Introduction
A detailed study of nighttime 4.3 µm emissions was conducted by
aimed at determining the dominant mechanisms of
exciting CO2(ν3), where ν3 is the asymmetric stretch mode
that emits 4.3 µm radiation. The
nighttime measurements of SABER channels 7 (4.3 µm), 8 (2.0 µm), and 9 (1.6 µm) for
geomagnetically quiet conditions were analyzed, where channels 8 and 9 are
sensitive to the OH (ν≤9) overtone radiation from levels ν=8–9
and ν=3–5, respectively. showed a positive
correlation between 4.3 µm and both OH channel radiances at a
tangent height of 85 km. This correlation was associated with the transfer
of energy of the vibrationally excited OH(ν) produced
in the following chemical reaction (hereafter “direct” mechanism):
H+O3→O2+OH(ν≤9)
first to N2(1)
OH(ν≤9)+N2(0)↔OH(ν-1)+N2(1),
and then further to CO2(ν3) vibrations
N2(1)+CO2(0)↔N2(0)+CO2(ν3).
However, showed
that calculations based on the model do not reproduce the
4.3 µm radiances observed by SABER. Although accounting for energy
transfer from OH(ν) did provide a substantial enhancement to
4.3 µm emission, a 40 % difference between simulated and observed
radiance remained (for the SABER scan 22, orbit 01264, 77∘ N,
3 March 2002, which was studied in detail) for altitudes above 70 km. In
order to reproduce measurements these authors found that, on average,
2.8–3 N2(1) molecules (instead of Kumer's suggested value of 1) are
needed to be produced after each quenching of OH(ν) molecule in Reaction (R2). Alternative excitation mechanisms that were theorized to enhance the
4.3 µm radiance (i.e., via O2 and direct energy transfer from
OH to CO2) were tested but found to be insignificant.
Recently, suggested a new “indirect” mechanism of the OH
vibrational energy transfer to N2, i.e., OH(ν)⇒O(1D)⇒N2(ν). Accounting for this mechanism, but only
considering OH(ν=9), these authors performed simple model calculations to
validate its potential for enhancing mesospheric nighttime 4.3 µm
emission from CO2. They reported a simulated radiance enhancement
between 18 and 55 % throughout the MLT. In a latest study,
provided a definitive laboratory confirmation for the
validity of this new mechanism and measured its rates for OH(ν=9)+ O.
We studied in detail the impact of “direct” and “indirect” mechanisms
on the CO2(ν3) and OH(ν) vibrational level populations and
emissions and compared our calculations with (a) the SABER/TIMED nighttime
4.3 µm CO2 and OH 1.6 and 2.0 µm limb radiances of
MLT and (b) with the ground and space observations of the OH(ν)
densities in nighttime mesosphere.
The study was performed for quiet (non-auroral) nighttime
conditions to avoid accounting for interactions between charged
particles and molecules, whose mechanisms still remain poorly understood.
Non-LTE model
A non-LTE (non-local thermodynamical equilibrium) analysis was applied to CO2 and OH using the non-LTE ALI-ARMS
(Accelerated Lambda Iterations for Atmospheric Radiation and Molecular
Spectra) code package (), which
is based on the accelerated lambda iteration approach .
Our CO2 non-LTE model is described in detail by . We
modified its nighttime version to account for the “direct” mechanism,
Reactions (R1)–(R3), in a way consistent with that of
and added the “indirect” mechanism of
and as described in detail below.
Our OH non-LTE model resembles that of .
New mechanism of OH(ν) relaxation
suggested an additional mechanism that contributes to the
CO2(ν3) excitation at nighttime, and discussed in detail its
available experimental and theoretical evidence. According to this mechanism,
highly vibrationally excited OH(ν), produced by Reaction (R1), rapidly
loses several vibrational quanta in collisions with O(3P) through a
fast, spin-allowed, vibration-to-electronic energy transfer process that
produces O(1D):
OH(ν≥5)+O(3P)↔OH(0≤ν′≤ν-5)+O(1D).
Recently, presented the first laboratory
demonstration of this new OH(ν) + O(3P) relaxation pathway and
measured its rate coefficient for ν=9.
The production at nighttime of electronically excited O(1D) atoms in
Reaction (R4) triggers well-known pumping mechanism of the 4.3 µm
emission, which was studied in detail for daytime
(). Here O(1D) atoms are first quenched
by collisions with N2 in a fast spin-forbidden energy transfer process:
O(1D)+N2(0)↔O(3P)+N2(ν),
then
N2(ν) transfers its energy to ground state N2 via a very fast single-quantum
VV (vibrational–vibrational)
process:
N2(ν)+N2(0)↔N2(ν-1)+N2(1),
leaving N2 molecules with an average of 2.2 vibrational quanta, which is
then followed by Reaction (R3).
Collisional rate coefficients
We use, in our CO2 non-LTE model, the same VT (vibrational–translational) and VV collisional rate
coefficients for the CO2 lower vibrational levels as those of
. However, a different scaling of these basic rates
is applied for higher vibrational levels using the first-order perturbation
theory as suggested by .
The reaction rate coefficients applied in this study for modeling OH(ν)
relaxation transfer of OH(ν) vibrational energy to the CO2(ν3)
mode are displayed in Table 1. The total chemical production rate of
OH(ν) in Reaction (R1) was taken from and the
associated branching ratios for ν were taken from
. We treat Reaction (R2) both as a single (1Q,
ν′=1) and multi-quantum (MQ, ν′=2 or 3) quenching process. We use the
rate coefficient of this reaction (with associated branching ratios) taken
from Table 1 of and multiplied it by a low
temperature factor of 1.4 for MLT regions. The rate
coefficient for Reaction (R3) was taken from .
Significant collisional processes used in model.
Reaction
Reaction rate (cm3s-1)
Reference
(R1)
H + O3 ↔ OH(ν≤ 9) + O2
k1=fνa × 1.4 × 10-10 exp(-470/T)
,
(R2)
OH(ν≤9) + N2(0) ↔ OH(ν-ν′) + N2(ν′)
k2=fνb × 1.4 × 10-13
,
ν′=1,2,3
(R3)
N2(1) + CO2(0) ↔ N2(0) + CO2(ν3)
k3=8.91 × 10-12 × T-1
(R4)
OH(ν≥5) + O(3P) ↔ OH(0≤ν′≤ν-5) + O(1D)
k4=fνc × (2.3 ± 1) × 10-10
,
OH(ν< 5) + O(3P) ↔ OH(0) + O(3P)
k4 = 5.0 × 10-11
(R5)
O(1D) + N2(0) ↔ O(3P) + N2(ν)
k5=2.15×10-11 exp(110/T)
(R6)
OH(ν≤9)+ O2(0) ↔ OH(ν′)+ O2(1)
k6=fνd × 1.18 × 10-13
ν′=0,1,2,…ν-1
(R7)
OH(ν≤9) + O(3P)↔ OH(0) + O(3P)
k7=2.0×10-10
a fν(ν=5–9) = (0.01, 0.03, 0.15, 0.34,
0.47);
b fν(ν=1–9) = (0.06, 0.10, 0.17, 0.30, 0.52, 0.91, 1.6, 7,
4.8);
c fν(ν=5–9) = (0.91, 0.61, 0.74, 0.87, 1.0);
d fν(ν=1–9) = (1.9, 4, 7.7, 13, 25, 43, 102, 119, 309)
Following and , the rate
coefficient of Reaction (R4) was taken as
(2.3 ± 1) × 10-10 cm3s-1 for temperatures
near 200 K (with corresponding branching ratios for ν≥5).
Additionally, for OH(ν<5) collisions with O(3P), which are considered
completely inelastic, we used the rate coefficient
5 × 10-11 cm3s-1 from . The
rate coefficient for the reaction O(1D) + N2(0) (Reaction R5 in
Table 1) was taken from with accounting for the fact that
33 % of the electronic energy is transferred to N2
producing, on average, 2.2 N2 vibrational quanta. For the reaction
OH(ν≤ 9) + O2(0) (Reaction R6 in Table 1), we consider single
and multi-quantum quenching, using rate coefficients with associated
branching ratios taken from Tables 1 and 3 of ,
respectively. Rate coefficients are scaled by a factor of 1.18 to account for
MLT temperatures (). Lastly,
Reaction (R7) describes an alternative OH–O quenching mechanism which
previous studies () applied in
their OH models, where atomic oxygen completely quenches
(ν→ν=0) OH(ν) upon collision. For this reaction, we took
the vibrationally independent rate coefficient of
2.0 × 10-10 cm3s-1 from
.
Model inputs and calculation scenarios
The nighttime atmospheric pressure, temperature, and densities of trace gases
and main atmospheric constituents for calculations presented below were taken
from the WACCM (Whole Atmosphere
Community Climate Model) model .
The following sets of processes and rate coefficients were used in our
calculations:
(OH-N2 1Q)
& (OH-O2 1Q) &
Reaction (R7): this model accounts for Reactions (R1), (R2), (R3), (R6) and
(R7) from Table 1. Reaction (R2) is treated as a single-quantum (ν′=1)
process; Reaction (R6) is also treated as single-quantum (ν′=ν-1).
This model reproduces the initial model described by
.
(OH-N2 3Q) &
(OH-O2 1Q) &
Reaction (R7): same as model 1; however, Reaction (R2) is treated as the
three-quantum (ν′=3) process and Reaction (R6) is single-quantum (ν′=ν-1). As it is shown below, this version matches best with the final model
of , where the efficiency of reaction (R2) was
increased by a factor of 3.
(OH-N2 3Q) &
(OH-O2 MQ) &
Reaction (R7): same as model 2; however, Reaction (R6) is treated as
multi-quantum (any ν′≤ν-1) process.
(OH-N2 1Q) &
(OH-O2 MQ) &
Reactions (R4), (R5): Reactions (R1) through (R6) from Table 1 are included.
This is our basic model version with both “direct”, Reaction (R2), and
“indirect”, Reaction (R4) + (R5), mechanisms working together when
Reaction (R2) is treated as the single-quantum process (ν′=1) as was
suggested by , though Reaction (R6) is treated as the
multi-quantum process (any ν′≤ν-1). A new mechanism, Reactions (R4)
and (R5), replaces here Reaction (R7), which is used in other models
described above.
(OH-N2 3Q) &
(OH-O2 MQ) &
Reactions (R4), (R5): same as model 4, but “direct” process Reaction (R2)
is treated as the three-quantum process corresponding to its 3 times higher
efficiency suggested by .
(OH-N2 2Q) &
(OH-O2 MQ) &
Reactions (R4), (R5): same as model 5; however Reaction (R2) is treated as
the two-quantum process.
(OH-N2 1,2Q) &
(OH-O2 MQ) &
Reactions (R4), (R5): same as model 5; however Reaction (R2) is treated as
two-quantum process for highly resonant transitions
OH(9) + N2(0) → OH(7) + N2(2) and
OH(2) + N2(0) → OH(0) + N2(2), and as
single-quantum for all others.
Results
Vibrational temperatures of the CO2(ν3) levels
The vibrational temperature Tν is defined from the Boltzmann formula
nνn0=gνg0expEν-E0kTν,
which describes the excitation degree of level ν against the ground level
0. Here gν and Eν are the statistical weight and the energy of
level ν, respectively. If Tν=Tkin then level ν is in
LTE.
Nighttime vibrational temperatures of CO2(00011) of four
CO2 isotopes, CO2(01111) of main CO2 isotope, and of
N2(1) for SABER scan 22, orbit 01264, 77∘ N, 3 March 2002.
Solid lines: [(OH-N2 1Q) & (OH-O2 MQ) & Reactions (R4), (R5)]; dashed lines:
[(OH-N2 3Q) &
(OH-O2 1Q) &
Reaction (R7)]; see Sect. 2.3 for a description of calculation scenarios.
Figure 1 shows the vibrational temperatures of the CO2 levels of four
isotopes, giving origin to 4.3 µm bands, which dominate the SABER
nighttime signal . These results were obtained for
SABER scan 22, orbit 01264, 77∘ N, 3 March 2002. The same scan was
used for the detailed analysis presented in the work by
. The kinetic temperature retrieved for this scan
from the SABER 15 µm radiances (SABER data version 2.0) and
vibrational temperature of N2(1) are also shown. Solid lines in Fig. 1
represent simulations with our basic model
[(OH-N2 1Q) &
(OH-O2 MQ) &
Reactions (R4), (R5)], when both the “direct” process Reaction (R2) (in its
single-quantum version, as was suggested by ), and the new
“indirect” process, Reactions (R4) + (R5) are included. For comparison we
also show vibrational temperatures (dashed lines) for the model
[(OH-N2 3Q) &
(OH-O2 1Q) &
Reaction (R7)], where the “indirect” mechanism is off and the “direct”
process is treated as a three-quantum one, which is equivalent to the 3 times
higher efficiency suggested by .
Vibrational temperatures of CO2 levels and N2(1) depart from LTE
around 65 km. For both models, vibrational temperatures nearly coincide up
to 85–87 km; however, above this altitude, where the OH density is high,
vibrational temperatures for [(OH-N2 3Q) & ( OH-O2 1Q)] & Reaction (R7)] are a few kelvin lower
compared to those for [(OH-N2 1Q)& (OH-O2 MQ) & Reactions (R4), (R5)]. These vibrational
temperature differences explain differences of simulated CO2(ν3)
emission for both models shown in Fig. 2.
Left: measured and simulated SABER nighttime radiances in
channel 7 (4.3 µm) for SABER scan 22, orbit 01264, 77∘ N,
3 March 2002. SABER measured (black); see Sect. 2.3 for a description of
calculation scenarios displayed in the legend. Right: radiance relative
difference (simulated - measured)/measured in percent.
In both simulations, CO2(00011) of main isotope 626 and N2(1) have
almost identical vibrational temperatures up to ∼ 87 km which is
caused by an efficient VV exchange (Reaction R3).
The CO2 4.3 µm emission
Figure 2 displays our simulations of SABER channel 7 (4.3 µm)
radiances for inputs which correspond to the measurement conditions of the
SABER scan described in Sect. . The calculations also account for
the minor contribution in channel 7 radiation emitted by the OH(ν≤9)
vibrational levels.
Our simulation for this scan with the
[(OH-N2 1Q) &
(OH-O2 1Q) &
Reaction (R7)] set of rate coefficients is shown by the violet curve. The
turquoise curve displays our results for the rate coefficient set
[(OH-N2 3Q) &
(OH-O2 1Q) &
Reaction (R7)], which simulates the model suggested by
with the factor of 3 increased efficiency of
Reaction (R2). One may see that treating Reaction (R3) as a three-quantum VV
process strongly enhances the pumping of the CO2(ν3) vibrations and
that the 4.3 µm radiance is in agreement with
results.
Top: measured and simulated nighttime radiances for SABER channel 7
(4.3 µm). Middle: channel 7 radiance relative differences
(simulated-measured)/ measured in percent. Bottom: measured and simulated
SABER Volume Emissions Rate Ratios (channel 8 / channel 9). Four standard
atmospheres are displayed: (a) mid-latitude summer (MLS),
(b) tropical (TROP), (c) mid-latitude winter (MLW), and
(d) sub-Arctic winter (SAW) for selected SABER scans described in
Table 2. SABER measured with NER (noise equivalent radiance) (black);
[(OH-N2 1Q) &
(OH-O2 MQ) &
Reactions (R4), (R5)] (red); [(OH-N2 3Q) & (OH-O2 1Q) & Reaction (R7)] (turquoise); see Sect. 2.3 for
a description of calculation scenarios.
The blue curve in Fig. 2 displays our run with the model
[(OH-N2 3Q) &
(OH-O2 MQ) &
Reaction (R7)]. In this model Reaction (R6) is treated, following
, as a multi-quantum VV process. Compared to previous
model this run shows a significantly lower channel 7 signal. This is
obviously caused by a much more efficient removal of the OH(ν)
vibrational energy in the multi-quantum quenching by collisions with O2.
As a result, a significantly smaller part of this energy is collected by
N2(1) and delivered to the CO2(ν3) vibrations with the “direct”
mechanism Reaction (R2). To compensate this OH(ν) decay and keep the
transfer of energy to CO2(ν3) unchanged,
adjusted new, presumably higher OH(ν) to the SABER 1.6 and
2.0 µm radiances. In our study the higher channel 7 emission is,
however, restored when we include the Reactions (R4) and (R5) (“indirect”
mechanism of energy transfer from OH(ν) to CO2(ν3)) in the model,
but return Reaction (R2) to its single-quantum mode.
The red curve in Fig. 2, which corresponds to our
[(OH-N2 1Q) &
(OH-O2 MQ) &
Reactions (R4), (R5)] model is nearly overlapped by the turquoise curve of
the [(OH-N2 3Q) &
(OH-O2 1Q) &
Reaction (R7)] model. This demonstrates a very high efficiency of the
“indirect” channel compared to the “direct” one since it provides the
same pumping of CO2(ν3) even when OH(ν) energy is efficiently
removed in the multi-quantum version of Reaction (R6).
In Fig. 2, we show (black curve with diamonds) the channel 7 radiance profile
for the SABER scan specified in Sect. . Comparing turquoise and
red curves with this measurement, one may see that both the “direct”
mechanism alone in its three-quantum version and the combination of
“indirect” and single-quantum “direct” mechanisms are close to the SABER
radiance for this scan. However, to provide this pumping level, the
multi-quantum “direct” mechanism needs to be supported by the inefficient
single-quantum OH(ν) quenching in Reaction (R6) by collisions with O2,
which helps maintain a higher population of OH(ν). We also note here that
both our violet [(OH-N2 1Q) & (OH-O2 1Q) & Reaction (R7)] and turquoise
[(OH-N2 3Q) &
(OH-O2 1Q) &
Reaction (R7)] curves reproduce the corresponding results in Fig. 10 of
López-Puertas et al. (2004), short-dash and solid lines, respectively,
very well.
We also show in Fig. 2 our study of how both the “direct” and “indirect”
mechanisms work together when the “direct” process Reaction (R2) is treated
as a multi-quantum. The magenta curve in this plot is the result obtained
with the model [(OH-N2 3Q) & (OH-O2 MQ) & Reactions (R4), (R5)] when Reaction (R2) is
treated as a three-quantum process. This combination of both mechanisms
provides high CO2(ν3) pumping and subsequently strong channel 7
emission. The latter exceeds the turquoise and red curves by 20–45 % in
the altitude range considered and strongly deviates from the measured
radiance profile.
Two other results of this study are shown only on the right panel of this
plot for the signal differences. The light blue curve corresponds to
simulations with the [(OH-N2 2Q) & (OH-O2 MQ) & Reactions (R4), (R5)] model when the quantum
transfer in Reaction (R2) is reduced from 3 to 2. The dark green curve is
obtained for the case when Reaction (R2) is treated as two-quantum process
for highly resonant transitions OH(9) + N2(0)
→ OH(7) + N2(2) and OH(2) + N2(0)
→ OH(0) + N2(2), and as single-quantum for all other
vibrational levels. It is seen that both of these input versions bring the
calculated radiance closer to our result for a single-quantum “direct”
process Reaction (R2) (red curve) and to our simulation
[(OH-N2 3Q) &
(OH-O2 1Q) &
Reaction (R7)] of results obtained based on López-Puertas et al. (2004),
turquoise.
In Fig. 3 (upper and middle rows) we compare our simulation results for two
sets of rate coefficients: [(OH-N2 1Q) & (OH-O2 MQ) & Reactions (R4), (R5)] (red) and
[(OH-N2 3Q) &
(OH-O2 1Q) &
Reaction (R7)] (turquoise). The WACCM model nighttime inputs representing
four different atmospheric situations described in Table 2 were used for
these simulations. These inputs also match the measurement conditions of the
four SABER nighttime scans (solar zenith angle > 105∘) listed in
the Table 2. The corresponding 4.3 µm radiances from SABER-measured
channel 7 are shown in black as reference data.
One may see that in Fig. 3 the “direct” mechanism alone with three-quantum
efficiency for Reaction (R2), as well as both the “direct” (as
single-quantum) and “indirect” mechanisms together provide similar results
for all four atmospheric models, within a 10 to 30 % difference range. By
comparing these models to measured radiances, both calculations are close to
the observed signal down to 68 km for MLW and down to 75 km for SAW. For
MLS and TROP, the two-mechanism calculations are somewhat closer to
measurements than those for “direct” mechanism alone in altitude interval
75–90 km.
Comparison of OH vibrational populations with ground- and space-based observations
In Fig. we present relative OH(ν) populations calculated using
three different sets of rate coefficients discussed in the previous section,
which provided comparably high enhancement of the CO2(ν3) emission.
These calculations are compared with the vibrational populations derived from
ground (panel a) and space-based
(panel b) observations of OH
emissions.
Measured populations (black) displayed in panel (a) were recorded by
on 3 March 2000 using the echelle spectrograph and imager
(ESI) on the Keck II telescope at Mauna Kea (19.8206∘ N,
155.4681∘ W). The authors measured emission intensities of the 16 OH
Meinel bands which were converted into the OH(ν) column densities and
normalized to column density of OH(ν=9). Several observations of OH
emissions were recorded throughout the night. We display the average column
densities as well as their variation ranges for each vibrational level. The
three simulated distributions (red, turquoise, and magenta) in this panel are
modeled using WACCM inputs taken on 3 March 2000 at latitude 20∘ N
at local midnight.
Selected nighttime SABER scans.
Atmosphere
Latitude
Day
Orbit
Scan
(a)
mid-latitude summer (MLS)
37∘ S
26 Jan 2004
11 556
62
(b)
tropical (TROP)
6∘ N
20 Jan 2008
33 130
48
(c)
mid-latitude winter (MLW)
34∘ S
15 Jul 2010
46 594
90
(d)
sub-Arctic winter (SAW)
72∘ S
15 Jul 2010
46 585
78
Relative OH(ν) populations, normalized to OH(ν=9), for
measurements taken: (a) on 3 March 2000 at 20∘ N and
(b) in November 2009 between 38 and 47∘ N. Measured
populations (black with diamonds); simulations:
[(OH-N2 1Q) &
(OH-O2 MQ) &
Reactions (R4), (R5)] (red); [(OH-N2 3Q) & (OH-O2 1Q) & Reaction (R7)] (turquoise);
(OH-N2 3Q) &
(OH-O2 MQ) &
Reactions (R4), (R5)] (magenta). See Sect. 2.3 for a description of
calculation scenarios.
Measured densities displayed in panel (b) of Fig. 4 were taken from
, who analyzed VIRTIS (for Visible and
Infrared Thermal Imaging Spectrometer) measurements on board the Rosetta
mission. VIRTIS performed two limb scans of the OH Meinel bands from 87 to
105 km covering the latitude range from 38 to 47∘ N between 01:30
and 02:00 solar local time, in November 2009. To achieve a high
signal-to-noise ratio, 300 radiance spectra (OH Δν=1 and 2) were
collected and averaged. We show in panel (b) of Fig. 4 the OH(ν)
population distribution normalized to OH(ν=9) derived in this study as
well as corresponding uncertainties. The three simulated distributions (red,
turquoise, and magenta) in this panel are modeled using WACCM inputs taken on
22 November 2000 at latitude 45∘ N at local midnight.
To simulate the ground-based observations of Cosby and Slanger (2007), panel (a), the calculated relative
populations were integrated over the entire altitude range of our model
(30–135 km). For panel (b),
we have integrated calculated OH(ν) densities of 87 to 105 km as
observed by VIRTIS from to simulate mean
population distribution obtained in this study.
The turquoise profiles in both panels of Fig. 4 represent results obtained
with our set of rate coefficients [(OH-N2 3Q) & (OH-O2 1Q) & Reaction (R7)] similar to the one used in
, where the authors treated the
OH–N2 reaction with an efficiency increased by a factor of 3,
the OH–O2 reaction as single-quantum, and the OH–O reaction as a
“sudden death” quenching or chemical OH removal process (Reaction R7), with
ν independent rate coefficient of
2.0 × 10-10 cm3 s-1. In panel (a), the turquoise
profile shows higher relative populations compared to measurements for upper
vibrational levels ν>4, whereas in panel (b) this model shows populations
within the uncertainty range of measurements for ν>4. For lower
vibrational levels ν≤ 4, the populations calculated with this model
are, however, significantly lower than measured ones: by 30 % for ν=3
in the panel (a) and by up to 85 % for ν=1 in panel (b). A
significantly slower increase in populations calculated with the
[(OH-N2 3Q) &
(OH-O2 1Q) &
Reaction (R7)] model compared to measurements can be explained by the lack of
efficient mechanisms redistributing the OH(ν) energy from higher
vibrations levels to lower ones. The single-quantum OH–O2 reaction also
allows for more excited OH molecules in the upper vibrational levels relative
to a multi-quantum process. Additionally, a slower increase in calculated
populations with the ν decreasing compared to measured ones which is seen
in both panels, is the effect of the high quenching rate coefficient of the
Reaction (R7) for lower vibrational levels for which this reaction dominates
over the single-quantum O2 quenching.
The situation is different when our basic model
[(OH-N2 1Q) &
(OH-O2 MQ) &
Reactions (R4), (R5)] is applied (red curves). As discussed above in the
previous section, this model provides the same level of the CO2(ν3)
emission pumping as the extreme model of López-Puertas et al. (2004);
compare red and turquoise curves in Fig. 2. However, they demonstrate
significantly different population distributions. Relative OH population
distribution in panel (a)
shows our standard model in very good agreement with the results from
, falling completely within the variation range of these
measurements. Panel (b) also
shows excellent agreement between calculations and measurements, where the
former lie nearly completely within the measurement error bars for the
majority of vibrational levels. In both panels our results reproduce well the
steady upward trend in populations from upper to lower vibrational levels.
Significantly higher populations of lower OH levels in this model are the
result of redistribution of higher-vibrational-level energy to lower levels
due to two dominant multi-quantum quenching mechanisms, namely the new
Reaction (R4) and the multi-quantum version of Reaction (R6). We also note
that Reaction (R4) uses a lower rate coefficient than Reaction (R7) for
quenching the lower vibrational levels ν<5, which results in maintaining
their higher populations.
Measured OH(ν=3) (panel b)
was the only population which showed disagreement with our model. Various
reasons of increased measured population at ν=3 are discussed by
; however, no definitive conclusions were
given.
Above 90 km atomic oxygen density increases rapidly with the altitude. As a
result the role of Reaction (R4) in quenching higher OH vibrational and
pumping lower levels increases. This effect is easily seen in panel (b) of
Fig. 4, where mean OH(ν) densities for higher altitude region 87–105 km
are compared. The turquoise curve (no Reaction R4) in this panel shows lower
populations compared to those calculated with Reaction (R4) included.
The magenta profiles in both panels represent our calculations with the model
[(OH-N2 3Q) &
(OH-O2 MQ) &
Reactions (R4), (R5)], which is identical to our standard model except for
the Reaction (R2) treated as the three-quantum one. The multi-quantum
OH–N2 VV transfer provides faster quenching of excited OH,
hence, a lower overall population of the magenta profiles compared to red
profiles. Despite showing reasonable agreement with measurements in both
panels, this model caused, however, an excessive increase for the
4.3 µm emissions, as seen in Fig. 2.
OH 1.6 and 2.0 µm emissions
SABER channels 8 (2.0 µm), and 9 (1.6 µm) are dominated
by the OH(ν) emission from levels ν=8–9 and ν=3–5,
respectively. We simulated channel 8 and 9 radiances for four atmospheric
models from Table 2. Results are shown in Fig. 3, bottom row, as ratios of
volume emission rates for channels 8 and 9. Volume emission rate (VER) is defined as the sum
Rν=∑Aν,ν′[OH(ν)]=[OH]∑Aν,ν′pν,
over all transitions contributing to the channel, where
pν= [OH(ν)] / [OH] is the probability of the OH molecule to
be in the vibrational state ν. It follows from this expression that the
VER ratio does not depend on the total OH density and is, therefore,
convenient for analyzing impacts of various populating/quenching mechanisms
on OH(v) distribution. The calculations with our basic model
[(OH-N2 1Q) &
(OH-O2 MQ) &
Reactions (R4), (R5)] are shown in Fig. 3 in red, and those for the model
[(OH-N2 3Q) &
(OH-O2 1Q) &
Reaction (R7)] with the three-quantum mechanism, Reaction (R2), in turquoise.
Black curves in this plot display SABER-measured VER ratios, for which VERs
were obtained with the Abel inversion procedure, similar to that described by
, from the SABER channel 8 and 9 limb radiances for
scans listed in Table 2.
Comparing red and turquoise profiles in Fig. 3 (bottom row), one may see that
our standard model (red) shows significantly lower VER ratios for altitudes
85–100 km than the model of López-Puertas et al. (2004), turquoise.
These differences between ratios are a result of very different OH(ν)
population distributions (Fig. 4) for each model, which were discussed in the
previous section. The channel 8/channel 9 VER ratios reflect these
distributions very well since channel 8 is sensitive to the OH(ν)
emissions from higher levels 8 and 9, whereas channel 9 records emissions of
lower levels 3–5. A significantly higher population of lower vibrational
levels in our model (red curves in Fig. 4) explain low VER ratios. In
contrast, the model [(OH-N2 3Q) & (OH-O2 1Q) & Reaction (R7)], which underpredicts lower
level populations, provides VER ratios which significantly exceed both our
model results and measurements for altitudes above 90 km, where [O] density
rapidly increases with altitude. This comparison demonstrates the strong
impact of Reaction (R2), which provides efficient quenching of higher OH
vibrational levels in collisions with O(3P) atoms in this altitude
region.
Conclusions
first proposed the transfer of
vibrational energy from chemically produced OH(ν) in the nighttime
mesosphere to the CO2(ν3) vibration, OH(ν)⇒N2(ν)⇒CO2(ν3). The effect of this
“direct” mechanism on the SABER nighttime 4.3 µm emission was
studied in detail by , who showed that in order to
match observations, an additional enhancement is needed that would be
equivalent to the production of 2.8–3 N2(1) molecules instead of one
molecule for each quenching reaction OH(ν) + N2(0).
concluded that the required 30 % efficiency in
the OH(ν) + N2(0) energy transfer is, in principle, possible,
although the mechanism(s) whereby the energy is transferred is (are) not
currently known.
Recently, suggested a new efficient “indirect” channel
of the OH(ν) energy transfer to the N2(ν) vibrations,
OH(ν)⇒O(1D)⇒N2(ν)
and showed that it may provide an additional enhancement of the MLT nighttime
4.3 µm emission. provided a definitive
laboratory confirmation of new OH(ν) + O vibrational relaxation pathway
and measured its rate for OH(ν=9)+ O.
We included the new “indirect” energy transfer channel in our non-LTE
model of the nighttime MLT emissions of CO2 and OH molecules and studied
in detail the impact of “direct” and “indirect” mechanisms on simulated
vibrational level populations and radiances. The calculations were compared
with (a) the SABER/TIMED nighttime 4.3 µm CO2 and OH 1.6 and
2.0 µm limb radiances of MLT and (b) with the ground and space
observations of the OH(ν) densities in the nighttime mesosphere. We found
that new “indirect” channel provides significant enhancement of the
4.3 µm CO2 emission. This model also produces OH(ν)
density distributions which are in good agreement with both SABER limb OH
emission measurements and the ground and space observations in the
mesosphere. Similarly strong enhancement of 4.3 µm emission can
also be achieved with the “direct” mechanism alone assuming a factor of 3
increase in efficiency, as was suggested by . This
model does not, however, reproduce either the SABER-measured VER ratios of
the OH 1.6 and 2.0 µm channels or the ground and space measurements
of the OH(v) densities. This discrepancy is caused by the lack of efficient
redistribution of the OH(ν) energy from the higher vibrational levels
emitting at 2.0 µm to lower levels emitting at 1.6 µm in
the models based on the “direct” mechanism alone. In contrast, this new
“indirect” mechanism (Reactions R4 and R5 of Table 1), efficiently removes
at least five quanta in each OH(ν) + O(3P) collision from high OH
vibrational levels. Supported also by the multi-quantum OH(ν) + O2
quenching (Reaction R6 of Table 1), the new mechanism provides OH(ν)
distributions which are in agreement with both measured VER ratios and
observed OH(ν) populations.
The results of our study suggest that the missing nighttime mechanism of
CO2(ν3) pumping has finally been identified. This confidence is
based on the fact that the new mechanism accounts for most of the
discrepancies between measured and calculated 4.3 µm emission for
various atmospheric situations, leaving relatively little room for other
processes, among them the multi-quantum “direct” mechanism. The accounting
for the multi-quantum transfer in reaction OH(v) + N2 together with the
“indirect” mechanism has little influence on the OH(ν) population
distributions; however, it can enhance the 4.3 µm emission. Therefore,
further laboratory and/or theoretical investigation of this reaction is
needed to define its role. Further improvements for the new “indirect”
mechanism will require optimizing the set of rate coefficients used for
OH(ν) relaxation by O(3P) and O2 at mesospheric temperatures
and, in particular, understanding the dependence of the “indirect”
mechanism on the OH vibrational level. Relevant laboratory measurements and
theoretical calculations are sorely needed to understand these relaxation
rates and the quantitative details of the applicable mechanistic pathways.
Nevertheless, the results presented here clearly demonstrate significant
progress in understanding the mechanisms of the nighttime OH and CO2
emission generation in MLT.