Microwave rotational spectrometry is used to measure tropospheric carbonyl sulfide. The instrument and techniques for using it are described

Hofmann, D. J.

Rinehart, E. A.

English

NASA Technical Reports Server

General Disclaimer 
One or more of the Following Statements may affect this Document 
 
 This document has been reproduced from the best copy furnished by the 
organizational source. It is being released in the interest of making available as 
much information as possible. 
 
 This document may contain data, which exceeds the sheet parameters. It was 
furnished in this condition by the organizational source and is the best copy 
available. 
 
 This document may contain tone-on-tone or color graphs, charts and/or pictures, 
which have been reproduced in black and white. 
 
 This document is paginated as submitted by the original source. 
 
 Portions of this document are not fully legible due to the historical nature of some 
of the material. However, it is the best reproduction available from the original 
submission. 
 
 
 
 
 
 
 
Produced by the NASA Center for Aerospace Information (CASI) 
https://ntrs.nasa.gov/search.jsp?R=19820026044 2020-03-21T06:05:25+00:00Z
1	 ^"
	
^`	 r
1
Y
s
T
Measurement of Tropospheric Carbonyl
Sulfide by Microwave Spectrometry
V
D. J.o ma nH f n and E. A Rinehart
(NASA-CR-1'69417) MEASU R.E34ST OF	 N'82-33920
TROPOSPIi'ERIC CARBONYL SULeIDE BY MICRCWAVE
SPECTROMETRY Final Report (Wyoming Univ.)
13 p HC A02/MF A01	 CSCL 04A	 Unclas
63/46 35370
December 10, 1981
J
Final Report for NASA Grant NAG 2-33
`	 v 1y,,w .1
ftLL	 d
i
Although Microwave Rotational Spectrometry (MRS) has been used very
little in routine analysis, 'it has been shown (Varma and Hrubesh, 1979)
to be a viable method for analysis of polar gases. The particular
advantages of this form of spectrometry are 1) The extreme specificity,
2) wide dynamic range and 3) linear response with variation in
concentration,
The high specificity of MRS arises from the high resolution
(v/Av f^ 500,000) and the unique spectrum for each particular molecular
structure. The high r^.solution arises from the small doppler broadening
at these frequencies and from operation at low pressures (10--100 mtorr)
to reduce pressure broadening. The spectrum of a particular molecule is
determined by the three principal moments of inertia, the dipole moment
direction within the molecule and selection r=ules for transitions, hence
is unique for that molecule. For example, the strong line at 36488.812
MHz of OCS has a linewidth [full width at half maximum (FWHM)l of about
.120 MHz at 100 mtorr and the nearest known microwave spectral line of
another substance is about 1 MHz away. Similar conditions exist for S02
and other microwave absorbers, hence, the accurate determination of the
frequency of maximum absorption uniquely determines the identity of the
absorber.
The wide dynamic range and linear response are inherent in this
f
fo rnr of spectrometry. Varma and Hrubesh (1979) derive the expression:
1
p p = K Sp Av 3/2	 (1)T
where pp is the partial presure of a component in a mixture, K is a
proportionality constant, Sp is the peak absorption signal, Av is the
m
Ihalf width at half height of the absorption line, and a and A g are the
wavelength and guide wavelength respectively. Since K is actually
constant, a measurement of S p sand Qv and the total pressure give the
concentration of that component over many orders of magnitude.
The integrated absorption coefficient, a int , can be expressed	
ft.
(Varma and Hrubesh, 1979) as
Cxint	
1r U vn	 x • p
	
(?)
where U is a constant for a given transition and temperature, vo is the
center frequency of the absorption line, x is the mole fraction of the
molecular species, and p is the total'gas pressure. The determination
of the integrated intensity requires the measurement of the signal vs
frequency at many frequencies over the line profile. This can he done
easily under computer control, and allows the operator to observe the
line shape to confirm the existence and identity of the line. A further
advantage is a very large increase iii signal to noise ratio after
integration over the line profile. This method is not dependent upon an	 x
ideal Lorentzian line shape. The instrument used in this study consists
i
of a much modified Hewlett-Packard model 13400 a microwave rotational }
spectrometer which was interfaced to a Commodore Microcomputer shortly
before this work started. Programs written during this study step the
microwave source over the spectral line, acquire the signal and allow
for many such sweeps to be averaged. The result can be plotted on an
x-y plotter or printed. Such a plot is shown in Fig. 1 for a 1% sample
of OCS in N 2 , Both the signal and the "running sum" of the signal
values are plotted. The resulting sum is the integrated intensity of
C	 f
the, line, A region can be noted toward the left of the line center
where both the signal and sum are negative. This is an incompletely
resolved stark lobe. Because of this, subsequent data runs went only
from the line center upwards. This method was used for calibration, as
shown in figures 2 and 3. The integrated intensity can be seen, in Fig.
3, to be very linear with partial pressure of the absorber. The
intercept at zero pressure approaches zero signal as the mixing ratio is
reduced. Figure 4 is the integrated signal from a 9.7 ppmv test sample
at a cell pressure of 57.4 milli torr demonstrating the sensitivity.
This requires about 2 hours of averaging time, indicating that a
substantial preconcentration step was required.
Preconcentration methods which have been used with MRS include
cryogenic methods and adsorption concentration. In the first methods,
all condensible components are frozen out with 1.N 2 and the air then
pumped off. The remaining sample is allowed to return to room
temperature in the same volume. Pressure measurements before and after
this step give the percentage of condensibles. The MRS signal resulting
from an aliquot of this sample tF^en gives the partial pressure or 'mole
fraction of the species in question, and the concentration in the
original sample is found fran the pressure ratio previously recorded.
The adsorption concentration method consists of passing a large sample
of air through a small plug of adsorptive material at relatively low
temperature. The plug is then recovered and the adsorbed gases driven
off with heat.
	 ji
After examining and trying many adsorptive materials and various
techniques, the folloaing method evolved. A flowing air sample is
	
k	 .
passed first through a 
P205 drying tube, (since the large amount of
f4
C	 .,
1
I	 I
moisture collected will mask the OCS quantity in the cell',	 "" the low
measurement pressure required) then through a glass U tube containing
silica get. This U tube was chilled at 0°C to enchance the adsorption
efficiency. Tests, using the dynamic dilution system in Fig. 5
indicated no loss of OCS to the p2 05 . After a time determined by the
calibrated flow rate to pump 1000 liters of air through the sampler, it
was valved off and taken to the laboratory. The U tube was then
attached to a sample volumf, (of known volume) and dry, hot N Z passed
through it. A second U tube was maintained at liquid N 2 to trap the
sample. After about 30 min., flow was stopped and the N2 pumped away.
The sample the y+ was warmed to room temperature, and the pressure
measured. From the 'barometric pressure, and the sample volume pressure
and known volumes, a concentration ratio was determined.
Aliquots of the sample were then admitted to the microwave
absorption cell and the integrated intensity measured. The results of
one such measurement are shown in Fig. 6. From the amount of air passed
through the U tube (5.82 torr liters) and the measured amount of dry
condensate (.032 torr liters) a concentration ratio, for this sample,
was determined (1.820 x 167). The microwave" measurements, along with
previously determined calibration factors, resulted in a mixing ratio in
the dry condensate of 2.51 x 10 3 . This indicated that the !riginal air
sample mixing ratio was 1.37 x 10 -10 . This value for this particular
sample is confO derably lower than previous measurements using other
methods. This may be due to less than 100% adsorption efficiency in the
silica gel or to adsorption of OCS :within the system (although it was
passivateu with hexamethyldisilaxane). The previous tests with known
trace mixtures would indicate, however, that the ',w value is real..
}1	 [	 1
is
r
This sample was collected at the U.W. Balloon Lanuch Facility far from
possible anthropogenic sources of carbonylsulfide.
The results of this study do indicate that with proper sampling
procedures, the sensitivity is available for the measuremeot, of QCS
ambient levels. Since the sensitivity is depending upon v 
o
( among other
factors) higher frequency operation would allow much smaller samples to
be collected, and would eliminate some cf the sampling problems. To
completely eliminate these problems, some sort of "in-situ" method would
be preferred.
A
	
I
o
m
c
l
m
l
,
	
tv
O
F
 
p
O
o
R
 QUALM Y
E
w n s ^ o
 
-1 1 ^
 N nIj I ro
c
n
L)ED
-4cuI-
I
r cc
O
F
 
P
o
o
h
E
D
c
n
	
W
C14
S
.
v
s
iz
zE
D
CE:
C
C
Uc
n
UC
D
ii
ULI
J
Z
D
C
)
L
IJ
ryLL-
w
ns
	DNINNn^
,
-I
-
,
"
 
1
5
0
M
G
11 4)"AL 
P
A
I
I
.
,
X
a
O
F
 
P
O
O
R
 QUALITY
Ln
o
l
	
s
LuLnLnLLJCL
(ZH)W
J iIGN^IAI
MwS-tm
Ln
CDCDcr)
w
n
s
Ul.a_I
QWry
e
I\C7
c
n
UO
W
4.
arnw
h
s°
u
r
E
w
W
\
w
O
	
;
 
V
+
^
-*
	
y
+
0^
 V
v
O.
0^
-
^
q
tr
yN ( I
bR
 ..L
 'j^ti#,t
	
i.
	
s
	
.
.d
44i.
E
^/1
C
D
oE
O
O
2
0 
n
o
w
"
®
o=
 o
0
Z
 cry
o
^
U
c
* OmanE
_
.
.
_
.^.....
o
Q
c
C
°
'
	
^
O
v
0
a
 . K 
.^
L'1
U
^
.
oca
E
E
a^
O
LL.
V
_
fT
N
C
o
^-- U)
0
d
0
(n
4-4 -
%6
-
-
,O
O
0
Q
«0
L
O
c
N
`
w
a
^
E
o
.c
LU
V
C
?
a
 -v
^
`-N'c
3
Q
°
W
>
o
U)
o
O
Q
vQ
o
m
CL
Z
cr-
N
F
^
f
V
 ^
4
i
3
ws.rn
O
RIG
IN
A
L PA
O
Z IN
O
F PO
O
R
 QUALITY
jp
Figure Captions
Figure 1.
	
The microwave signal (SIC) and the running sum vs frequency
for carbony sulfide at a 1% mixing ratio. The negative going signal and
sum at a lower frequency than the linecenter is due to an incompletely
resolved stark lobe.
Figure 2.	 Running sums of the microwave signal for the upper half of
the carbanyl sul fide absorption line. The numbers to the rightrepresent
the cell pressure in milli tore. 8KG is the background signal
Figure 3.	 The calibration curve resulting from data in Fig. Z. With
reduced mixing ratios, the zero pressure intercept approaches the
origin, but the slope remains constant at .488 V Khz/mtorr.
Figure 4.	 The resulting integrated intensity for a 9 . 7 ppm sample of
carbonyl, sulfide in nitrogen.
rigure 5,	 Dilution systems used to produce standard gas mixtures.
Figure 6.	 The signal and sum for the dry condensable fraction of a 1000 C
sample collected west of Laramie, WY.


Measurement of tropospheric carbonyl sulfide by microwave spectrometry

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server