Indian Journal of Radio & Space Physics
VoL25, Febnwy 1996, pp. 1-7
Balloon-borne cryogenic air sampler experiment for the study of
atmospheric trace gases
Shyam Lall, Y B Acharya I, P K Patra I, P Rajaratnam2, B H Subbarayal & S Venkataramanil
IPhysical Research Laboratory, Ahmedabad 380 009
2Indian Space Research Organisation, Antriksh Bhavan, Bangalore 560 094
Received 20 June'1995; revised received 27 September 1995
A balloon.,.bome cryogenic air sampler for collecting tropospheric and lower stratospheric ambient air has been developed indigenously. Jt has 16 stainless steel sampling tubes,- each having a
motor driven metal bellow valve, operated by telecommand. A balloon flight cimying..this air sam~
pier was made on 16 Apr. 1994 from the TIFR (Tata Institute of Fundamental.R~ch)
balloon
launch facility, Hyderabad (17.5°N). Fifteen air samples were collected during the ascent and slow
valve controlled descent of the balloon in the 8-37 kIn altitude region. T.hese collected samples were
analysed at Physical Research Laboratory (PRL), Ahmedabad, using gas chromatographic techniques
for Irian) of the trace source gases. The vertical distribution profiles of N20, CFC-12, and Halon"
1211 thus obtained were compared with the measurements made earlier. The vertical distribution
profiles of N20 and CFC-12 show discernible features in the stratosphere compared to earlier measurements during 1987 and 1990 from Hyderabad. Tropospheric growth rates of gases like CFC-12
and Halon-1211 during tlle period 1990-94 appear to be lower than those measured during 1987-
·90..
1 Introduction
Stratospheric ozone precludes the harmful solar
ultraviolet radiations reaching the earth's atmosphere. A large decreasing trend in total ozone
over the Antarctic and even at Arctic has been
noticed 1,2. Recently decreasing trend has also
been obse~ed over middle latitudes3• The loss of
ozone due to catalytic reactions witlr OH, NO,
Br, I, etc. is well known4-6 and their relative contributions to the stratospheric chemical ozone loss
have been quantified'. In addition to these gas
phase reactions, a plausible scheme of heterogeneous reactiop.s.· with much faster ozone loss
rates has also been proposed8,9. Ozone depleting
radicals are produced in the. stratosphere mainly
due to the photodissociation. by solar UV radiation (170-300 nm) and partly due to the reaction
with 010 from their respective trace gases, also
kno~
as source gases. Most of these source
gases are mainly of anthropogehical origin; however,. natural sources also exists for some of these
species. Many of the CFCs are solely man-made,
e.g. CFC-ll and CFC-12, which are used as aerosol propellent, in refrigeration, as foam blowing
agents, etc., CC14 and CH3CC13 are commonly
used as chemical solvents. Halons are widely used
as fire extinguisher. CH4,· N20. CH1Cl, CH3Br.
a,
etc .. have both natural
and anthropogenic
sour~esl0-12.
It has also been realized that CH4, N20, CFCs,
and ffillons have strong infrared absorption bands
in the atmospheric window region (7-13 ,urn) and
as a result they have tremendous global warming
potential (GWP)12. Present estimates indicate that
due to long atmospheric lifetimes· of these halocarbons, their abundances will attain maximum
during 2000 (ReL13) even though some of these
gases are being phased out since 1990. It is therefore of great interest to study these consequences.
Limited· number of techniques are available for
comprehensive measurements of vertical distribution of trace gases. Satellite-based remote atmospheric sensors can make measureqlents_ of ~Jew
gas speCIes like 03, H20, CO2, N20, N02,
CH4, CION02, HO, etc;'14-1liIn certain altitude
ranges but with poor altitude resolution.
VerticaJ. mixing ratio profiles for cemiin gases
can also be deduced from the column abundances
measured using ground-base4 infrared observations1'. This technique is applicable only to compounds having strong and -well isolated infrared
absorption band. Aircraft measuremeJ$ are also
being. made onlv.1nthe .lower stratosphenc and.
tropospheric altitudesQ;I8. The crvO"sampUng, in
",
2
INDIAN J RADIO & SPACE PHYS, FEBRUARY
,contrast, can provide adequate sample amount
and it is possible to measure verti~al' distribution
profiles of a large group of source gases of interest in the troposphere as well as in the stratosphere (up to about 37 km). A general ovei-view of
in situ techniques used for stratospheric measurements has been elaborated by various authors19,20.
A number ot measurements of trace gases are
available at mid- and high-latitude regions21 - 24.
But measurements
over tropical regions are
scarce24 - 27 even though tropical regions play vital
role in transporting these trace gases into the global stratosphere due to the strong upwelling motions
prevailing there. A programme has been initiated
at PRL to study the vertical distribUtion of various trace source gases over this region. A cryosampler has been developed indigenously and a
successful balloon flight was conducted on 16
Apr. 1994 from Hyderabad. This paper describes
the technique used and results obtained from this
flight.
1996
MOTOR
VAlVE
--,
MANIFOLD
Ne EXHAUST
SAMPLING
TUBE
CRYOTHERM
2 Balloon-borne cryo-sampler experiment
2.1 Cryo-sampler
A cryo-sampler has been developed indigenously. It consists of sixteen electropolished, vacuum
backed, stainless steel tubes of volume 400 ml
each. Since the trace gases are in very low concentration levels (parts per trillion by volume,
pptv), the quality of tubes play an important role
in their measurements. To remove impurities even
at very small scale, each tube was baked several
times at high temperature and high vacuum. This
process is commonly known as vacuum baking.
All the sampling tubes were assembled on a common manifold and each connected to bellowsealed stainless steel Nupro valves (Fig. 1) which
could be operated simultaneously at high vacuum
and high pressure. The manifold was alliO connected to a - 2 m air intake tube to avoid sampling near the balloon gondola which could be contaminated due to degas ing of .the balloon gondola.
The valves are coupled with permanent magnet
d.c. motors (12V; 3000 rpm; torque: 3 Ncm)
which in turn are coupled with planetary gears of
reduction ratio 288:1 and each motor drives its
valve through a clutch which ensures proper
torque for leak tight closure. The manifold was
kept on an interface which was attached to a cryotherm of 25 litre capacity so that tubes were immersed in liquified neon (- 246°C). A vapour
pressure of 1.5 bar inside the cryotherm was
maintained throughout the flight using a safety release valve. This motor valve assembly was enclosed in a thermally insulated, double walled
I
i<
I
"I
I'
"n'I"'"
'II
"I
Fig. I-Mechanical
drawing of the indigenously developed
cryo-sampler showing various components
chamber and was covered with an insulating
rings
transparent perspex plate on the top with
to avoid the ambient cool air circulation, A hermetically sealed 50-pin connector was mounted
on the plate which carried power for all the sixteen motors and thermistor connections. Cryosampler was completely enclosed in a specially
designed light weight aluminium cage. This arrangement makes it convenient to handle during
integration and also prevents cryo-sampler from
any damage during landing. The cryo-sampler
valves were operated (opening or closing) by using a cryo-control unit through telecommands as
discussed in the next section.
0
2.2 Cryo-control unit (CCU) and its operation
The electronic control unit basically generates
signals which can be communicated through telecommands to open or close the valve of the desired sampling tube. The block diagram of the
cryo-control unit (CeU) is shown in Fig. 2. Four
bits of one data cOrrllnands are used to address
16 valves. Six ON/OFF commands, namely, arm,
safe, open, close, high current, normal current,
are used to control the entire unit. High current
option to motors is kept as standby for normal
current, in case motor does not move to open or
close the valve due to large friction which could
increase the normal <tperating. current of the' mo-
II 111'''' 1I11~11'II
III,t
III I
<II '11"11 110 q
,
I
\
1
~
SHYAM
lAL
et aL: BAlLOON-BORNE
CRYOGENIC AIR SAMPLER FOR lRACE
GASES
3
Fig. 2-Block diagram of the cryo-control unit.
,
..
.tors, especially at low temperatures. The unit generates status information for open/close, armIsafe,
high current/normal current, motor address numbers, motor current value in analog (0-9 V) as
well as in digital form (12 bits), and on motor
ON/OFF. The ecu was qualified at all enVironmental specifications needed for space-borne applications.
Initially the instrument is in safe and normal
current ·condition. The operation -of the instNment starts with an arm command. After receipt
of this command by ecu, the arm output status
becomes logical one and safe.status becomes logical zero. The deSIred sampling tube to the opened
or closed is selected through a data command
which is decoded by the 'Va,lye address demultiplexer circuit. This i~ confirmed by the 4 bit valve
address in the telemetry output. The evacuated
sampling tubes are kept closed before the launch
of the balloon. The valve of the sampling tube is
then opened with the help of open command. The
status of the valve and motor are confirmed
through the status information. After collecting
the air sample for a predetermined period, the
,valve is closed and necessary status information
verified. This process is repeated for au the
sampling tubes. After completing the sampling in
all the tubes, the instrument is put in the safe
mode.
2.3 Balloon gondola
A 1,46,000 m3
size balloon made out of low
density polyethylene was launched. from Hyderabad, carrying cryo-sampler payload, cryo-control
unit, telemetry/telecommand, .a radio frequency
link, etc. Telemetry transmits electrical signals
corresponding to the various physical quantities
such. as pressure, temperature and various mechanical and electrical parameters regarding
valves and motors status. The telecommand system
provides a number of ON/OFF commands required for cryo-sampling process as well as (or
balloon flight operations which include command
channels for ballasting, valving, and flight termination. The flight was terminated by separating the
parachute together with the payload from the balloon.
2.4 Air sampling
The balloon from TIFR balloon facility at Hyderabad was launched on 16 Apr. 1994 at 0506
hrs 1ST.The sampling tubes were maintained at a
vacuum of about 10-6 Torr using a turbo molecular pump for more than a week and all the valves
were closed just before the filling of liquid neon
in the cryotherm prior to flight. The balloon take
off was normal and intake tube was deployed immediately after that by using a telecommand: Air
samples were collected at desired altitudes by
opening and closing the valves through telecom~
mands for certain duration which was decided
prior to flight depending on the altitude, during
both ascent and apex valve controlled descent of
the balloon. The air inside the manifold and intake
"
4
INDIAN J RADIO
& SPACE
I
PHYS, FEBRUARY
1996
40
10
~-
30
~
~
E
'--'
Q)
"'0
20
Q)
I
.3
:oJ
9
<
\
10
f'O'
J
Hyderabod
16 Aprr 1994
o
10 •
0500
0700
0900
Time
1100
1300
(1ST)
Fig. 3-Trajectory
of the balloon shoWing the altitude attained as a'function of local time (1ST) during the
flight conducted from Hyderabad (17''N) on 16 Apr. "1994. The boxes drawn/ along the trajectory indicate the valv~ opening time spans and altitude ranges atwhich air samples were collected.
tube was flushed with ill situ air by opening a
Ian
number
range
11
11.79-12.26
34.78-32.39
30.78-29.88
25.1826.18-25.67
21.77-23.77
7.96-8.21
12
14
10*
98pressure
6547321of
21.34-23.09
10.65-12.15
42.13-31.03
24.91-17.11
2105-72.08
4.33-4.39
358-341
201-186
7.64sampling
29.91-37.37
37.06-36.96
Tube
16.48-18.3
28.22-27.11
26.97-26.~
25.62-25.36
25.67-25.11
19.24-20.05
13
23.09-25.18
15.65-18.52
18.93-20.97
23.26-24.18
62.34-54.97
12.1-4.13
15
5.97-8.41
Atmospheric
Height
range
Table I-Details
air
mb
spare tube (#and16)
before
collecting
sample
in
each
hrs
1ST
time
instrument.
closing
09:38:23-10:19:27
06:41
05:54:33-05:56:02
12:06:05-12:16:59
:15-06:49:16
06:31
10:34:39-11
12:17:10-12:19:46
:34-06:37:00
:00:26
06:18:20-06:22:58
11:44:22-11
11:31
:10-11
:44:
:54:42
10
08:01
06:55:48-07:20:14
06:07:36-06:14:29
11:54:52-12:05:46
:45-08:56:45
tube. Seven samples were collected during the as- Valve opening
05:37:27-05:37:50
air sample was found inside
to malfunctioning of the
cent. The*Noballoon
reached a highestdue(ceiling)
altitude of 37 km. The balloon was then kept floating at the same altitude for about an hour. The
balloon was then controlled to descent slowly by
releasing the lift gas through an apex valve. Eight
air samples were collected during. the controlled
descent of the balloon down to 25 km after which
the flight was terminated. The trajectory of the
balloon and altitude ranges at which the samples
were collected are depicted in Fig. 3. The valve
ppening time varied from about 0.5 min at
around 8 km to about an hour at the ceiling altitude of 37 km (Fig. 3). These valve opening time
spans and altitude ranges at which samples were
collected are listed in Table 1. The flight was
terminated at 1220 hrs 1ST after collecting all the
samples at 15 different altitudes. The cryo-sampIer, telemetry, telecommand units and ~pex valve
were successfully recovere4. The amount of ~a!!lpIe in these tubes varied from 30 litre to 6 litre at STP.
2.5 Sample analysis
The air samples
I
I
I'
were analysed at PRL using
1
111
ill
I'
during this balloon flight
f
SHYAM LAL et al.: BALLOON-BORNE
CRYOGENIC
gas' chromatographic
(GC) technique. Different
GC column materials and detectors were selected
based on the physical and chemical properties of
gases under study. CH4 and CO were separated
using a 4 m x kin. o.d. stainless steel column
packed with molecular sieve, maintaiI).ed at 25°C
throughout the analyses and detected by flame
ionization detector (FID). The amount of sample
required for each analysis for these gases was
about 4 mI. Analyses for NzO were made by injectmg an amount of about 4 mI of air sample into a GC column packed with Porapak-Q and was
detected by an electron capture detectror (ECD).
Column oven temperature for these analyses was
maintained isothermally at 40°C. Separation of
many of the CFCs and Halon-1211 was obtained
using a 5 m x! in.o.d. stainless steel GC column
filled with OV-10! on Chromosorb WHP. A few
hundred millilitres of sample was preconcentrated
in a sample injection loop by pumping away Oz
and Nz while holding other gases iIi a glass bead
packed loop at -195.8°C (liqu.id nirogen temperature). The column temperature during analysis
ranged from - 60°C to lOO°C at a programmed
temperature rise of 8°C/min. A chromatogram
showing the separation of various CFCs and Hal-
AIR SAMPLER FOR lRACE
5
GASES
on-1211 is shown in Fig. 4. Absolute concentra,tions were estimated using a laboratory standard
calibrated a priori with respect to synthetic gas
mixture, made using in-house static volume dilution system. The precision of the instrument was
about 1% for tropospheric samples and less than
3% for stratospheric samples. Accuracy of these
measurements ~ in the range 5-10%.
3 Results and discussion
3.1 Vertical distributions
of trace gases
Vertical distributions of Ntq, CFC-12, and
Halon-1211 are discussed here for which the analysis work has been completed.
Vertical profile of NzO is depicted in Fig. 5. A
comparison of the vertical profiles is made with
the earlier measurements made over Hyderabad
jointly by PRL and Max-Planck-Institut fUr Aeronomie, Germany, during 1987 and 1990 (Ref.
26). Mixing ratio remains constant in the troposphere because of very long lifetime (-106 years)
(Ref. 28) and complete mixing in this region and
starts decreasing in the stratosphere. The vertical
distribution profiles of 1987 and 1994 show almost similar decreasing trend in the stratosphere
apart from a kink seen at around 25 kID in 1987,
but the 1990 profile shows a faster decrease rate
in the same altitude range (20-34 km). This decrease is due to photodissociation by solar UV radiation (170-250 nm) and'reaction with OlD.
The vertical distribution of CFC-12, which is
the most abundant chlorofluorocarbon and one of
the major· contributors to free chlorine radicals, is
Hyderabad
40
o
30
N
'j
u
~
u
~
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%
II
o
0
o
't>
o
o
2
•
o
••
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~
U
-I~
uu
~~
uu
o
•olb
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o
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o
N20
27 Mar. 1987
9 Apr. 1990
16 Apr. 1994
•••
000
coo
Q
J
25
o6
RETENTION
TIME
(min)
Fig. 4-A typical gas chromatogram obtained for the analyses
of 'Various' halocarbons using a 5 m x in. stainless steel column packed with OV~101 and an electron capture detector.
i
.
100
•
200
Mixing Ratio
.
300
.
460
(ppbv)
Fig. 5-Vertical profile of NJ.O measured from the sllJuples
collected during the balloon flight on 16 Apr. 1994. £ar~r'
measurements are also shown for comparison.
6
INDIAN J RADIO & SPACE PHYS, FEBRUARY
Hyderabad
1996
Hyderabad
40
40
o
•
o
30
000
.•...
DC
o
•
"" •0
o~ '-"
000
••• .::t-Halon-1211
':J1)
'0
o
Q) 20
«
o
••
•
••
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.•..•
20
*
0 'J' 1994
* * 027
0 Mar.
1987
16
Apr.
30
1990
09
E oo [JJ
00
* 0 I *0* 00
* 0o
0
~il
o
,'00
o 0
o
,ee
•
.0
«
10
CFC
-
•oo
12
•••
27 Mar. 1987
o 0 0 9 Apr. 1990
ODD
16 Apr. 1994
o
162
01&
163
Mixing ratio (pptv)
shown in Fig. 6 for the years 1987. (Ref. 27),
1990 (Borchers, private communication, 1995)
and 1994. The absolute values of mixing ratios
were c<;dculatedusing calibrated PRL standard.
The mixing ratio of CFC-12 in the troposphere is
constant as· it has been seen for N20 and it also
has a long atmospheric lifetime of about 90
years28, which is much higher than the tropospheric mixing time. 'Fhe tropospheric abundance
~scalculated to Ibe 50S pptv. It decreases in .the
stratosphere because of the loss due to photodissociation by solar UV radiation (170-~50 run). Its
concentration reduces to 338 pptv and 75 pptv at
altitudes of 26.4 and 37 kIn respectively. The
comparison with two earlier profiles also shows
similar characteristics in the stratosphere as it has
been seen for N20.
Figure 7 shows the distribution profile of Halon-1211 (CBrF2Cl) along with the profiles of earlier I1'leasurements29 for comparison. Halon-1211
is one of the major contributors to the production
of bromine radicals in the stratosphere in spite of
its much lower concentration compared to other
bromine containing species25• Since it has long atmospheric lifetime (- 40 years), it shows a constant mixing ratio in the region of 8-17 km. Trace
gases of long lifetimes are transported to the stratosphere without much loss in the troposphere
md they release catalytic ozone depleting radicals
1;
I
I
'I
I' I'
.
, ""10-1
'
,
Mixing ratio
Fig. 6-Vertical distributions of CFC-12 have been depicted
for three different measurements (two earlier, during 1987
and 1990, and the present one of 1994). These results show
discernible differences for various measurements in the stratosphere.
I
~OL2
I' ~I" II'II 'II I"
'''''','
,.,
""1b
(pptv)
Fig. 7-Vertical distribution of Halon-1211 obtained on 16
Apr. 1994 over Hyderabad. Earlier measurements made from
the same location are also presented for a comparison.
in the stratosphere. The rate of dissociation of
these compounds is the measure of rate of release
,of the radicals. Mixing ratio of.Halon-1211 decreases rapidly from a troposphere value of 2.15
pptv to 0.07 pptv at 26.4 km height. This species
can be dissociated by lower energy solar radiations in the wavelength range of 190-300 run as
compared to those ofN20 and CFC-12.
3.2 Troposphen't: trends
We have co.mpared the concentrations of N20,
CFC-12, and Halon-1211 measured during this
flight with the previous measurements made during 1987 and 1990 to comprehend their trends.
No significant change in the growth rates of N20
concentrations has been observed during the different periods of measurements. However, growth
rates for CFC-12 and Halon-1211 during the period 1990-94 are found to be 1.6 ± 0.2 and
4.5 ± 0.5%/year respectively, which were 4 and
15%/year29 during 1987-90. During J982-83 the
growth rate of Halon-1211' was reported to be
20%/year23• The present results of lower growth
'rates are commensurate with recent surface measurements, giving a growth of - 2%/yr in 1993
for CFC-12 (Ref. 30). This decrease in growth
rate is due to phasing 9ut of these gases under the
Montreal protocol.
Aeknowledeements
This project is a part of the ISRO/DOS-
SHY AM LAL et al.: BAlLOON-BORNE
CRYOGENIC
Geosphere-Biosphere
Programme. We sincerely
thank the Chairman, ISRO and other ISRO- DOS
authorities for encouragement and support for
this project. Continued
interaction
with the
MPAE scientists specially with Prof P Fabian and
Dr R Borchers was very helpful to some of us in
the indigenous development of the cryo-sampler.
The then director of PRL, Prof. R K Verma, has
also taken a keen interest in this development. We
sincerely thank all oLthem. We also thank other
members of PRL who have directly or indirectly
helped in this programme. We are grateful to Prof.
Darnle, Mr Joshi, Mr Shurpali, Mr Sreenivasan,
Mr Sreekumar and all other members of TIFR
balloon facility and the members of the Balloon
Board for conducting a successful balloon flight.
~I
'.'
References
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AIR SAMPLER FOR TRACE GASES
7
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