vibrationally excited ozone in the middle atmosphere

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Journal of Atmospheric and Solar-Terrestrial Physics 68 (2006) 202–212

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Vibrationally excited ozone in the middle atmosphere

M. Kaufmanna,b,�, S. Gil-Lopezb, M. Lopez-Puertasb, B. Funkeb,M. Garcıa-Comasb, N. Glatthorc, U. Grabowskic, M. Hopfnerc, G.P. Stillerc,

T. von Clarmannc, M.E. Koukoulid, L. Hoffmanna, M. Riesea

aResearch Center Julich, Institute of Chemistry and Dynamics of the Geosphere, Julich, GermanybInstituto de Astrofısica de Andalucıa, CSIC, Granada, Spain

cResearch Center Karlsruhe, Institute for Meteorology and Climate Research Karlsruhe, GermanydAristotle University of Thessaloniki, Laboratory of Atmospheric Physics, Thessaloniki, Greece

Received 6 April 2005; received in revised form 15 September 2005; accepted 1 October 2005

Available online 28 November 2005

Abstract

Daytime mesospheric limb emission spectra of ozone in the 4:8mm regime are analyzed with respect to vibrational

excitation and relaxation processes. The data, which was obtained by the MIPAS (Michelson Interferometer for Passive

Atmospheric Sounding) instrument on board ESA’s Environmental Satellite, is simulated by means of a non-local

thermodynamic equilibrium (non-LTE) model utilizing O3-abundance, temperature, and pressure data from simultaneous

retrievals in other spectral regions. The vibrational states of ozone depart from LTE due to the absorption of radiation

from the lower atmosphere and due to the production of excited states in the OþO2 þM! O3ðvÞ þM recombination

reaction. The energy flow into the ozone molecule as well as the collisional relaxation are highly uncertain. Model

calculations that assume ozone formation at energies larger than 5000 cm�1 underestimate the measured radiances by a

factor of 2–3 in the 50–75 km altitude regime, if the nominal relaxation scheme is assumed. Agreement between measured

and modeled radiances is achieved, if the collisional rates for the transformation of hot band stretching to bending quanta

are reduced by about a factor of three, or if the quasi-nascent distribution of ozone favors vibrational states in the

3000 cm�1 region.

r 2005 Elsevier Ltd. All rights reserved.

Keywords: Ozone; Stratosphere; Mesosphere; MIPAS; Non-LTE remote sensing

1. Introduction

In this study we analyze the spectral region from2080 to 2130 cm�1 that covers ozone hot bandtransitions from the v1 þ v3X2 vibrational states.

e front matter r 2005 Elsevier Ltd. All rights reserved

stp.2005.10.006

ing author. Research Center Julich, Institute of

Dynamics of the Geosphere, Julich, Germany.

615250; fax: +49 2461 615346.

ess: [email protected] (M. Kaufmann).

These states depart from local thermodynamicequilibrium (LTE) during daytime above thestratopause due to chemical pumping through thehighly exothermic OþO2 recombination reaction:

OþO2 þM�!O3ðvÞ þMþ 8547 cm�1. (1)

Therefore, emissions from the v1 þ v3X2 vibra-tional states are a valuable proxy for the quasi-nascent distribution of ozone vibrational states

.

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ARTICLE IN PRESSM. Kaufmann et al. / Journal of Atmospheric and Solar-Terrestrial Physics 68 (2006) 202–212 203

formed in this reaction and/or the collisionalrelaxation scheme.

An improved knowledge of the production andrelaxation mechanisms will advance the modeling ofozone 10mm radiation and the retrieval of meso-spheric ozone. This is especially true for broadbandinstruments that cannot avoid contributions fromhigher excited vibrational states. For example, theSABER mid-infrared ozone channel has a spectralbandpass of 1010–1140 cm�1 and contains at least19 bands of ozone (Mlynczak and Zhou, 1998).Uncertainties in the chemical pumping and in thecollisional quenching prevent the measurement ofozone from the required 15% accuracy and asignificantly improved understanding of the energyflow into the ozone molecule upon recombination isrequired (Mlynczak and Zhou, 1998).

Laboratory studies of the OþO2 recombinationreaction indicate that ozone is produced with morethan one quantum of vibrational energy. vonRosenberg and Trainor (1973, 1974, 1975) showedthat about 50% of the exothermicity of the OþO2

reaction apparently ends up in vibrational excita-tion. Similar results were obtained from Kleindienstand Bair (1977), who suggest that 5750 cm�1, or70% of the exothermicity, appears as vibrationalexcitation, mainly in the n3 mode. The flashphotolysis measurements of Joens et al. (1982) alsoindicate the production of multiply excited ozoneand that stretching states are populated withdecreasing sequence indicating a stepwise model ofvibrational relaxation. Rawlins and Armstrong(1987) and Rawlins et al. (1987) analyzed O3

chemiluminescence by means of the COCHISEfacility and observed ozone n3 vibrational statesup to v3 ¼ 5.

Several remote sensing instruments for the atmo-sphere found steady state distributions of ozone thatare similar in shape and extend of excitation to thelaboratory results. Green et al. (1986) analyzedSPIRE (Spectral Infrared Rocket Experiment)9–12mm atmospheric limb emission spectra indicat-ing the existence of highly excited ozone vibrationalstates up to v3 ¼ 6. A detailed analysis of SPIREand HIRIS (High Resolution Interferometer Spec-trometer) data with respect to the quasi-nascentdistribution and the collisional relaxation modelwas carried out by Rawlins (1985). In this study azero surprisal distribution of vibrationally excitedozone after recombination, pumping primarily thelowest vibrational levels, was assumed. However,both studies utilized model atmospheres for the

major atmospheric parameters which does not allowfor a detailed quantitative comparison of modeledand measured data.

Adler-Golden and Smith (1990) investigatedatmospheric emission spectra of ozone n3 hot bandsfrom the rocket-borne interferometer SPIRIT 1(SPectral InfRared Interferometric Telescope).Vibrational states with up to 7 vibrational quantahave been detected in the 780–980 cm�1 wavenum-ber range at altitudes of 70–80 km. SPIRIT dataexhibits a significant population of bending-excitedstates containing at least one v2 quantum.

The collisional relaxation model is an importantparameter for the study of multiply excited ozone. Areview of the kinetics and spectroscopy of ozonewas published by Steinfeld et al. (1987) andSteinfeld and Gamache (1998). Basically there arefour major vibrational–translational relaxationchannels for ozone (Fig. 1):

(1)
An intermode transfer process (v12v3) equili-brating n1 and n3 vibrational states. The strongCoriolis interaction coupling of these levels,which are close in energy (DE ¼ 61 cm�1 for thefundamental modes), cause a very high effi-ciency of this interaction, which is typically 2–3order of magnitude faster than all otherchannels (10�11 cm3=s at 300K; Doyennetteet al., 1990). Due to this strong couplingdyads/polyads of near resonant states arecollisionally equilibrated to the kinetic tem-perature and the ozone molecule can be con-sidered as a system with two modes: nd ¼ n1 þ n3and n2.
(2)
The relaxation of the stretching modes appearsprimarily in a stretching-to-bending intermodetransfer (v1;32v2). The conversion of stretchingto bending vibrations is far from resonance(DE ¼ 3402400 cm�1 for the fundamentalstates). The transfer rate constants forð100 001Þ ! ð0 1 0Þ at 300K are 3:625�10�14 cm3=s for O2 (Adler-Golden and Steinfeld,1980; Menard-Bourcin et al., 1991; West et al.,1978; Zeninari et al., 2000) and 2:723:9�10�14 cm3=s for N2 (Menard-Bourcin et al.,1991; Zeninari et al., 2000). Joens et al. (1982)measured a significantly lower value (1�10�14 cm3=s) for ð100 001Þ þO2! ð0 1 0Þ þO2
in comparison with the other measurements.
(3) The pure loss of one stretching quantum
(v1;32v1;3 � 1) releases about 1000 cm�1 (for

ARTICLE IN PRESS

000

200 101 002

300

201

102 003

100 001

210

111 012

010

110 011

120 021

020

130 031

030

040

1000

2000

3000

Fig. 1. Vibrational levels of ozone up to 3200 cm�1 (thick horizontal bars) and their quantum numbers versus vibrational energy (in

cm�1). Blue dotted lines indicate v1;3 ! v2 transitions, red solid lines v2 ! ðv2 � 1Þ, and green dashed lines v1;3 ! ðv1;3 � 1Þ transitions.

The v12v3 intermode transfer is not shown.

M. Kaufmann et al. / Journal of Atmospheric and Solar-Terrestrial Physics 68 (2006) 202–212204

fundamental band transitions) and is at leastone order of magnitude slower than the stretch-ing-to-bending transfer (Menard-Bourcin et al.,1991; Zeninari et al., 2000).

(4)
Rate constants for the relaxation of bendingquanta (v22v2 � 1) exhibit ranges of 223:3�10�14 cm3=s for O2 and 2:123� 10�14 cm3=s forN2 at 300K for the fundamental band transition(Menard-Bourcin et al., 1991; Zeninari et al.,2000).
The temperature dependence of the relaxation ratesof ozone in O3–O2 and O32N2 gas mixtures wasmeasured in the 200–300K temperature range byMenard et al. (1992). Upschulte et al. (1994)measured the relaxation of O3 stretching modes at90K. Their O2 relaxation rate coefficient for (100;

001) lies significantly above an extrapolation of theformula by Menard et al. (1992) whereas it agreesfor N2 within the experimental uncertainty.

Very few measurements exist for the quenching ofmultiply excited ozone vibrational states. Menard-Bourcin et al. (1996, 1994) derived collisional ratesfor the triad ½ð200Þð101Þð002Þ� and their temperaturedependence in O3–N2 and O3–O2 mixtures bymeans of double-resonance measurements andsemiclassical calculations. Their measurementscould be explained using a vibrational relaxationscheme considering 29 vibrational relaxation pro-cesses with O3, N2 and O2 applying a Landau–Tellertype scaling law, that scales the fundamental bandrates proportional to the vibrational quantum

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2080 2081 2082 2083 2084 2085

0.11

10

0.11

10

2080-2085

2085-2090

M. Kaufmann et al. / Journal of Atmospheric and Solar-Terrestrial Physics 68 (2006) 202–212 205

numbers. In this case, the relaxation rates for thetriad ½ð200Þð101Þð002Þ� are two times larger than forthe dyad ½ð100Þð001Þ�. Similar results were obtainedby Upschulte et al. (1994) for up to three stretchingquanta.

0.11

10

0.11

10

0.11

10

0.11

10

0.11

10

0.11

10

0.11

10

2125 2126 2127 2128 2129 2130

Wavenumber [cm-1]

0.11

10R

adia

nce

[nW

/ cm

2 sr

cm-1

]

2090-2095

2095-2100

2100-2105

2105-2110

2110-2115

2115-2120

2120-2125

2125-2130

Fig. 2. Simulated daytime limb radiances including non-LTE at

60 km altitude. The spectrum covers 2080–2130 cm�1 and is

divided into spectral windows covering 5 cm�1 each; the spectral

ranges (in cm�1) are given in the upper right corner of each plot.

The different emitters are marked by colors: Ozone (red shading),

CO (blue cross hatching), CO2 (green diagonal cross hatching),

H2O (black hatching) and the sum (black). The thick horizontal

lines mark the spectral microwindows used in this study. More

details in the text.

2. The MIPAS measurements

The Michelson Interferometer for Passive Atmo-spheric Sounding (MIPAS) is part of the corepayload of ESA’s Environmental Satellite (ENVI-SAT), which was successfully launched in March,2002. ENVISAT’s orbit is nearly sun-synchronous(98:6� inclination) and takes 100min at an altitudeof 800 km. The local time at the equator for thedescending node is 10 a.m. MIPAS is a Fouriertransform spectrometer for the measurement ofmid-infrared emission spectra at the Earth’s limb.The measurements cover 680–2410 cm�1 except forsmall band gaps (e.g. 970–1020 cm�1) in five spectralbands (Table 1) at a spectral resolution of0:035 cm�1 (unapodized). The noise equivalentsignal radiance (NESR, unapodized) spans4–50 nW=ðcm2 sr cm�1). A single interferometerstroke is performed at each tangent altitude andtakes about 4.5 s. The data analyzed here stemsfrom the Upper Atmosphere 1 (UA1) measurementmode. It covers altitudes from 18 to 100 km with avertical increment of 3 km in the stratosphere and5 km in the mesosphere.

In this study MIPAS daytime radiances in the4:8mm spectral region are analyzed. Emissions inthis wavelength region stem mainly from ozone andCO. Other emitters are CO2 and H2O (Fig. 2).Hydroxyl ro-vibrational lines are negligible in the4:8mm region below 75 km, because the abundanceof vibrationally excited OH below 75 km as well ascontributions from higher altitudes in the line-of-sight column densities are very small. The contribu-

Table 1

The MIPAS spectral bands and typical noise equivalent signal

radiance (NESR) for unapodized spectra

Band Spectral range NESR

ðcm�1Þ ðnW=ðcm2 sr cm�1ÞÞ

A 685–970 50

AB 1020–1170 24

B 1215–1500 12

C 1570–1750 4

D 1820–2410 4

tion of all gases was analyzed on the basis offorward calculations, including all species emittingin this region by means of the Karlsruhe Optimizedand Precise Radiative transfer Algorithm (KOPRA;Stiller et al., 2002), which has been cross-validatedfor non-LTE applications (von Clarmann et al.,2002). Non-LTE was considered for O3, CO, CO2,NO, and H2O. For a detailed description of theindividual non-LTE models see Lopez-Puertas andTaylor (2001).

The chemical excitation of ozone was modeledassuming a zero surprisal distribution, and for thecollisional relaxation nominal rate constants incombination with a Landau–Teller type scalinglaw are applied. For details see below. For CO the2–1 and 1–0 transitions are considered; CO minorisotope emissions are not shown in Fig. 2, but theiremissions are less than 1% of the total radiance inthe microwindows used in this study (see below).

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0.01 0.1 1 10 100

Radiance [nW / cm2 sr cm-1]

50

60

70

80

90

100

Tan

gent

Hei

ght [

km]

2080-2130 cm-1

1020-1040 cm-1

day

night

night

day

Fig. 4. Wavenumber-averaged radiance profiles for the selected

microwindows in the 2080–2130 cm�1 region (solid lines) and for

1020–1040 cm�1 (dashed lines) from MIPAS UA 1 measurement

mode for 11 June 2003 (135 profiles). The NESR for single

profiles is indicated by the vertical lines.

M. Kaufmann et al. / Journal of Atmospheric and Solar-Terrestrial Physics 68 (2006) 202–212206

In total, 24 vibrational bands of CO2 emitting inthe 4:8 mm region are modeled.

Up to 50% of the total daytime radiance in the2080–2130 cm�1 interval stem from CO, dependingon the ozone model. CO2 contributes 5% or less.Emissions from H2O can be neglected except for the2091 cm�1 region. The high spectral resolution ofthe MIPAS instrument allows for the separation ofemissions from other gases from that of ozone. Forthis analysis, 19 spectral windows with a spectralbandpass larger than 0:5 cm�1 were selected, whereozone constitutes at least 80% of the total radiance.This number of microwindows is a balance betweengood signal-to-noise ratio and computation time.This set of microwindows is illustrated in Fig. 2 byblack horizontal bars.

The strongest vibrational transition of ozoneemitting in the 2080–2130 cm�1 wavelength regionis the 101–000 band. At 60 km it constitutes about80% to the total radiance (Fig. 3), applying a zerosurprisal quasi-nascent distribution for ozoneformed in the recombination reaction and thenominal relaxation model. Other bands emitting inthis regime are 111–010 (15%), 201–100 (3%), and002–000 (3%).

Wavenumber-averaged radiance profiles for theselected microwindows in the 2080–2130 cm�1

region (‘4:8mm’) in comparison with 10mm ra-diances are plotted in Fig. 4. The 4:8mm radiancesshow a very clear daytime enhancement at altitudes

2080 2100 2120 2140

Wavenumber [cm-1]

0.01

0.1

Rad

ianc

e [n

W /

cm2 s

r cm

-1]

sum

101-000

002-000

111-010

201-100

60 km

Fig. 3. Ozone vibrational bands contributions to daytime limb

radiance at 60 km tangent altitude; for details see text.

below 90 km, which is opposite to the diurnalvariation of ozone volume mixing ratio. Nighttimeozone abundance is up to a factor of 10 larger incomparison to daytime and this behavior is, at leastpartially, reflected in the 10mm radiances. Thereason for the daytime enhancement in the 4:8mmradiances is the strong non-thermal excitation ofozone vibrations due to the recombination reactionbetween atomic and molecular oxygen. Since atomicoxygen is virtually absent during nighttime in thelower and middle mesosphere nighttime 4:8mmradiances are decreased.

Average intensities in the 4:8mm region are below1nW=ðcm2 sr cm�1Þ in the altitude region of interest,and special care has to be taken to the propercharacterization of instrumental effects such asradiometric offset or possible straylight effectswithin the optical system.

Looking at deep space enables to determine theoffset caused by the instrument self-emission. Deepspace measurements are made frequently, one everyfour elevation scans, in order to account forchanging instrument self emission due to tempera-ture variations along the orbit. For the 1890–1900 cm�1 spectral region, deep space radiancesvary within �0:1 nW=ðcm2 sr cm�1Þ (A. Kleinert,personal communication, 2004). However, the


changing offset is already considered in MIPASL1B data and the residual offset error isp0:05 nW=ðcm2 sr cm�1Þ (A. Kleinert, personalcommunication, 2004). Since the data consideredin this study stems from a different wavelengthregion within MIPAS channel D, the residual offsetwas verified by analyzing mean intensities in themesopause region, where emissions should be verylow. Averaged altitude profiles show values as lowas 0:01 nW=ðcm2 sr cm�1Þ around 100 km (Fig. 4)indicating that the residual offset is below this value.

Another source of offset could be straylightwithin the optical system, which may vary withelevation. Its characterization is difficult sincespectral windows without any atmospheric emis-sions are required at each tangent altitude. In factthere are no such regions within the set ofmicrowindows we are using, but there are regionsof very low intensities within channel D at lowerand higher wavenumbers. We averaged 2800 spectrafrom the nominal measurement mode in the1878–1880 and 2155–2157 cm�1 intervals (Fig. 5).The radiances are similar to the NESRð0:08 nW=ðcm2 sr cm�1ÞÞ indicating that the offsetis below this value. Since there is no reason toassume that the offset might increase between thesespectral windows, we conclude that for the micro-windows used in this study the offset is below0:1 nW=ðcm2 sr cm�1Þ.

1878 1879 1880

0

1

2

2155 2156 2157

0

1

2

Rad

ianc

e [n

W /

cm2 s

r cm

-1]

Wavenumber [cm-1]

Fig. 5. Mean spectra around 60.5 km tangent altitude, recorded

from 5 to 11 June 2003 (2800 profiles). The NESR for the mean

spectra is indicated by the thick horizontal line. The prominent

signatures at 1879.3 and 2145:36 cm�1 are H2O and CO lines,

respectively.

Other instrumental effects that may affect theaccuracy of the radiances are uncertainties in theradiometric calibration of MIPAS such as blackbody temperature or non-linearity corrections.These errors are estimated to be about0.3–0:45 nW=ðcm2 sr cm�1Þ þ 1% of the radiance inchannel D [M. Birk, personal communication, 2005;L. Moreau, personal communication, 2005]. Rele-vant for this study is the constant component,though it can be further reduced by the determina-tion of the radiance offset during atmosphericmeasurements. To verify the given instrumentaluncertainties, two studies are performed:

(1)
Measured and modeled ozone radiances arecompared for nighttime conditions, using thesame spectral regions as for the daytime data.During nighttime the ozone (1 0 1) vibrationalstate is in LTE up to 70 km, so that measuredand modeled radiances should agree within20–30%, if ozone abundance and kinetic tem-perature are taken from the simultaneousretrievals. In fact, measured radiances in the0–2 nW=ðcm2 sr cm�1Þ interval are about0:1 nW=ðcm2 sr cm�1Þ þ 20% larger than themodel results. The constant shift is consistentwith the offset determination presented above,and the relative difference of 20% is compatiblewith uncertainties of retrieved ozone abundanceand temperature (see below). For radiancesX2 nW=ðcm2 sr cm�1Þ (resp. lower altitudes),the relative difference of 20% vanishes.
(2)
A similar study to (1) was performed for theCO2 emissions in the 2080–2090 cm�1 spectralregion. In this case, modeled radiances deviatein a similar manner from the measurements.
3. The non-LTE model

Part of the energy transferred to O3 after OþO2

recombination is directly converted to heat duringthe stabilization of the molecule immediately afterrecombination. Another part is used for the inter-nal, vibrational excitation of ozone. The earlieststate for which collisional re-dissociation of ozone isnegligible in comparison to radiative and collisionaldeactivation is called the quasi-nascent distributionof vibrational states and is not well known.Different models for the quasi-nascent distributionare assumed in this study. All of these models

ARTICLE IN PRESSM. Kaufmann et al. / Journal of Atmospheric and Solar-Terrestrial Physics 68 (2006) 202–212208

consider 251 ozone vibrational states up to thedissociation limit at 8547 cm�1.

The most simple quasi-nascent distribution modelis the prior or zero surprisal distribution (Levineand Bernstein, 1974). In this case, the probabilityPðvÞ populating the various vibrational states v

follows the density of states, only, and is given by

PðvÞ ¼ð1� EðvÞ=EDÞ

1:5

Pv ð1� EðvÞ=EDÞ

1:5,

where ED is the total energy available, i.e. thedissociation energy of ozone, and EðvÞ is thevibrational energy of a specific vibrational state.This distribution favors the population of lowenergy vibrational states. The total energy Etotal ¼P

v PðvÞEðvÞ released into the vibrational excitationof ozone is 4219 cm�1 in this case.

Other quasi-nascent distribution models used inthis study assume that only the v3 ¼ 3; 5 or 8vibrational levels are excited. Here, 3046, 4919 or7601 cm�1 are released into the vibrational excita-tion of ozone.

Collisional relaxation with N2 and O2 appears inthe four channels described in the introduction. Therate constants are adapted from Menard-Bourcin etal. (1996) and Menard et al. (1992). The values forhigher excited states are adjusted using a Land-au–Teller type scaling law. For intra-mode transi-tions (e.g. v2! v2 � 1) the rates scale linearly in v;for intermode transfers (e.g. ðv1; v2; v3Þ !ðv1; v2 þ 1; v3 � 1Þ) they scale with the product ofthe quantum numbers changed (v3 � ðv2 þ 1Þ in thiscase) (Schwartz et al., 1952).

Interactions with atomic oxygen are includedconsidering chemical and collisional deactivation ofozone. Rate constants are taken from West et al.(1978) applying the scaling rule of Rawlins andArmstrong (1987) for higher excited states. How-ever, collisions with atomic oxygen are negligible inthe altitude range considered in this study.

Full inter-layer radiative transfer is considered forall transitions listed in the current HITRANmolecular data base (Rothman et al., 2003) andextensions from J.M. Flaud (personal communica-tion, 2003). The uncertainty of line intensities in the1850–2300 cm�1 spectral range are of particularimportance for the limb radiance modeling. Theyhave a precision of 3.5% and an accuracy of 6%(Barbe et al., 1994). The accuracy of the linepositions are of minor importance, because of thebroad microwindows used in this study.

Absorption of solar radiation is considered for alltransitions listed in HITRAN. However, the che-mical excitation is much stronger, and the vibra-tional temperatures do not change by more than0.5K, if the direct solar excitation is neglected.

4. Modeling of MIPAS 4:8mm radiances

The modeling of MIPAS 4:8mm radiances re-quires the knowledge of ozone, atomic oxygen,temperature, pressure, and line-of-sight informa-tion. All of these quantities are determined fromsimultaneous retrievals of MIPAS radiances atdifferent wavelengths by means of the IMK/IAAdata processor.

Temperature, pressure, and line-of sight arederived from CO2 14.8 and 4:3mm radiances(Lopez-Puertas et al., 2004; von Clarmann et al.,2003). Temperature errors are 2K below 60 km andin the order of 5K between 60 and 80 km. Retrievedtangent altitudes are usually 1–3 km lower thanESA’s initial pointing data. In the mesosphere theiraccuracy is better than 200m. Pressure is calculatedon the basis of the hydrostatic approximation usingthe retrieved temperature and tangent altitude andone pressure data point from ECMWF (EuropeanCentre for Medium-Range Weather Forecasting) at20 km.

Ozone abundance is retrieved from MIPAS 14.8and 9:6mm radiances (Gil-Lopez et al., 2005). Itsuncertainty is 0.2 ppmv (20%) in the lower meso-sphere, 0.1–0.2 ppmv at 70–80 km, and about 30%in the upper mesosphere. The effect of non-LTEprocesses onto the ozone abundance retrieval isnegligible at altitudes below 70 km (Gil-Lopez et al.,2005).

Atomic oxygen is derived from ozone duringdaytime assuming steady state conditions. Assum-ing a 15% uncertainty in the Hartley band photo-lysis rates, atomic oxygen densities are uncertain by20–30% in the lower and middle mesosphere.

Contaminant gases such as CO and H2O are alsotaken from simultaneous retrievals of MIPAS data.However, the choice of special microwindowssuppresses emissions from these gases below 1% inthe 60–70 km altitude region. Therefore, uncertain-ties in these gases can be neglected.

5. Results

A complete orbit of MIPAS daytime radiances at4:8mm recorded in the UA1 measurement mode on


June 11, 2003 was modeled. All modeled profilesshow a significant and very similar underestimationof the wavenumber-averaged radiances by a factorof 2–3. Fig. 6 illustrates this difference for a typicalmid-latitude profile. At 60 km the calculated radi-ance is about 50% lower than the measurement. At70 km the difference is a factor 3, if no offset isassumed, or a factor of 2, if the maximum offset of0:1 nW=ðcm2 sr cm�1Þ is considered. The choice ofthe offset is not important at 60 km, becauseradiances are a factor of 5 larger here.

Uncertainties in the atmospheric state, i.e. mainlyin the retrieved ozone, temperature/line-of-sight,affect 4:8mm radiances by 10–20% each, i.e. thedifference between measured and modeled radiancesis significant for altitudes larger than 55 km. Here,non-LTE begins to affect the emitting vibrationalstates. Therefore, the adjustment of non-LTE

0.1 1 10Radiance [nW / cm2 sr cm-1]

30

40

50

60

70

Tan

gent

Hei

ght [

km]

MIPASν3=5

06691_20030611T125657Z

Fig. 6. Wavenumber-averaged radiances for the selected micro-

windows of MIPAS scan 06691_20030611T125657Z (lon ¼

�40:6�, lat ¼ 49:3�, solar zenith angle ¼ 33:3�, measured at 11

June 2003, 12:56) in comparison with a non-LTE forward

calculation assuming that ozone is produced at level v3 ¼ 5 after

the recombination reaction (red curve with triangles). The

vertically hatched area indicates the uncertainties of the calcula-

tion due to ozone abundance errors; the horizontally hatched

area due to kinetic temperature errors. The red shaded area

marks the total uncertainty including O3 and kinetic temperature

errors, and neglecting uncertainties in the non-LTE model.

Original MIPAS data is indicated by the thick solid line. The

black shaded area represents the range of the MIPAS data due to

uncertainties in the calibration and offset determination (assum-

ing an offset of 0:1nW=ðcm2 sr cm�1Þ).

processes provides a possibility to improve theagreement between measured and modeled data.

One non-LTE process, which affects the vibra-tional excitation of O3ð1 0 1Þ, a prominent emitter of4:8mm radiance at 60 km altitude, is the quasi-nascent distribution model. Assuming that ozone isproduced at v3 ¼ 5 after recombination, modelcalculations underestimate the measurements con-siderably, because a significant fraction of themolecules excited at v3 ¼ 5 thermalize withoutexciting O3ð1 0 1Þ. Hence, producing O3 at v3 ¼ 8decreases the limb intensities at 70 km by another30% (Fig. 7). Assuming a zero surprisal distributiongives very similar results to the v3 ¼ 5 case. Toincrease the radiance, ozone has to be produced atvibrational levels whose energy is closer to theemitting levels, i.e., closer to (1 0 1). Populating v3 ¼

3 directly enhances modeled radiances more than

0.1 1 10

Radiance [nW / cm2 sr cm-1]

40

50

60

70

Tan

gent

Hei

ght [

km]

06691_20030611T125657Z

Fig. 7. Wavenumber-averaged radiances for the selected micro-

windows of MIPAS scan 06691_20030611T125657Z (lon ¼

�40:6�, lat ¼ 49:3�, solar zenith angle ¼ 33:3�, measured at 11

June 2003, 12:56) in comparison with non-LTE forward

calculations assuming different quasi-nascent distributions:

Ozone is produced at level v3 ¼ 8, v3 ¼ 5, v3 ¼ 3, or applying a

zero surprisal distribution. For all cases the standard relaxation

scheme is applied, except for one case ðv3 ¼ 5Þ where the

collisional rates for hot band transitions converting stretching

to bending quanta are reduced by a factor of 3 (‘kd2=3’). Original

MIPAS data is indicated by the thick solid line. The black shaded

area represents the combination of uncertainties in the MIPAS

data as well as in the model calculations due to O3 and

temperature errors.


50% and gives a close agreement with the measure-ment.

Another possibility to increase modeled radiancesis to change the collisional coupling of the emittingstates. Non-LTE model calculations, that vary themagnitude of the different relaxation channels, areperformed to study this effect quantitatively. Thequenching constants for fundamental band transi-tions are always kept unchanged in these simula-tions. In general, a decrease of the collisional ratesthermalizes the molecule less strongly. In case of thev1;3 ! v2 (kD2) relaxation channel, a decrease of therate by a factor of 3–4 has to be applied to increase4:8mm radiances by a factor of 2 (Fig.7) to bringmeasured and modeled radiances into agreement.

However, the opposite effect is observed for thev2! ðv2 � 1Þ (‘k2’) relaxation channel, becauseO3ð1 0 1Þ marks the end of a n2 relaxation chainðð200; 101; 002Þ ð210; 111; 012Þ ð220; 121; 022Þ. . .; cf. Fig. 1), i.e. an increase of k2 increases thepopulation but not the loss of O3 ð101Þ. Indeed, thiseffect is not very strong, because an increase of k2

reduces the population of the ð210; 111; 012Þ triad,which is the reservoir of ð200; 101; 002Þ vibrationalstates within the v2! ðv2 � 1Þ relaxation chain, too.Quantitatively, a factor of three increase of k2

increases the vibrational temperature of O3 ð101Þ by4K at 60 km or 7K at 70 km, whereas a factor ofthree decrease of kD2 increases the vibrationaltemperature of O3 ð101Þ by 16K at 60 km or 22Kat 70 km.

6. Conclusions

MIPAS daytime limb measurements at 4:8mm inthe 50–75 km altitude region were analyzed withrespect to various non-LTE excitation and de-excitation mechanisms. The high spectral resolutionof the MIPAS instrument allows for the separationof ozone radiation from strong emissions of othersgases, such as CO. Non-LTE model calculations,that assume an ozone quasi-nascent distribution athigh energies (X5000 cm�1) and utilize the nominalrelaxation model, underestimate MIPAS measure-ments by a factor of two or three.

Uncertainties in the atmospheric state, which isretrieved simultaneously from MIPAS measure-ments at other wavelengths not affected by non-LTE, may account for 20–30% of the difference.

The radiometric accuracy of MIPAS, which wasverified by the analysis of ozone nighttime emissionsin the 50–70 km altitude regime not affected by non-

LTE, may account for another 10% of thedifference in the 60 km altitude regime.

Another possible explanation for the differencebetween measured and modelled radiances may bemissing species in the limb forward modeling. But tothe authors’ knowledge, there are no further emittersin addition to O3, CO, CO2, and H2O in the60–70 km regime that might significantly increase the4:8mm radiances (see, e.g. the measurements of theCIRRIS-1A experiment; Wise et al., 2001).

The enhancement of the highly uncertain vibra-tional excitation of the O3 vibrational states 101111, and 201 is another option. Basically there aretwo possibilities to compensate the differencebetween MIPAS 4:8mm measurements and non-LTE model calculations:

(1)
The quasi-nascent distribution: O3 vibrationallevels with v3 ¼ 3 instead of v3X5 have to beproduced directly after recombination. If O3 isproduced at higher vibrational states too littleenergy reaches the levels of interest. The sameoccurs for the zero surprisal distribution.
(2)
The stretching-to-bending intermode transfer:This process constitutes the most important lossmechanism of stretching vibrations. The colli-sional rates for hot band transitions ðkD2Þ haveto be reduced by a factor of three or four to fitthe MIPAS measurements.
Both cases constitute rather strong changes of theozone excitation and relaxation model, so that acombination of the various sources of uncertaintytogether with a moderate modification of the O3

non-LTE model seems to be the most likely way tofit MIPAS 4:8mm radiances.

Changes in the quasi-nascent distribution or thecollisional relaxation model have implications forthe energy budget of the mesopause region, becausethey control the amount of chemical energy which islost by chemiluminescence. Heating rates canchange several Kelvin in the mesopause regiondepending the non-LTE model (cf. Mlynczak,1991). A quantitative analysis is currently underinvestigation.

Further analyses of ozone infrared data undernon-LTE conditions as well as laboratory and/ortheoretical studies are recommended to achieve abetter understanding of the energy flow into theozone molecule upon recombination and to improveour knowledge of the collisional relaxation of ozonevibrational states.


Acknowledgements

The IAA team has been partially supported bySpanish Ministerio de Educacion y Ciencia underProjects REN2001-3249/CLI and ESP2004-01556,and by the European Community Marie Curie HostFellowship HPMD-CT-2000-40 (SIESTA).

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