vertical distribution of minor constituents in the tropical middle...

Indian Journal of Radio & Space PhysicsVol. 16, February 1987, pp. 25-35

Vertical Distribution of Minor Constituents in the TropicalMiddle Atmosphere

B H SUBBARAYAPhysical Research Laboratory, Ahmedabad 380009

Received 30 October 1986

During the last ten years there has been a great improvement in the status of our knowledge of the vertical distribution ofminor constituents in the tropical middle atmosphere. Rocket and balloon measurements have been made of the verticalprofiles of ozone, nitric oxide, water vapour as well as the chlorofluoromethanes from the Thumba rocket range (8SN)and the Hyderabad Balloon Facility (17SN). An attempt has been made to compile and present in this review the resultsfrom these measurements.

1 IntroductionIt has been recognized for a long time now that eventhough molecular nitrogen and oxygen constitute thebulk of our atmosphere, a number of other specieswhich exist only as trace constituents, in parts per mil-lion by volume or even less, play major roles in thephysical, chemical and radiative processes that takeplace in the atmosphere. Consequently, minor consti-tuent study has constituted an important part of aer-onomy research. While the minor constituents are im-portant at all altitudes in the earth's atmosphere, theyplay a dominant role in the so called middle atmos-phere - thealtituderegionoflOkm to about 100km.Ozone, water vapour and carbon dioxide are some ofthe more important trace constituents that playa di-rect role in the middle atmosphere chemistry and ra-diation budget. Many other trace species become im-portant for the role they play in the chemistry of ozoneand water vapour. Many of these trace gases emanatefrom the biosphere and the troposphere, due to natu-ral causes as well as due to human activities. Whenthey enter into the stratosphere they have long resid-ence times, liberate a number of chemically very ac-tive species due to photodissociation and chemicalreactions and form a complex photochemical systemwith important consequences to the radiation bal-ance, energy budget and the dynamical regime in themiddle atmosphere.

The tropical regions contribute almost 50% of theglobal budget of these trace gases. The upward mov-ing branch of the Hadley circulation cell reaches ro al-titudes which are well within the stratosphere. Henceat low latitudes, vertical motions carry troposphericgases into the stratosphere where they spread meridi-on ally to the middle latitudes (Fig. 1).This intrusion oftrace species from the tropics into the midlatitude

stratosphere seems to be the major source for many ofthese species. Hence the global content as well as thevertical distribution of these long-lived trace gases inthe low latitudes control their global stratosphericbudget. Further, a comparison of the vertical distribu-tion over the tropics with that over the midlatitudesseems to provide an important method of testing the2D and the 3D numerical models which includephotochemistry and dynamics. While there has beena good deal of observational data for the midlatituderegions, data over the tropical regions have been re-latively sparse. Ozone has been monitored systemati-cally by the Indian Dobson network for several de-cades. But rocket measurements of the ozone verticaldistribution up to mesospheric altitudes have been arelatively recent development. Only a few rocket andballoon measurements of the other trace species,such as nitric oxide, water vapour and the chloroflu-

30

E..>C 20UJo:::JI-

5<{

J~EHADLEY ~

CIRCULATION

10

o~--------~-- ~ ~o 30

LATITUDE (de-g)

Fig. 1 - Intrusion of the tropicaltropospheric air into the rnidlat-.itude stratosphere via the Hadley circulation

60 90

25

INDIAN J RADIO & SPACE PHYS, VOL 16, FEBRUARY 1987

oromethanes have been made in India from theThumba rocket range and the Hyderabad BalloonFacility in recent years. An attempt has been made topresent an up-to-date status report on these low lati-tude measurements in this review.

2 OzoneOzone is the most important minor constituent of

the earth's middle atmosphere. Many other trace con-stituents acquire their importance because of the rolethey play in the chemistry of ozone. The great signific-ance of ozone in atmospheric processes became evi-dent when Hartley'< in 1881 discovered the ozoneabsorption bands in the ultraviolet and identified theatmospheric limit of the solar spectrum with ozoneabsorption. Solar radiation absorption by atmos-pheric ozone is one of the major sources of energy inthe earth's middle atmosphere and controls its ener-getics, dynamics and radiation balance. By virtue ofits ability to absorb solar ultraviolet radiation it alsoprovides a protective shield to plant and animal life onearth. The recent recognition that this protectiveshield is vulnerable to attack by a variety of chemicalsthat are being released regularly into the atmosphereby natural causes as well as by human activities andthat the latter could, with lapse of time, provide a sign-ificant perturbation of the overhead ozone distribu-tion with possible impact on the atmospheric pro-cesses, climate and even biological life, has given anincreased importance to ozone studies.

Fabry and Buisson ' made the first measurement ofatmospheric ozone during the year 1920 and estimat-ed the total overhead ozone to have an average valueof 0.3 cm STP. In the next few years GEM Dobsondeveloped the first version of the now famous Dob-son spectrophotometer and undertook a study of itsglobal distribution using a network of six stations, outof which Kodaikanal was one. These represent thefirst observations of ozone in the tropics. Systematicobservations of ozone in India were initiated when aDobson spectrophotometer was set up in Pune in theearly forties. During the IGY, Prof. K R Ramanathanset up the present Indian network of six Dobson spec-trophotometers at Kodaikanal (loo13'N, 7r28'E),Pune (18°31 'N), Mt. Abu/Ahmedabad (24°36'N,72°39'E), New Delhi (28°35'N, 77° 13'E), Srinagar(34°08'N, 74°50'E)and Varanasi(25°19'N, 82°52'E).Since then systematic observations of both totalozone as well as the vertical distribution of ozone bythe Umkehr technique are being made at thesestations.

A balloon ozonesonde was also developed by theIndia Meteorological Department" and regular bal-loon soundings are being made from three stations,Trivandrum, Pune and New Delhi since the early sc-

26

venties. The Indian balloon ozonesonde has also par-ticipated in several WMO sponsored balloon ozone-sonde intercomparison experiments. These balloonsgenerally reach a peak altitude of about 30 km. The al-titude region above 30 km is the region that is of realinterest in ozone chemistry since it is only aboveabout 30 km that ozone is photochemically con-trolled. Below 30 km, the ozone distribution is con-trolled by dynamical factors rather than photochem-istry. Furthermore, the altitude region of 40 km is re-cognized to be the crucial region for studying theproblem of long term depletion of ozone due to an-thropogenic causes. Hence, a rocket programme wasundertaken to measure the vertical distribution ofozone in the atmosphere in the altitude region of 15km to about 70 km. A solar MUV photometer and alunar MUV photometer to make daytime and night-time measurements of the vertical distribution ofozone were developed at the Physical Research La-boratory (PRL), Ahmedabad. After a few preliminarymeasurements during the late seventies, specificscientific programmes to study the changes in ozonevertical distribution during a solar eclipse and day-night variations in ezone at different altitudes wereundertaken during 1980-81. The solar MUV photo-meter also participated in the NASNWMO spon-sored rocket ozonesonde intercomparison experi-ment at Wallops Islands in 1979. A similar instrumentwas also developed at the National Physical Labora-tory (NPL), New Delhi, and a few measurements havebeen made from Thumba. A fairly large size experi-ment was conducted atThumba in March 1983 underan Indo-Soviet collaborative scheme with differenttypes of Indian and Soviet rocket ozonesondes, bal-loon ozone sondes and ground-based Dobson ob-servations. Near simultaneous measurements weremade at different times of the day as well as night usingdifferent rocket-borne and balloon-borne sensorswith the objective of intercomparing the differentsensors and obtaining a mean reference vertical dis-tribution of ozone for the tropical site, Thumba.These rocket experiments have yielded a data set ofabout 20 ozone profiles over the Thumba site. Thesedata have been used to obtain a mean reference pro-file for the ozone vertical distribution in the tropics"and to study short-term changes in the ozone verticaldistribution such as changes during a solar eclipse"and day-to-night as well as day-to-day variations.These results are summarized below.

..

2.1 Mean Vertical Distribution of Ozone over Thumba

A mean reference vertical distribution profile ofozoncovcr Thumba has been constructed using rock-.r-bornc and balloon-borne sensors for the altituderegion of 0 to 60 km (Fig. 2). The data are also shown

SUBBARAYA: MINOR CONSTITUENTS IN TROPICAL MIDDLE ATMOSPHERE

in Table 1both in terms of number densities.as well asmixing ratios. A total of 19 ozone profiles obtainedduring 1980 to 1984 period using the rocket-bornesensors are used for constructing the reference pro-

60e.l<

w 40a::>....5<f

20

OZONE NUMBER DEN~TY (cm-3)Fig. 2 - Mean vertical distribution of ozone over Thumba(8.5°N) based on the Thumba rocket experiments and the Trivan-

drum balloon ozonesonde data-----------_.------

file for the altitude region of 20 to 60 km. The ozonemeasurements made prior to 1980 have not been in-cluded as the instrument and the analysis procedureswere standardized only in 1979-80. For the altituderegion of 0 to 20 km the mean of several ozone pro-files obtained from the balloon ozonesonde overTrivandrum by IMD is used. Eventhough the rocketmeasurements start from about 16 km,duetocompa-ratively larger errors the rocket data between 16 to 20km were not used. Similarly, the balloon measure-ments above 20 km are not used since the ozone con-centrations are overestimated if the balloon does notreach the ozone peak levels 7 below the peak height of27 krn. The peak ozone concentration over Thumbais found at 27 km with a value of 3.45 x 1012 mole-cules/ cc while over rnidlatitude this is found at 22 kmwith a value of 4.86 x 1012 molecules/em! (Ref. 8).The ozone mixing ratios are of the order of3-4 x 10 - 2

ppmv in the lower troposphere and exhibit a high var-iability in the tropopause region. In the stratosphere,above about 20 km the measurements show a syste-

Table 1 - Mean Vertical Distribution ofOzoneoverThumba(8SN, 76.9°E)

Altitude Ozone concentration Volume mixing ratio Altitude Ozone concentration Volume mixing ratiokm km

1(0)) Standard (03) Standard n(03) Standard (0)) Standardcrn ":' deviation ppmv deviation cm-) deviation ppmv deviation

cm-3 ppmv cm-3 ppmv

0 8.6(11 ) 2.3( 11) 3.6(-2) 1.0( - I) 31 2.48(12) 5.47(11) 7.9(0) 1.8(0)1 7.7 2.2 3.5 1.1 32 2.15 5.00 8.0 1.92 7.0 1.9 3.5 1.0 33 1.82 4.28 7.9 1.83 6.2 1.8 3.4 0.9 34 1.55 3.60 7.8 1.64 5.5 1.5 3.4 0.9 35 1.33 3.06 7.8 1.75 5.0 1.2 3.4 0.8 36 1.12 2.66 7.6 1.76 4.4 1.1 3.3 0.8 37 9.56( 11) 2.28 7.5 2.27 3.8 1.1 3.2 0.8 38 7.97 1.98 7.2 2.38 3.5 9.0(10) 3.3 0.9 39 6.70 l.69 7.0 1.89 3.3 l.O( 11) 3.4 l.0 40 5.42 1.41 6.6 1.710 3.2 1.0 3.7 l.2 41 4.57 1.30 6.4 1.811 3.1 l.l 4.0 1.4 42 3.68 1.09 5.9 2.012 3.1 1.2 4.5 1.7 43 3.10 9.48( 10) 5.7 1.913 3.2 1.2 5.2 2.0 44 2.48 7.50 5.2 l.814 3.4 1.2 6.2 2.5 45 2.04 6.16 4.9 l.515 4.1 1.1 8.6 2.2 46 1.63 4.91 4.4 1.316 5.0 1.0 1.2( - I) 2.4 47 1.32 3.90 4.1 1.217 8.2 1.0 2.4 4.0 48 1.04 3.06 3.7 1.118 1.14{l2) 1.6 4.1 6.0( -2) 49 8.46{lO) 2.54 3.4 1.119 1.46 5.4 6.3 2.0( -1) 50 6.84 2.06 3.1 1.020 1.78 8.64 9.7 4.8 51 5.89 1.95 3.0 1.121 2.06 8.88 1.3(0) 5.5 52 5.12 1.63 2.9 1.222 2.36 9.57 1.7 7.0 53 4.21 l.46 2.7 1.423 2.66 9.07 2.3 8.0 54 3.44 1.27 2.5 1.224 2.91 8.90 3.0 9.0 55 2.86 1.14 2.3 1.225 3.14 8.20 3.8 9.0 56 2.33 9.97(9) 2.1 1.126 3.35 8.04 4.8 1.1(0) 57 2.00 9.85 2.1 1.027 3.45 7.86 5.8 1.3 58 1.67 8.65 2.0 0.928 3.35 7.54 6.7 1.4 59 1.55 7.80 2.0 1.029 3.08 6.61 7.2 1.4 60 1.25 6.82 1.9 0.930 2.81 6.03 7.9 1.7

Example: 8.6( 11) is 8.6 X 10"-- -- -_.- _ ..,_ .. --. -- - -_ ..- -- - '- --_. .. --------_.- -- --. -- - ---- -- -.- -_. ---.

27


o O~~~~~~-L~~ __ ~-L~ __ ~~10

9 IdO10" 10'2 '109 '0'0 10" 1012

OZONE DENSITY (cm-3) OZONE DENSITY(cm-3)

Fig. 3 - Day-to-night changes in the ozone concentrations at different altitudes obtained from the Sep. 1980 andNov. 1981 rocket experiments using optical ozonesonde

matic behaviour and the mixing ratio profile shows abroad maximum in the 30-35 km region with a valueof 8.0 ± 1.8 ppmv. Above the maximum, the mixingratio falls rather slowly with altitude and reaches va-lues of 2.0 ± 1.0 ppmv in the 55-60 km region. Thesemesospheric values are larger than those encoun-tered in the middle latitude's" larger by a factor ofabout 2.2.2 Short-term and Transient Effects in the Ozone Verticill Dis-tribution over Thumbll

2.2.1 Day-night variations - The first successfulattempt to study the day-night variations in ozone wasmade on 24/25 Sep. 1980 and 12 Nov. 1981 usingrocket-borne optical sensors designed to measureozone concentrations during daytime and nighttime.For nighttime measurement, moon is used as thesource of radiation. The results obtained are shown inFig. 3. It is seen that while day-night changes are notthe same in the two sets of experiments below about27 km, above this altitude the nighttime values arehigher at all altitudes except around 47-48 km region.The nighttime increase is found to be altitude depend-

0-

X<!) 20wx

o 0 0 M-08-55625 SEP. 19800300 hrs 1ST

E 40.l<

••• M-08-55424 Sf P. 19801500 hrs 1ST

ent with typical values of 8% at 45 km and 55 km and25% and 60% at altitudes 50 and 60 km, respectively(Table 2).

During the March 1983 ozonesonde intercompar-ison experiment, three sets of daytime and nighttimemeasurements were made on 23 March, 28129March and 31 March 1983. Fig. 4 shows a comparis-on of the mean daytime vertical distribution with thatof the nighttime vertical distribution for each of the

Table 2 - Percentage Increase in the Ozone Concentr-ations from Day-to- Night Measured Over Thumba Com-

pared with Model Predictions

N(03) Night/N(03) DayAltitudekm

Modelpredictions 10

1980-81 March 1983expt. "'pt. (mean of

3 sets)40 - (very small)45 8% 28% 3%50 25% 32% 13%55 8% 45% 25%60 60% 56%

60'.....

....-,

E~ 400-

X\!)

wx 20

-,• • • M-08-622

12 NOV. 19811500 h rs 1ST

'.,.,..:>

.' 0I'..

o 0 0 M-08-62112 NOV. 19810305 hrs 1ST

23 MAR 1983 28MAR.198370,------,,------.-------. ,-------,------,-------,,-------,------.-------,60

r,

OL-----~------~------~ L -L ~ ~~ ~ ~ ~

1010

Esx:

c,'0.'0

-c,'0

'0'0

~ 40=>I--

I--

-'<l: 20,

-+- DAY-0- NIGHT

'0'0

'0,- ,0_

'0,-c.

31 MAR. 1983

1011 12 13 10

10 10 10

OZONE NO DENSITY(cm-3),

Fig. 4 - Day-to-night changes in the ozone concentrations at different altitudes obtained from the daytime optical rocketozonesondes and nighttime chemiluminescent rocket ozonesondes during the March 1983 Indo-Soviet ozonesonde inter-

comparison experiment

28


three salvos. In each case while the nighttime profile isobtained from a single instrument, the Soviet chemil-uminescent ozonesonde, the daytime profile is themean of several measurements made with differentoptical ozonesondes. All three sets of data show qual-itatively similar features. The nighttime measure-ments are lower than the daytime mean values belowan altitude of about 30 km. Above this altitude, thenighttime values are larger than the daytime mean.This difference increases with increasing altitude andthe values are different for the different sets. The aver-age differences are shown below and compared withthe estimates from a 1D photochemical model".

It is seen that the observed differences are muchlarger than the model predictions. Further, the datafrom the 1980-81 day-night experiment are closer tothe model predictions than the 1983 data. It is notedthat both the instruments used in the 1980-81 experi-ment are optical instruments and that similar data an-alysis procedure is used in both cases, while duringthe 1983 experiment the instrumentation for thenighttime measurements is different from those usedfor the daytime measurements. It is not clear whetherthe observed larger values for the nighttime increasesduring 1983 experiment are due to a systematic in-strument bias or they represent a genuine day-to-night variation in ozone.

2.2.2 Changes in the ozone vertical distributionduring a solar eclipse - During the solar eclipse of 16Feb. 1980, three rocket flights were conducted tostudy the changes in the ozone concentrations at dif-ferent altitudes in the stratosphere and mesosphereproduced by a solar eclipse. Two rockets werelaunched during the eclipse period at 40% and 70%solar obscuration. The third rocket was launched as acontrol flight on the following day. Ozone concentr-ations were obtained in the altitude region of 15 to 65krn. The results are shown in Fig. 5. It is found thatwhile the ozone concentrations are nearly same on allthe three flights in the altitude region of 25 to 35 km,there are changes both below and above this altituderegion. Below 25 krn, the eclipse day values are lowerthan the control day values as the eclipse progressesfrom 40% to 70% solar obscuration. Above about 35krn, however, an increase is seen during the eclipseperiod. At 40% eclipse conditions, ozone concentr-ations are larger than the control day values by about15% in the 45-50 km range, and 60% at 55 km. Above55 km, a large increase in the ozone concentrations isseen during this flight. In the altitude region of 35 to50 km, ozone concentration increases systematicallyfrom the 40% solar obscuration conditions to the70% solar obscuration conditions. However, above55 km, ozone values measured at 40% eclipse are

o~----~~----~--~~=- __~109 10'° 10'I 10'2 10'3

OZONE CONCENTRATION (mol. cm-3)Fig. 5 - Vanations in the ozone concentration profiles over

Thumba observed during the Feb. 1980 solar eclipse

~ 40::>f--

5<{ 30

70

oooo

ooo

x rHUMBA SOLAR ECLIPSE

X 01 16fEB.1980,1454hrSIST6 02 16 FEB. 1980.152 2 hrs 1ST

• 03 17 FEB. 1980,1522 hrs 1ST

o MEAN TRIVANORUMBALLOONSONOE

6•

60

50

20

10

larger than those measured at 70% eclipse eventhough both are larger than the control day values. At60 km altitude, the 40% eclipse value is twice the 70%eclipse value and 3.3 times the normal day value. Sim-ilarly, at 65 km the 40% eclipse value is 3.8 times the70% eclipse value and 5.5 times the normal day value.The 70% eclipse values are larger than the normal dayvalues by a factor of about 1.6 throughout this heightrange".

A model study of the solar eclipse induced var-iations in meso spheric ozone concentrations wasconducted using a simple time dependent one-di-mensional photochemical model which included theHO x chemistry in addition to the Chapman reactions.Table 3 shows a comparison of the observations withmodel predictions. It is seen that in general the mea-sured .ncreases are larger than the model predictions.Further, the large increase measured at 40% obscura-tion level in the altitude region of 55 to 65 km and thesubsequent decrease at 70% eclipse levels are notreproduced in the model calculations. Large scale ad-vective effects and/or very large values of the verticaleddy diffusion coefficient during the initial stages ofthe eclipse will have to be invoked to explain the ob-served transient changes.

29

INDIAN J RADIO & SPACE PHYS,VOL 16,FEBRUARY 1987

2.3 Ozone Vertical Distribution over HyderabadRecently two balloon measurements have been

made of the ozone vertical distribution from the Hy-derabad Balloon Facility (17 SN) up to an altitude ofabout 35 km using a sun-tracking multichannel radi-ometer. These need special mention because the bal-loons have reached a peak altitude beyond the ozonemaximum and the measurement technique does notrequire any calibration and yields absolute values ofthe ozone number densities; also when compared to

Table 3 - Rocket Measurement of the Fractional In-crease in Ozone Concentrations during the Solar Eclipseof February 1980 Compared with the Model PredictionsAltitude Measured ozone increase

km

404550556065

Predictions increase________ from model calculations"

40% eclipse 70% eclipse40% solareclipse1.031.15l.20l.703.305.50

70% solareclipse

1.11.4l.6l.61.71.4

1.1l.22 1.4l.61 2.2

w 30o::>I-

5<!

40

x

x

\"- xx

x x--o X

1012 Id3

OZGNE NUMBER DENSIT Y (cm-3)

27 MAR.85} HYDERA8AD22 OCT .85

MEAN THUMBA

o 0 0 10 - 20' N ANNUAL MEANMe PETERS .t 01. (1984)

X X X KRUEGER AND MINZER 11976)

20

10

O~ __ ~ __ -L __ L-~ L- __ -L~~~ro"

Fig. 6 - Verticaldistribution of ozone over Hyderabad (17SN)obtained using a balloon-borne sun-tracking multichannel radi-ometer [The Hyderabad data are compared with mean ThumbaprofileII,middle latitutde modelprofile" and availablesatelliteda-

tafor 100-200N(Ref.12).]

30

Table 4 - Ozone Measurements from the HyderabadBalloon Facility (17SN) Compared with the MeanThumba Profile and the Midlatitude Model of Krueger

and Minzner 1976

Altitude Hyderabad balloon Thumba Midlatitudekm measurements meanII model"

27 Mar. 22 Oct.1985 1985

10 l.5( 12) 3.00(11) 1.13(12)12 8.1(11) 2.95(11) 2.02(12)14 9.5(11) 3.40(11) 2.35(12)16 4.1(11) 1.12(12) 5.00(11) 2.95(12)18 8.0(11) 1.85(12) 9.50(11) 4.04(12)20 2.25(12) 3.25(12) 1.78(12) 4.77(12)22 4.25(12) 4.30(12) 2.38(12) 4.86(12)24 4.60(12) 4.60(12) 2.91(12) 4.54(12)26 3.95(12) 3.95(12) 3.35(12) 4.03(12)28 3.30(12) 3.12(12) 3.35(12) 3.24(12)30 2.70(12) 2.42(12) 2.81(12) 2.52(12)32 2.00(12) 2.06(12) 2.15(12) 2.03(12)34 1.60(12) 1.55(12) 1.58(12)

the rocket measurements they have much better alti-tude resolution ( < 250 m). Further, as a suntracker isused, the ozone number densities are determinedwith high accuracies. Measurement errors are lessthan ± 2% in the altitude region of 20 to 35 km (Ref.11 ).The data are shown in Fig. 6 along with the meanThumba profile and typical midlatitude referencemodel of Krueger and Minzner" and is also shown inTable 4.

The Hyderabad measurements show that the maxi-mum in ozone vertical distribution is reached at an al-titudeof24 kmwith avalueof(4.6 ± 0.2) X 1012mole-culesl em- when compared to the mean Thumba pro-file which reaches the maximum at 27 km altitudewithavalueof(3.46 ± 0.78) x 1012molecules/cc. Theobserved peak level over Hyderabad is intermediatebetween that over Thumba (26-27 km) and the mid-latitude value of 21- 22 km. The ozone concentrationsat the level of the peak and the profile above the peakare closer to the midlatitude model than the Thumbaprofile. In the region below the peak, while there aresome differences between the two Hyderabad pro-files the observed values tend to lie in between theThumba values and the midlatitude values. A com-parison of the Hyderabad balloon data with the Nim-bus-7 satellite data for 200N (Ref. 12) shows verygood agreement in the entire altitude region of 20 to35km.

3 Chlorofluoromethanes and Related TraceSpeciesThe chlorofluoromethanes have gained great im-

portance during the last decade because they are thesource for the CIOx species which constitute an im-

SUBBARAYA: MINOR CONSTITIJENTS IN TROPICAL MIDDLE ATMOSPHERE

portant catalytic loss chain for stratospheric ozone!',They are essentially of anthropogenic origin, themore important being the freons, CF2Cl2, (CFC-12)and CFCI3, (CFC-ll) which are used in aerosolsprays and in refrigerants. Their lifetimes in the stra-tosphere are large (Table 5) and they do not have anyknown natural sinks other than photodissociation.Hence their atmospheric content is expected to in-crease with time and constitute a potential long termthreat to the environment". In the troposphere, theirmixing ratios are nearly constant with altitude. Theyget photo dissociated in the stratosphere releasingCIO x species which are responsible for the ozone lossand the mixing ratios decrease with increase in height.

During the last few years, there has been a signifi-cant effort at establishing the global budget of thesespecies and many measurements have been madeover the middle latitudes 15.But there have been only afew measurements in the tropics. In 1985, a balloon-borne cryogenic gas sampling experiment was con-ducted by the Physical Research Laboratory, Ah-medabad, in collaboration with the Max Planck Insti-tute for Aeronomie at Lindau, FRG, under an ISRO-DFVLR exchange programme from the HyderabadBalloon Facility. Measurements were made of thevertical distribution of several of the chlorofluorome-thane family, CH4, CFC-ll (CCI3F), CFC-12(CCI2F2), CFC-1l3 (CCI2F CClF2) and also the bro-mine compound CBr ClF2 (CFC-12B 1) for the alti-tude region of 10 to 25 km. A comparison of the Hy-derabad measurements with typical midlatitude data(Figs 7-9) shows that while in the troposphere the dis-tributions are nearly same, there is a marked differ-ence in the stratosphere in all cases. The decrease inmixing ratio with increase in altitude starts at a higherlevel over Hyderabad than over the midlatitudes andthe slope is generally smaller!", The mixing ratio ofCFC-ll decreases from a tropospheric value of 195pptv (1 ppt = 10-12) to a value of 124 pptv at 25 km.Similarly, the mixing ratio of CFC-12 decreases froma tropospheric value of 358 pptv to about 190 pptv at25 km. These decreases are much slower than what isobserved at rnidlatitudes. Similar features are seen inthe vertical distribution of other halogenated hydro-carbons also (Fig. 10). The observed differences be-

Table 5 - Typical Life Times of the Chlorofluororne-thanes in the Stratosphere

Chemical species LifetimeYr8-1175

11030-40

CFC-llCFC-12CFC-113CFC-12BI

30

20

10

E.x: 0.s:

30

2~

10

a1

HYDERABt.DI'

,,,,,

HYDERABAO/,

,,,,,,

10 100 1000VOLUME MIXING RATIO(pptv)

Fig. 7 - Vertical distribution of the halogenated hydrocarbons,CFC-ll and CFC-12 obtained over the tropical site, Hyderabad(17.5°N) during the March 1985 ISRO-DFVLR collaborative ex-periment [The Hyderabad data (---) are also compared with

the mean mid-latitude data( -----) (Fabian et al.'s ).J

30 -, •...•... •... •... ,-,,-, -, •... •... •...,, CCI2F-(Cl F2

«(F(-1I3)

10

0~'----------~'0---L----~1~OO~------~1000VOLUME MIXING RATIO (pptv)

Fig. 8-Same as Fig. 7 but for CFC-1l3 instead of "CFC-ll andCFC-12"

30

" ,,,II,,c s- (I F2

«(F(-12BO

10

0·1 1VOLUME MIXING RATIO (pptv)

Fig. 9-Same as Fig. 7 but for "CFC-12BI" instead of "CFC-lland CFC-12"

(}()I 10

31


30\\," ,,,,,,,

,~ 20

,"

10

00 0-5 1·0 1·5 2.0 2·5VOLUME MIXING RATIO(ppmv)

Fig. JO-Same as Fig. 7 but for "CH." instead of "CFC-ll andCFC-12"

tween the Hyderabad data and the midlatitude dataare attributed to the effect of upwelling in the tropicsand the differences have been found to be in agree-ment with a 20-model prediction of the differencebetween equatorial and midlatitudes. These are theonly measurements available over Hyderabad so far.There is a need to make a few more measurementsand obtain statistically significant data set.for the.dis-tribution of the source gases over the tropics.

4 Nitric OxideNitric oxide is the most important nitrogen com-

pound in the earth's atmosphere. In the stratosphere,it is the major destroyer of ozone. Together with halo-carbons it accounts for almost 70% of the total chemi-cal ozone loss 17. At these altitudes, even though somenitric oxide is also produced by galactic cosmic raysand during solar proton events's"? most of it is pro-duced by the dissociation of nitrous oxide by the ex-cited oxygen in the 10 state?".

N20+0(10)-2NO

Nitrous oxide itself is of biospheric origin 21. It is pro-duced principally during the nitrification and denitri-fication of soils and it is injected into the stratospherevia the troposphere where it is present in abundancesof the order of 300 ppbv.

In the mesosphere, nitric oxide is important be-cause it is the major source of ionization+.

NO + HLy-a( 1216A) - NO + + eNitric oxide in the mesosphere is mostly due to trans-port from the thermosphere where it is produced by amulti-step process initiated by the ionization of mo-lecular nitrogen by solar radiation (A. < 796A) and thesubsequent production of atomic nitrogen by the re-combination process

N2+hv(),<796A)-N{ + e... (1 )

32

The atomic nitrogen so produced could be in the 2Dstate or the 4S state. Both of them combine with mo-lecular oxygen to produce nitric oxide.

N(2D)+02-NO+0N(4S)+ O2 - NO+O ... (2)

But the reaction of O2 with the nitrogen in the 2D stateis very rapid. Some nitric oxide is also formed by thereactions

0; +N2-NO+ +NONO++e-NO ... (3)

It was earlier believed that reaction (3) was the majorsource of nitric oxide in the lower thermosphere-V".It is now clear that this is only a minor source and thereaction scheme (2) originally proposed by Nortonand Barth" is the major source of neutral nitric oxidemolecules in the lower thermosphere. Nitric oxide istransported into the mesosphere from the thermos-phere both by eddy diffusion as well as horizontalwinds. The photochemical time constant for nitric ox-ide is comparable to the time constant for transport byeddy diffusion, as well as by zonal and meridionalwinds throughout the altitude region of70 to 110 km(e.g. Solomon et al.26). Hence nitric oxide concentr-ations in the lower thermosphere and the mesosphereare strongly dependent on solar and geomagnetic ac-tivity and also exhibit a.strong latitudinal variation.

4.1 Thumba Measurements

Only two measurements have been made of thevertical distribution of nitric oxide over Thumba. Thefirst experiment was conducted by the University ofTokyo jointly with the National Physical Laboratory,New Delhi. A Centaur rocket was launched on 26Mar. 1976 in the morning time at a solar zenith angleof 82° (Ref. 27). The second experiment was conduct-ed under an ISRO-OFVLR collaborative pro-gramme jointly by the Instiuute for Optoelectronicsof DFVLR and the Physical Research Laboratory".This rocket was launched on 11 Mar. 1982 again inthe morning hours at a solar zenith angle of 80°. Eventhough both measurements were made in the sameseason of the year and at the same local time, the solaractivity conditions were very much different. The firstmeasurement was made under solar quiet conditions(Rz = 42) while the second measurement was madeunder condition of high solar activity i R, = 118). TheFIO.7 indices for the two days were 84.1 and 178.2, re-spectively. The nitric oxide concentrations were ob-tained for the altitude region of 90 to 140 km from therocket flight of 1976 whereas the 1982 rocket flightyielded the concentration profile for the altitude re-gion of 60 to 120 km (Fig. 11). A comparison of thetwo profiles shows that the nitric oxide concentr-


130~----,-- ---.- ~

___ 110

E~- 100wo::::Jf- 90

54

THUMBA

-- - 26 MAR'76

- II MAR.'82

120

80

70

60~106~--~~--~~-~~,A101 Id' ir

NUMBER DENSITY (cm3)

Fig. 11 - Nitric oxide measurements overThumba(8SN)usingrocket-borne sensors

ations obtained from the second flight are higher by afactor of about two in the entire overlapping altituderegion of 90 to 120 km. While this does illustrate thesolar activity control on the nitric oxide concentr-ations at these altitudes and the observed factor of 2difference is consistent with the theoretical predic-tions, such as those of Solomon et ali", it may be re-marked that the absolute values of the 1976 measure-ments were considered to be lower than what are re-quired from ionospheric considerations ".5 Water Vapour

Water vapour is important to the earth atmospheresystem in a variety of ways. Apart from the fundamen-tal role it plays in the biosphere, the role in infrared ra-diation balance and the consequent control of thetemperature structure in the troposphere and lowerstratosphere, it is also important in the chemistry ofthe middle atmosphere. Photodissociation of watervapour in the mesosphere gives rise to the hydroxylradical which dominates meso spheric chemistry andis responsible for a variety of airglow emissions.

HzO + hv --+OH + H(A.< 2000A)

In the mesosphere the HOx chemistry provides themajor loss process for ozone'", At lower wavelengthsaround Lyman alpha photodissociation into Hz and 0is also possible

HzO+ hvt): < 1216A)--+ Hz + 01(D)

In the stratosphere and lower mesosphere water va-pour reacts with O(lD) and gives rise to OH radicalwhich plays a fundamental role in the chemistry atthese altitudes. Water vapour also plays an importantrole in ionization phenomena. It leads to the forma-tion of the hydrated cluster ions, H+(HzO)1l whichhave been observed by rocket borne mass-spec-trometers " and the resulting ion composition is thekey to the final equilibrium electron and ion densitiesat these altitudes.

Till recently it was believed that there is no majorsource or sink for water vapour in the stratosphereand it was customary to assume a constant mixing ra-tio for water vapour in the stratosphere and mesos-phere. However, with the recent recognition of therole of methane oxidation in the stratosphere+ watervapour mixing ratios are expected to show an increasewith altitude above about 35 km. This has indeedbeen observed in many measurements and there is anincreased interest in accurate measurements of thevertical distribution of water vapour in the stratos-phere and mesosphere. Further, in view of the ex-tremely low temperatures encountered in the equato-rial tropopause most of the water vapour upwellingfrom the troposphere in the tropics is expected tofreeze and the tropical stratosphere is expected to beextremely dry'",

During the late seventies a number of rocket mea-surements of water vapour in the mesosphere weremade at Thumba by the Soviet scientists under an In-do-Soviet collaborative programme. Observationsare generally available in the 20 to 60 km altituderange". Fig. 12 shows the average mixing ratios ob-tained from these experiments compared with data

60•.•..•.• HEISS ISLAND

0--0 VOLGOGRAD

0--<> THUMBA

so

UIg 40•....

5-<

30

WATER VAPOUR MIXING RATIO (g/g)

Fig. 12 - Mean water vapour mixing ratios in the middle atmos-phere over Thumba (8SN) obtained by the Soviet rocketbornecoulometric hygrometers compared with the data obtained usingsimilar instruments from the mid latitude station, Volgograd

(48°41 'N) and the high latitude station, Heiss Island (80037N)

33

60r----------,----r--,~,_--------__.


E.x:

----. THUMBA 22.02.80

50

-510

uro::Jl-

I--'«

40

0--0 HEISS ISLAND 29.02.80

30 0-0 VOLGOGRAD 20.02.80

WATER VAPOUR MIXING RATIO (g/ g)

Fig. 13 - Near simultaneous measurements of water vapour dis-tribution over the equatorial middle atmosphere with that overmiddle and high latitudes made in 1980 under an Indo- Soviet col-

laborative programme

obtained from similar instruments over Volgogradand Heiss Island stations. A set of near simultaneousmeasurements obtained during 1980 are shown inFig. 13. While these two figures do show lower watervapour mixing ratios over Thumba than over thehigher latitudes, the absolute values of the water va-pour mixing ratio obtained in the altitude region of 40to 60 km over Thumba are in the range 4-7 x 10 - 6

ug/ g and these values are somewhat larger than whathas been measured by other techniques even at mid-dle and higher latitudes [e.g.Mastenbrook 1974 (Ref.35)]). It is therefore believed by many workers that theSoviet rocket measurements overestimate the watervapour concentrations. However, some studies andrecent ground-based microwave measurements" - 3R

show evidence of a significant variability of the me-sospheric water vapour content and some of these va-lues are comparable to the Soviet rocket data. There isa need to make measurements of water vapour mixingratio in the tropical stratosphere and mesospherewith independent techniques.

AcknowledgementThe author would first like to express his thanks to

the editorial board of this special issue for having giv-en him the opportunity to pay tribute to one of the

34

most dynamic and versatile figures of the present dayIndian scientific scene. Even though the author waspersonally associated with most of the scientific pro-grammes whose results are being presented in this re-view, these results are due to the efforts of a number ofscientists and engineers in different institutions bothwithin India and outside and the number is too largefor individual mention. Acknowledgements are alsodue to the author's colleagues, Drs Shyam Lal and AJayaraman who helped in the preparation of thismanuscript.

-I,.10

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SUBBARAYA: MINOR CONSTITUENTS IN TROPICAL MIDDlE ATMOSPHERE

31 Narcisi R S & Bailey A C, ] Geophys Res (USA), 70 (1965)3687.

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vertical distribution of minor constituents in the tropical middle...

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