric lines.
A balloon-borne microwave limb sounder 409
Table WThe stratospheric species abundances used Tor calculations shown in Fig. 1. Abundances here are
relative to the total number density by volume. Species are listed in order or increasing mass.
SPECIES
H20
HDD
HCN
CO
H2CO
NO
°2
HO,
160180
H2°2
NjO
NO,
°3
LOG (ABUNDANCE)
-5.3
-9.1
-9.8
-7.0
-10.0
-8.0
-0.7
-9.7
-3.1
-9.5
-7.0
-8.0
-5.2
SPECIES
I6o17o'8o
16o16o17o
160180160
160160.180
CJO
HOCM
37CJO
H037CJ
ocs
HNO3
SO,
"BrO
8Vo
LOG (ABUNDANCE)
-8.7
-8.3
' -7.8
-7.5
-9.4
-9.8
-9.9
-10.3
-10.7
-8.7
-11.0
-11.1
-11.1
20
10
-20
-30
LINE SHAPE WF
(GROUND-BASED AND AIRCRAFT)
h = 10-15 km
0.1 0.2
WEIGHTING RJNCTION VALUE Own"1)-I,
0.3
Fig. 2. Weighting functions for limb and spectral line shape microwave measurements. These functions
describe the vertical resolution of the technique: the measured signal is proportional to the integral,
through the observed atmospheric path, of the product of this function and the mixing ratio of the species
being measured. The curves shown here have been normalized to unity area. The limb curve includes
smearing by the BMLS antenna when pointed at IS km tangent height from a float height of 40 km; for
higher tangent heights this curve is narrower. The line shape curve is calculated for infinitesimally spaced
spectral channels with infinitesimal resolution.


Microwave, mm, and sub-mm measurement capability

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server