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Excitation processes of infrared atmospheric emissions
Planet.
Space Sci., Vol. 24, pp.
EXCITATION
149
to
756.
Pergamon
PROCESSES
Press,
1976.
Printed
in Northern
Ireland
OF INFRARED TOSHIHIRO
ATMOSPHERIC
EMISSIONS
OGAWA
Geophysics Research Laboratory, The University of Tokyo, Tokyo 113, Japan (Received 13 January 1976) Abstract-Excitation rates of the infrared emissions which are likely to occur in the mesosphere and thermosphere are quantitatively evaluated. They include the 9.6 pm band of OS, the 15 and 4.3 pm bands of CO? and the 5.3 and 2.8 pm bands of NO. These emissions may be excited through nonthermal processes such as chemiluminescent reactions and resonant fluorescence in the thermosphere, whereas they are of thermal origin in the stratosphere and mesosphere. Increase of the non-thermal excitation rate caused bi precipitating electrons could be responsible for the enhancement of the 4.3 pm band of COz, and the 5.3 and 2.8 pm bands of NO observed in the aurora1 thermosphere. 1. INTRODUCTION
Atmospheric emission spectra in the infrared of wavelengths longer than 4 pm have been obtained in the stratospheric height range (MacDonald et al., 1968; Bunn and Gush, 1970; Murcray et al., 1973; Huppi et al., 1974) and at heights up to the lower thermosphere (Stair el al., 1974, 1975; Baker ef al., 1974). The major emission bands have been identified as belonging to the vibrational transitions of CO, at 4.3 and 15 pm, H,O at 6.3 pm, CH, at 7.7 pm, O3 at 9.6 pm and HNO, at 11.3 pm. The weak emissions at 5.3 and 2.8 pm were also deiected at mesospheric heights under aurora1 conditions (Stair et al., 1975); these are tentatively identified as originating from NO. Since the emissions from H,O, CH, and HNO, have been detected only in the stratosphere, their excitation would be caused by purely thermal processes. Other emissions measured above the stratosphere such as those from CO, and 0, would also be excited through thermal processes. However, there is a possibility that non-thermal excitation is important at heights above the mesopause. As a matter of fact, the vibrational excitation of Nz by photoelectron impact, as well as the energy transfer from O(‘D), and the mutual combination of N and NO results in the vibrational temperature of N, being higher than the gas kinetic temperature (Walker, 1968; Walker et al., 1969; Bauer et al., 1971; Kummler and Bortner, (Walker, 1968; Walker et al., 1969; Bauer et al., 1971; Kummler and Bortner, 1972), and the subsequent vibrational energy transfer from N, to CO* will produce the non-thermal emission of the 4.3 pm band of CO*. An alternative non-thermal process in the upper atmosphere is the direct vibrational excitation of the products in chemical reactions; some processes
have been proved to occur in the laooratory system. Earlier works on this subject are found in the papers by Dalgarno (1963, 1969). Recent laboratory measurements of rate coefficients and quantum yields for these reactions, together with the model distribution of various atmospheric constituents, enable us to evaluate quantitatively non-thermal excitation rates for the vibrational states of some molecules which lead to the infrared emissions. 2. OZONE The principal infrared emission from 0, occurs at the 9.6 pm band of the vg vibrational mode. The vg mode of O3 is excited through the translationvibration (TV) energy transfer 0,+M~0,(001)+M-1042cm~‘,
(1)
where the O3 not specified for the vibrational state denotes the ground state O,(OOO), and M= N2 or 0,. The excitation to the v2. mode of O3 via the similar TV transfer O,+M+=
O,(OlO)+M-701
cm-’
(2)
occurs more rapidly than process (l), because of its lower excitation energy than process (1). The subsequent vibration-vibration (W) transfer through O,(OlO)+O,+
O,+O,(OOl)-341
cm-’
(3)
leads to the production of the vg mode, although almost all O,(OlO) may be de-excited via the inverse reaction of (2) or via the spontaneous emission O,(OlO) + O,+ hv (14.3 pm),
A, = 0.25 set-‘, (4)
749
750
TOSHIHIRO OGAWA
where A4 is the transition probability calculated on the basis of the band strength measured by McCaa and Shaw (1968). Although no direct measurement of the rate coefficients for the processes (l), (2) and (3) is available, it is possible to evaluate these rate coefficient to be (1-2)x lo-l4 cm3/sec for M=02 or Nz (Kurylb et al., 1974, 1975; Rosen and Cool, vibrational relaxation of O3 indicate the rate coefficient to be (l-2) x lOmE4cm3/sec for M = O2 or N, (Kurylo et al., 1974, 1975; Rosen and Cool, 1973, 1975; von Rosenberg and Trainor, 1974). This value may be applicable for the VT transfer of the manifold of the v, and vg modes, or for the VT transfer of v, preceded by the VV transfer from vt or v, to v, (Taylor, 1974). Recently, Hui et al. (1975) found the VV transfer from v1 or vg to vZ to occur with a rate coefficient larger by a factor of 2 or 3 than that of the VT transfer from iv,, v2 or vg. On the basis of this information, the rate coefficients for the inverse reactions of (l), (2) and (3) are assumed to’ be 2~ 10-14, 2 k lo-l4 and 6 x lo-l4 cm3/sec, respectively. The rate coefficients for (l), (2), and (3) may be estimated by means of detailed balancing; k, = 2 x 10-‘4e-‘Soo’T, kz = k,e -‘“‘“r,
r, =
[co,l [Ml ’ k5 = 1 x lo-l3 cm3/sec is an estimated value based on the measured rate coefficient for the inverse reaction of (5) at 300 K by Rosen and Cool (1975). r,=3x lo-” for [CO,]/[M]=3x lo-” in the height range concerned; process (5) is ne~i~ble in the excitation of O,(OlO), compared with process (2) and also (1). One of the non-thermal excitations may occur through chemical reactions. The reaction (7) which is believed to be the only reaction to form the atmospheric ozone where
0+0,-l-M+O,+M,
k7=l.l~10-34e510’T (7)
k3= 6 x 10-‘4e-4w’T, and kz- =t2 x 1()-‘4,
where k, denotes the rate coefficient for the process (i) in cm3/sec, kiL denotes the rate coefficient for the inverse reaction, and T denotes the temperature in K. The reiative magnitude for the rate of process (1) to process (2) followed by (3) is rz =
the relative population of C02(010) to total COZ is given as [CO,(OlO)j/[CO,] = e-960’T according to a Boltzmann distribution. Consequently, the relative excitation rate for process (5) to (2) is represented as
k,[OJMl x
m 2.4
1 U031 b-CM!+ A,’ k,~CMCiW k&3 II031 Z---X k, h-EM] + -%
for [M] >>A,/k,x
lo-‘“[O,]
= 2.5
x
can yield the V~mode of OS, where the rate coef?icient was measured by Huie et al. (1972). The quantum yield for the vibrationally excited O3 was also measured to be about unity, and invariant for M =N, or O2 (Bevan and Johnson, 1973; von Rosenberg and Trainor, 1974). The height profiles for the excitation rate of the O,(OOl) through the chemiluminescent process (7), together with those through the thermal process (1) are calculated with the aid of the 0, model distribution by Ogawa and Shimazaki (1975), and depicted in Fig. 1. This
lOI cmY3,
L
for [Ml<< 2.5 x 1Ol3 cmm3,
100
/
o+o,~O~cooH
where [ ] denotes the number density of molecules designated in the bracket. Since r, does not exceed 1 x lo-“, process (2) can be neglected in the excitation to the vj mode. The VV transfer from the v, mode of CO* COz(OIO)+O,+
CO,+O,(OlO)-34
cm-’
(5)
may also lead to the v2 mode of 03. Since it is a reasonable assumption that the population of CO,(OlO) is determined by the VT exchange process COZ + M * COZ(OIO) + M -667 cm-‘,
(6)
40 103
10'0 RATE. ~rn-~sec-'
FIG. 1. EXCITATION RATES TO 03(001) DUE TO THE TV TRANSFER
AND CHEMILUMINESCE~
PROCESSES.
Profiles ai noon are indicated by solid curves, and those at midnight by dashed curves.
7.51
Excitation processes of infrared atmospheric emissions figure indicates that the excitation rate due to the chemilumines~ent reaction (7) dominates over that due to the thermal process (1) above the heights of 75 - 80 km. Resonant absorption of the terrestrial radiation from the underlying atmosphere may be another candidate for the non-thermal excitation of the v3 mode: O,+hv(9.6
~m)-*03(OOl),
(8)
the fluorescence coefficient of which can be evaluated to be 1.0~ lo-’ photonslsec per 0, molecule for the terrestrial radiation with the aid of the band strength measured by McCaa and Shaw (1968). Estimation of the excitation rate due to this process depends on the solution of the radiative transfer equation for the 9.6 pm band, because the atmosphere is optically thick for the 9.6 pm radiation. According to a preliminary result from solving the 9.6 pm radiative transfer equation by Tohmatsu (1975, private ~mmunication), the terrestrial radiation originating from the underlying atmosphere is more important than the solar radiation as a radiative source. Again process (8) becomes important in the heights above -70 km, but the excitation rate for process (8) is not larger than that for process (7) in an optimum condition. Comparison of the present calculation with the observed emission intensity by Stair et al. (1974) is beyond the scope of this study, because it needs radiative transfer analysis. However, the hump found at the height of 80 km in the height-radiance profiles measured by them would be ascribed to the predominance of the chemiluminescent reaction (7). The VV energy transfer CO,(OOl)+O,
+ CO~(lOO)+O~(OOl)-81 kg = 1 x lo-”
cm-‘,
cm3/sec,
(9)
where the rate coefficient was measured at 300 K by Cool and Airey (1973) and von Rosenberg and Lowenstein (1973), could be of importance in the O,(OOl) excitation under the aurora1 condition. Collisions by secondary electrons produced by precipitating electrons would cause an enhanced population of the vibrationally excited state N,(v = 1) (e.g. Walker et al., 1969; Schunk and Hays, 1971), and the subsequent VV energy transfer to CO,(OOl) could produce an appreciable amount of co,(oo1): N2(u = l)+CO,
it N,+CO,(OOl)-
18 cm-‘, (10)
klo = 6 x lo-” e1J.27fT-‘e for Tc600K (Taytor, 1974). The estimation of the excitation rate for process (9) requires knowledge of the population of the v3 mode of CO*. Since the VV exchange (10) occurs very rapidly, it will control the population of CO,(OOl). However, the spontaneous emission where
C02(O01) -+ CO,+ hv(4.3 pm),
A,, = 400 see-‘, (11)
where the transition probability has been calculated from the band strength by McClatchey et al. (1973), may also affect the population of C02(O01) in the lower thermosphere. Again, since the atmosphere at mesospheric heights is optically thick for the 4.3 pm radiation, the effect of radiation imprisonment occurs, and a precise estimation for the population of the vj mode depends on solving the radiative transfer equation (e.g. James and Kumer, 1973; Kumer and James, 1974), together with the calculation of the primary excitation rate for N,(u = 1) (e.g. Stolarski, 1968). It must be noted that an enhancement of the 4.3 Ic.rn radiation from CO,(OOl) actually occurs in the aurora1 atmosphere (Stair et ai., 1975). The fact that the Iarger 9.6 Frn radiance observed by Stair et al. (1974) was in the aurora1 condition rather than that in the non-aurora1 condition is suggestive of the importance of process (9) in cases of aurora1 disturbances. 3. CARBON DIOXIDE
Prominent features of the infrared COz emission are the v, and vg bands at 15 and 4.3 &m, respectively. The v2 mode is excited mainly by the TV energy transfer, the forward reaction of (6). The rate coefficient for the backward reaction of (6) has been reviewed by Taylor (1974), who gave it to be k,- = 6.69 x lo-” e-s4~07T~‘~ for 111= Nz or 02_ The rate coefficient for the forward reaction of (6) can be evaluated with the aid of the realation kg= kCe-960iT. The VV transfer from O,(OlO), the inverse reaction of (5) can contribute to the excitation of CO,(OlO) at a rate about 3 orders of magnitude smaller than the forward reaction of (6), since almost all O,(OlO) is de-excited through the process (4) or the inverse reaction of (2). The forward reaction of (6) is likely to be the only important collisional process in the excitation of the vz mode of co,.
752
TOSHIHIRO OGAWA
An alternative candidate for the excitation of CO,(OlO) is the resonant absorption of the terrestrial radiation originating from the underlying atmosphere. A quantitative estimation of the excitation rate for this process must wait solving the transfer equation for the 15 pm radiation. Another prominent emission of CO2 at 4.3 pm originating from the v, mode is excited by the backward reaction of the VV exchange (10) as stated in the previous section, the TV transfer CO,+M+CO,(OOl)+M-2349cm-r,
(12)
and the resonant absorption of the solar 4.3 ir_m radiation (i.e. the inverse of reaction 11). The resonant fluorescence induced by the solar 2.7 or 2.8 pm radiation may also emit the 4.3 pm (James and Kumer, 1973; Kumer and James, 1974): CO,+ hv(2.8 pm) + CO,(O21) -+ CO,(O20) + hv(4.3 ym),
+ CO&OO) + hv(4.3 em).
(15)
and the VV energy transfer NOi-N2(u=1)-+NO(u=1)+N,+455cm-‘, (161 and NO+0,(u=1)+NO(u=1)+0,-320cm-1.
(14)
At heights below the mesopause the population of N,(v = 1) and CO,(OOl) may be determined by thermal collisions via the TV transfer, whereas a higher vibrational population of N, than that expected for the thermal equilibrium with the gas kinetic temperature can occur in the thermosphere due to the vibrational excitation of Nz by photoelectron impact, the mutual combination of N and NO, and the energy transfer from the electronically excited O(lD) to the vibrational energy of Nz (Walker, 1968; Walker et al., 1969; Bauer et al., 1971; Kummler and Bortner, 1972). The secondary electrons produced by precipitating electrons in the aurora1 atmosphere will also cause the vibrational excitation of N1. The peak at around 93 km in the height profile of the 4.3 pm radiance measured by Stair et al. (1975) in the aurora1 condition may be ascribed to the enhancement of the 4.3 pm emission due to this excitation process, together with the radiation i&prisonment. Solving the radiative transfer equation for the 4.3 pm is a future problem in urder to make a quantitative comparison of this theory with observations. Again, during the daylight hours and in the low and middle latitudes an enhancement of the 4.3 pm emission similar to that of the aurora1 atmosphere is expected. 4.NITRICOXIDE
fundamental (Au = 1) transitions in the vibrational levels of the ground electronic state of NO The
NO+~~NO(~=l)+~-1876cm-1,
(13)
and CO,+ hv(2.7 pm) -+ CO,(lOl)
emit the radiation at around 5.3 pm, and the first overtone (ho = 2) at around 2.8 +m. Among them, (1,O) and (2,0) transitions may be most intense in the 5.4 and 2.8 pm bands. Various mechanisms for the vibrational excitation of NO in the u = 1 level have been discussed by Dalgarno (1963) and Degges (1971). They include chemical reactions, and resonant absorption of the solar and terrestrial radiation, as well as thermal collision processes. The followings are potential thermal excitation processes: the TV energy transfer
(17) The rate coefhcients for (1.5) through (17) can be evaluated with the aid of those for the inverse reactions of (15) through (17) determined experimentally from the quenching rate of NO(u = 1). The rate coefficient for the NO(u = 1) quenching was measured at 300 K to be 1 x lo-l6 cm3/sec for NZ, 2 x lo-l4 cm3/sec for Oz, 3 x 10-r’ cm3/sec for COz, and 2 x lo-” cm3/sec for H,O (Basco et ab, 1961; Stephenson, 1973, 1974; Murphy et al., 1975). These values may be representative of the rate coefficient for the VV exchange rather than that for the TV exchange process (15). Though some rate coefficient were changed later, Taylor (1974) evaluated a small rate coefficient for the inverse reaction of (15) with M = 02; k,,- = 4,QX 10-loe-lomT-~~~ for M= Oz or N,. He also listed such a rate coefficient for the inverse reaction of (15) with M= COz or Hz0 as in agreement with the measured rate coefficient for the NO(v = 1) quenching, i.e. klS- = 1.32 x ~0-8e-‘00.0T-“” for M = COz, and k,,- = 7.47 x 10-‘“e-40~‘9r-“3 for M = H20. Since it is reasonably assumed in the heights below the mesopause that the vibrational population of NZ or O7 is determined by a Boltzman distribution in equilibrium with the gas kinetic temperature, the excitation rates of NO(o = 1) due to the processes (15) through (17) are written as follows: q15= k&NO][M]
= k~5e-2700’T[NO][M]
q16= klb[NO][N2(u = l)] = k16[NO]e-3350’T[N,J = k,e- 2700’T[NO][N,],
Excitation processes of infrared atmospheric emissions
753
and q,7 = k,,[NO][Oz(u
= l)] = k,,[NO] e--22401r[02]
= k,7-e- 2700’T[N0][0,], where k,* = k16-e+650’T and k,, = k,,-e‘460iT, with k16- = 1 x lo-l6 cm3/secand k17- = 2 x 10-‘4cm3/sec, An alternative expression for kt6 is k16= 4.42 x 10Vio e--86~3sT-“3 given by Taylor (1974). By the use of above values for the rate coefficient, the relative magnitude of q15 to qr, is evaluated to be T,~= q,Jq,, = 3 x 10e2, 7~ IO-‘, 3 x 10m4 and 2 x 10m3at 300 K for M = N,, 02, CO* and HzO, respectively, where [co,]/[o,J = 1.6 x 1o-3 and [HzO]/[0,]=2 x 10M5 are assumed. rr5 does not exceed unity even at 600K. Also we can evaluate T16= q*dql, = 2 x lo-* at 300 K; rr6 becomes unity at 600 K. As a result, it may be concluded that process (17) is the most important thermal excitation of NO(u = 1). Dalgarno (1963) has suggested that the atom interchange reaction O’+NO-+NO’+O
(18)
leads to the vibrational excitation of NO. Using the isotopic ‘*O as a tracer, the rate coefficient for (18) was measured to be k18= 1.8~ lo-r2 cm”/sec at 300 K by Herron and Klein (1964). Degges (1971) estimated the activation energy for (18) to be between 0 and 1000 Cal/mole. Assuming a unit quantum yield and zero activation energy for (IQ, we obtain the rate coefficient far the excitation of NO(u = 1) to be k&v = 1) = 1.8 X 10-‘Ze-*900’T, where the exponential term arises from the excitation energy of NO(u = 1). Chemicaf reactions which are major sources of NO in the mesosphere and lower thermosphere such as NO,+O+NO+O, (19) and N(2D)+0,-+NO+0
(20)
are exothermic enough to excite NO vibrationally. In evaluating the excitation rates due to these reactions, the rate coefficient is taken from the experiments by Davis et al. (1973) and Husain et al. (1972): k,, = 9.1 x lo-” cm3/sec and kzO= 9.3 X lo-l2 cm’lsec. A constant value for k,, or k,,
in the temperature range concerned in this study is supported by the experiments. Hushfar et al. (1971, 1972) measured the quantum yield of total NO(v > 2) for the reaction N+O,-+NO+O,
(21)
20
NO 5.3 pm
FIG. 2. EXCITATIONRATESTO
NO (u = 1).
A ~bratio~aliy excited state of a molecule is marked by a double dagger. Solid curves represent the noon profiles, and dashed curves the midni~t ones. The emission rates from CO,(O30) at 5.2 pm are also shown by dot-dashed
curves. and obtained a value of 0.018. The rate coefficient for this reaction is k,, = 1.1 X lo-l4 Te-3150’T,which is taken from the evaluation by BauIch et al. (1973), and gives a value of 3 X lo-l9 at 300 K. The excitation of NO(u = 1) through the reaction (21) can be neglected, in as far as the quantum yield of NO(u = 1) is not much larger than unity. Resonant absorption of the solar and terrestrial radiation is also responsible for the excitation of NO(u = 1): NO+ hv(5.3 pm) ---, NO(u = 1).
(22)
Using the band strength measul;t?d by Abel anfi Shaw (1966), the absorption cross section of NO for the 5.3 pm band is evaluated to be 1.0~ lo-l5 cm2 at 273 K, the fluorescence coefficient to be 1 .OX 10m4 and 1.4 x 10m4 photons/set per NO molecule for the solar and terrestrial radiation, respectively. Since the optical thickness for the 5.3 pm radiation does not exceed unity above the height of 50 km, the excitation rate can be rigorously evamated by the use of single scattering approximation. The height variations of the excitation rate of NO(u = 1) arising from various processes are calculated based on the model of nitrogen and oxygen compounds by Ogawa and Shimazaki (1975), and they are depicted in Fig. 2. In the stratosphere and mesosphere, the principai excitation is caused by the thermal process (17). The chemiluminescent reaction (19) would be of some importance. However, this process does not occur at night, because atomic oxygen is almost completely removed. In the thermosphere the resonant absorption (22) results in the dominant excitation, and processes (18)
TOSHIHIROOGAWA
754
and (20) would also be important above 100 km. These three processes belong to the non-thermal excitation, and the total emission rate from these processes amounts to 1 x 10” photonslcm’sec or 0.1 Mega-Rayleigh (MR) above 80 km. It must be noted that process,(22) due to solar radiation and process (20) are effective only during the sunlit hours. Stair et al. (1975) measured the radiance of the 5.3 and 2.8 pm bands in the thermosphere during aurora1 night, and found the apparent emission rates for these bands to be 1 MR at the heights above 80 km in an IBC II aurora. They also found a remarkable peak at the height around 84 km in an IBC III aurora, with the peak emission rate of 8 MR. These enhancements under aurora1 conditions may be a consequence of the increase of NO and N(*D) densities, which is caused by the additional production of odd nitrogen due to precipitating electrons associated with the aurora. The precipitating electrons give rise to the dissociation of NZ, and produce N(2D) in the lower the~osphere. This results in an increase of the N(2D) density, together with an enhancement of the rate for process (20), and then increases of the NO density. A calculation (Kondo and Ogawa, 1976) shows that for an IBC II aurora the NO density becomes 3 times larger than that in the mid-latitude thermosphere, and that the production rate of N(*D) becomes 100 times as large as that produced in the sunlit atmosphere. Therefore, the 5.3 pm radiation of about 1 MR measured in an IBC II aurora by Stair et a& (1975) is reasonably ascribed to the emission from the (1,O) band of NO. Similarly, the 2.8 Frn radiance observed under aurora1 conditions can be interpleted by the (2,0) band of NO, unless the quantum yield of NO(u =2) in process (20), which is the only important excitation mechanism for the NO(u = 2) excitation, would be much less than 0.1. In an actual observation of the 5.3 pm radiation from NO, the 5.2 pm emission from the 3~ mode of CO, may contaminate the 5.3 pm emission. It is interesting to evaluate the emission rate from the 3~ mode of CO*. The resonance scattering CO,+ kv(5.2 r*m) * CO,(O30)
(23)
and the forward reaction of the TV exchange CO,+M~C0,(030)+M-1932cm-’
(24)
may be responsible for the excitation of CO,(O30). By the use of the band strength by McClatchey et al. (1973), the absorption cross section of CO2 at
the 5.2 pm band is calculated to be 1.1 X lo-l9 cm* at 273 K, and the transition probability for the spontaneous emission from c02(030) to be Aa3= 1.2 x lo-’ set-“. Since the optical thickness for the 5.2 pm radiation of CO, is less than unity above the height of 45 km, the emission rate from the resonant scattering can be evaluated from single scattering approximation in almost all the height range concerned, adopting the scattering coefficient of 1 x 10m8 photonslsec per CO* molecule for the solar or terrestrial radiation. Alternatively, if the population of C02(030) is determined by an equilibrium between the TV exchange (24), the excitation rate to the thermal emission originating from process (24) is given by AZ3[C02(030)]= A&27s0iT [CO,]. The dash-dot curves in Fig. 2 represent the calculated emission rates for the 5.2 pm from CO,(O30). The 5.2 pm band of CO1 is found to affect the 5.3 pm band of NO. 5.
NITROGEN
Laboratory experiments chemical reactions
DIOXIDE
suggested
NO+G3+N02+0z,
that
the (25)
and O+NO+(M)+
NO,+(M),
(26)
which are the main sources of NO:! in the upper atmosphere, produced the luminescence in a broad wavelength region centered at 3.6 pm (Stair and Kennealy, 1967; Clough and Thrush, 1969; Golde et al., 1973; Golde and Kaufman, 1974). The luminescence is likely to originate from the it1+ us and other modes of NO*, although the quantum yield of the 3.6 km luminescence in (25) and (26) has not been measured. The reactions (25) and (26) could also produce NO,(OOl) which emits in the 6.2 pm band, but the quantum yield for this band has not been measured. The excitation rate is evaluated by assuming unit quantum yield, and by the use of the model distribution of odd nitrogen by Ogawa and Shimazaki (1975). Reaction (26) is the principal process for the NO, production in the thermosphere, while reaction (25) dominates over reaction (26) in the stratosphere. Reaction (25) occurs only during daytime, because NO disappears at night in the stratosphere and mesosphere. The resonant absorption of the solar and terrestrial radiation will also be responsible for the excitation of the 3.6 and 6.2 pm bands of NO,. The major part of this excitation will occur in the stratosphere, since NO, occurs mainly in this height region. On the basis of the band strength measured
Excitation processes of infrared atmospheric emissions by Guttman (1962), the fluorescence coefficient of the 6.2 pm band from the vg mode can be calculated to be 1.5 x 10m3 and 6.8 x 10m3 photonslsec per NO, molecule’ for the solar and terrestrial radiation, respectively, and the 3.6 pm band from the v, + v3 mode to be 6.9 x lo-’ and 6.2X 10m7 photons/set per NO, molecule for the solar and terrestrial radiation, respectively. Since the absorption cross section for the 6.3 pm band is calculated to be 2.55~lo-l4 cm2 at 300 K from the band strength measured, the optical depth is estimated to be less than unity above the height of about 50 km. The optical depth for the 3.6 pm band is about 2 orders of magnitude smaller than that of the 6.3 pm band. At around the wavelength of 6.2 pm band of NO* there must occur an intense emission from H,O(OlO) due to thermal collisions in the stratosphere and mesosphere. The 6.3 pm band from the v2 mode of Hz0 would conceal the 6.2 Frn emission from NOz. Again the thermal collision could also be important for the excitation of both v3 and v,+ vj modes of NO* in the stratosphere and mesosphere. The last process that is interesting from the observational point of view is reaction (26). The total emission rate amounts to 1 x lo9 photons/cm*sec at the heights around 80 km for both the 6.3 and 3.6 pm bands, and an emission rate of 1 x 10” photonslcm’sec are probable in the thermosphere under intense aurora1 conditions. As a result, it might be said that dbservation of the NO2 emissions in the upper atmosphere is very difficult, because of the weak emission intensity, or because of hindrance by the H,O and other thermal emissions.
6. CONCLUDING
RFMARRS
The prominent infrared emissions in the upper atmosphere belong to the rotation-vibration bands of minor constituents such as CO2 at 15 and 4.3 Frn, HN03 at 11.3 pm, O3 at 9.6 pm, Hz0 at 6.3 pm, CH, at 7.7 pm and NO at 5.3 Fm. These emissions are excited mainly through thermal collision processes in the stratosphere and mesosphere, whereas in the thermosphere they may be excited mainly through non-thermal processes such as chemiluminescent reactions, resonant absorption of the solar and terrestrial radiation, and excitation transfer. The non-thermal excitation rate would be enhanced in the aurora1 thermosphere due to the action of precipitating electrons. This results in the aurora1 enhancement of the 5.3 and 2.8 pm bands
755
of NO, and of the 4.3 pm band of CO2 observed by Stair etal. (1974,1975). Since the upper atmosphere is optically thick for almost all of the infrared bands, detailed comparison of the theory with observations must await a solution for the radiative transfer equation specific to each emission band. REFERENCES
Abels, L. L. and Shaw, J. H. (1966). Widths and strengths of vibration-rotation lines in the fundamental band of nitric oxide. J. molec. Spectrosc. 20, 11-28. Baker. D. J.. Wvatt. C. L.. Pendleton. W. R. and Ulwick, J. C!. (1974). high Altitude Effects ‘Simulation (HAES) Program, Rep. No. 1. Rocket launch of a SWIR spectrometer into an aurora (ICECAP 72). Environmental Research Papers No. 466, Air Force Cambridge Res. Labs. AFCRL-TR-74-0077. Basco, N., Callear, A. B. and Norrish, R. G. W. (1961). Fluorescence and vibrational relaxation of nitric oxide studied bv kinetic sodctroscoov. Proc. R. Sot. (A) 260., 459-474: a *Bauer, E., Kummler, R. and Bortner, M. H. (1971). Internal energy balance and energy transfer in the lower thermosphere. Appl. Opt. 10,1861-1869. Baulch, D. L., Drysdale, D. D., Horne, D. G. and Lloyd, A. C. (1973). Evaluated Kinetic Data for High Temperature Reactions, Vol. 2. Homogenkous Gas P&e Reactions of the H,-N,-0, Svstem. CRC Press, Cleveland, OH. Bevan, P. L. T. and Johnson, G. R. A. (1973). Kinetics of ozone formation in the pulse radiolvsis of oxygen gas. Faraday
Trans. them. Sbc. 69, 21&227.
_-
-
Bunn. F. E. and Gush. H. P. (1970). Snectrum of the airhow between 4 and 8 mi&ons.’ Can. J. Phys. 48, 98-100. Clough, P. N. and Thrush, B. A. (1969). Vibrational emission by NO2 in reaction of nitric oxide with ozone. Trans. Faraday Sot. 65, 23-31.
Cool, T. A. and Airey, J. R. (1973). Vibrational deactivation of CO,(OO”l) molecules by ONF, COFz and 03. Chem. Phys. L&t. 20, 67-70. Dalgarno, A. (1963). Vibrationally excited molecules in atmospheric reactions. Planet. Space Sci. 10,19-28. Dalgarno, A. (1969). Infrared day and night airglow of the earth’s upper atmosphere. Phil. Trans. 264, 153160. Davis, D. D., Herron, J. T. and Huie, R. E. (1973). Absolute rate constants for the reaction 0(3P) +NO, + NO+O, over the temperature range 230-339 K. 1 &em. Phys. 58, 530-535. Deeees. T. C. (1971). Vibrationallv excited nitric oxide in &s upper atmosphere. Appl. O$. 10,1856-1860. Golde, M. F., Roche, A. E. and Kaufman, F. (1973). Absolute rate constant for the O+NO chemiluminescence in the near infrared. J. them. Phys. 59, 39533959.
Golde, M. F.. and Kaufman, F. (1974). Vibrational emission of NO2 from the reaction of NO with 03. Chem Phys. Lett. 29, 480-485.
Guttman, A. (1962). Absolute infrared intensity measurements on nitrogen dioxide and dinitrogen tetroxide. J. quant. Spectrosc. rad. Transfer 2, l-15.
Herron, J. T. and Klein, F. S. (1964). Mass spectrometric
756
TOSHIHIROOGAWA
study of the isotopic exchange rate of oxygen atoms with 02, NO, and NOz, J. &em. Phys. 40, 2731. Hui, K. K., Rosen, D. I. and Cool, T. A. (1975). Intermode energy transfer in vibrationally excited 03. Chem. Phys. I&t. 32, 141-143. Huie, R. E., Herron, J. T. and Davis, D. D. (1972). Absolute rate constants for the reaction 0 + O2 + M + O,+M over the temperature range 200-346 K. J. phys. Chem 76, 2653-2658. H;p$i, E. R., Rogers, J. W. and Stair, A. T. (1974). Aircraft observations of the infrared emission of the atmosphere in the 700-28OOcn-’ region. Appl. Opt. 13, 1466-1476. Husain, D., Kirsch, L. J. and Wiesenfeld, J. R. (1972). Collisional quenching of electronically excited nitrogen atoms, N(2’D,, 2’P,) by time-resolved atomic absorption spectroscopy. Faraday Disc. them. Sot. 53, 201210. Hushfar, F., Rogers, J. W. and Stair, A. T. (1971). Infrared chemiluminescence of the reaction N+O, + NO+O. Appl. Opt. 10, 1843-1847. Hushfar, F., Rogers, J. W. and Stair, A. T. (1972). Further. comments on the chemiluminescent reaction N+O, + NO+0 and its overtone photon efficiency. Appl. Opt. 11, 1656-1657. James, T. C. and Kumer, J. B. (1973). Fluorescence of CO, near 4.3 microns: Application to daytime limb radi&e calculations. J. ge&hys. Res. 78, 8320-8329. Kondo. Y. and Oaawa, T. (1976). Odd nitrogen in the lower thermosphere under aurora1 perturbations. J. Geomagn. Geoelectr. 28 In press. Kumer, J. B. and James, T. C. (1974). C02(O01) and N; vibrational temperatures in the 50~ z S 130 km altitude range. J. geophys. Res. 79, 638-648. Kummler, R. H. and Bortner, M. H. (1972). Vibrational temperatures in the E and F regions. Space Res. 12, 711-719. Kurylo, M. J., Braun, W. and Kaldor, A. (1974). A laser enhanced reaction technique for the measurement of V+ T deactivation rates: Deactivation of vibrationallv excited OS. Chem. Plays. Lett. 27, 249-253. Kurvlo. M. J.. Braun. W.. Xuan. C. N. and Kaldor. A. (i975). Infrared la&r enhanced’reactions: Temperature resolution of the chemical dynamics of the 03++N0 reaction system. J. them. Phys. 62, 2065-2071. MacDonald, R., Buijs, H. L. and Gush, H. P. (1968). Spectrum of the night airglow between 3 and 4 microns. Can. J. Phys. 46, 2575-2578. McCaa, D. J. and Shaw, J. H. (1968). The infrared spectrum of ozone. J. molec. Spectrosc. 25, 374-397. McClatchey, R. A., Benedict, W. S., Clough, S. A., Burch, D. E., Calfee, R. F., Fox, K., Rothman, L. S. and Garing, J. S. (1973). AFCRL atmospheric absorption line parameters compilation. Environ. Res. Papers No. 434, Air Force Cambridge Res. Labs. AFCRL-TR73-0096.
Murcray, D. G., Goldman, A., Csoeke-Poeckh, A., Murtray, F. H., Williams, W. J. and Stocker, R. N. (1973). Nitric acid distribution in the stratosphere. J. geophys. Res. 78, 7033-7038. Murphy, R. E., Lee, E. T. P. and Hart, A. M. (1975). Quenching of vibrationally excited nitric oxide by molecular oxygen and nitrogen. J. them. Phys. 63, 2919-2925. Ogawa, T. and Shimazaki, T. (1975). Diurnal variations of odd nitrogen and ionic densities in the mesosphere and lower thermosphere: Simultaneous solution of photochemical-diffusive equations. J. geophys. Res. 80, 3945-3960. Rosen, D. I. and Cool, T. A. (1973). Vibrational deactivation of O,(lOl) molecules in gas mixtures. J. them Phys. 59, 6097-6103. Rosen, D. I. and Cool, T. A. (1975). Vibrational deactivation of O3 molecules in gas mixtures. II. J. them. Phys. 62, 466-476. Schunk, R. W. and Hays, P. B. (1971). Theoretical N2 vibrational distribution in an aurora. Planet. Space Sci. 19, 1457-1461. Stair, A. T. and Kennealy, J. P. (1967). Infrared chemiluminescence and vibraluminescence in the NOO-NO2 reaction system. J. chim. Phys. 64, 124-128. Stair, A. T., Ulwick, J. C., Baker, D. J., Wyatt, C. L. and Baker, K. D. (1974). Altitude profiles of infrared radiance of 03(9.6 pm) and COZ(15 pm). Geophys. Res. I&t. 1, 117-118. Stair, A. T., Ulwick, J. C., Baker, K. D. and Baker, D. J. (1975). Rocketborne observations of atmospheric infrared emissions in the aurora1 region. In Atmospherk ofEarth and the Planets, (Ed. by l&Cormac, B. k. pp. 335-346. Reidel, Dordrecht, Holland. Stephenson, J. C. (1973). Vibrational energy transfer in NO. J. &em. Phvs. 59. 1523-1527. Stephenson, J. C. (1974j. Vibrational relaxation of NO X’Il (v = 1) in the temperature range 100-300 K J. them. Phys. 60,4289-4294. Stolarski, R. S. (1968). Calculation of aurora1 emission rates and heating effects. Planet. Space Sci. 16, 12651276. Taylor, R. L. (1974). Energy transfer processes in the stratosohere. Can. J. Chem. 52. 1436-1451. von Rosdnberg, C. W. and Lowenitein, A. (1973). Vibrational relaxation of COZ(v3) by ozone. J. them. Phys. 59, 2751. von Rosenberg, C. W. and Trainor, D. W. (1974). Vibrational excitation of ozone formed by recombination. J. &em. Phys. 61, 2442-2456. Walker, J. C. G. (1968). Electron and nitrogen vibrational temperature in the E-region of the ionsophere. Planet. Space Sci. 16, 321-327. Walker, J. C. G., Stolarski, R. S. and Nagy, A. F. (1969). The vibrational temperature of molecular nitrogen in the thermosphere. Annls Ghphys. 25, 831-839.
Space Sci., Vol. 24, pp.
EXCITATION
149
to
756.
Pergamon
PROCESSES
Press,
1976.
Printed
in Northern
Ireland
OF INFRARED TOSHIHIRO
ATMOSPHERIC
EMISSIONS
OGAWA
Geophysics Research Laboratory, The University of Tokyo, Tokyo 113, Japan (Received 13 January 1976) Abstract-Excitation rates of the infrared emissions which are likely to occur in the mesosphere and thermosphere are quantitatively evaluated. They include the 9.6 pm band of OS, the 15 and 4.3 pm bands of CO? and the 5.3 and 2.8 pm bands of NO. These emissions may be excited through nonthermal processes such as chemiluminescent reactions and resonant fluorescence in the thermosphere, whereas they are of thermal origin in the stratosphere and mesosphere. Increase of the non-thermal excitation rate caused bi precipitating electrons could be responsible for the enhancement of the 4.3 pm band of COz, and the 5.3 and 2.8 pm bands of NO observed in the aurora1 thermosphere. 1. INTRODUCTION
Atmospheric emission spectra in the infrared of wavelengths longer than 4 pm have been obtained in the stratospheric height range (MacDonald et al., 1968; Bunn and Gush, 1970; Murcray et al., 1973; Huppi et al., 1974) and at heights up to the lower thermosphere (Stair el al., 1974, 1975; Baker ef al., 1974). The major emission bands have been identified as belonging to the vibrational transitions of CO, at 4.3 and 15 pm, H,O at 6.3 pm, CH, at 7.7 pm, O3 at 9.6 pm and HNO, at 11.3 pm. The weak emissions at 5.3 and 2.8 pm were also deiected at mesospheric heights under aurora1 conditions (Stair et al., 1975); these are tentatively identified as originating from NO. Since the emissions from H,O, CH, and HNO, have been detected only in the stratosphere, their excitation would be caused by purely thermal processes. Other emissions measured above the stratosphere such as those from CO, and 0, would also be excited through thermal processes. However, there is a possibility that non-thermal excitation is important at heights above the mesopause. As a matter of fact, the vibrational excitation of Nz by photoelectron impact, as well as the energy transfer from O(‘D), and the mutual combination of N and NO results in the vibrational temperature of N, being higher than the gas kinetic temperature (Walker, 1968; Walker et al., 1969; Bauer et al., 1971; Kummler and Bortner, (Walker, 1968; Walker et al., 1969; Bauer et al., 1971; Kummler and Bortner, 1972), and the subsequent vibrational energy transfer from N, to CO* will produce the non-thermal emission of the 4.3 pm band of CO*. An alternative non-thermal process in the upper atmosphere is the direct vibrational excitation of the products in chemical reactions; some processes
have been proved to occur in the laooratory system. Earlier works on this subject are found in the papers by Dalgarno (1963, 1969). Recent laboratory measurements of rate coefficients and quantum yields for these reactions, together with the model distribution of various atmospheric constituents, enable us to evaluate quantitatively non-thermal excitation rates for the vibrational states of some molecules which lead to the infrared emissions. 2. OZONE The principal infrared emission from 0, occurs at the 9.6 pm band of the vg vibrational mode. The vg mode of O3 is excited through the translationvibration (TV) energy transfer 0,+M~0,(001)+M-1042cm~‘,
(1)
where the O3 not specified for the vibrational state denotes the ground state O,(OOO), and M= N2 or 0,. The excitation to the v2. mode of O3 via the similar TV transfer O,+M+=
O,(OlO)+M-701
cm-’
(2)
occurs more rapidly than process (l), because of its lower excitation energy than process (1). The subsequent vibration-vibration (W) transfer through O,(OlO)+O,+
O,+O,(OOl)-341
cm-’
(3)
leads to the production of the vg mode, although almost all O,(OlO) may be de-excited via the inverse reaction of (2) or via the spontaneous emission O,(OlO) + O,+ hv (14.3 pm),
A, = 0.25 set-‘, (4)
749
750
TOSHIHIRO OGAWA
where A4 is the transition probability calculated on the basis of the band strength measured by McCaa and Shaw (1968). Although no direct measurement of the rate coefficients for the processes (l), (2) and (3) is available, it is possible to evaluate these rate coefficient to be (1-2)x lo-l4 cm3/sec for M=02 or Nz (Kurylb et al., 1974, 1975; Rosen and Cool, vibrational relaxation of O3 indicate the rate coefficient to be (l-2) x lOmE4cm3/sec for M = O2 or N, (Kurylo et al., 1974, 1975; Rosen and Cool, 1973, 1975; von Rosenberg and Trainor, 1974). This value may be applicable for the VT transfer of the manifold of the v, and vg modes, or for the VT transfer of v, preceded by the VV transfer from vt or v, to v, (Taylor, 1974). Recently, Hui et al. (1975) found the VV transfer from v1 or vg to vZ to occur with a rate coefficient larger by a factor of 2 or 3 than that of the VT transfer from iv,, v2 or vg. On the basis of this information, the rate coefficients for the inverse reactions of (l), (2) and (3) are assumed to’ be 2~ 10-14, 2 k lo-l4 and 6 x lo-l4 cm3/sec, respectively. The rate coefficients for (l), (2), and (3) may be estimated by means of detailed balancing; k, = 2 x 10-‘4e-‘Soo’T, kz = k,e -‘“‘“r,
r, =
[co,l [Ml ’ k5 = 1 x lo-l3 cm3/sec is an estimated value based on the measured rate coefficient for the inverse reaction of (5) at 300 K by Rosen and Cool (1975). r,=3x lo-” for [CO,]/[M]=3x lo-” in the height range concerned; process (5) is ne~i~ble in the excitation of O,(OlO), compared with process (2) and also (1). One of the non-thermal excitations may occur through chemical reactions. The reaction (7) which is believed to be the only reaction to form the atmospheric ozone where
0+0,-l-M+O,+M,
k7=l.l~10-34e510’T (7)
k3= 6 x 10-‘4e-4w’T, and kz- =t2 x 1()-‘4,
where k, denotes the rate coefficient for the process (i) in cm3/sec, kiL denotes the rate coefficient for the inverse reaction, and T denotes the temperature in K. The reiative magnitude for the rate of process (1) to process (2) followed by (3) is rz =
the relative population of C02(010) to total COZ is given as [CO,(OlO)j/[CO,] = e-960’T according to a Boltzmann distribution. Consequently, the relative excitation rate for process (5) to (2) is represented as
k,[OJMl x
m 2.4
1 U031 b-CM!+ A,’ k,~CMCiW k&3 II031 Z---X k, h-EM] + -%
for [M] >>A,/k,x
lo-‘“[O,]
= 2.5
x
can yield the V~mode of OS, where the rate coef?icient was measured by Huie et al. (1972). The quantum yield for the vibrationally excited O3 was also measured to be about unity, and invariant for M =N, or O2 (Bevan and Johnson, 1973; von Rosenberg and Trainor, 1974). The height profiles for the excitation rate of the O,(OOl) through the chemiluminescent process (7), together with those through the thermal process (1) are calculated with the aid of the 0, model distribution by Ogawa and Shimazaki (1975), and depicted in Fig. 1. This
lOI cmY3,
L
for [Ml<< 2.5 x 1Ol3 cmm3,
100
/
o+o,~O~cooH
where [ ] denotes the number density of molecules designated in the bracket. Since r, does not exceed 1 x lo-“, process (2) can be neglected in the excitation to the vj mode. The VV transfer from the v, mode of CO* COz(OIO)+O,+
CO,+O,(OlO)-34
cm-’
(5)
may also lead to the v2 mode of 03. Since it is a reasonable assumption that the population of CO,(OlO) is determined by the VT exchange process COZ + M * COZ(OIO) + M -667 cm-‘,
(6)
40 103
10'0 RATE. ~rn-~sec-'
FIG. 1. EXCITATION RATES TO 03(001) DUE TO THE TV TRANSFER
AND CHEMILUMINESCE~
PROCESSES.
Profiles ai noon are indicated by solid curves, and those at midnight by dashed curves.
7.51
Excitation processes of infrared atmospheric emissions figure indicates that the excitation rate due to the chemilumines~ent reaction (7) dominates over that due to the thermal process (1) above the heights of 75 - 80 km. Resonant absorption of the terrestrial radiation from the underlying atmosphere may be another candidate for the non-thermal excitation of the v3 mode: O,+hv(9.6
~m)-*03(OOl),
(8)
the fluorescence coefficient of which can be evaluated to be 1.0~ lo-’ photonslsec per 0, molecule for the terrestrial radiation with the aid of the band strength measured by McCaa and Shaw (1968). Estimation of the excitation rate due to this process depends on the solution of the radiative transfer equation for the 9.6 pm band, because the atmosphere is optically thick for the 9.6 pm radiation. According to a preliminary result from solving the 9.6 pm radiative transfer equation by Tohmatsu (1975, private ~mmunication), the terrestrial radiation originating from the underlying atmosphere is more important than the solar radiation as a radiative source. Again process (8) becomes important in the heights above -70 km, but the excitation rate for process (8) is not larger than that for process (7) in an optimum condition. Comparison of the present calculation with the observed emission intensity by Stair et al. (1974) is beyond the scope of this study, because it needs radiative transfer analysis. However, the hump found at the height of 80 km in the height-radiance profiles measured by them would be ascribed to the predominance of the chemiluminescent reaction (7). The VV energy transfer CO,(OOl)+O,
+ CO~(lOO)+O~(OOl)-81 kg = 1 x lo-”
cm-‘,
cm3/sec,
(9)
where the rate coefficient was measured at 300 K by Cool and Airey (1973) and von Rosenberg and Lowenstein (1973), could be of importance in the O,(OOl) excitation under the aurora1 condition. Collisions by secondary electrons produced by precipitating electrons would cause an enhanced population of the vibrationally excited state N,(v = 1) (e.g. Walker et al., 1969; Schunk and Hays, 1971), and the subsequent VV energy transfer to CO,(OOl) could produce an appreciable amount of co,(oo1): N2(u = l)+CO,
it N,+CO,(OOl)-
18 cm-‘, (10)
klo = 6 x lo-” e1J.27fT-‘e for Tc600K (Taytor, 1974). The estimation of the excitation rate for process (9) requires knowledge of the population of the v3 mode of CO*. Since the VV exchange (10) occurs very rapidly, it will control the population of CO,(OOl). However, the spontaneous emission where
C02(O01) -+ CO,+ hv(4.3 pm),
A,, = 400 see-‘, (11)
where the transition probability has been calculated from the band strength by McClatchey et al. (1973), may also affect the population of C02(O01) in the lower thermosphere. Again, since the atmosphere at mesospheric heights is optically thick for the 4.3 pm radiation, the effect of radiation imprisonment occurs, and a precise estimation for the population of the vj mode depends on solving the radiative transfer equation (e.g. James and Kumer, 1973; Kumer and James, 1974), together with the calculation of the primary excitation rate for N,(u = 1) (e.g. Stolarski, 1968). It must be noted that an enhancement of the 4.3 Ic.rn radiation from CO,(OOl) actually occurs in the aurora1 atmosphere (Stair et ai., 1975). The fact that the Iarger 9.6 Frn radiance observed by Stair et al. (1974) was in the aurora1 condition rather than that in the non-aurora1 condition is suggestive of the importance of process (9) in cases of aurora1 disturbances. 3. CARBON DIOXIDE
Prominent features of the infrared COz emission are the v, and vg bands at 15 and 4.3 &m, respectively. The v2 mode is excited mainly by the TV energy transfer, the forward reaction of (6). The rate coefficient for the backward reaction of (6) has been reviewed by Taylor (1974), who gave it to be k,- = 6.69 x lo-” e-s4~07T~‘~ for 111= Nz or 02_ The rate coefficient for the forward reaction of (6) can be evaluated with the aid of the realation kg= kCe-960iT. The VV transfer from O,(OlO), the inverse reaction of (5) can contribute to the excitation of CO,(OlO) at a rate about 3 orders of magnitude smaller than the forward reaction of (6), since almost all O,(OlO) is de-excited through the process (4) or the inverse reaction of (2). The forward reaction of (6) is likely to be the only important collisional process in the excitation of the vz mode of co,.
752
TOSHIHIRO OGAWA
An alternative candidate for the excitation of CO,(OlO) is the resonant absorption of the terrestrial radiation originating from the underlying atmosphere. A quantitative estimation of the excitation rate for this process must wait solving the transfer equation for the 15 pm radiation. Another prominent emission of CO2 at 4.3 pm originating from the v, mode is excited by the backward reaction of the VV exchange (10) as stated in the previous section, the TV transfer CO,+M+CO,(OOl)+M-2349cm-r,
(12)
and the resonant absorption of the solar 4.3 ir_m radiation (i.e. the inverse of reaction 11). The resonant fluorescence induced by the solar 2.7 or 2.8 pm radiation may also emit the 4.3 pm (James and Kumer, 1973; Kumer and James, 1974): CO,+ hv(2.8 pm) + CO,(O21) -+ CO,(O20) + hv(4.3 ym),
+ CO&OO) + hv(4.3 em).
(15)
and the VV energy transfer NOi-N2(u=1)-+NO(u=1)+N,+455cm-‘, (161 and NO+0,(u=1)+NO(u=1)+0,-320cm-1.
(14)
At heights below the mesopause the population of N,(v = 1) and CO,(OOl) may be determined by thermal collisions via the TV transfer, whereas a higher vibrational population of N, than that expected for the thermal equilibrium with the gas kinetic temperature can occur in the thermosphere due to the vibrational excitation of Nz by photoelectron impact, the mutual combination of N and NO, and the energy transfer from the electronically excited O(lD) to the vibrational energy of Nz (Walker, 1968; Walker et al., 1969; Bauer et al., 1971; Kummler and Bortner, 1972). The secondary electrons produced by precipitating electrons in the aurora1 atmosphere will also cause the vibrational excitation of N1. The peak at around 93 km in the height profile of the 4.3 pm radiance measured by Stair et al. (1975) in the aurora1 condition may be ascribed to the enhancement of the 4.3 pm emission due to this excitation process, together with the radiation i&prisonment. Solving the radiative transfer equation for the 4.3 pm is a future problem in urder to make a quantitative comparison of this theory with observations. Again, during the daylight hours and in the low and middle latitudes an enhancement of the 4.3 pm emission similar to that of the aurora1 atmosphere is expected. 4.NITRICOXIDE
fundamental (Au = 1) transitions in the vibrational levels of the ground electronic state of NO The
NO+~~NO(~=l)+~-1876cm-1,
(13)
and CO,+ hv(2.7 pm) -+ CO,(lOl)
emit the radiation at around 5.3 pm, and the first overtone (ho = 2) at around 2.8 +m. Among them, (1,O) and (2,0) transitions may be most intense in the 5.4 and 2.8 pm bands. Various mechanisms for the vibrational excitation of NO in the u = 1 level have been discussed by Dalgarno (1963) and Degges (1971). They include chemical reactions, and resonant absorption of the solar and terrestrial radiation, as well as thermal collision processes. The followings are potential thermal excitation processes: the TV energy transfer
(17) The rate coefhcients for (1.5) through (17) can be evaluated with the aid of those for the inverse reactions of (15) through (17) determined experimentally from the quenching rate of NO(u = 1). The rate coefficient for the NO(u = 1) quenching was measured at 300 K to be 1 x lo-l6 cm3/sec for NZ, 2 x lo-l4 cm3/sec for Oz, 3 x 10-r’ cm3/sec for COz, and 2 x lo-” cm3/sec for H,O (Basco et ab, 1961; Stephenson, 1973, 1974; Murphy et al., 1975). These values may be representative of the rate coefficient for the VV exchange rather than that for the TV exchange process (15). Though some rate coefficient were changed later, Taylor (1974) evaluated a small rate coefficient for the inverse reaction of (15) with M = 02; k,,- = 4,QX 10-loe-lomT-~~~ for M= Oz or N,. He also listed such a rate coefficient for the inverse reaction of (15) with M= COz or Hz0 as in agreement with the measured rate coefficient for the NO(v = 1) quenching, i.e. klS- = 1.32 x ~0-8e-‘00.0T-“” for M = COz, and k,,- = 7.47 x 10-‘“e-40~‘9r-“3 for M = H20. Since it is reasonably assumed in the heights below the mesopause that the vibrational population of NZ or O7 is determined by a Boltzman distribution in equilibrium with the gas kinetic temperature, the excitation rates of NO(o = 1) due to the processes (15) through (17) are written as follows: q15= k&NO][M]
= k~5e-2700’T[NO][M]
q16= klb[NO][N2(u = l)] = k16[NO]e-3350’T[N,J = k,e- 2700’T[NO][N,],
Excitation processes of infrared atmospheric emissions
753
and q,7 = k,,[NO][Oz(u
= l)] = k,,[NO] e--22401r[02]
= k,7-e- 2700’T[N0][0,], where k,* = k16-e+650’T and k,, = k,,-e‘460iT, with k16- = 1 x lo-l6 cm3/secand k17- = 2 x 10-‘4cm3/sec, An alternative expression for kt6 is k16= 4.42 x 10Vio e--86~3sT-“3 given by Taylor (1974). By the use of above values for the rate coefficient, the relative magnitude of q15 to qr, is evaluated to be T,~= q,Jq,, = 3 x 10e2, 7~ IO-‘, 3 x 10m4 and 2 x 10m3at 300 K for M = N,, 02, CO* and HzO, respectively, where [co,]/[o,J = 1.6 x 1o-3 and [HzO]/[0,]=2 x 10M5 are assumed. rr5 does not exceed unity even at 600K. Also we can evaluate T16= q*dql, = 2 x lo-* at 300 K; rr6 becomes unity at 600 K. As a result, it may be concluded that process (17) is the most important thermal excitation of NO(u = 1). Dalgarno (1963) has suggested that the atom interchange reaction O’+NO-+NO’+O
(18)
leads to the vibrational excitation of NO. Using the isotopic ‘*O as a tracer, the rate coefficient for (18) was measured to be k18= 1.8~ lo-r2 cm”/sec at 300 K by Herron and Klein (1964). Degges (1971) estimated the activation energy for (18) to be between 0 and 1000 Cal/mole. Assuming a unit quantum yield and zero activation energy for (IQ, we obtain the rate coefficient far the excitation of NO(u = 1) to be k&v = 1) = 1.8 X 10-‘Ze-*900’T, where the exponential term arises from the excitation energy of NO(u = 1). Chemicaf reactions which are major sources of NO in the mesosphere and lower thermosphere such as NO,+O+NO+O, (19) and N(2D)+0,-+NO+0
(20)
are exothermic enough to excite NO vibrationally. In evaluating the excitation rates due to these reactions, the rate coefficient is taken from the experiments by Davis et al. (1973) and Husain et al. (1972): k,, = 9.1 x lo-” cm3/sec and kzO= 9.3 X lo-l2 cm’lsec. A constant value for k,, or k,,
in the temperature range concerned in this study is supported by the experiments. Hushfar et al. (1971, 1972) measured the quantum yield of total NO(v > 2) for the reaction N+O,-+NO+O,
(21)
20
NO 5.3 pm
FIG. 2. EXCITATIONRATESTO
NO (u = 1).
A ~bratio~aliy excited state of a molecule is marked by a double dagger. Solid curves represent the noon profiles, and dashed curves the midni~t ones. The emission rates from CO,(O30) at 5.2 pm are also shown by dot-dashed
curves. and obtained a value of 0.018. The rate coefficient for this reaction is k,, = 1.1 X lo-l4 Te-3150’T,which is taken from the evaluation by BauIch et al. (1973), and gives a value of 3 X lo-l9 at 300 K. The excitation of NO(u = 1) through the reaction (21) can be neglected, in as far as the quantum yield of NO(u = 1) is not much larger than unity. Resonant absorption of the solar and terrestrial radiation is also responsible for the excitation of NO(u = 1): NO+ hv(5.3 pm) ---, NO(u = 1).
(22)
Using the band strength measul;t?d by Abel anfi Shaw (1966), the absorption cross section of NO for the 5.3 pm band is evaluated to be 1.0~ lo-l5 cm2 at 273 K, the fluorescence coefficient to be 1 .OX 10m4 and 1.4 x 10m4 photons/set per NO molecule for the solar and terrestrial radiation, respectively. Since the optical thickness for the 5.3 pm radiation does not exceed unity above the height of 50 km, the excitation rate can be rigorously evamated by the use of single scattering approximation. The height variations of the excitation rate of NO(u = 1) arising from various processes are calculated based on the model of nitrogen and oxygen compounds by Ogawa and Shimazaki (1975), and they are depicted in Fig. 2. In the stratosphere and mesosphere, the principai excitation is caused by the thermal process (17). The chemiluminescent reaction (19) would be of some importance. However, this process does not occur at night, because atomic oxygen is almost completely removed. In the thermosphere the resonant absorption (22) results in the dominant excitation, and processes (18)
TOSHIHIROOGAWA
754
and (20) would also be important above 100 km. These three processes belong to the non-thermal excitation, and the total emission rate from these processes amounts to 1 x 10” photonslcm’sec or 0.1 Mega-Rayleigh (MR) above 80 km. It must be noted that process,(22) due to solar radiation and process (20) are effective only during the sunlit hours. Stair et al. (1975) measured the radiance of the 5.3 and 2.8 pm bands in the thermosphere during aurora1 night, and found the apparent emission rates for these bands to be 1 MR at the heights above 80 km in an IBC II aurora. They also found a remarkable peak at the height around 84 km in an IBC III aurora, with the peak emission rate of 8 MR. These enhancements under aurora1 conditions may be a consequence of the increase of NO and N(*D) densities, which is caused by the additional production of odd nitrogen due to precipitating electrons associated with the aurora. The precipitating electrons give rise to the dissociation of NZ, and produce N(2D) in the lower the~osphere. This results in an increase of the N(2D) density, together with an enhancement of the rate for process (20), and then increases of the NO density. A calculation (Kondo and Ogawa, 1976) shows that for an IBC II aurora the NO density becomes 3 times larger than that in the mid-latitude thermosphere, and that the production rate of N(*D) becomes 100 times as large as that produced in the sunlit atmosphere. Therefore, the 5.3 pm radiation of about 1 MR measured in an IBC II aurora by Stair et a& (1975) is reasonably ascribed to the emission from the (1,O) band of NO. Similarly, the 2.8 Frn radiance observed under aurora1 conditions can be interpleted by the (2,0) band of NO, unless the quantum yield of NO(u =2) in process (20), which is the only important excitation mechanism for the NO(u = 2) excitation, would be much less than 0.1. In an actual observation of the 5.3 pm radiation from NO, the 5.2 pm emission from the 3~ mode of CO, may contaminate the 5.3 pm emission. It is interesting to evaluate the emission rate from the 3~ mode of CO*. The resonance scattering CO,+ kv(5.2 r*m) * CO,(O30)
(23)
and the forward reaction of the TV exchange CO,+M~C0,(030)+M-1932cm-’
(24)
may be responsible for the excitation of CO,(O30). By the use of the band strength by McClatchey et al. (1973), the absorption cross section of CO2 at
the 5.2 pm band is calculated to be 1.1 X lo-l9 cm* at 273 K, and the transition probability for the spontaneous emission from c02(030) to be Aa3= 1.2 x lo-’ set-“. Since the optical thickness for the 5.2 pm radiation of CO, is less than unity above the height of 45 km, the emission rate from the resonant scattering can be evaluated from single scattering approximation in almost all the height range concerned, adopting the scattering coefficient of 1 x 10m8 photonslsec per CO* molecule for the solar or terrestrial radiation. Alternatively, if the population of C02(030) is determined by an equilibrium between the TV exchange (24), the excitation rate to the thermal emission originating from process (24) is given by AZ3[C02(030)]= A&27s0iT [CO,]. The dash-dot curves in Fig. 2 represent the calculated emission rates for the 5.2 pm from CO,(O30). The 5.2 pm band of CO1 is found to affect the 5.3 pm band of NO. 5.
NITROGEN
Laboratory experiments chemical reactions
DIOXIDE
suggested
NO+G3+N02+0z,
that
the (25)
and O+NO+(M)+
NO,+(M),
(26)
which are the main sources of NO:! in the upper atmosphere, produced the luminescence in a broad wavelength region centered at 3.6 pm (Stair and Kennealy, 1967; Clough and Thrush, 1969; Golde et al., 1973; Golde and Kaufman, 1974). The luminescence is likely to originate from the it1+ us and other modes of NO*, although the quantum yield of the 3.6 km luminescence in (25) and (26) has not been measured. The reactions (25) and (26) could also produce NO,(OOl) which emits in the 6.2 pm band, but the quantum yield for this band has not been measured. The excitation rate is evaluated by assuming unit quantum yield, and by the use of the model distribution of odd nitrogen by Ogawa and Shimazaki (1975). Reaction (26) is the principal process for the NO, production in the thermosphere, while reaction (25) dominates over reaction (26) in the stratosphere. Reaction (25) occurs only during daytime, because NO disappears at night in the stratosphere and mesosphere. The resonant absorption of the solar and terrestrial radiation will also be responsible for the excitation of the 3.6 and 6.2 pm bands of NO,. The major part of this excitation will occur in the stratosphere, since NO, occurs mainly in this height region. On the basis of the band strength measured
Excitation processes of infrared atmospheric emissions by Guttman (1962), the fluorescence coefficient of the 6.2 pm band from the vg mode can be calculated to be 1.5 x 10m3 and 6.8 x 10m3 photonslsec per NO, molecule’ for the solar and terrestrial radiation, respectively, and the 3.6 pm band from the v, + v3 mode to be 6.9 x lo-’ and 6.2X 10m7 photons/set per NO, molecule for the solar and terrestrial radiation, respectively. Since the absorption cross section for the 6.3 pm band is calculated to be 2.55~lo-l4 cm2 at 300 K from the band strength measured, the optical depth is estimated to be less than unity above the height of about 50 km. The optical depth for the 3.6 pm band is about 2 orders of magnitude smaller than that of the 6.3 pm band. At around the wavelength of 6.2 pm band of NO* there must occur an intense emission from H,O(OlO) due to thermal collisions in the stratosphere and mesosphere. The 6.3 pm band from the v2 mode of Hz0 would conceal the 6.2 Frn emission from NOz. Again the thermal collision could also be important for the excitation of both v3 and v,+ vj modes of NO* in the stratosphere and mesosphere. The last process that is interesting from the observational point of view is reaction (26). The total emission rate amounts to 1 x lo9 photons/cm*sec at the heights around 80 km for both the 6.3 and 3.6 pm bands, and an emission rate of 1 x 10” photonslcm’sec are probable in the thermosphere under intense aurora1 conditions. As a result, it might be said that dbservation of the NO2 emissions in the upper atmosphere is very difficult, because of the weak emission intensity, or because of hindrance by the H,O and other thermal emissions.
6. CONCLUDING
RFMARRS
The prominent infrared emissions in the upper atmosphere belong to the rotation-vibration bands of minor constituents such as CO2 at 15 and 4.3 Frn, HN03 at 11.3 pm, O3 at 9.6 pm, Hz0 at 6.3 pm, CH, at 7.7 pm and NO at 5.3 Fm. These emissions are excited mainly through thermal collision processes in the stratosphere and mesosphere, whereas in the thermosphere they may be excited mainly through non-thermal processes such as chemiluminescent reactions, resonant absorption of the solar and terrestrial radiation, and excitation transfer. The non-thermal excitation rate would be enhanced in the aurora1 thermosphere due to the action of precipitating electrons. This results in the aurora1 enhancement of the 5.3 and 2.8 pm bands
755
of NO, and of the 4.3 pm band of CO2 observed by Stair etal. (1974,1975). Since the upper atmosphere is optically thick for almost all of the infrared bands, detailed comparison of the theory with observations must await a solution for the radiative transfer equation specific to each emission band. REFERENCES
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