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The excitation of the infrared atmospheric oxygen bands in the nightglow
F’hef.
Spce
Q l’iqamon
Sci., Vol. Press Ltd.,
26, pp. 533 to 538. lW8.
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lmhd
THE EXCITATION OF THE INFRARED ATMOSPHERIC OXYGEN BANDS IN THE NIGHTGLOW E.1.LLEWELLYN
aad B. IL SOLHEIM
Institute of Space and Atmospheric Studies, University of Saskatchewan, Saskatoen, Sask, Canada (Received 20 October 1977) Abstract-Simultaneous observations of the nightglow emission protlles of &(*A) and the OH MeineI bands have been used to show that the excitation mechanism for O,(‘A) in the night-time is through the reaction between OH* and atomic oxygen and the recombination of atomic oxygen. The-se reactions, and the proposed rate constants, have been used to derive the atomic oxygen profile appropriate to the observations. It is suggested that the atomic oxygen profile may exhibit significant
structure near the mesopause at high latitude. It is also suggested that the extent of this structure may be influenced by transport effects related to stratospheric warming events. 1. INTRODUCIION
The Oz(u’A, -X3&-) i.r. atmospheric system in the airglow has been extensively studied since Valiance-Jones and Harrison (19.58) first detected an emission feature at 1.58~, corresponding to the (O-l) band, in the evening twilight. The (O-O) band at 1.27~ was subsequently identified in the day and twilight airglow by Noxon and Valiance-Jones (1962) who used an aircraft platform to reduce the effect of absorption by molecular oxygen below the emitting region. Since these early observations the diurnal variation and the concentration height profile of OZ(‘Ak) molecules in the atmosphere have been intensively investigated and the results reviewed by Llewellyn et al. (1973). It is now recognized that the photolysis of ozone within the Hartley band is the main source of O&As) emission in the day and early twilight airglow. However, the source of excitation for the nightglow has not been fully explained. Gattinger (1968) proposed that the recombination of atomic oxygen, reaction 1, O+O+W+O,(‘A,)+~
Evans et al. (1971) and by Gattinger and VallanceJones (1973) during the NASA 1969 airborne auroral expedition. Evans ef al. concluded that there was a tendency for the molecular oxygen emission, the OH Meinel emissions and the nightglow component of A5577(0I) to covary. This result was confirmed by the spectrometer results of Gattinger and Valiance-Jones (1973). These latter authors also noted that when the nightglow green line intensity was low the intensity variations of the O,(‘As) emissions and the OH Meinel emissions correlated and when the green line intensity was high its variations correlated with the Ol(‘Ahg) emission. Additional evidence for the suggested correspondence of the OH and O,(‘A) emissions has been provided by the measured altitude distributions of these emissions in the nightglow (Evans et al., 1972, 1973). Other experimenters have also commented on the correlation between hydroxyl and excited molecular oxygen and Han et al. (1973) noted that the reaction between vibrationally excited OH and atomic oxygen,
(1)
was the most probable production mechanism, although to provide agreement with the observed nightglow intensities almost all recombinations were required to yield the ‘A, state. More recently Moreels et al. (1977) have re-examined this reaction. These authors have concluded that the rate constant would have to be unacceptably large to explain the observed emission intensity and that reaction (1) can only account for about one-quarter of the typical 100 kR nightglow emission. A study of the nightglow variations of the (O-O) band of the O,(‘&) emission was undertaken by
GH*+O+H+O,, “a*
(2)
was energetically capable of exciting the Ol(rAg) state. Recently Llewellyn er al. (1978) have investigated the problem of the quenching of OH* in the atmosphere and have concluded that reaction (2) does not have to be suppressed to provide agreement with the observed Meinel airglow emission. These authors also suggested that the reaction might provide the required OZ(‘A) emission in the
533
534
E. J. LLIWELLYN
and B. H. SOLHEIM
nightglow. In the present paper we report calculations relating to reaction (2) and conclude that this reaction, together with reaction (l), does satisfy the known parameters for the ni~t~ow emission. We have also used previously published height profiles of the molecular oxygen and hydroxyl emissions to derive the atomic oxygen profile appropriate to the observations. 2. RICAClXON
scxmulx
The
production and loss mechanisms for O&A) which have been considered in the present study are listed in Table 1 together with the appropriate rate constants. Reaction (2) for the production of O,(‘A) is exothermic for all vibrational levels u 2 1 so that we may assume that all vib~tion~ly excited OH levels can contribute to G,(‘A,) production. However, for u 24 reaction (2) is energetically capable of producing the O,(‘Z,+) state so that the reaction rate for O,(‘A) production may decrease for t) 24. In their analysis of OH* quenching Llewellyn et cri. (1978) noted that the ground state reaction between OH and 0, which yields the 0,(X3&-) state, is exothermic and conserves spin and has a fast rate constant (Westenberg et al., 1970). These authors also noted that reaction (2) for the production of Ot(‘Ag) satisfies both of these conditions so that the total rate coefficients for the reaction between OH and 0 proposed by Llewellyn et al. (1977) are probably appropriate. These rate constants are given in Table 2. For both reactions (1) and (2) the efficiency for the production of OJ’A,J is not necessarily unity, Reaction (1) may yield any, or all, of the X”Z,, A38”“, b’Z,+ and ai& states and other investigators have suggested an efficiency of 20%, or greater, for the production of the a’A, state. For the present calculations we have adopted an efficiency of 25% in agreement with the work of Gattinger (1971) and Moreels et al. (1977). For reaction (2), the reaction of vibrationally excited
hydroxyl with atomic oxygen, we have adopted an efficency of 100% for the u = 1 level, 80% for the u = 2 level, 75% for the u = 3 level, SO% for the 2, = 4 level, and no ~n~bution from the higher vibrational levels. These values correspond to an efficiency of lOO%, at least for the lower levels, if the total rate constant for reaction (2) remains at its measured ground state value, and were derived from the requirement that the total production from reactions (I) and (2) should yield an emission intensity of 100 kR in the nightglow. The assumption of a negligible contribution to O,(‘A,) production from reaction (2) for u zc 5 should not invalidate the adopted reaction scheme as the atmospheric column concentrations of these vibrational levels are very much less than for v = 1. Thus, even for unit quantum yield of O&A) from these high vibrational levels the effect on the calculated nightglow emission will be less than a factor 1.5. Reaction (6) has not been considered previously and no rate constant has been measured. However, we may estimate the rate constant from observations of the twilight emission. These have indicated that in late twilight the intensity decays with a time constant closely equal to the radiative lifetime (Evans et al., 1970). Thus, the loss rate from reaction (6) cannot exceed that from other collisional losses; for an assumed H atom concentration of 10” cmws3at 85 km the rate constant for reaction (6) cannot be larger than 1 X lo-l3 cme3 s-l. The observed covariation of the OH Meinel band emissions and Oz(‘A,) is readily apparent from the inclusion of reaction (2). For an increased OH emission, and thus OH, population, there is an increased production of O,(“A). However, as the primary losses of OH,,, for all levels L,>O, are through collisional relaxation and radiation this increased loss rate through reaction (2) causes a negligible modification of OH emission. The covariation of the green line emission and O,(‘A) is provided through oxygen recombination, reaction
TABLE 1. CHEMICALREACHONSCHEMEUSEDINTHEPRESENTWORK
Rate Constant
Reaction (1).
O+O+M-+O,+M
(la). (2).
-+O,(‘A,) + M O+O~*~O~(‘As)~H
(3).
0z(‘A,)+0,+02+02 O,(‘Ah,)fN,+O,+Nz O,(‘A\,)+O+O,+O O,(‘As)+H-+OH+O
k, =9.4x
Reference
1O-34exp (484/T)
k,, = Sk, =0.25 k,
&k,, k, = 2.2 x lo’~‘* (773OO)“.78 k,e 1O-2” kg=1.3~10-‘6
k,s:1x10-‘3
All rate constants are given in the ~ntime~e-molecule-se~nd
system.
Campbell and Gray (1973). Gattinger (1971). See Table 2. Findfay and Snelling (1971). Becker et al. (1971). Clarke and Wayne (1969). See text.
535
Excitatian of infrared atmospheric oxygen bands Tastn
2.
REACnCM
Vibrational LeVCitl
u
RATES MR
Total Rate Constant
THE OH*+0
Efficiency for 0#A,f Production
k 2s
&
4 x 10-l” 5 x lo-‘% 4x10-‘” 7x10-“” 9 x 10-i” io-‘0 1.3 X lo-lo 1.4x Io-10 1.5x 10-l”
: 3 4 : 7 8 9
REACXION
:8 0:75 0.5 8 : 0
All rate constants are given in the centimetre-moleculesecond system. (1). However, wo have not considered the difliculties associated with explaining the observed green line emission using the most recent rate constants for the Chapman mechanism (Slanger and Black, 1976). 3. RlEsuLTs
For a steady-state model and the reaction scheme given in Table 1 the volume emission profile for the i.r. atmospheric oxygen emission is given by,
0
I
2
3
Volume Rc.1.
4
5
In this equation .$ and 5, are the respective efficiencies of reaction (I) and (2) for the production of U&A), sifXi] is the loss rate of O&A) through reaction with specie-s X, and A is the spontaneous transition probability (Badger et al., l%S). The fight-tic profiles required in the evaiuation of equation (7) were taken from a number of different sources. The profiles for MY 02, Nz and temperature were from the mean CIBA (1972) for a latitude of 45”N and those for H, 0 and G3 were the d~s~butions calculated by Moreels Edal. (1977) in a dynamical model of the oxygen-hydrogen atmosphere. For vibrationally excited hydroxyl we have adopted the profiles calculated by Llewellyn et al. (1978) in an analysis of OH* quenching in the atmosphere. The result of this lactation is shown in Fig. 1 together with the experimental results of Evans et al. (1972) and Bishop et al. (1972). The calculated height profile is peaked near g7 km and has a 10 km half-width. Unfortunately the height resolution of the Bishop et al. measurement was rather poor so that the single Gaussian layer fitted by these authors may only be representative of an approximate nightglow layer near 90 km. It should also be noted that these observations were made during an aurora1 display so that the height profile may not be the true nightglow profile. The altitude distribution measured by Evans et al. (1972) indicated two definite layers which are certainly not
6
Emission
7
Rote
~~A~~~R~~AN~~AL~~~A~DVOLL~MEEM~~O~~EIGHTPRO~L~S
8
9
IO
II
(kR/km) OF
?3J1A) IN~~NI~~GLO~.
Curve A: Measurement, 58”N February, 1972 (Evans et al., 1972). Curve B: Measurement, WN March, 1969 (Bishop et al., 1972). Curve C: Calculated total volume emission profile. Curve D: Calculated volume emission profile from reaction (2). Curve E: Calculated votume emission profile from reaction (1). Curves C, D and E were calculated using the constituent profiles of More& et al. (1977) as described in the text.
536
E.
J. LLEWELLYN
reproduced in the present calculation. However, it should be noted that the measured integrated intensity on that occasion was only 63 kR compared to the 100 kR in the present calculation. In addition these latter authors also suggested that the two layer structure might merge to form a single layer under different atmospheric conditions. In the measurement of Evans et al. (1973) it is apparent that the Meinel hydroxyl layer was similar in shape to the lower O,(‘A) layer at the time of the measurement. This characteristic is also reproduced in our calculations where there is a strong similarity between the observed lower layer and one produced by vibrationally excited hydroxyl, curve D in Fig. 1. Thus, it would suggest that the i.r. atmospheric oxygen emission in the nightglow is probably due to reactions (1) and (2) and that the double structure observed by Evans et al. (1972) must have occurred under different conditions than those adopted in the present calculations. If we assume that the excitation mechanism has been correctly identified and that the adopted rate constants are appropriate then we may use the measured profiles of O,(‘A) and OH* (Evans et al., 1973) to estimate the atomic oxygen profile at the time of the measurement. Unfortunately the OH* measurements do not directly provide the necessary altitude profiles for all required vibrational levels. However, if we employ the results recently obtained by Llewellyn et al. (1978) for the quenching of OH* in the atmosphere we may use the measurements to estimate the height profiles for all vibrational levels. To derive these concentration
and B. H.
profiles we have assumed that the distribution of the vibrational level population, at any altitude, was the same as that calculated by Llewellyn et al. (1978). Thus the emission signals measured in the OH channels may be interpreted in terms of the concentration of one vibrational level and the entire altitude distribution of all vibrational levels derived. In determining these concentration profiles we have used an average of the up-leg and downleg data presented by Evans et al. (1973). The atomic oxygen concentration is then given by the solution of equation (S),
= PM’a)X-k:~, + kdCM+k&Y+ W-CD, (8) where each term has its usual meaning. The resulting atomic oxygen concentration profile is shown in Fig. 2. It is immediately apparent that the profile is double structured with a peak concentration near 98 km. The concentration at this altitude is 1 X 10” cmW3which is larger than that obtained in the Moreels et al. (1977) model but in reasonable agreement with other measurements. The uncertainty in these profiles has been derived from the uncertainties in the measured O,(‘A) and OH* profiles. 4. DISCUSSION
If the atomic oxygen profile at the time of the measurement was as shown in Fig. 2 then the contribution from reaction (l), at the upper peak, was only 40% of the total O,(‘A) production. Thus,
IO”
IO'0
Concentration FIG. 2. ATOMKOXYOENPROFILESDERIVED
FROM
SOLHEIM
(cmS3)
THEPRESENTWORKANDCOMPAR~~ONPROFILES.
Excitation of infrared atmospheric oxygen bands if the green line emission results from the recombination of oxygen atoms a covariation between the green line nightglow and the O,(“A) emission would be expected. However, because of the dominant contribution of reaction (2) to the integrated intensity of the O&A) emission in the nightglow, this covariation would not be readily detected in the presence of large fluctuations in the OH emissions. This is in agreement with the observations of Gattinger and Valiance-Jones (1973). We have also compared our derived atomic OXygen profile with those reported by other investigators; these latter profiles are also included in Fig. 2. It is apparent that both the profiles of Dickinson et al. (1974) and Evans and Lleweliyn (1973) indicate structure near 94 km and a peak oxygen wncentration near 98 km. This latter is in excellent agreement with the present calculation although the extent of the structure below the peak is much smaller in the observations. It is of value to note that Donahue et al. (1973) in their analysis of the data from the OGO6 green line photometer also suggested a significant structure in the atomic QXygen profile at high latitude. However, the recent measurements of O(lS) production rates from atomic oxygen recombination (Slanger ‘and Black, 1976) make the satellite analysis somewhat uncertain. Despite these uncertainties it would appear that structure in the atomic oxygen profile may be a normai situation, at least at high latitudes, and that the smooth profiles derived in the model atmosphere calculations are the exception. However, there is some evidence to suggest that the structure in the atomic oxygen profile derived from the O,(‘Af and OH Meinel bands may have been associated with a stratospheric warming event. In an extensive study of the mesospheric winds associated with stratospheric warming events Gregory and Manson (1975) have indicated that there was a wind reversal in the upper mesosphere, typical of a warming event, during the period around the nightglow measurements. In an attempt to obtain additional information relating to these effects we have derived an atomic oxygen profile from the ozone profile observations of Llewellyn and Witt (1977). These measurements were made at high latitude (58”Nf in the evening twilighf during a stratospheric warming event. If we assume that for the altitude regime above 90 km full daytime conditions existed at the time of their measurement then the atomic oxygen concentration is eiven bv.
537
In this equation kz is the total rate constant for the reaction between ozone and atomic hydrogen, .I3 is the ozone photolysis rate and klz is the rate constant for ozone formation. ~~fo~unatel~, the atomic hydrogen concentrations at the time of the ozone measurement are unknown so that we must resort to model values. However, as the contribution to the total ozone loss rate from the reaction with atomic hydrogen is quite small a large error in the H atom wncemration should not give an unacceptably large error in the derived 0 atom concentration. Thus to evaluate equation (9) we used an H atom profile and rate constants, k2 and k12, from Moreels et al. (1977). The remaining parameters were those used by Llewellyn and Witt (1977). The atomic oxygen concentrations obtained from these ozone measurements are also indicated in Fig. 2 by the individual data points. The uncertainty associated with these concentrations could be as large as 50% because of the uncertainty in the ozone concentrations. However, it is readily apparent that these atomic oxygen concentrations also exhibit a trough at 92 km, similar to that derived from the nightglow O,(‘A) and OH Meinel band observations. Thus, the extreme nature of the depletion of the atomic oxygen profile at 92 km in the nightglow measurements is probably associated with a transport change, which is retated to a stratospheric warming, rather than an error in the chemical model scheme, The identification of the nightglow excitation as reactions (1) and (2) with the dominant production being through reaction (2) also removes one of the difficulties associated with the dayglow observation of O#A). These observations have failed to indicate the presence of an obvious n~~t~ow layer during the daytime (Evans and Lleweilyn, 1970). However, with the nightglow mechanism being essentially through the hydrogen-ozone mechanism the daytime contribution from this source would also occur in the region of the upper ozone layer and would not be observed as a separate peak. The small wntribution from the atomic oxygen recombination would, of wurse, still be observed. However, the small signal-to-noise ratio available in the measurements at the appropriate altitude has made the detection of this contribution rather uncertain.
The identification of the presence of the atomic oxygen-excited hydroxyl reaction in the atmosphere has suggested that this reaction may be a significant source of O,(‘A) in the night-time. It has been shown that this reaction together with the atomic oxygen recombination reaction can explain
538
E. J. Ltnwnrxrx
the observed infrared atmospheric oxygen emission in the nightglow. These probable reactions have aiso been used to derive the atomic oxygen profile in the atmosphere and it is suggested that the atomic oxygen profile significant structure near the may exhibit mesopause. It is possible that part of this structure may be associated with transport effects related to warming even&The peak concentration in the derived atomic oxygen profile was 1 x 10” cm-‘, this is in close agreement with that measured by Evans and Llewellyn (1973) using an independent technique. Thus the simultaneous measurements of the O&A) and OH Meinel band profiles in the atmosphere may provide a convenient method of measuring the atomic oxygen profile in the mesosphere.
Acknowledgements-This work has been supported by Grants-in-Aid From the National Research Council of Canada. One of us (B.H.S.) wishes to acknowledge the University of Saskatchewan for the award of the Herzberg Scholarship. We are indebted to Drs. W. F. J. Evans, A. VallanceJones and R. L. Gattinger for valuable discussions. REFERFNCES
Badger, R. M., Wright, A. C. and Whitlock, R. F. (1965). J. ckem. Pkys. 43, 4325. Becker, K. H., Groth, W. and Schurath, U. (1971). Ckem. pkys. Let&. 8, 259. Bishop, R. II., Baker, K. D. and Han, R. Y. (1972). J. a&s. temst. Pkys. 34, 1477. Camobell. I. M. and Grav. ukvs. _, C. N. (1973). Ckem. _ I_.&. lb, 607. CIRA. (1972). Cospar Znternational Reference Armospkere. Akademie-Verlag, Berlin. Clarke, I. D. and Wayne, R. P. (1969). Ckem. pkys. I.&s. 3, 405.
and B. H. Sotxm~ Dickinson, P. II. G., Bolden, R. C. and Young, R. A. (1974). Nature, Land. 252, 289. Donahue, T. M., Guenther, B. and Thomas, R. J. (1973). J. geopkys. Res. 78, 6662. Evans, W. F. J. and Llewellyn, E. J. (1970). Annls Gopkys. 26, 167. Evans, W. F. J., Wood, H. C. and Llewellyn, E. J. (1970). Planet. Space Sci. 18,1065. Evans, W. F. J., Llewellvn. E. J., Moreels, G. and Blamant,. J. (1971). Am. deopkys. Union 52, 482. Evans. W. F. J.. Liewellvn. E. J. and ValIanceJones. A. (19i2). J. ge~pkys. Rei. 77, 4899. Evans, W. F. J. and Llewellyn, E. J. (1973). J. geopkys. Res. 78, 323. Evans, W. F. J., Llewellyn, E. J. and Valiance-Jones, A. (1973). Can. J. Pkys. 51,1288. Findlay, F. D. and Snelling, D. R. (1971). J. ckem. Pkys. 54, 2750.
Gattinger, R. L. (1968). Can. J. Pkys. 46, 1613. Gattinger, R. L. (1971). In: ‘I&e Radjaring A~osp~~ (Ed. B. M. McCormac), p. 51. D. Reidel. Dordrecht, Holland. Gattinger, R. L. andVallance-Jones, A. (1973). Pkysicsand Chemistry of Upper Atmospheres. (Ed. B. M. McCormat), p. 184. D. Reidel, Dordrecht, Holland. Gregory, J. B. and Mattson, A. H. (1975). J. Atmos. Sci. 32, 1676. Llewellyn, E. J., Evans, W. F. J. and Wood, H. C. (1973). Physics and Chemistry of Upper Afmospkeres. (Ed. B. M. McCormac), p. 193. D. Reidel, Dordrecht, Holland. Llewellyn, E. J., Long, B. H. and Solheim, B. II. (1978). Planet. Space Sci. 26, 527.
Llewellyn, E. J. and Witt, G. (1977). Planet. Space Sci. 25. 165.
Moreels, G., Megie, G., Valiance-Jones, A. and Gattinaer. R. L. (1977). .I. atmos. terresf. Pkvs. 39. 551. Noxo”n, ‘J. F. and Valiance-Jones, A. (l-%2). Nature, Lend. 1%, 157. Slanger, T. G. and Black, G. (1976). J. ckem. Pkys. 64, 3767.
Valiance-Jones,
A. and Harrison, A. W. (1958). J. atmos.
terrest. Phys. 13, 45.
Westenberg, A. A., de Haas, N. and Roscoe, (1970). J. pkys. Ckem. 74, 3431.
J. M.
Spce
Q l’iqamon
Sci., Vol. Press Ltd.,
26, pp. 533 to 538. lW8.
Printed
in Northern
lmhd
THE EXCITATION OF THE INFRARED ATMOSPHERIC OXYGEN BANDS IN THE NIGHTGLOW E.1.LLEWELLYN
aad B. IL SOLHEIM
Institute of Space and Atmospheric Studies, University of Saskatchewan, Saskatoen, Sask, Canada (Received 20 October 1977) Abstract-Simultaneous observations of the nightglow emission protlles of &(*A) and the OH MeineI bands have been used to show that the excitation mechanism for O,(‘A) in the night-time is through the reaction between OH* and atomic oxygen and the recombination of atomic oxygen. The-se reactions, and the proposed rate constants, have been used to derive the atomic oxygen profile appropriate to the observations. It is suggested that the atomic oxygen profile may exhibit significant
structure near the mesopause at high latitude. It is also suggested that the extent of this structure may be influenced by transport effects related to stratospheric warming events. 1. INTRODUCIION
The Oz(u’A, -X3&-) i.r. atmospheric system in the airglow has been extensively studied since Valiance-Jones and Harrison (19.58) first detected an emission feature at 1.58~, corresponding to the (O-l) band, in the evening twilight. The (O-O) band at 1.27~ was subsequently identified in the day and twilight airglow by Noxon and Valiance-Jones (1962) who used an aircraft platform to reduce the effect of absorption by molecular oxygen below the emitting region. Since these early observations the diurnal variation and the concentration height profile of OZ(‘Ak) molecules in the atmosphere have been intensively investigated and the results reviewed by Llewellyn et al. (1973). It is now recognized that the photolysis of ozone within the Hartley band is the main source of O&As) emission in the day and early twilight airglow. However, the source of excitation for the nightglow has not been fully explained. Gattinger (1968) proposed that the recombination of atomic oxygen, reaction 1, O+O+W+O,(‘A,)+~
Evans et al. (1971) and by Gattinger and VallanceJones (1973) during the NASA 1969 airborne auroral expedition. Evans ef al. concluded that there was a tendency for the molecular oxygen emission, the OH Meinel emissions and the nightglow component of A5577(0I) to covary. This result was confirmed by the spectrometer results of Gattinger and Valiance-Jones (1973). These latter authors also noted that when the nightglow green line intensity was low the intensity variations of the O,(‘As) emissions and the OH Meinel emissions correlated and when the green line intensity was high its variations correlated with the Ol(‘Ahg) emission. Additional evidence for the suggested correspondence of the OH and O,(‘A) emissions has been provided by the measured altitude distributions of these emissions in the nightglow (Evans et al., 1972, 1973). Other experimenters have also commented on the correlation between hydroxyl and excited molecular oxygen and Han et al. (1973) noted that the reaction between vibrationally excited OH and atomic oxygen,
(1)
was the most probable production mechanism, although to provide agreement with the observed nightglow intensities almost all recombinations were required to yield the ‘A, state. More recently Moreels et al. (1977) have re-examined this reaction. These authors have concluded that the rate constant would have to be unacceptably large to explain the observed emission intensity and that reaction (1) can only account for about one-quarter of the typical 100 kR nightglow emission. A study of the nightglow variations of the (O-O) band of the O,(‘&) emission was undertaken by
GH*+O+H+O,, “a*
(2)
was energetically capable of exciting the Ol(rAg) state. Recently Llewellyn er al. (1978) have investigated the problem of the quenching of OH* in the atmosphere and have concluded that reaction (2) does not have to be suppressed to provide agreement with the observed Meinel airglow emission. These authors also suggested that the reaction might provide the required OZ(‘A) emission in the
533
534
E. J. LLIWELLYN
and B. H. SOLHEIM
nightglow. In the present paper we report calculations relating to reaction (2) and conclude that this reaction, together with reaction (l), does satisfy the known parameters for the ni~t~ow emission. We have also used previously published height profiles of the molecular oxygen and hydroxyl emissions to derive the atomic oxygen profile appropriate to the observations. 2. RICAClXON
scxmulx
The
production and loss mechanisms for O&A) which have been considered in the present study are listed in Table 1 together with the appropriate rate constants. Reaction (2) for the production of O,(‘A) is exothermic for all vibrational levels u 2 1 so that we may assume that all vib~tion~ly excited OH levels can contribute to G,(‘A,) production. However, for u 24 reaction (2) is energetically capable of producing the O,(‘Z,+) state so that the reaction rate for O,(‘A) production may decrease for t) 24. In their analysis of OH* quenching Llewellyn et cri. (1978) noted that the ground state reaction between OH and 0, which yields the 0,(X3&-) state, is exothermic and conserves spin and has a fast rate constant (Westenberg et al., 1970). These authors also noted that reaction (2) for the production of Ot(‘Ag) satisfies both of these conditions so that the total rate coefficients for the reaction between OH and 0 proposed by Llewellyn et al. (1977) are probably appropriate. These rate constants are given in Table 2. For both reactions (1) and (2) the efficiency for the production of OJ’A,J is not necessarily unity, Reaction (1) may yield any, or all, of the X”Z,, A38”“, b’Z,+ and ai& states and other investigators have suggested an efficiency of 20%, or greater, for the production of the a’A, state. For the present calculations we have adopted an efficiency of 25% in agreement with the work of Gattinger (1971) and Moreels et al. (1977). For reaction (2), the reaction of vibrationally excited
hydroxyl with atomic oxygen, we have adopted an efficency of 100% for the u = 1 level, 80% for the u = 2 level, 75% for the u = 3 level, SO% for the 2, = 4 level, and no ~n~bution from the higher vibrational levels. These values correspond to an efficiency of lOO%, at least for the lower levels, if the total rate constant for reaction (2) remains at its measured ground state value, and were derived from the requirement that the total production from reactions (I) and (2) should yield an emission intensity of 100 kR in the nightglow. The assumption of a negligible contribution to O,(‘A,) production from reaction (2) for u zc 5 should not invalidate the adopted reaction scheme as the atmospheric column concentrations of these vibrational levels are very much less than for v = 1. Thus, even for unit quantum yield of O&A) from these high vibrational levels the effect on the calculated nightglow emission will be less than a factor 1.5. Reaction (6) has not been considered previously and no rate constant has been measured. However, we may estimate the rate constant from observations of the twilight emission. These have indicated that in late twilight the intensity decays with a time constant closely equal to the radiative lifetime (Evans et al., 1970). Thus, the loss rate from reaction (6) cannot exceed that from other collisional losses; for an assumed H atom concentration of 10” cmws3at 85 km the rate constant for reaction (6) cannot be larger than 1 X lo-l3 cme3 s-l. The observed covariation of the OH Meinel band emissions and Oz(‘A,) is readily apparent from the inclusion of reaction (2). For an increased OH emission, and thus OH, population, there is an increased production of O,(“A). However, as the primary losses of OH,,, for all levels L,>O, are through collisional relaxation and radiation this increased loss rate through reaction (2) causes a negligible modification of OH emission. The covariation of the green line emission and O,(‘A) is provided through oxygen recombination, reaction
TABLE 1. CHEMICALREACHONSCHEMEUSEDINTHEPRESENTWORK
Rate Constant
Reaction (1).
O+O+M-+O,+M
(la). (2).
-+O,(‘A,) + M O+O~*~O~(‘As)~H
(3).
0z(‘A,)+0,+02+02 O,(‘Ah,)fN,+O,+Nz O,(‘A\,)+O+O,+O O,(‘As)+H-+OH+O
k, =9.4x
Reference
1O-34exp (484/T)
k,, = Sk, =0.25 k,
&k,, k, = 2.2 x lo’~‘* (773OO)“.78 k,e 1O-2” kg=1.3~10-‘6
k,s:1x10-‘3
All rate constants are given in the ~ntime~e-molecule-se~nd
system.
Campbell and Gray (1973). Gattinger (1971). See Table 2. Findfay and Snelling (1971). Becker et al. (1971). Clarke and Wayne (1969). See text.
535
Excitatian of infrared atmospheric oxygen bands Tastn
2.
REACnCM
Vibrational LeVCitl
u
RATES MR
Total Rate Constant
THE OH*+0
Efficiency for 0#A,f Production
k 2s
&
4 x 10-l” 5 x lo-‘% 4x10-‘” 7x10-“” 9 x 10-i” io-‘0 1.3 X lo-lo 1.4x Io-10 1.5x 10-l”
: 3 4 : 7 8 9
REACXION
:8 0:75 0.5 8 : 0
All rate constants are given in the centimetre-moleculesecond system. (1). However, wo have not considered the difliculties associated with explaining the observed green line emission using the most recent rate constants for the Chapman mechanism (Slanger and Black, 1976). 3. RlEsuLTs
For a steady-state model and the reaction scheme given in Table 1 the volume emission profile for the i.r. atmospheric oxygen emission is given by,
0
I
2
3
Volume Rc.1.
4
5
In this equation .$ and 5, are the respective efficiencies of reaction (I) and (2) for the production of U&A), sifXi] is the loss rate of O&A) through reaction with specie-s X, and A is the spontaneous transition probability (Badger et al., l%S). The fight-tic profiles required in the evaiuation of equation (7) were taken from a number of different sources. The profiles for MY 02, Nz and temperature were from the mean CIBA (1972) for a latitude of 45”N and those for H, 0 and G3 were the d~s~butions calculated by Moreels Edal. (1977) in a dynamical model of the oxygen-hydrogen atmosphere. For vibrationally excited hydroxyl we have adopted the profiles calculated by Llewellyn et al. (1978) in an analysis of OH* quenching in the atmosphere. The result of this lactation is shown in Fig. 1 together with the experimental results of Evans et al. (1972) and Bishop et al. (1972). The calculated height profile is peaked near g7 km and has a 10 km half-width. Unfortunately the height resolution of the Bishop et al. measurement was rather poor so that the single Gaussian layer fitted by these authors may only be representative of an approximate nightglow layer near 90 km. It should also be noted that these observations were made during an aurora1 display so that the height profile may not be the true nightglow profile. The altitude distribution measured by Evans et al. (1972) indicated two definite layers which are certainly not
6
Emission
7
Rote
~~A~~~R~~AN~~AL~~~A~DVOLL~MEEM~~O~~EIGHTPRO~L~S
8
9
IO
II
(kR/km) OF
?3J1A) IN~~NI~~GLO~.
Curve A: Measurement, 58”N February, 1972 (Evans et al., 1972). Curve B: Measurement, WN March, 1969 (Bishop et al., 1972). Curve C: Calculated total volume emission profile. Curve D: Calculated volume emission profile from reaction (2). Curve E: Calculated votume emission profile from reaction (1). Curves C, D and E were calculated using the constituent profiles of More& et al. (1977) as described in the text.
536
E.
J. LLEWELLYN
reproduced in the present calculation. However, it should be noted that the measured integrated intensity on that occasion was only 63 kR compared to the 100 kR in the present calculation. In addition these latter authors also suggested that the two layer structure might merge to form a single layer under different atmospheric conditions. In the measurement of Evans et al. (1973) it is apparent that the Meinel hydroxyl layer was similar in shape to the lower O,(‘A) layer at the time of the measurement. This characteristic is also reproduced in our calculations where there is a strong similarity between the observed lower layer and one produced by vibrationally excited hydroxyl, curve D in Fig. 1. Thus, it would suggest that the i.r. atmospheric oxygen emission in the nightglow is probably due to reactions (1) and (2) and that the double structure observed by Evans et al. (1972) must have occurred under different conditions than those adopted in the present calculations. If we assume that the excitation mechanism has been correctly identified and that the adopted rate constants are appropriate then we may use the measured profiles of O,(‘A) and OH* (Evans et al., 1973) to estimate the atomic oxygen profile at the time of the measurement. Unfortunately the OH* measurements do not directly provide the necessary altitude profiles for all required vibrational levels. However, if we employ the results recently obtained by Llewellyn et al. (1978) for the quenching of OH* in the atmosphere we may use the measurements to estimate the height profiles for all vibrational levels. To derive these concentration
and B. H.
profiles we have assumed that the distribution of the vibrational level population, at any altitude, was the same as that calculated by Llewellyn et al. (1978). Thus the emission signals measured in the OH channels may be interpreted in terms of the concentration of one vibrational level and the entire altitude distribution of all vibrational levels derived. In determining these concentration profiles we have used an average of the up-leg and downleg data presented by Evans et al. (1973). The atomic oxygen concentration is then given by the solution of equation (S),
= PM’a)X-k:~, + kdCM+k&Y+ W-CD, (8) where each term has its usual meaning. The resulting atomic oxygen concentration profile is shown in Fig. 2. It is immediately apparent that the profile is double structured with a peak concentration near 98 km. The concentration at this altitude is 1 X 10” cmW3which is larger than that obtained in the Moreels et al. (1977) model but in reasonable agreement with other measurements. The uncertainty in these profiles has been derived from the uncertainties in the measured O,(‘A) and OH* profiles. 4. DISCUSSION
If the atomic oxygen profile at the time of the measurement was as shown in Fig. 2 then the contribution from reaction (l), at the upper peak, was only 40% of the total O,(‘A) production. Thus,
IO”
IO'0
Concentration FIG. 2. ATOMKOXYOENPROFILESDERIVED
FROM
SOLHEIM
(cmS3)
THEPRESENTWORKANDCOMPAR~~ONPROFILES.
Excitation of infrared atmospheric oxygen bands if the green line emission results from the recombination of oxygen atoms a covariation between the green line nightglow and the O,(“A) emission would be expected. However, because of the dominant contribution of reaction (2) to the integrated intensity of the O&A) emission in the nightglow, this covariation would not be readily detected in the presence of large fluctuations in the OH emissions. This is in agreement with the observations of Gattinger and Valiance-Jones (1973). We have also compared our derived atomic OXygen profile with those reported by other investigators; these latter profiles are also included in Fig. 2. It is apparent that both the profiles of Dickinson et al. (1974) and Evans and Lleweliyn (1973) indicate structure near 94 km and a peak oxygen wncentration near 98 km. This latter is in excellent agreement with the present calculation although the extent of the structure below the peak is much smaller in the observations. It is of value to note that Donahue et al. (1973) in their analysis of the data from the OGO6 green line photometer also suggested a significant structure in the atomic QXygen profile at high latitude. However, the recent measurements of O(lS) production rates from atomic oxygen recombination (Slanger ‘and Black, 1976) make the satellite analysis somewhat uncertain. Despite these uncertainties it would appear that structure in the atomic oxygen profile may be a normai situation, at least at high latitudes, and that the smooth profiles derived in the model atmosphere calculations are the exception. However, there is some evidence to suggest that the structure in the atomic oxygen profile derived from the O,(‘Af and OH Meinel bands may have been associated with a stratospheric warming event. In an extensive study of the mesospheric winds associated with stratospheric warming events Gregory and Manson (1975) have indicated that there was a wind reversal in the upper mesosphere, typical of a warming event, during the period around the nightglow measurements. In an attempt to obtain additional information relating to these effects we have derived an atomic oxygen profile from the ozone profile observations of Llewellyn and Witt (1977). These measurements were made at high latitude (58”Nf in the evening twilighf during a stratospheric warming event. If we assume that for the altitude regime above 90 km full daytime conditions existed at the time of their measurement then the atomic oxygen concentration is eiven bv.
537
In this equation kz is the total rate constant for the reaction between ozone and atomic hydrogen, .I3 is the ozone photolysis rate and klz is the rate constant for ozone formation. ~~fo~unatel~, the atomic hydrogen concentrations at the time of the ozone measurement are unknown so that we must resort to model values. However, as the contribution to the total ozone loss rate from the reaction with atomic hydrogen is quite small a large error in the H atom wncemration should not give an unacceptably large error in the derived 0 atom concentration. Thus to evaluate equation (9) we used an H atom profile and rate constants, k2 and k12, from Moreels et al. (1977). The remaining parameters were those used by Llewellyn and Witt (1977). The atomic oxygen concentrations obtained from these ozone measurements are also indicated in Fig. 2 by the individual data points. The uncertainty associated with these concentrations could be as large as 50% because of the uncertainty in the ozone concentrations. However, it is readily apparent that these atomic oxygen concentrations also exhibit a trough at 92 km, similar to that derived from the nightglow O,(‘A) and OH Meinel band observations. Thus, the extreme nature of the depletion of the atomic oxygen profile at 92 km in the nightglow measurements is probably associated with a transport change, which is retated to a stratospheric warming, rather than an error in the chemical model scheme, The identification of the nightglow excitation as reactions (1) and (2) with the dominant production being through reaction (2) also removes one of the difficulties associated with the dayglow observation of O#A). These observations have failed to indicate the presence of an obvious n~~t~ow layer during the daytime (Evans and Lleweilyn, 1970). However, with the nightglow mechanism being essentially through the hydrogen-ozone mechanism the daytime contribution from this source would also occur in the region of the upper ozone layer and would not be observed as a separate peak. The small wntribution from the atomic oxygen recombination would, of wurse, still be observed. However, the small signal-to-noise ratio available in the measurements at the appropriate altitude has made the detection of this contribution rather uncertain.
The identification of the presence of the atomic oxygen-excited hydroxyl reaction in the atmosphere has suggested that this reaction may be a significant source of O,(‘A) in the night-time. It has been shown that this reaction together with the atomic oxygen recombination reaction can explain
538
E. J. Ltnwnrxrx
the observed infrared atmospheric oxygen emission in the nightglow. These probable reactions have aiso been used to derive the atomic oxygen profile in the atmosphere and it is suggested that the atomic oxygen profile significant structure near the may exhibit mesopause. It is possible that part of this structure may be associated with transport effects related to warming even&The peak concentration in the derived atomic oxygen profile was 1 x 10” cm-‘, this is in close agreement with that measured by Evans and Llewellyn (1973) using an independent technique. Thus the simultaneous measurements of the O&A) and OH Meinel band profiles in the atmosphere may provide a convenient method of measuring the atomic oxygen profile in the mesosphere.
Acknowledgements-This work has been supported by Grants-in-Aid From the National Research Council of Canada. One of us (B.H.S.) wishes to acknowledge the University of Saskatchewan for the award of the Herzberg Scholarship. We are indebted to Drs. W. F. J. Evans, A. VallanceJones and R. L. Gattinger for valuable discussions. REFERFNCES
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