- Email: [email protected]
The measurement of ozone concentrations at high latitude during the twilight
Phut.
s@ace Sci., Vol. 25, jw. 165 to 172. Permmon Prim, 1977. Printed in Northern Ireland
THE
~EA~URE~~NT OF OZONE CONCENTRATIONS LATITUDE DURING THE TWILIGHT
AT
HIGH
E. J. LLEWELLYN
Institute of Space and Atmospheric Studies, University of Saskatchewan, Saskatoon, Canada
Meteorological Institute, University of Stockholm, Sweden (Receiwd 26 July 1976) AbHra&-The ozone height profIIe in the Arctic, at the end of the winter, has been measured up to an altitude of 100 km using a combined s&u occultation and 1.27~ oxygen emission technique. The typical two layer structure has been observed with a high altitude minimum near 80 km and a m&mum at 86 km. The measured concentration in this ozone bulge was 5.1 x 10’ cme3, typical of that measured at 52W for the summer months. It is suggested that this reduced ozone concentration may have been associated with a stratospheric warming event that was in progress at the time of the measurement.
1. INTRDDUCFION The
measurement of the ozone concentration in the upper atmosphere was first reported by Johnson et al. (1951) and has subsequently been measured by a number of investigators (e.g. Weeks and Smith, 1968; Carver e# af., 1972). However, the variability of these concentrations with season and geographic latitude is still only available to a limited extent. The extensive measurements reported by Hays and Roble (1973), using a stellar occultation technique from the OAO-2 satellite, were restricted to latitudes equatorward of 43”. In these measurements, which were taken at various times of the year under both quiet and disturbed conditions, there is an almost constant ozone concentration, 10’ cmW3, in the bulge near 85 km. The mesospheric ozone concentration may also be determined from the height profile of the O&‘&J emission. The 0riginaI observations of Evans et uL (1968) were shown to be in good agreement with an excitation through the photolysis of ozone (Valiance Jones and Gattinger, 1963) and Evans et al. suggested that a secondary maximum existed in the ozone height profile near 85 km. This secondary maximum is the bulge observed by Hays and Roble (1973) and is indicated by the original satellite data of Rawcliffe et al. (1963). Subsequent simultaneous measurements of the OZ(‘A,) emission and the ozone height profile have been reported by Llewellyn and Evans (1971) and h&her and Ryder (1973) and these have shown that the height profile of the infrared emission may be
used to infer the mesospheric ozone profile directly. An extensive series of ground based observations, at 52W, of the oxygen emission has been presented by Evans and Llewellyn (1972) and these authors have concluded that the bulge in the ozone concentration above 80 km exhibited a seasonal dependence with a maximum in the concentration, 1.3 x I@ cmW3,occurring during the winter months. However, these ground based studies also indicated that concentrations more typical of the summer regime, 3 x 10’ cm-‘, occurred during a stratospheric warming event. As part of an extensive investigation of minor constituents in the Arctic stratosphere and mesosphere, at the end of the winter, we have obtained a complete determination of the ozone height profile between the ground and the mesopause during the evening twilight. In the present paper we present this height profile, which was obtained from concurrent rocket and balioon frights, launched from Kiruna, Sweden in March, 1975 and suggest that the measured concentrations support the hypothesis of a reduction in the ozone bulge concentration during a stratospheric warming event, 2.
EXPERXMENTAL TECHNIQUE
The ozone concentrations reported here were obtained from three independent instruments which provided data over different height ranges, ahhough in the case of the two rocket instruments it has been possible to extrapolate for an altitude overlap of approximatefy 8 km. For the high
16.5
166
E. J. LLEWELLYN and G. Wrrr
altitude, above 60 km, part of the profile we have used a single channel infrared photometer, mounted with its optic axis parallel to the rocket axis, measuring the molecular oxygen emission at 1.27~. Previous measurements with a two channel photometer (Evans et al., 1968; Wood et al., 1970) have indicated that the contribution to the measured signal from the Rayleigh scattering of sunlight is negligibly small, approximately 10 kR at 50 km, so that a single channel instrument will not significantly increase the error in the measured profile. The calibration of the instrument followed the procedure described in detail by Evans et al. (1969) and included a determination of the instrumenta sensitivity as a function of detector temperature. The performance of the photometer during the initial part of the rocket flight, prior to clamshell ejection, was monitored through an internal catibration light and the detector temperature was recorded throughout the entire flight. In this way any change in the instrumental performance could be compensated for during the data analysis although in practice no correction factors were needed. For the altitude region from 30 to 70 km the solar occultation technique was used. The instrumentation consisted of six wavelength selective telescopes, centred at 254, 284, 316, 450, 480 and 481 nm respectively, mounted with their optic axes perpendicular to the rocket axis and having a single common photomultiplier detector. Thus as the rocket spins the transmitted solar flux in each wavelength channel is measured. F&r each telescope the spectral isolation was achieved by a double quartz diffusing screen and an interference filter of spectral width 8 nm, except for the 480 and 481 nm filters which were less than 1 nm. The double d&sing screen ensured that the filter transmission was independent of the angle between the solar vector and the filter normal. To prevent saturation of the photomultiplier tube by the full Sun signal in any wavelength channel a wire mesh attenuator was included between the two diffusing screens. The angular response of the instrument was determined from an extensive series of ground calibrations and using the 450 nm channel, which is outside the Hartley-Huggins bands of ozone, the measured solar flux could be corrected for variations in the solar aspect angle during the flight. A further check on these corrections was possible through the wavelength channels at 480 and 481 nm. For the present meas~ements the rocket employed a high spin rate, 7 Hz, so that the assumption of a constant solar aspect angle during a single spin period introduced a negligible error into these corrections.
For the low altitude data, below 30 km, we have used a chemiluminescent balloon-sonde launched from the Kiruna Geophysical Observatory. 3. OBSERVATIONS
AND
RESULTS
The relevant details for the rocket flight are given in Table 1. The solar occultation telescopes were exposed at 37 seconds after launch while the nose-cone covering the forward-looking experiments was ejected at 60 seconds. Both instruments provided continuous data until 230 seconds when the rocket re-entered and overturned. TABLE ~.DETAILSOF
THE
Launch site Date Time (UT) Apogee Solar zenith angle
FLIGHT(S~~/~)
Kiruna, Sweden March 13,1975 1702 106 km 94.1”
3.1. Infrared photometer The overhead emission intensity, corrected to the zenith, recorded by the infrared photometer is shown in Fig. 1. These data have been obtained from a running average of 15 consecutive data points equally spaced in time and occupy approximately one second. Thus, for the lower altitudes
-i
I
k
co55
I
0
I
I 200
Overhead
I
I
I
400
emission
I
< 800
600
intensity,
kR
FIG. 1. Oz(*Ah,) OVERHEAD EMISSION INTENSITY CORREClZDTOTHEZJ3NlTH.
Ozone concentrations at high latitude during twilight ble oxygen concentration
167 is given by:
$ (O,*) = J3[031-(k[Ml+A)n(02*),
(1)
where each term has its usual meaning. (1) may be integrated to yield,
Equation
n(O,*)=exp(k[Mj+A)r [Oj(f’)]JX(t’) exp (k[M]+A)t’dt’
(2)
where [O,( t’)] = the ozone concentration at time t’, J3( t’) = the ozone dissociation coefficient at time t’, C=constant of integration, the dayglow concentration of metastable oxygen,
1
0
I
I
IO
20
Volume
emission
FIG. 2. 02(rAg)
VOLUME
I
30 rote,
I
40
I
50
I
kR/km
EMISSION HEIGHT RIVEDFROMFIG. 1.
PROFILE DE-
the smoothing interval is 1 km while at 95 km the smoothing interval is only 0.3 km. It was assumed that the required correction for the reduction of the measured intensity to the zenith is given by the cosine of the zenith distance of the rocket axis. The necessary vehicle aspect was determined from onboard magnetometers and solar aspect sensors, although an independent check of the vehicle aspect was made through an analysis of the phase delay between the response of the magnetometer and the 450 nm channel of the solar occultation photometer. The attitude corrected data shown in Fig. 1 has been differentiated to give the volume emission height profile shown in Fig. 2. The numerical differentiation technique was equivalent to fitting a least squares straight line to a 7 km data set and advancing by 1 km steps; this process results in a height resolution of 3.5 km in the altitude distribution (Wood et al., 1970). It is readily apparent from Fig. 2 that the two layer structure seen on previous rocket flights (Wood et al., 1970) was present although there was also a third layer at 99 km. The derivation of the ozone concentration responsible for the oxygen emission is straightforward when the atmosphere is under full dayglow conditions. However, for a twilight observation it is necessary to consider the decreasing solar flux during the twilight and the departure from photochemical equilibrium. The rate of change of the metasta-
and t is the time since the atmosphere, at the height considered, was under full dayglow illumination. If it is assumed that the time dependent ozone dissociation coefficient can be represented by a linear function (Plemel, 1974), and that the ozone wncentration is time independent, the integral in equation (2) may be evaluated explicitly and the local ozone concentration is given by, l
Lo
n(Oz*){k[M+A) .I30
=
3
a k[M]+A
x(1-exp
(-k[M]-A)t}&
(3)
where J3( t) = J3&1- at). In Fig. 3(a) we show the ozone concentration I
I
I
I
I
I (a) j
go-
5
65-
$
80-
3
.?Y
2
75-
7065'. r-s;-l '. '\
6055 106
I IO' Ozone
I 100 concentration,
I
I
109
-
I
10'0 cm-3
FIG. 3(a). OZONE CONCENTRATION HEIGHT PROFILE DERIVED FROMTHE OZ(‘Ag) HEIGHTPROFILESHOWNIN FIG.~.
168
E. J. LLEWELLYNand G. Wm
height profile derived from the infrared photometer data using equation (3). The error bars in this figure represent the uncertainty in the ozone concentration due to the uncertainty in the metastable oxygen concentration; the dashed part of the profile is a simple extrapolation, at constant scale height, of the derived concentrations at higher altitudes. In the evaluation of equation (3) the solar flux values of Ackerman (1970), Arvessen et al. (1969), Broadfoot (1972) and Widing et al. (1970) have been used together with the absorption crosssections of Ackerman (1970) and Hudson and Mahle (1972) for molecular oxygen, and Inn and Tanaka (1953) for ozone to determine the ozone dissociation coefficient. The required ozone height profile for altitudes below 70 km was taken from the average ozone distribution derived by Evans (1967). This profile is in reasonable agreement with that obtained from the solar occultation technique in the present experiment so that no additional corrections were applied to the calculated JsOvalues. The spontaneous transition probability, A, was taken to be 2.8 x 10e4 s-l (Badger et al., 1965) and the adopted value of the collisional quenching coefficient, k, was 4.4 x lo-l9 cm3 s-‘, in agreement with the measurements of Clark and Wayne (1969). The atmospheric profile was assumed to correspond to the 1966 U.S. Standard Atmosphere supplement for winter at 75”N. 3.2. Solar occultation photometer The signal from the solar occultation photometer was a series of regular spaced pulses with amplitudes, in the absorbing channels, that increased with altitude indicating the decrease in the attenuation of the solar flux. These signals were corrected for solar aspect using the control channel at 450 nm, and corrected for amplifier drift by comparing the control channel signal at those times in the flight when the rocket had the same solar aspect angle. The derivation of the ozone height profile from these observations has used a two step procedure that involved the determination of the slant path column concentration and its subsequent inversion. The measured attenuation of the solar flux was assumed, in the first approximation, to be given by, I = I, exp -Nao,
(4)
where I, is the full-Sun signal measured at apogee and (TVis a mean absorption coefficient over the spectral passband of the filter. The value of N obtained from equation (4) was then used as an
initial value in an improved attenuation I/Z, = +(h)e-x”‘“‘-““’
dA,
equation (5)
where $(A) is the spectral distribution of the solar flux weighted by the filter transmission curve, and T(A) is the slant path optical depth for molecular scattering. Equation (5) was solved by an iterative procedure and was assumed to have converged when the change in X from successive iterations was less than 0.1%. In the solution of this equation the required spectral distribution of the solar flux was taken from Ackerman (1970) and the ozone absorption coefficients were from Inn and Tanaka (1953). To determine the local ozone number density as a function of height from the derived slant path column concentrations, the atmosphere was divided into thin spherical shells, each 1 km thick, and within which the ozone number density is a linearly varying quantity. The integral relating the slant path column concentrations and the local number density equations was transformed into a set of linear equations with coefficients determined by the geometry of the experiment and the finite size of the Sun (Miller and Ryder, 1973; Swider, 1964). This latter will cause a smearing of the derived concentration profile over a height interval in km approximately equal to the solar depression at the payload in degrees. The resulting matrix equation was solved by successive substitution with the assumption that the concentration was zero at the upper edge of the highest shell. This assumption leads to some error in the ozone concentration determined from those tangent ray heights for which the attenuation of the solar flux is small; however, as noted by Miller and Ryder (1973), any error associated with this assumption is rapidly damped out and is negligible 3 km below the tangent ray height at which it is assumed X = 0. A further uncertainty in these derived ozone concentrations, Fig. 3(b), is contained in the assumption of horizontal homogeneity for the ozone distribution along the line of sight from the sensor to the Sun. This is obviously incorrect for those altitudes where the ozone concentration changes during the twilight, and at 65 km the derived concentration is approximately 15% larger than that corresponding to 0” solar depression. 3.3. Balloon-sonde
data
The very low altitude, below 30 km, ozone concentrations shown in Fig. 3(b) are those derived from the chemiluminescent balloon-sonde. The plotted values are the measured values multiplied
Ozone concentrations
at high latitude during twilight
169
a-254nm
l-284nm f-316 x -
“m Balloon
Sonde
Ozone
FIG.
3(b).
concentrotm.
cm-3
OZONECONCENTRATIONHEIGHTPROFILEDERIVEDFROMTHESOLAROCCULTATIONPHOTOMETERANDBALLOON-SONDEOBSERVATIONS.
by a factor of 1.7; this latter factor was determined from a comparison of the total zone amount measured by the sonde with that measured from the recording stations at Murmansk (68”N) and Leningrad (60”N) as shown in Fig. 4. The individual
observations shown in Fig. 4 have simply been joined by straight lines to facilitate representation and the dashed curve in this figure represents the balloon-sonde data multiplied by a factor 1.7. It is readily apparent that the recorded amounts track each other during the period around the rocket launch although the balloon data are consistently low. As a telemetry modification was included in the balloon-sondes and resulted in measured temperatures much below those determined independently at ESRANGE it is believed that the correction procedure is appropriate. 4.
3 20
March
1975
(date)
FIG. 4. TOTAL OS MEASURED AT MURMANSK LENINGRAD (60"N) AND KIRUNA (68”N) FOR THE AROUND THE ROCKET LAUNCH. TI-IBDASHED LINE FIGURE IS THE CORRECTED DATA AS DESCRIBED TEKT. 5
(68"N), PERIOD IN THIS IN THE
DISCUSSION
A comparison of Figs: 3(a) and (b) indicates that the ozone concentrations derived from the infrared photometer are, in the overlap region, consistently higher than those derived from the solar occultation technique. A reconsideration of equation (2) indicates that the contribution to the measured metastable oxygen concentration from the decaying dayglow emission is extremely small. The effective lifetime of the O,(‘A,) molecules is only 15 min at 70 km. Thus the ozone concentrations derived from the infrared photometer are appropriate to the solar depression angle at the rocket rather than the tangent ray points and afford direct confirmation of the time varying ozone concentrations predicted by extensive model calculations (Thomas and Bowman, 1972). For a simple oxygen atmosphere it can
170
E. J. LLEWELLYN and G. Wrrr
be shown that during the twilight the time dependent ozone concentrations is, to a good approximation, represented by a simple exponential function. Hence equation (2) may be rewritten as,
concentration, 5 x lo7 cm-“, is typical of that reported by Evans and Llewellyn (1972) for summertime conditions at 52“N. However, these latter authors noted that the normal winter-time high altitude bulge concentration, 1.5 x 10’ cmm3, decreased during a stratospheric warming event to a tvpical summertime value. The present rocket measurement also corresponds to a stratospheric warming event, although in this case it was the final warming associated with the complete establishment of the summer circulation (Labitzke, 1976). Thus the present measurements suggest a seasonal variation in the mesospheric ozone concentration at high latitude similar to that observed at 52”N. A comparison of the present observations with predicted values shows excellent agreement with the extensive model calculations of Bowman and Thomas (1974). However, these authors noted that to obtain agreement between calculation and previously reported O,(‘A,) concentration profiles (Evans and Llewellyn, 1970) a large increase, a factor 2-6, in the 85 km ozone concentration would be required. Such an increase in the ozone wncentration is possible through the reduction in the value of adopted eddy diffusion coefficient and the water vapour mixing ratio. Thus the present observations of the ozone concentration suggest an increase in
n(O,*)=exp-(k[w+A)t
C+ 3x0(1 - at’) I I’I’=0 x[03],exp(k[M]+A+P)t’dt’ (6) I
where [O,& = daytime ozone concentration, l/p = the time constant for the change ozone concentration.
in the
We have applied equation (6) to metastable oxygen profile shown in Fig. 2 to determine the daytime ozone profile. The resulting ozone profile is shown in Fig. 5 and is in very good agreement with that derived from the solar occultation data. In Fig. 5 we have also compared the measured ozone profile with that obtained from a previous observation (March, 1970) with the solar occultation photometer and the measurements of other investigators (Miller, 1976; Miller and Ryder, 1973). It is readily apparent that the March 1975 profile exhibits a slight reduction in the wncentration near 60 km and a major reduction for the altitude region near 85 km. In this upper region the
80-
?O-
60-
5 d 3 .g 4
!5O-
40-
0 680N
February
1970 (Mi
. %PN
September 1969 (Miller and Ryder)
a 6WN
March
1970
x (1.27~) derived ozone
30-
o (1.27~) derived ozone corrected to 0”solar depression
20-
+ Solar occulation 0 Balloon
6EPN, March 1975
‘.. ..o
Soride
IO-
ol I06
1
1 III IO'
1
1 10s
Ozone
FIG.~. A
10'0
I09
concentration,
COMPARISONOFTHEMEASUREDOZONEPROFILEWITHTHAT MJwlX
IO"
10'2
cmm3 OBTAINEDFROM
OTHEREXPERI-
Ozone concentrations
at high latitude during twilight
the water vapour mixmg ratio and the eddy diffusion coefhcient from that usuahy associated with the high latitude winter-time condition. There is some support for this hypothesis from a measurement of the water vapour mixing ratio made from the same rocket, these observations indicated a value near the stratopause in excess of 8 ppm (Evans, 1976). Further support is also afforded from a measurement of the Meinel OH (8-3) band obtained from a rocket launched on 7 March 1975. The observed band intensity was consistent with a total system emission of 1 MR so that if the ozone concentrations recorded one week later are appropriate to this observation an H atom concentration of 10’ cm-” is required at the peak of the emission Iayer. This concentration is a factor three larger than that estimated by Evans and Lieweliyn (I973) from a rocket observation at Churchih (WN) but is in good agreement with that caIculated by Hunten and Strobe1 (1974). Thus, there is some reason to suggest a significant increase in the eddy diffusion coefficient during a stratospheric warming event. In Fig. 5 the ozone profile in the 99 km region has been derived under the assumption that the entire signaI is due to the dissociation of ozone. However, a comparison of the present observation with that reported by Evans et al. (1972) for the U2ffAJ nightgIow emission suggests that this third layer is more probabfy the upper layer nightgIow. Thus, the apparent third iayer in the ozone concentration profile may not be correct. If it is assumed that the observed emission in this region is due to the three body recombination of atomic oxygen then the rate constant for the process can he estimated. For an atomic oxygen inundation of 3 x 101’ cm+, measured on 7 March 1975, the required rate constant is 9x 10d3’ cm’ see-‘. This value is larger than that measured for the total yield from the reaction at a temperature of 200K (Campbeil and Thrush, 1967). Hence if this third Iayer is due to the n~ght~ow the efficient excitation mechanism remains unidentified.
5. CONCLUSIONS AND SUMMARY The present observations have provided the ozone profile for altitudes up to 100 km at the end of the Arctic winter. The excellent agreement between the concentrations derived from the solar occultation technique and from the height profile of the 1.27~ emission provide a direct confirmation of the validity of using the oxygen emission to measure ozone concentrations. In the region of the ozone buIge;e, near 85 km, the derived nnrnber densities are 5.1 x 10’ cmM3, typicaI of that meas-
171
ured during the summer months at 52”N. It is suggested that this reduced ozone concentration is consistent with an increased eddy diffusion coefficient and an increased water vapour mixing ratio in the mesopause region during a stratospheric warming event. Acknowledgements-We wish to acknowledge the assistance of Mr. S. Grahn in the evaluation of the solar occultation data and to thank Dr. W. F. J. Evans, Dr. K. Labitzke and Dr. D. E. Mier for kindly agreeing to the use of data prior to publication. This work was supported through grants in aid from the National Research Council (Canadayand the Space Board (Sweden). We wish to thank Mr. P. Schlyter and Mr. B. Long for their heip with the computer programs. REFERENCES
Ackerman, M. (1970).Aeron. Acra 77, 1. Arvessen, J. C., Griffin, R. N. and Pearson, B. D. (1969). AppL Optics 8, 2215. Badger, R. M., Wright, A. C. and Whitlock, R. P. (1965). 3. Chem. Pfiys. 43, 4345. Bowman, M. R. and Thomas, L. (1974). .I. atmos. tew. Pkys. 36, 657. Broadfoot, A. L. (1972). Astmphys. J. 173,681. Campbell, I. M. and Thrush, B. A. (1967). Proc. R. Sac. (A)2%, 220. Carver, 3. H., Horton, B. H., O’Brien, R. S. and Rofe, B. (1372). Plenef. Space Sci. 20, 217. Clark, I, D. and Wayne, R. P. (1959). Pmt. R. Sec. (A)314, 111. Evans, W. F. J. (1967). Ph.D. Thesis, University of Saskatchewan. Evans, W. F. J. (1976). Private communication. Evans. W. F. J.. Hunten. D. M.. Llewellvn. E. J. and Valiance Jones., A. (1968). .I. geopkys. R& 73, 2885. Evans, W. F. J, and Llewellyn, E. J. (1970). Ann, Geophys. 26, 167. Evans, W. F. J. and Llewellyn, E. J. (1972). Radio Sci. 7, 45. Evans, W. F. J, and Llewellyn, E. 3. (1973). J. geophys. Res. 78, 323. Evans, W. F. J., Llewellyn, E. J. and Valiance Jones, A. (1969). Planet. Space Sci. 17, 933. Evans, W. F. J., Lleweliyn, E. 3. and Valiance Jones, A. (1972). .I. geophys_ Res. 77, 4899. Hays, P. B. and Robie, R. G. (1973). Plarzet.Space SC% 21, 273. Hudson, R. D. and Mahle, S. H. (1972). .I. geopkys. Res. 77, 2902. Hunten, D. M. and Strobel, D. F. (1974). .I. atmos. Sci. 31, 305. Inn, E. C. Y. and Tanaka, Y. (1953). .I. opt. Sot. Am. 43, 870. Labitzke, K. (1976). Private ~mmuni~tion. ’ Llewellyn, E. J. and Evans, W. F. J. (1971). Tire Radiating Atmosphere (Ed. B. McCormac), p. 17. Reidel, Dordrecht, Holland. Miller, D. E. (1976). Private communication. Miller, D. E. and Rvder, P. (1973)., Planet. Space Sci. 2%. . 9631 PIemeL R. A. (1974). M.Sc. Thesis? XIniversity of Saskatchewan.
172
E. J. LLE~ELLYNand G. WITT
Rawcliffe, R. D., Meloy, G. E., Friedman, R. M. and Rogers, F. H. (1963). J. geophys. Res. 68, 6425. Swider, W. (1964). Planet. Space Sci. 12, 761. Thomas. L. and Bowman. M. R. (1972). .T. atmos. ten. Phys.‘34, 1843. Valiance Jones, A. and Gattinger, R. L. (1963). Planet. Space Sci. 11, 961.
Weeks, L. H. and Smith, L. G. (1968). Planet. Space Sci. 16, 1189. Widing, K. G., Purcell, J. D. and Sandiin, G. D. (1970). Solar Phvs. 12, 52. Wood, H. -C., I&IS, W. F. J., Llewellyn, E. J. and Valiance Jones, A. (1970). Can J. Phys. 48,862.
s@ace Sci., Vol. 25, jw. 165 to 172. Permmon Prim, 1977. Printed in Northern Ireland
THE
~EA~URE~~NT OF OZONE CONCENTRATIONS LATITUDE DURING THE TWILIGHT
AT
HIGH
E. J. LLEWELLYN
Institute of Space and Atmospheric Studies, University of Saskatchewan, Saskatoon, Canada
Meteorological Institute, University of Stockholm, Sweden (Receiwd 26 July 1976) AbHra&-The ozone height profIIe in the Arctic, at the end of the winter, has been measured up to an altitude of 100 km using a combined s&u occultation and 1.27~ oxygen emission technique. The typical two layer structure has been observed with a high altitude minimum near 80 km and a m&mum at 86 km. The measured concentration in this ozone bulge was 5.1 x 10’ cme3, typical of that measured at 52W for the summer months. It is suggested that this reduced ozone concentration may have been associated with a stratospheric warming event that was in progress at the time of the measurement.
1. INTRDDUCFION The
measurement of the ozone concentration in the upper atmosphere was first reported by Johnson et al. (1951) and has subsequently been measured by a number of investigators (e.g. Weeks and Smith, 1968; Carver e# af., 1972). However, the variability of these concentrations with season and geographic latitude is still only available to a limited extent. The extensive measurements reported by Hays and Roble (1973), using a stellar occultation technique from the OAO-2 satellite, were restricted to latitudes equatorward of 43”. In these measurements, which were taken at various times of the year under both quiet and disturbed conditions, there is an almost constant ozone concentration, 10’ cmW3, in the bulge near 85 km. The mesospheric ozone concentration may also be determined from the height profile of the O&‘&J emission. The 0riginaI observations of Evans et uL (1968) were shown to be in good agreement with an excitation through the photolysis of ozone (Valiance Jones and Gattinger, 1963) and Evans et al. suggested that a secondary maximum existed in the ozone height profile near 85 km. This secondary maximum is the bulge observed by Hays and Roble (1973) and is indicated by the original satellite data of Rawcliffe et al. (1963). Subsequent simultaneous measurements of the OZ(‘A,) emission and the ozone height profile have been reported by Llewellyn and Evans (1971) and h&her and Ryder (1973) and these have shown that the height profile of the infrared emission may be
used to infer the mesospheric ozone profile directly. An extensive series of ground based observations, at 52W, of the oxygen emission has been presented by Evans and Llewellyn (1972) and these authors have concluded that the bulge in the ozone concentration above 80 km exhibited a seasonal dependence with a maximum in the concentration, 1.3 x I@ cmW3,occurring during the winter months. However, these ground based studies also indicated that concentrations more typical of the summer regime, 3 x 10’ cm-‘, occurred during a stratospheric warming event. As part of an extensive investigation of minor constituents in the Arctic stratosphere and mesosphere, at the end of the winter, we have obtained a complete determination of the ozone height profile between the ground and the mesopause during the evening twilight. In the present paper we present this height profile, which was obtained from concurrent rocket and balioon frights, launched from Kiruna, Sweden in March, 1975 and suggest that the measured concentrations support the hypothesis of a reduction in the ozone bulge concentration during a stratospheric warming event, 2.
EXPERXMENTAL TECHNIQUE
The ozone concentrations reported here were obtained from three independent instruments which provided data over different height ranges, ahhough in the case of the two rocket instruments it has been possible to extrapolate for an altitude overlap of approximatefy 8 km. For the high
16.5
166
E. J. LLEWELLYN and G. Wrrr
altitude, above 60 km, part of the profile we have used a single channel infrared photometer, mounted with its optic axis parallel to the rocket axis, measuring the molecular oxygen emission at 1.27~. Previous measurements with a two channel photometer (Evans et al., 1968; Wood et al., 1970) have indicated that the contribution to the measured signal from the Rayleigh scattering of sunlight is negligibly small, approximately 10 kR at 50 km, so that a single channel instrument will not significantly increase the error in the measured profile. The calibration of the instrument followed the procedure described in detail by Evans et al. (1969) and included a determination of the instrumenta sensitivity as a function of detector temperature. The performance of the photometer during the initial part of the rocket flight, prior to clamshell ejection, was monitored through an internal catibration light and the detector temperature was recorded throughout the entire flight. In this way any change in the instrumental performance could be compensated for during the data analysis although in practice no correction factors were needed. For the altitude region from 30 to 70 km the solar occultation technique was used. The instrumentation consisted of six wavelength selective telescopes, centred at 254, 284, 316, 450, 480 and 481 nm respectively, mounted with their optic axes perpendicular to the rocket axis and having a single common photomultiplier detector. Thus as the rocket spins the transmitted solar flux in each wavelength channel is measured. F&r each telescope the spectral isolation was achieved by a double quartz diffusing screen and an interference filter of spectral width 8 nm, except for the 480 and 481 nm filters which were less than 1 nm. The double d&sing screen ensured that the filter transmission was independent of the angle between the solar vector and the filter normal. To prevent saturation of the photomultiplier tube by the full Sun signal in any wavelength channel a wire mesh attenuator was included between the two diffusing screens. The angular response of the instrument was determined from an extensive series of ground calibrations and using the 450 nm channel, which is outside the Hartley-Huggins bands of ozone, the measured solar flux could be corrected for variations in the solar aspect angle during the flight. A further check on these corrections was possible through the wavelength channels at 480 and 481 nm. For the present meas~ements the rocket employed a high spin rate, 7 Hz, so that the assumption of a constant solar aspect angle during a single spin period introduced a negligible error into these corrections.
For the low altitude data, below 30 km, we have used a chemiluminescent balloon-sonde launched from the Kiruna Geophysical Observatory. 3. OBSERVATIONS
AND
RESULTS
The relevant details for the rocket flight are given in Table 1. The solar occultation telescopes were exposed at 37 seconds after launch while the nose-cone covering the forward-looking experiments was ejected at 60 seconds. Both instruments provided continuous data until 230 seconds when the rocket re-entered and overturned. TABLE ~.DETAILSOF
THE
Launch site Date Time (UT) Apogee Solar zenith angle
FLIGHT(S~~/~)
Kiruna, Sweden March 13,1975 1702 106 km 94.1”
3.1. Infrared photometer The overhead emission intensity, corrected to the zenith, recorded by the infrared photometer is shown in Fig. 1. These data have been obtained from a running average of 15 consecutive data points equally spaced in time and occupy approximately one second. Thus, for the lower altitudes
-i
I
k
co55
I
0
I
I 200
Overhead
I
I
I
400
emission
I
< 800
600
intensity,
kR
FIG. 1. Oz(*Ah,) OVERHEAD EMISSION INTENSITY CORREClZDTOTHEZJ3NlTH.
Ozone concentrations at high latitude during twilight ble oxygen concentration
167 is given by:
$ (O,*) = J3[031-(k[Ml+A)n(02*),
(1)
where each term has its usual meaning. (1) may be integrated to yield,
Equation
n(O,*)=exp(k[Mj+A)r [Oj(f’)]JX(t’) exp (k[M]+A)t’dt’
(2)
where [O,( t’)] = the ozone concentration at time t’, J3( t’) = the ozone dissociation coefficient at time t’, C=constant of integration, the dayglow concentration of metastable oxygen,
1
0
I
I
IO
20
Volume
emission
FIG. 2. 02(rAg)
VOLUME
I
30 rote,
I
40
I
50
I
kR/km
EMISSION HEIGHT RIVEDFROMFIG. 1.
PROFILE DE-
the smoothing interval is 1 km while at 95 km the smoothing interval is only 0.3 km. It was assumed that the required correction for the reduction of the measured intensity to the zenith is given by the cosine of the zenith distance of the rocket axis. The necessary vehicle aspect was determined from onboard magnetometers and solar aspect sensors, although an independent check of the vehicle aspect was made through an analysis of the phase delay between the response of the magnetometer and the 450 nm channel of the solar occultation photometer. The attitude corrected data shown in Fig. 1 has been differentiated to give the volume emission height profile shown in Fig. 2. The numerical differentiation technique was equivalent to fitting a least squares straight line to a 7 km data set and advancing by 1 km steps; this process results in a height resolution of 3.5 km in the altitude distribution (Wood et al., 1970). It is readily apparent from Fig. 2 that the two layer structure seen on previous rocket flights (Wood et al., 1970) was present although there was also a third layer at 99 km. The derivation of the ozone concentration responsible for the oxygen emission is straightforward when the atmosphere is under full dayglow conditions. However, for a twilight observation it is necessary to consider the decreasing solar flux during the twilight and the departure from photochemical equilibrium. The rate of change of the metasta-
and t is the time since the atmosphere, at the height considered, was under full dayglow illumination. If it is assumed that the time dependent ozone dissociation coefficient can be represented by a linear function (Plemel, 1974), and that the ozone wncentration is time independent, the integral in equation (2) may be evaluated explicitly and the local ozone concentration is given by, l
Lo
n(Oz*){k[M+A) .I30
=
3
a k[M]+A
x(1-exp
(-k[M]-A)t}&
(3)
where J3( t) = J3&1- at). In Fig. 3(a) we show the ozone concentration I
I
I
I
I
I (a) j
go-
5
65-
$
80-
3
.?Y
2
75-
7065'. r-s;-l '. '\
6055 106
I IO' Ozone
I 100 concentration,
I
I
109
-
I
10'0 cm-3
FIG. 3(a). OZONE CONCENTRATION HEIGHT PROFILE DERIVED FROMTHE OZ(‘Ag) HEIGHTPROFILESHOWNIN FIG.~.
168
E. J. LLEWELLYNand G. Wm
height profile derived from the infrared photometer data using equation (3). The error bars in this figure represent the uncertainty in the ozone concentration due to the uncertainty in the metastable oxygen concentration; the dashed part of the profile is a simple extrapolation, at constant scale height, of the derived concentrations at higher altitudes. In the evaluation of equation (3) the solar flux values of Ackerman (1970), Arvessen et al. (1969), Broadfoot (1972) and Widing et al. (1970) have been used together with the absorption crosssections of Ackerman (1970) and Hudson and Mahle (1972) for molecular oxygen, and Inn and Tanaka (1953) for ozone to determine the ozone dissociation coefficient. The required ozone height profile for altitudes below 70 km was taken from the average ozone distribution derived by Evans (1967). This profile is in reasonable agreement with that obtained from the solar occultation technique in the present experiment so that no additional corrections were applied to the calculated JsOvalues. The spontaneous transition probability, A, was taken to be 2.8 x 10e4 s-l (Badger et al., 1965) and the adopted value of the collisional quenching coefficient, k, was 4.4 x lo-l9 cm3 s-‘, in agreement with the measurements of Clark and Wayne (1969). The atmospheric profile was assumed to correspond to the 1966 U.S. Standard Atmosphere supplement for winter at 75”N. 3.2. Solar occultation photometer The signal from the solar occultation photometer was a series of regular spaced pulses with amplitudes, in the absorbing channels, that increased with altitude indicating the decrease in the attenuation of the solar flux. These signals were corrected for solar aspect using the control channel at 450 nm, and corrected for amplifier drift by comparing the control channel signal at those times in the flight when the rocket had the same solar aspect angle. The derivation of the ozone height profile from these observations has used a two step procedure that involved the determination of the slant path column concentration and its subsequent inversion. The measured attenuation of the solar flux was assumed, in the first approximation, to be given by, I = I, exp -Nao,
(4)
where I, is the full-Sun signal measured at apogee and (TVis a mean absorption coefficient over the spectral passband of the filter. The value of N obtained from equation (4) was then used as an
initial value in an improved attenuation I/Z, = +(h)e-x”‘“‘-““’
dA,
equation (5)
where $(A) is the spectral distribution of the solar flux weighted by the filter transmission curve, and T(A) is the slant path optical depth for molecular scattering. Equation (5) was solved by an iterative procedure and was assumed to have converged when the change in X from successive iterations was less than 0.1%. In the solution of this equation the required spectral distribution of the solar flux was taken from Ackerman (1970) and the ozone absorption coefficients were from Inn and Tanaka (1953). To determine the local ozone number density as a function of height from the derived slant path column concentrations, the atmosphere was divided into thin spherical shells, each 1 km thick, and within which the ozone number density is a linearly varying quantity. The integral relating the slant path column concentrations and the local number density equations was transformed into a set of linear equations with coefficients determined by the geometry of the experiment and the finite size of the Sun (Miller and Ryder, 1973; Swider, 1964). This latter will cause a smearing of the derived concentration profile over a height interval in km approximately equal to the solar depression at the payload in degrees. The resulting matrix equation was solved by successive substitution with the assumption that the concentration was zero at the upper edge of the highest shell. This assumption leads to some error in the ozone concentration determined from those tangent ray heights for which the attenuation of the solar flux is small; however, as noted by Miller and Ryder (1973), any error associated with this assumption is rapidly damped out and is negligible 3 km below the tangent ray height at which it is assumed X = 0. A further uncertainty in these derived ozone concentrations, Fig. 3(b), is contained in the assumption of horizontal homogeneity for the ozone distribution along the line of sight from the sensor to the Sun. This is obviously incorrect for those altitudes where the ozone concentration changes during the twilight, and at 65 km the derived concentration is approximately 15% larger than that corresponding to 0” solar depression. 3.3. Balloon-sonde
data
The very low altitude, below 30 km, ozone concentrations shown in Fig. 3(b) are those derived from the chemiluminescent balloon-sonde. The plotted values are the measured values multiplied
Ozone concentrations
at high latitude during twilight
169
a-254nm
l-284nm f-316 x -
“m Balloon
Sonde
Ozone
FIG.
3(b).
concentrotm.
cm-3
OZONECONCENTRATIONHEIGHTPROFILEDERIVEDFROMTHESOLAROCCULTATIONPHOTOMETERANDBALLOON-SONDEOBSERVATIONS.
by a factor of 1.7; this latter factor was determined from a comparison of the total zone amount measured by the sonde with that measured from the recording stations at Murmansk (68”N) and Leningrad (60”N) as shown in Fig. 4. The individual
observations shown in Fig. 4 have simply been joined by straight lines to facilitate representation and the dashed curve in this figure represents the balloon-sonde data multiplied by a factor 1.7. It is readily apparent that the recorded amounts track each other during the period around the rocket launch although the balloon data are consistently low. As a telemetry modification was included in the balloon-sondes and resulted in measured temperatures much below those determined independently at ESRANGE it is believed that the correction procedure is appropriate. 4.
3 20
March
1975
(date)
FIG. 4. TOTAL OS MEASURED AT MURMANSK LENINGRAD (60"N) AND KIRUNA (68”N) FOR THE AROUND THE ROCKET LAUNCH. TI-IBDASHED LINE FIGURE IS THE CORRECTED DATA AS DESCRIBED TEKT. 5
(68"N), PERIOD IN THIS IN THE
DISCUSSION
A comparison of Figs: 3(a) and (b) indicates that the ozone concentrations derived from the infrared photometer are, in the overlap region, consistently higher than those derived from the solar occultation technique. A reconsideration of equation (2) indicates that the contribution to the measured metastable oxygen concentration from the decaying dayglow emission is extremely small. The effective lifetime of the O,(‘A,) molecules is only 15 min at 70 km. Thus the ozone concentrations derived from the infrared photometer are appropriate to the solar depression angle at the rocket rather than the tangent ray points and afford direct confirmation of the time varying ozone concentrations predicted by extensive model calculations (Thomas and Bowman, 1972). For a simple oxygen atmosphere it can
170
E. J. LLEWELLYN and G. Wrrr
be shown that during the twilight the time dependent ozone concentrations is, to a good approximation, represented by a simple exponential function. Hence equation (2) may be rewritten as,
concentration, 5 x lo7 cm-“, is typical of that reported by Evans and Llewellyn (1972) for summertime conditions at 52“N. However, these latter authors noted that the normal winter-time high altitude bulge concentration, 1.5 x 10’ cmm3, decreased during a stratospheric warming event to a tvpical summertime value. The present rocket measurement also corresponds to a stratospheric warming event, although in this case it was the final warming associated with the complete establishment of the summer circulation (Labitzke, 1976). Thus the present measurements suggest a seasonal variation in the mesospheric ozone concentration at high latitude similar to that observed at 52”N. A comparison of the present observations with predicted values shows excellent agreement with the extensive model calculations of Bowman and Thomas (1974). However, these authors noted that to obtain agreement between calculation and previously reported O,(‘A,) concentration profiles (Evans and Llewellyn, 1970) a large increase, a factor 2-6, in the 85 km ozone concentration would be required. Such an increase in the ozone wncentration is possible through the reduction in the value of adopted eddy diffusion coefficient and the water vapour mixing ratio. Thus the present observations of the ozone concentration suggest an increase in
n(O,*)=exp-(k[w+A)t
C+ 3x0(1 - at’) I I’I’=0 x[03],exp(k[M]+A+P)t’dt’ (6) I
where [O,& = daytime ozone concentration, l/p = the time constant for the change ozone concentration.
in the
We have applied equation (6) to metastable oxygen profile shown in Fig. 2 to determine the daytime ozone profile. The resulting ozone profile is shown in Fig. 5 and is in very good agreement with that derived from the solar occultation data. In Fig. 5 we have also compared the measured ozone profile with that obtained from a previous observation (March, 1970) with the solar occultation photometer and the measurements of other investigators (Miller, 1976; Miller and Ryder, 1973). It is readily apparent that the March 1975 profile exhibits a slight reduction in the wncentration near 60 km and a major reduction for the altitude region near 85 km. In this upper region the
80-
?O-
60-
5 d 3 .g 4
!5O-
40-
0 680N
February
1970 (Mi
. %PN
September 1969 (Miller and Ryder)
a 6WN
March
1970
x (1.27~) derived ozone
30-
o (1.27~) derived ozone corrected to 0”solar depression
20-
+ Solar occulation 0 Balloon
6EPN, March 1975
‘.. ..o
Soride
IO-
ol I06
1
1 III IO'
1
1 10s
Ozone
FIG.~. A
10'0
I09
concentration,
COMPARISONOFTHEMEASUREDOZONEPROFILEWITHTHAT MJwlX
IO"
10'2
cmm3 OBTAINEDFROM
OTHEREXPERI-
Ozone concentrations
at high latitude during twilight
the water vapour mixmg ratio and the eddy diffusion coefhcient from that usuahy associated with the high latitude winter-time condition. There is some support for this hypothesis from a measurement of the water vapour mixing ratio made from the same rocket, these observations indicated a value near the stratopause in excess of 8 ppm (Evans, 1976). Further support is also afforded from a measurement of the Meinel OH (8-3) band obtained from a rocket launched on 7 March 1975. The observed band intensity was consistent with a total system emission of 1 MR so that if the ozone concentrations recorded one week later are appropriate to this observation an H atom concentration of 10’ cm-” is required at the peak of the emission Iayer. This concentration is a factor three larger than that estimated by Evans and Lieweliyn (I973) from a rocket observation at Churchih (WN) but is in good agreement with that caIculated by Hunten and Strobe1 (1974). Thus, there is some reason to suggest a significant increase in the eddy diffusion coefficient during a stratospheric warming event. In Fig. 5 the ozone profile in the 99 km region has been derived under the assumption that the entire signaI is due to the dissociation of ozone. However, a comparison of the present observation with that reported by Evans et al. (1972) for the U2ffAJ nightgIow emission suggests that this third layer is more probabfy the upper layer nightgIow. Thus, the apparent third iayer in the ozone concentration profile may not be correct. If it is assumed that the observed emission in this region is due to the three body recombination of atomic oxygen then the rate constant for the process can he estimated. For an atomic oxygen inundation of 3 x 101’ cm+, measured on 7 March 1975, the required rate constant is 9x 10d3’ cm’ see-‘. This value is larger than that measured for the total yield from the reaction at a temperature of 200K (Campbeil and Thrush, 1967). Hence if this third Iayer is due to the n~ght~ow the efficient excitation mechanism remains unidentified.
5. CONCLUSIONS AND SUMMARY The present observations have provided the ozone profile for altitudes up to 100 km at the end of the Arctic winter. The excellent agreement between the concentrations derived from the solar occultation technique and from the height profile of the 1.27~ emission provide a direct confirmation of the validity of using the oxygen emission to measure ozone concentrations. In the region of the ozone buIge;e, near 85 km, the derived nnrnber densities are 5.1 x 10’ cmM3, typicaI of that meas-
171
ured during the summer months at 52”N. It is suggested that this reduced ozone concentration is consistent with an increased eddy diffusion coefficient and an increased water vapour mixing ratio in the mesopause region during a stratospheric warming event. Acknowledgements-We wish to acknowledge the assistance of Mr. S. Grahn in the evaluation of the solar occultation data and to thank Dr. W. F. J. Evans, Dr. K. Labitzke and Dr. D. E. Mier for kindly agreeing to the use of data prior to publication. This work was supported through grants in aid from the National Research Council (Canadayand the Space Board (Sweden). We wish to thank Mr. P. Schlyter and Mr. B. Long for their heip with the computer programs. REFERENCES
Ackerman, M. (1970).Aeron. Acra 77, 1. Arvessen, J. C., Griffin, R. N. and Pearson, B. D. (1969). AppL Optics 8, 2215. Badger, R. M., Wright, A. C. and Whitlock, R. P. (1965). 3. Chem. Pfiys. 43, 4345. Bowman, M. R. and Thomas, L. (1974). .I. atmos. tew. Pkys. 36, 657. Broadfoot, A. L. (1972). Astmphys. J. 173,681. Campbell, I. M. and Thrush, B. A. (1967). Proc. R. Sac. (A)2%, 220. Carver, 3. H., Horton, B. H., O’Brien, R. S. and Rofe, B. (1372). Plenef. Space Sci. 20, 217. Clark, I, D. and Wayne, R. P. (1959). Pmt. R. Sec. (A)314, 111. Evans, W. F. J. (1967). Ph.D. Thesis, University of Saskatchewan. Evans, W. F. J. (1976). Private communication. Evans. W. F. J.. Hunten. D. M.. Llewellvn. E. J. and Valiance Jones., A. (1968). .I. geopkys. R& 73, 2885. Evans, W. F. J, and Llewellyn, E. J. (1970). Ann, Geophys. 26, 167. Evans, W. F. J. and Llewellyn, E. J. (1972). Radio Sci. 7, 45. Evans, W. F. J, and Llewellyn, E. 3. (1973). J. geophys. Res. 78, 323. Evans, W. F. J., Llewellyn, E. J. and Valiance Jones, A. (1969). Planet. Space Sci. 17, 933. Evans, W. F. J., Lleweliyn, E. 3. and Valiance Jones, A. (1972). .I. geophys_ Res. 77, 4899. Hays, P. B. and Robie, R. G. (1973). Plarzet.Space SC% 21, 273. Hudson, R. D. and Mahle, S. H. (1972). .I. geopkys. Res. 77, 2902. Hunten, D. M. and Strobel, D. F. (1974). .I. atmos. Sci. 31, 305. Inn, E. C. Y. and Tanaka, Y. (1953). .I. opt. Sot. Am. 43, 870. Labitzke, K. (1976). Private ~mmuni~tion. ’ Llewellyn, E. J. and Evans, W. F. J. (1971). Tire Radiating Atmosphere (Ed. B. McCormac), p. 17. Reidel, Dordrecht, Holland. Miller, D. E. (1976). Private communication. Miller, D. E. and Rvder, P. (1973)., Planet. Space Sci. 2%. . 9631 PIemeL R. A. (1974). M.Sc. Thesis? XIniversity of Saskatchewan.
172
E. J. LLE~ELLYNand G. WITT
Rawcliffe, R. D., Meloy, G. E., Friedman, R. M. and Rogers, F. H. (1963). J. geophys. Res. 68, 6425. Swider, W. (1964). Planet. Space Sci. 12, 761. Thomas. L. and Bowman. M. R. (1972). .T. atmos. ten. Phys.‘34, 1843. Valiance Jones, A. and Gattinger, R. L. (1963). Planet. Space Sci. 11, 961.
Weeks, L. H. and Smith, L. G. (1968). Planet. Space Sci. 16, 1189. Widing, K. G., Purcell, J. D. and Sandiin, G. D. (1970). Solar Phvs. 12, 52. Wood, H. -C., I&IS, W. F. J., Llewellyn, E. J. and Valiance Jones, A. (1970). Can J. Phys. 48,862.