AMTAtmospheric Measurement TechniquesAMTAtmos. Meas. Tech.1867-8548Copernicus PublicationsGöttingen, Germany10.5194/amt-11-2187-2018MIPAS observations of ozone in the middle atmosphereMIPAS middle atmospheric ozoneLópez-PuertasManuelpuertas@iaa.eshttps://orcid.org/0000-0003-2941-7734García-ComasMayahttps://orcid.org/0000-0003-2323-4486FunkeBerndhttps://orcid.org/0000-0003-0462-4702GardiniAngelaStillerGabriele P.https://orcid.org/0000-0003-2883-6873von ClarmannThomasGlatthorNorbertLaengAlexandraKaufmannMartinhttps://orcid.org/0000-0002-1761-6325SofievaViktoria F.https://orcid.org/0000-0002-9192-2208FroidevauxLucienWalkerKaley A.https://orcid.org/0000-0003-3420-9454ShiotaniMasatoInstituto de Astrofísica de Andalucía, CSIC, Granada, SpainKarlsruhe Institute of Technology, Institute of Meteorology and Climate Research, Karlsruhe, GermanyInstitute for Energy and Climate Research, Research Centre Jülich, Jülich, GermanyFinnish Meteorological Institute, Earth Observation, Helsinki FinlandJet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USADepartment of Physics, University of Toronto, Toronto, Ontario, CanadaResearch Institute for Sustainable Humanosphere, Kyoto University, Uji, Kyoto, JapanManuel López-Puertas (puertas@iaa.es)18April20181142187221222December201718March20189March20189January2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://amt.copernicus.org/articles/11/2187/2018/amt-11-2187-2018.htmlThe full text article is available as a PDF file from https://amt.copernicus.org/articles/11/2187/2018/amt-11-2187-2018.pdf
In this paper we describe the stratospheric and mesospheric
ozone (version V5r_O3_m22) distributions retrieved from
MIPAS observations in the three middle atmosphere modes (MA, NLC, and UA) taken with an unapodized spectral resolution of
0.0625 cm-1 from 2005 until April 2012. O3 is retrieved from microwindows in the 14.8 and
10 µm spectral regions and requires non-local thermodynamic equilibrium (non-LTE) modelling of the O3v1 and v3 vibrational
levels. Ozone is reliably retrieved from 20 km in the MA mode (40 km for UA and NLC) up to ∼ 105 km during dark
conditions and up to ∼ 95 km during illuminated conditions. Daytime MIPAS O3 has an average vertical
resolution of 3–4 km below 70 km, 6–8 km at 70–80 km, 8–10 km at 80–90, and 5–7 km at the secondary maximum
(90–100 km). For nighttime conditions, the vertical resolution is similar below 70 km and better in the upper
mesosphere and lower thermosphere: 4–6 km at 70–100 km, 4–5 km at the secondary maximum, and 6–8 km at
100–105 km. The noise error for daytime conditions is typically smaller than 2 % below 50 km, 2–10 % between
50 and 70 km, 10–20 % at 70–90 km, and ∼ 30 % above 95 km. For nighttime, the noise errors are very
similar below around 70 km but significantly smaller above, being 10–20 % at 75–95 km, 20–30 % at
95–100 km,
and larger than 30 % above 100 km. The additional major O3 errors are the spectroscopic data uncertainties
below 50 km (10–12 %) and the non-LTE and temperature errors above 70 km. The validation performed suggests that
the spectroscopic errors below 50 km, mainly caused by the O3 air-broadened half-widths of the v2 band, are
overestimated. The non-LTE error (including the uncertainty of atomic oxygen in nighttime) is relevant only above
∼ 85 km with values of 15–20 %. The temperature error varies from ∼ 3 % up to 80 km to 15–20 %
near 100 km. Between 50 and 70 km, the pointing and spectroscopic errors are the dominant uncertainties. The validation
performed in comparisons with SABER, GOMOS, MLS, SMILES, and ACE-FTS shows that MIPAS O3 has an accuracy better
than 5 % at and below 50 km, with a positive bias of a few percent. In the 50–75 km region, MIPAS O3 has
a positive bias of ≈ 10 %, which is possibly caused in part by O3 spectroscopic errors in the
10 µm region. Between 75 and 90 km, MIPAS nighttime O3 is in agreement with other instruments by
10 %, but for daytime the agreement is slightly larger, ∼ 10–20 %. Above 90 km, MIPAS daytime O3
is in agreement with other instruments by 10 %. At night, however, it shows a positive bias increasing from
10 % at 90 km to 20 % at 95–100 km, the latter of which is attributed to the large atomic oxygen abundance
used. We also present MIPAS O3 distributions as function of altitude, latitude, and time, showing the major
O3 features in the middle and upper mesosphere. In addition to the rapid diurnal variation due to photochemistry,
the data also show apparent signatures of the diurnal migrating tide during both day- and nighttime, as well as the
effects of the semi-annual oscillation above ∼ 70 km in the tropics and mid-latitudes. The tropical daytime
O3 at 90 km shows a solar signature in phase with the solar cycle.
Introduction
Ozone is a key constituent of the atmosphere playing a major role in its energy budget and chemistry, particularly in the
stratosphere and upper mesosphere . Typical ozone profiles show two clearly distinguished maxima, one
located between 10 and 35 km, the so-called ozone layer, where most ozone resides, and a secondary maximum around the
mesopause (∼ 90–95 km). Ozone has been extensively measured in the stratosphere and also in the mesosphere using
different techniques (see e.g. for measurements taken prior to 2003 and for the
most recent observations, mainly from satellite instruments). Measurements that cover the atmosphere from the lower
stratosphere up to the lower thermosphere, both at day- and nighttime conditions and with global latitudinal coverage
are, however, not very frequent. MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) is a high-resolution
limb sounder on board the Envisat satellite, launched on 1 March 2002 and taking measurements until 8 April 2012, when the
Envisat satellite failed. The satellite was inserted into a polar Sun-synchronous orbit at an inclination of
81.5∘ and altitude of about 800 km. The orbital period was about 101 min, achieving a global coverage of the
Earth with 14.3 daily orbits, with the descending (north-to-south) part crossing the Equator at approximately 10:00 LT
(local time) and the ascending part crossing it at approximately 22:00 LT. MIPAS had a wide spectral coverage and a high spectral resolution,
operating at 0.025 cm-1
during 2002–2004 and 0.0625 cm-1 from 2005 until 8 April 2012 . MIPAS operated with
a global latitude coverage (pole-to-pole) and performed measurements during day and night. The instrument spent most of
the time observing in the 6–68 km altitude range (the nominal mode, NOM), but it also regularly looked at higher
altitudes in its middle atmosphere (MA), noctilucent (NLC), and upper atmosphere (UA) modes
. The retrieval of ozone from the NOM mode has been carried out, among others, by
the Institute of Meteorology and Climate Research and Instituto de Astrofísica de Andalucía (IMK/IAA)
. In that inversion local thermodynamic equilibrium (LTE) is assumed, which is a good approximation up
to about 50 km and a reasonable approach up to about 60 km . MIPAS took a few spectra of the
middle atmosphere (only half a day of measurements on 11 June 2003) with its full spectral resolution mode. Ozone
concentrations were retrieved from those spectra from the low stratosphere up to the lower thermosphere, including the
non-LTE effects . In this paper we focus on the inversion of ozone from the bulk of MIPAS
observations of the middle atmosphere taken from 2005 until April 2012 in three different modes (MA, NLC, and UA) with the
optimized spectral resolution of 0.0625 cm-1. Part of this dataset (2008–2009), version V4O_O3_m02, was
previously retrieved and used in some studies e.g.. Here we describe the inversion of the entire
MA/UA/NLC period, 2005–2012, version V5r_O3_m22, derived from V5 L1b spectra. The method is essentially described in
. Here we review several aspects of the retrieval baseline including the changes in the non-LTE
modelling of O3 and the new microwindows (MWs) used. In addition, an assessment of the quality of the middle
atmosphere O3 data, a comparison with the previous version V4O_O3_m02 (sometimes referred to as V4O_502), and
a validation with recent middle atmosphere ozone measurements taken by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER),
Global Ozone Monitoring by Occultation of Stars (GOMOS),
Microwave Limb Sounder (MLS),
Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) and
Atmospheric Chemistry Experiment's Fourier Transform Spectrometer (ACE-FTS) are also
presented. Finally we describe the major features of the O3 database focusing on the mesosphere.
O3 non-LTE modelling
As mentioned below most of the MWs used in the retrieval of O3 in the mesosphere are located in the
9.6 µm region, where the v1 and v3 fundamental, combinational, and hot bands arise. We discuss here the
non-LTE modelling of those vibrational levels. The v2 fundamental band is used only at the lower altitudes, below
50 km, where it is in LTE . We should clarify that the non-LTE populations of O3 used in the
retrieval have been calculated with the Generic RAdiative traNsfer AnD non-LTE population algorithm (GRANADA) (see
Sect. ). In this section we describe a simplified formulation of non-LTE for O3 with the aim of
showing the quantities required for performing the retrieval.
The major processes affecting the populations of the vibrationally excited O3(v1,v2,v3) levels are
discussed in detail by and are listed in Table . The collisional processes (1–4)
have been adapted from Table 7 in , to which we have added the simplified radiative processes of (a) the
absorption of radiation from the layers below (process 5) and (b) the spontaneous emission (process 6).
From the statistical equilibrium assumption and the processes listed in Table , the population of the
vibrationally excited O3(v3) levels (we focus here on v3, from whose emission we are retrieving O3)
can be obtained, approximately, by
[O3(v3)][O3]=JEarth+pt+pntA+kvt,M[M]+kvt,O[O]+kchem[O],
where the brackets denote concentrations,
JEarth is the excitation rate of O3(v3) due to the
absorption of ν3 photons coming from the lower atmospheric regions (upwelling flux), A is the Einstein coefficient
of the ν3=1 band, [O] the atomic oxygen concentration, [M] the air molecules concentration (sum of
N2 and O2), and the other collisional rates are defined in Table . pt is the
specific thermal production of O3(v3) given by
pt=kvt,M[M]exp[-E(v3)/kT],
where T is the kinetic temperature, k the Boltzmann constant and E(v3) the energy of the O3(v3)
level. The specific production due to non-thermal processes (chemiluminescence, process 1 in Table ),
pnt, is given by
pnt=k1[O2][M]([O]/[O3])ϕ(v3),
where ϕ(v3) is the fraction of the O3 molecules produced by process 1, which finally end excited in the
O3(v3) level. In practice, ϕ(v3) depends not only on the nascent distribution of reaction 1 but also, and
mainly for the lower v3 levels, on the radiative and collisional relaxations of the more energetic (v1,v2,v3)
levels, i.e. on A,kvt,M,kvt,O, and kchem for those levels and also
on [M] and [O]. From all those parameters, the only unknown in our case is the atomic oxygen concentration, [O].
Collisional and radiative processes affecting the O3
vibrational levels.
No.RateProcess1k1O2+O+ M →O3(v1,v2,v3)2kvt,MO3(v1,v2,v3) + M ⇌O3(v1′,v2′,v3′) + M3kchemO3(v1,v2,v3) + O →O2+O24kvt,OO3(v1,v2,v3) + O →O3(v1′,v2′,v3′) + O5JEarthO3+ hν→O3(v1,v2,v3)6AO3(v1,v2,v3) →O3(v1′,v2′,v3′) + hν
Note that the energy of the O3(v1,v2,v3) level is larger
than that of O3(v1′,v2′,v3′).
For the retrieval of O3, one needs the [O3(v3)] / [O3] ratio of Eq. (),
which is proportional to the O3(v3) vibrational temperature and hence to the measured O3 emission, to
be the least dependent as possible on kinetic rates and other atmospheric parameters. For the MIPAS O3 retrieval,
all the parameters in Eqs. , , and are known, or measured simultaneously
by MIPAS, except [O].
Because of the rapid timescales for ozone production and loss in the non-LTE region, the assumption of photochemical
equilibrium above around 60 km is valid. Thus, from the major photochemical reactions affecting O3 in the
mesosphere and lower thermosphere (see Table ), the non-thermal production, pnt, and [O]
are given for daytime conditions, respectively, by
pnt, day=JSunϕ(v3)and[O]day≈JSun[O3]k1[O2][M],
where the chemical losses (k2 and k3) have been neglected. In this way, we have no major limitation of unknown
atmospheric parameters in the O3 non-LTE retrieval during daytime if we update the non-LTE model with the
O3 abundance in each iteration.
Major photochemical reactions affecting O3 in the mesosphere
and lower thermosphere.
No.RateProcess1k1O2+O+ M →O3(v1,v2,v3)2k2H+O3→OH*(v) +O23k3O +O3→O2+O24JSunO3+ hν→O2+ O
During nighttime, assuming also photochemical equilibrium and the processes in Table , the non-thermal
production and [O] are given, respectively, by
pnt, night=k2[H]1-k3[O3]k1[O2][M]and[O]night≈k2[H][O3]k1[O2][M]-k3[O3].
In this case, even iterating, we still have the unknown of the atomic hydrogen concentration, [H], which has to be
taken from models or from parameterization. As we see, the non-thermal production in nighttime is directly proportional to
[H]. Then, in order to estimate the errors introduced by the uncertainty in [H], it is worthwhile to
estimate the importance of the contribution of the non-thermal term, pnt, night in Eq. (), in
comparison with the radiative and thermal contributions, JEarth, and pt in
Eq. (). Figure show the calculations of these terms computed with the GRANADA model for a few
nighttime profiles for 2 days typical of equinox and solstice. The figure shows that the contribution of the non-thermal
term to the excitation of O3(v3= 1) is, in the worst case, a factor of 5 smaller than the radiative and
thermal excitations. We then expect that the uncertainties in the inputted [H] from the US
Naval Research Laboratory Mass Spectrometer and Incoherent Scatter Radar model
(NRLMSISE-00)
see below would not introduce large uncertainties in the retrieved O3. This effect, however,
is larger for higher O3 vibrational levels because for these states the radiative term is negligible and the
non-thermal part is larger. Thus the uncertainty in [H] might introduce a larger error for the case of wide band
radiometers measuring a significant contribution of the hot bands' emission.
Specific production terms of the population of O3(001) in
nighttime for 2 days typical of equinox (black) and solstice (red). Dotted
lines are the specific non-thermal productions of O3(v3= 1)
(Eq. ), and solid lines are the sum of the radiative
absorption and thermal collisions, JEarth and pt in
Eq. ().
Ozone non-LTE retrieval
As described above, we used MIPAS spectra taken in the MA, NLC, and UA modes. In the MA mode, the spectra are taken at
limb tangent heights from about 20 up to 102 km with a vertical spacing of 3 km. The UA mode
ranges from about 42 up to 172 km, which has a 3 km vertical sampling up to 102 and
5 km above. The NLC mode is a variant of the MA mode specifically tailored for measuring the NLCs during the
summers . In this mode the spectra are taken at tangent heights from 39 up to
78 km at 3 km steps, from 78 up to 87 km at 1.5 km steps, and from 87 up to 102 km again in
3 km steps. MIPAS horizontal field of view (FOV) is approximately 30 km.
The method used for the inversion of ozone under non-LTE conditions is essentially described in . In
that work the retrieval was adapted for the very few (only 26 orbits) MIPAS data of the upper atmosphere taken on
June 2003 at the full spectral resolution of 0.025 cm-1. Here we briefly summarize the approach, discuss the
new MWs used in the inversion of the MA, NLC, and UA observational modes (taken at 0.0625 cm-1), and review the
changes in the non-LTE modelling of O3 vibrational levels.
The MIPAS V5r_O3_m22 ozone retrieval is based on a constrained multilinear least-squares fitting of non-LTE limb
radiances. That is performed using the IMK/IAA Scientific Processor extended with the
non-LTE GRANADA algorithm , which is able to cope with non-LTE emissions. describe the
peculiarities of the retrievals under consideration of non-LTE. The basic retrieval equations, the methods for
characterization of results through error estimates and vertical and horizontal averaging kernels, the iteration and
convergence criteria, and the regularization method are described in . Version V5
(5.02/5.06) of the ESA-calibrated L1b spectra was used seeand references therein.
The inversion is performed after a retrieval of the residual spectral shift and the non-LTE retrieval of temperature
. The IMK/IAA processor simultaneously retrieves, besides the O3
abundance, microwindow- and altitude-dependent continuum radiation and zero level calibration corrections (the latter,
assumed constant with altitude).
The retrievals are performed from the surface to 120 km over a fixed altitude grid of 1 up to 50 km, at 72–75 km,
and at 77–88 km; of 2 km at 50–72 km, 75–77 km, and 88–102 km; and of 5 km from 105 up to 120 km. The grids
have been selected as a trade-off between higher accuracy and computational efficiency. The forward calculations are
performed using the same grid. The oversampled retrieval grid, finer than the MIPAS vertical sampling of 3 km, makes
necessary the use of a regularization in order to obtain stable solutions. We used here a Tikhonov-type first-order
smoothing constraint . The numerical integration of the signal over the 3 km FOV is done using five
pencil beams. The selected width of the integration window (apodized instrument line shape (ILS) function) avoids channel border
effects. Forward model calculations along the line of sight (LOS) are done considering horizontal gradient
corrections. Thus, they account for changes in the populations (either in LTE or in non-LTE) of the emitting O3
levels along the LOS due to kinetic temperature variations.
Microwindows and altitude ranges used in the retrieval of MIPAS
ozone V5r_O3_m22.
No.Wave number (cm-1) Altitudes (km) MinimumMaximum182124273033363942454851545760636669727578–1021687.6875688.6875▪▪▪▪▪▪▪▪▪▪▪▪▪▪2689.3125691.8750▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪3692.2500695.1875▪▪▪▪▪▪▪▪▪▪▪▪▪▪4707.1250710.0625▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪5712.3125713.4375▪▪▪▪▪6713.5000716.4375▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪7716.5000719.4375▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪8720.7500723.6875▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪9728.5000729.3750▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪10730.0625730.5000▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪11731.9375732.8750▪▪▪▪▪▪▪▪▪▪▪▪12734.0000734.7500▪▪▪▪▪▪▪▪▪▪13736.4375739.3750▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪14739.4375741.9375▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪15745.2500745.6875▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪16746.6875747.1250▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪17747.6250748.3750▪▪▪▪▪▪▪▪▪▪▪▪▪18749.5625752.5000▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪19752.9375755.8750▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪20758.3750759.4375▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪21759.5000761.8750▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪22765.0000765.6250▪▪▪▪▪▪▪▪▪▪▪▪▪▪23767.5000768.0000▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪24771.8750772.1250▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪25774.2500774.5625▪▪▪▪▪▪▪▪▪▪26776.5000776.7500▪▪▪▪▪▪▪▪27780.2500781.9375▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪28788.9375789.6875▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪29790.7500791.0000▪▪30791.1875791.5625▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪311034.12501034.3750▪▪▪▪▪321034.43751035.0000▪▪▪▪▪▪▪▪▪▪331038.18751039.0000▪▪▪▪▪▪▪▪▪▪341040.00001040.8125▪▪▪▪▪▪▪▪▪▪351048.81251049.5000▪▪▪▪▪▪361050.62501051.8125▪▪▪▪▪▪371053.31251053.8125▪▪▪▪▪▪381054.68751055.5000▪▪▪▪▪▪
Examples of MIPAS L1b apodized single spectra in
channel A (d) and in channel AB (a–c) at several tangent
heights (as labelled) used in the inversion of O3. The geolocation of
the spectra is given in panel (a). The radiance in the 1030–1060 cm1
spectral region is dominated by O3 features. However, in
channel A (d), it is dominated by CO2 emissions and the
O3 contribution is hardly noticed.
Ozone is reliably retrieved from 20 km in the MA mode (40 km for the UA and NLC modes) up to ∼ 105 km during dark
conditions and up to ∼ 95 km during illuminated conditions. The logarithm of the volume mixing ratio (VMR) is
retrieved from MWs covering rotational–vibrational emissions of the O3 main isotope. They have been selected from a broad
spectral region, covered by channel A (685–970 cm-1) and channel AB (1020–1170 cm-1), and
they vary with tangent altitudes in order to optimize computation time and minimize systematic errors
. The MWs used are listed in Table , and Fig. shows some examples of
single MIPAS spectra in the A and AB channels. The MWs are an extension of the set used for the ozone retrieved
from the MIPAS NOM mode of observation, covering altitudes below 70 km . Below 50 km, we use the
same MWs as in the NOM retrieval, located all in channel A. Above 50 km, the MWs used are mainly located in channel AB
and are strongly affected by non-LTE.
The MIPAS V5r_O3_m22 ozone retrieval setup uses the following inputs. The O3 a priori is taken from a MIPAS
dedicated climatology similar to that described in. Pressure, LOS, temperature, and temperature
horizontal gradients are taken from MIPAS T-LOS retrieval V5r_T_m21 . They have been retrieved
from the CO2 emission near 15 µm and recorded in channel A, accounting for the non-LTE effects. The
detailed description of the method and the characterization of the inverted pressure–temperature profiles are described
in . The upgrades in the retrieval of the temperature used here (V5r_T_m21) and a validation of
the results are reported by . This version of MIPAS temperatures correct the main systematic errors
of the previous version and have, in general, a remarkable agreement with the measurements taken by ACE-FTS, MLS, OSIRIS,
SABER, SOFIE, and the Rayleigh lidars at Mauna Loa and Table Mountain. In the region of interest here, however, there are
still significant differences, with the MIPAS mesopause differing by 5–10 K from the other instruments, being
warmer than SABER, MLS, and OSIRIS and colder than ACE-FTS and SOFIE.
The O3 spectroscopic data were taken from the HITRAN 2008 database . A test performed using the
HITRAN 2016 O3 database has shown that the radiances are just marginally larger (only 0.5 %
in channel A and 0.25 % in channel AB).
The O3 non-LTE vibrational populations were computed online in each iteration of the inversion by using the
GRANADA model . As described above, the O3 non-LTE model requires several inputs. The set of
O3 vibrational levels, the non-LTE collisional scheme and the rate constants are based on the GRANADA model except
as noted below.
The exponent a in the rate k1=6.0×10-34(T/300)a of reaction
O+O2+M→O3(v1,v2,v3) +M has been updated
from 2.3 to 2.4 following . This has a small effect on the O3v1 and v3 vibrational
temperatures and also on the retrieved O3.
The collisional deactivation of O3(v1,v2,v3) (process 2 in Table ) with Δv1 or
Δv3=-1 and Δv2= 1 (process 2c, kd, in Table 7 of ) was erroneously
implemented in the previous version with a rate of 3.1 × 10-15(T/300)1/2. Now we include the
expression in Table 7 of but limited to a minimum value of
4 × 10-16cm3s-1 at temperatures below 200 K . Although this change in
the rate coefficient is rather large, its impact on the O3(v3) vibrational temperature is very small, as it is
dominated by the relaxation to v2. This leads to an O3 VMR about 5 % smaller between 70 and 85 km.
The chemical quenching of vibrationally excited O3 by O,
O3(v1,v2,v3) +O→O2+O2, has been neglected in this
version. This implies larger O3(v3) vibrational temperatures and hence smaller retrieved O3
abundances. Note, however, that the total quenching of O3(v1,v3) by O, including also collisional relaxation
by O (process 4) in Table ), is still within the measurement errors of the deactivation of
O3(v1,v3) see discussion below.
Latitude–altitude cross sections of MIPAS MA daytime
ozone (a, d), its vertical resolution (b, e), and single
profile noise error (1σ, c, f) for solstice
(December–January–February: DJF) (a–c) and equinox
(March–April–May; MAM) (d–f). All measurements from 2005 to 2012
are included. White areas denote regions where the retrieved O3 is
not significant. Contour lines are marked in the colour bar scale.
As shown in Sect. , the inversion of O3 under non-LTE conditions requires knowledge of the atomic
oxygen concentration, [O]. In our retrievals, we constrain the O abundance by employing photochemical equilibrium
with the O3 abundance retrieved in the previous iteration. This photochemical constraint for
[O]/[O3] is an optional feature in GRANADA which has been switched on in this retrieval below 97 km.
At night, the non-thermal production and O depends not only on O3 but also on hydrogen, H (see
Eqs. and ). For this reason, we use the H abundance from the NRLMSISE-00 model
in the photochemical equilibrium computation. Above 97 km we use the atomic oxygen from the Whole
Atmosphere Community Climate Model with specified dynamics (SD-WACCM) simulations spanning over the time period of the
measurements . To be more precise, both profiles were merged in the 92–102 km region using a hyperbolic
tangent function centred at 97 km. SD-WACCM (in the following just WACCM) is constrained with output from NASA's
Modern-Era Retrospective Analysis (MERRA) below approximately 1 hPa. The main reason for including
WACCM about that altitude is motivated by the lack of or incorrect latitudinal and seasonal variation of atomic oxygen in the
NRLMSISE-00 model.
The forward model also includes the contribution (as a potential overlap with O3 lines) of CO2 lines. As
for O3, we also include the emissions by CO2 bands in non-LTE. A detailed description of the CO2
non-LTE model used and all the required input parameters can be found in .
Characterization of the retrieved O3 and error analysis
Figures and show seasonal zonal means of O3 retrieved from MIPAS and the mean of
the noise error (1σ) and vertical resolution for the MA mode for daytime (10:00) and
nighttime (22:00) conditions, respectively. The results for the UA and NLC modes are very similar except that the
O3 is retrieved above 40 km instead of 20 km. The ozone fields are included in these figures only for
a reference to the noise and vertical resolution; its major features and the vertical and latitude distributions are
discussed in Sect. below. Two seasons are shown; the noise error and vertical resolution for the other two
seasons are very similar.
As Fig. but for nighttime conditions.
The vertical resolution of the MIPAS retrieved ozone is given by the full width at half maximum of the averaging kernels
rows. For daytime, the O3 average vertical resolution is 3–4 km below 70 km, 6–8 km at 70–80 km, 8–10 km
at 80–90 km (although it can be coarser in tropical regions), and 5–7 km at the secondary maximum (90–100 km). For
nighttime conditions the vertical resolution is similar below 70 km, and it is better in the upper mesosphere and lower
thermosphere. It is 4–6 km at 70–100 km (except for a narrow region near 80 km where it takes values of 8–10 km),
4–5 km at the secondary maximum, and 6–8 km at 100–105 km.
Two criteria are recommended to be used to screen the retrieved O3 version V5r_O3_m22 data in order to guarantee
the quality of the profiles. First, the retrieved O3 values of individual profiles where the diagonal value (or the
mean diagonal value when averaging) of the averaging kernel is less (in absolute value) than 0.03 are considered
non-trustful. Second, those values corresponding to altitudes not sounded by MIPAS (below the lowermost tangent altitude)
and flagged by the visibility flag should not be used.
The error budget described here considers the propagation of the measurement noise and of the uncertainties of model
parameters onto the retrieved ozone abundances. Noise-induced retrieval errors (as well as the vertical resolution; see
above) are estimated routinely for each individual profile by the retrieval algorithm. The ozone noise error is calculated
assuming a wavelength-dependent noise-equivalent spectral radiance which has approximated average values of 17 and
10 nW (cm2 cm-1 sr)-1 for channels A and AB, respectively.
Typical values (1σ) for daytime (see the
right column of Fig. ) are smaller than 2 % below 50 km, 2–10 % between 50 and 70 km,
10–20 % at 70–90 km, and about 30 % above 95 km. For nighttime (see right column of Fig. ), the
noise errors are very similar below around 70 km but significantly smaller above, being 10–20 % at 75–95 km,
20–30 % at 95–100 km, and larger than 30 % above 100 km.
Errors of the retrieved O3 VMR for daytime (a) and nighttime (b) in relative values. The different sources are described in the legend of Table .
Summary of main errors (1σ) of ozone VMR in percent. Errors
refer to nighttime conditions with daytime values, when different, in
parentheses. “NLTE” includes errors due to uncertainties in the collisional
rates of the non-LTE model and in the atomic oxygen (see text). “Total
(Syst.)” is the root sum square of all systematic errors (noise is not
included). LOS stands for line of sight, “Tem” for temperature, ILS for
instrument line shape, and “Spec.” for spectroscopy.
Errors related to the mapping of uncertain model parameters on the retrieved ozone VMR are estimated for representative
profiles for daytime and nighttime conditions. We name these “systematic” errors to distinguish them from the noise
error, although they are not purely systematic but in some cases they also have a random component. We have calculated
them by comparing the retrieved profiles obtained with the nominal parameter and with the perturbed parameter.
Figure shows those errors (1σ), including also mean profiles of the noise errors shown in
Figs. and , and Table lists them. The uncertainties assumed for the
estimation of the systematic errors are 1 % for gain calibration and 3 % for the ILS. For
the elevation pointing (LOS) we have assumed an error of 150 m below 60 km, where we have information on the relative
pointing from the temperature–LOS retrieval . Above that height, where we obtain the
LOS information from the engineering tangent altitudes of MIPAS (adjusted with the LOS retrieved below), we assumed an
error of 300 m. For temperature, the errors have been considered as 0.5 K below 50 km, 1 K at 50–70 km, 2 K at
70–80 km, 5 K at 80–100 km, and 10 K above .
The spectroscopic errors have been estimated by J.-M. Flaud (personal communication, 2008). The errors assumed for the
line intensities of the fundamental v1, v2, and v3 bands (those mainly used here) are 2 % for the strongest
lines, 5 % for the moderate lines, and 10 % for the weaker lines. For the lines of the hot bands with a lower
state of (010), (020), (100), or (001), the error is in the range of 4–25 %, increasing as the line intensity
decreases. The errors considered for the more excited hot bands are larger, up to 30 %, but their contributions to the
selected MWs is small. The error assumed for the O3 air-broadened half-widths is 12 %. The larger O3
spectroscopic errors below 50 km (see Fig. ) are mainly due to the error in the half-widths. The
contribution of the error in the line intensities is only 2–3 %.
The modelling of the non-LTE populations of the O3 vibrational levels emitting near 9.6 µm is an
important source of the MIPAS ozone systematic error above the mid-mesosphere. Non-LTE errors are dominated by the
uncertainties in the collisional rates used in the non-LTE model. The three major sources are (1) the error in the
three-body reaction rate of O3 formation, (2) the thermal relaxation of the O3(v1,v3) levels with
N2 and O2, and (3) the collisional relaxation and/or chemical reaction of O3(v1,v3) levels
with atomic oxygen. According to the current literature, we have considered uncertainties of 10 % for the first,
20 % for the second, and 50 % for the third. For the latter, derived experimentally the
deactivation rate of O3(v1,v3) by O and provided rates for both the thermal relaxation,
O3(v1,v3) +O→O3+O, and the chemical reaction,
O3(v1,v3) +O→ 2 O2, assuming each one alone was responsible for the total
deactivation. These authors could not estimate the relative contribution of each process and the error associated with
these two rates is about 50 %. further reported that the deactivation is most likely to occur through
the thermal relaxation. Hence, we have considered that the deactivation takes place through the thermal relaxation
(process 2 in Table ) and assumed the measured error of 50 %.
Comparison of O3 abundance retrieved in the current
V5r_O3_m22 with the previous V4O_O3_m02 version for daytime conditions
(10:00) (a, b) and nighttime (c, d). In absolute
values (a, c) and in percent (b, d) of the older version
(V5r_O3_m22-V4O_O3_m02)/V4O_O3_m02, for all data taken in 2009.
We have not considered additional errors due to the uncertainty in the atomic oxygen (daytime) or atomic hydrogen
(nighttime) below 95 km (the error due to the O3 error itself is already taken into account). During daytime,
this is reasonable because atomic oxygen is derived from the photochemical equilibrium with the retrieved O3. At
night, however, when the retrieval depends on the atomic hydrogen concentration, if the error in H is significantly
smaller than the 50 % assumed for the deactivation of O3(v1,v3), its contribution will not be
significant. If comparable, however, we might be underestimating the non-LTE errors. Above 97 km, we have assumed an
uncertainty in the WACCM atomic oxygen of 50 %, leading to an error of about 10 % in the retrieved O3 in
that region (see Fig. ). This error has been added, quadratically, to the other components of the non-LTE
uncertainties discussed above.
The uncertainty of the first of those three processes on O3 is only important above ∼ 85 km, reaching
a maximum of 5 % in the retrieved O3. The second one is significant only from ∼ 60 up to
∼ 80 km, introducing an error of 2–5 % in ozone for nighttime, and of 2–10 % for daytime, including polar
summer, where it might be slightly larger near 80 km. The third one, including the error in the atomic oxygen, is
only significant above ∼ 85 km but it is the largest, with an estimated error in O3 of 15–20 % at
85–100 km. Overall, the non-LTE errors are typically negligible below 60 km, 2–10 % at 60–80 km, and
15–20 % above 85 km.
The overall systematic component of the O3 abundance error is dominated by the spectroscopic data uncertainty
below 50 km and by the non-LTE and temperature errors above about 70 km. Between 50 and 70 km, the pointing (LOS) and
the spectroscopic errors are the dominant uncertainties. Our validation studies suggest, however, that the spectroscopic
errors below 50 km are overestimated (see Sect. ).
Differences between current V5r_O3_m22 and previous V4O_O3_m02 versions
Since the previous V4O_O3_m02 version of MIPAS O3 has been used in some previous studies,
e.g., it is interesting to compare those results with the VMRs of this new V5r_O3_m22
version. The V5r_O3_m22 ozone retrieval setup has been improved with respect to the earlier in the following aspects.
First, the MIPAS L1b spectra have been updated from version 4.61/62 to version V5 (5.02/5.06). The spectroscopic data were
upgraded from the HITRAN database version of 2004 to that of 2008. The retrieved kinetic temperature from MIPAS has also
been changed to the temperature version of v5r_TLOS_m21 (see major differences discussed above). We have also improved
the width of the integration window of the apodized ILS function. The uppermost altitude of the
continuum retrieval has been expanded from 30 to 50 km. The regularization scheme has been updated in order to make it
compatible with that used in the nominal O3 retrieval in the common altitude range (up to
70 km). The MWs have also been revised so that below 50 km we only use MWs located in channel A (see Table ),
like in NOM retrievals. We use a different distribution of the CO2 abundance, now taken from WACCM (see above).
The merging altitude of the daytime atomic oxygen derived from the ozone retrieval (assuming photochemical equilibrium) to
the model (supplied for the region above) has been changed from 95 to 97 km. At night we need, additionally, the
H concentration, which has been taken from the NRLMSISE-00 model . In the previous version we took the
nighttime O from the NRLMSISE-00 model in the whole altitude range. Above 97 km we use now the atomic oxygen from WACCM
instead of that of NRLMSISE-00. An important update is the recalculation of the O3 vibrational temperatures
during the iterations in the inversion process, following the update of the atomic oxygen. The O3 non-LTE model
has also been improved, as described previously in Sect. .
The average impact on the ozone retrieval after those changes (see Fig. ) is an increase of 2–3 %
(0.2–0.5 ppmv) below around 40 km (except in the polar winter, where there is a decrease). The clear increase
around 40 km is due to the inclusion of the retrieval of the continuum for this altitude.
There is a clear decrease by ∼ 5–10 % (∼ 0.4–0.5 ppmv) between 40 and 50 km, principally induced by the
use of MWs in channel A only (MWs in channel AB for these altitudes were removed). In the lower and middle
mesosphere, 50–80 km, there is an increase of about 2–5 % (0.1–0.2 ppmv). In the upper mesosphere, there
is a general decrease of ∼ 0.5–1 ppmv in nighttime conditions, principally caused by neglecting the removal
of the excited O3(v3) by chemical reaction with atomic oxygen. This produces a larger population of
O3(v3) and hence less O3. In daytime the effect is much smaller and has a smaller impact on the
retrieved ozone (in absolute values). At altitudes above around 95 km, the O3 in the new version is larger by
about 5–10 %. This is caused by the use of the atomic oxygen from the WACCM model, which is larger than in NRLMSISE-00
and even overcomes the smaller relaxation of O3(v3) due to the chemical relaxation being ignored.
Validation
We have compared MIPAS V5r_O3_m22 ozone retrievals with co-located measurements from SABER, GOMOS, MLS, SMILES, and
ACE-FTS. Comparisons for GOMOS are only for night conditions and in number density. For ACE-FTS, because it is an
occultation instrument and O3 has very large diurnal variations around the terminator in the middle and upper
mesosphere, we compare ACE sunset and sunrise with MIPAS observations with solar zenith angles (SZA) in the range of 88 to
92∘. In order to select a pair of profiles to compare, we have selected measurements with universal time
differences smaller than 2 h and distances smaller than 1000 km. Considering an additional criterion of 1 h local time
difference did not change the results significantly.
InstrumentsSABER
SABER is a broadband radiometer flying on board the
NASA's Thermosphere–Ionosphere–Mesosphere Energetics and Dynamics (TIMED) satellite, launched in December 2001 and
starting operations in January 2002 . SABER measurements cover 83∘ S to 52∘ N
and from 52∘ S to 83∘ N, alternatively every 2 months. A 24 h local time coverage is completed
approximately in 60 days. SABER observes the daytime and nighttime ozone limb emission at 9.6 µm, from which
the ozone concentration is retrieved under non-LTE conditions from 10 to 100 km. We use version 2.0 ozone here, publicly
available at http://saber.gats-inc.com (last access: December 2017). The non-LTE model used in SABER ozone retrievals is described in
and references therein. Similar to MIPAS, the retrieval of O3 from SABER requires knowledge
of pressure, temperature, and atomic oxygen. The first two are taken from SABER retrievals of simultaneous measurements at
15 µm. The atomic oxygen for SABER ozone retrievals is taken from NRLMSISE-00 . SABER ozone retrieval
additionally needs to include the contribution of one CO2 laser band emission in the ozone channel. For that it
uses the CO2 vibrational temperatures computed during the SABER temperature retrieval. The vertical resolution of
SABER ozone is approximately 2 km. Given MIPAS O3 coarser vertical resolution, particularly in the mesosphere, we
used the MIPAS averaging kernels and a priori O3 to smooth SABER O3 profiles. report
SABER ozone precision of ≈ 1–2 % in the stratosphere and ≈ 3–5 % in the lower mesosphere. The
systematic errors range from 22 % in the lower stratosphere to ≈ 10 % in the lower mesosphere.
Previous comparisons between SABER v1.07 and MIPAS V4O_O3_m02 ozone VMRs in the mesosphere were performed by
. These showed the largest differences at the secondary maximum, which were attributed to the coarser
MIPAS vertical resolution. They also mentioned that differences arose from the different SABER and MIPAS
pressure/temperature profiles which affected conversion from density to VMR. The differences between SABER version 1.07
used in and version 2.0 (used here) are small .
GOMOS
GOMOS was a stellar occultation spectrometer on board the ESA's
Envisat space platform . It operated from August 2002 to April 2012. Ozone density profiles are derived
from GOMOS UV–visible measurements at 250–692 nm from 10 to 110 km. GOMOS provides nighttime observations that are
performed at around 22:00–23:00 LST at low latitudes. The latitudinal coverage eventually reaches the poles and slightly
varies throughout the year due to varying distribution of stars used in the observations. The dataset used here was
retrieved using ESA Instrument Processor Facility (IPF) version 6.01, described in and
, and is available under registration at the ESA Earth online portal
(https://earth.esa.int, last access: December 2017). Unreliable profiles in the dataset have been screened out following recommendations of the
GOMOS/6.01 Level 2 Product Quality Readme file. The user-friendly, open-access version of the GOMOS dataset
HARMOZ,, which is screened for invalid data, is accessible at the Ozone_cci web page,
http://www.esa-ozone-cci.org/?q=node/161 (last access: December 2017). The vertical resolution of GOMOS ozone varies from 2 km in the lower
stratosphere to 3 km in the upper stratosphere and above. Being better than the MIPAS vertical resolution, we have used
MIPAS averaging kernels to smooth GOMOS profiles. Since GOMOS provides O3 number density but not O3 VMR, we
compare GOMOS and MIPAS O3 number densities. Random errors due to measurement noise and scintillations are
0.5–4 % in the stratosphere and 2–10 % in the mesosphere. Systematic errors are smaller than 2 % and they
are mainly due to O3 cross sections .
MLS
The MLS was launched on July 2004 on the NASA's Earth Observing System Aura satellite
. The equatorial crossings occur at 01:43/13:43 LT and the latitudinal coverage is between
82∘ S and 82∘ N. Daytime and nighttime ozone profiles are derived from measurements of its thermal limb
emission at 235.71 GHz from the troposphere to ≈ 90 km. However, its usable range is up to 0.02 hPa or
∼ 72 km (see https://mls.jpl.nasa.gov/products/o3_product.php, last access: December 2017). The ozone dataset used here is version 4.2,
downloaded from GES DISC and described in and references therein. The vertical
resolution is 3 km in the stratosphere, 6 km in the middle mesosphere, and 9 km in the upper mesosphere. MLS ozone has
generally indicated 5–10 % agreement with other datasets in the stratosphere. The estimated systematic uncertainty is
5–10 % in the stratosphere, 10–20 % in the lower mesosphere, and 20–50 % in the middle mesosphere
. Due to the larger MLS O3 vertical resolution in the mesosphere, we have applied MLS
averaging kernels and a priori information to the MIPAS ozone.
SMILES
SMILES was attached to the Exposed Facility of the JAXA's
Japanese Experiment Module (JEM) of the International Space Station (ISS) and operated between October 2009 and April 2010
. It measured ozone profiles from 16 to 85 km during daytime and to 96 km during nighttime, derived
from measurements between 625 and 651 GHz using the technique described in and .
The latitudinal coverage is 38∘ S and 65∘ N. We use version 3.2 of the data here,
available at the Data Archives and Transmission System (DARTS) site (http://darts.isas.jaxa.jp/stp/smiles/, last access: December 2017). The
vertical resolution is 3 km in the stratosphere, 4 km in the lower and mid-mesosphere, and 6 km at 95 km. MIPAS and
SMILES O3 vertical resolutions are similar and hence we did not apply averaging kernels to any of them. Previous
versions of SMILES O3(v2.2) agree with other measurements within 10 % in the stratosphere and 30 % in the
mesosphere .
ACE-FTS
The FTS is an infrared solar occultation Michelson interferometer flying on the CSA's
ACE, also called Science Satellite (SciSat), launched in August 2003
. It measures atmospheric absorption from the cloud top to 150 km during sunrise and sunset. The ACE
orbit's high inclination results in coverage of the tropical, mid-latitude, and high-latitude regions over approximately
3 months. Ozone profiles are derived from measurements at several MWs between 829 and 2673 cm-1
using the methodology described in . We use version 3.5 of the data here. The retrievals are limited to the
altitude range of 5 to 95 km. The vertical resolution is 3–4 km. The ACE-FTS version 3.5 O3 product agrees
with MLS measurements to within +4 % from 20 to 45 km. Compared to MLS, it exhibits a positive bias of up to 18 %
between 45 and 60 km and an increasing negative bias at higher altitudes .
Mean of the daytime O3 VMR differences (MIPAS–instrument)
in % of MIPAS between co-located pairs of measurements of MIPAS (MA mode)
with ACE-FTS (green), SABER (red), MLS (purple), and SMILES (magenta) for
spring (MAM for NH and SON for SH) (a), autumn (SON for NH and MAM
for SH) (b), summer (JJA for NH and DJF for SH) (c), and
winter (DJF for NH and JJA for SH) (d). The symbols indicate the
mean altitude of the MIPAS O3 VMR primary (diamonds) and secondary
(circles) maxima coincident with the respective instrument. The numbers of
coincidences are indicated in the subscripts. The grey shaded area shows the
MIPAS systematic errors. The colour-shaded areas (hardly noticeable in many
cases) are the standard errors of the mean of the differences.
As Fig. but for nighttime O3. Instrument colours are the same except that ACE-FTS is replaced by GOMOS (light blue).
Global mean (for all latitudes and seasons) of the daytime
O3 VMR differences (MIPAS–instrument) in percent of MIPAS between
co-located pairs of measurements of MIPAS (MA mode) with ACE-FTS (green),
SABER (red), MLS (purple), and SMILES (magenta). For more details, see caption
of Fig. .
As Fig. but for nighttime O3. Instrument colours are the same except that ACE-FTS is replaced by GOMOS (light blue).
Results of comparisons
Figures and show the mean daytime and nighttime differences, respectively, between
MIPAS and the different instruments (MIPAS- instrument) for the four seasons, grouped in four latitude bins. In
addition, Figs. and show the global means, for all latitudes and
seasons, for daytime and nighttime, respectively. The number of co-located pairs included in each mean difference profile
is indicated as a subindex in the instrument's label. The mean altitude of the MIPAS O3 VMR primary and secondary
maxima, coincident with the respective instrument, is also plotted as additional information.
In general, the agreement with all instruments, except SABER, is better than 5 % below 50 km in all seasons for both
daytime and nighttime, MIPAS O3 being larger (see Figs.
and ). The differences around the ozone primary maximum (diamonds in the figures) are smaller
than 5 %. These differences are well within MIPAS systematic errors (see Table ). However, MIPAS
(as the other instruments) measures less ozone than SABER below 50 km, with values of 10–20 % from 30 to 50 km and
5–15 % at the stratospheric O3 maximum.
At altitudes from 50 to 65–70 km, we also find a good general agreement with all instruments, except SABER at
60–70 km, and MLS at 65–70 km in some latitudes and/or seasons. The differences are smaller than 5–10 % in all seasons
both during daytime and nighttime, with MIPAS O3 generally being larger (see Figs.
and ). These differences are within or just at the edges of the MIPAS systematic errors (grey
shading in the figures). Opposite to that behaviour, MIPAS is smaller (5–10 %) than ACE-FTS in the altitude range of
45–55 km (see Fig. ). This is likely the effect of the known positive bias of ACE-FTS
O3 in this region . MIPAS measures up to 10 % (nighttime) and up to 20 % (daytime) less
ozone than SABER from 50 to 60 km. These differences increase above 60 km and are discussed below.
Exceptions to that general behaviour are the larger differences found with MLS at 65–70 km in daytime in spring (at
30–50∘) and autumn and winter for most latitudes. Differences with SMILES are also exceptionally larger at
60–80 km during nighttime in winter at 70–90∘ but, with only eight coincidences, they are not statistically
significant.
A positive bias in SABER stratospheric O3, version 1.07, was previously reported by . Differences
in temperature cannot explain this bias because they are 1–2 K larger than MIPAS below 30 km (which would result in
less O3) and are in excellent agreement (within 1 K) from 30 to 85 km . Some tests have
shown the retrieval of MIPAS O3 at these altitudes using MWs in the AB channel, e.g. the 10 µm
spectral region where SABER measures, results in an 10 % ozone increase (see Sect. and
Fig. ). Thus, O3 spectroscopic errors in the 10 µm region could be the reason for
the larger SABER stratospheric ozone. Indeed, MIPAS and SABER measurements have a better agreement at 50 km (altitude
above which MIPAS uses AB channel MWs) than at lower altitudes, whereas MIPAS differences with instruments other than
SABER increase to 10 % above that altitude.
Between 50 and 85 km MIPAS and SABER ozone profiles show similar vertical gradients but shifted (not shown). In this
region ozone decreases with altitude to very small values and hence the relative differences are rather large. At
60–85 km they are within 20–80 % in daytime and 10–40 % at night, MIPAS ozone being smaller. A daytime
SABER ozone overestimation at these altitudes was already reported by . A likely explanation for the
larger SABER O3 values could be the faster deactivation of the O3v1 and v3 manifold by N2
and O2, kvt,M in Table . According to , the SABER retrieval
uses the values reported by , which are about a factor of 2 faster than those measured by
, which are used here. A faster collisional rate gives rise to smaller O3(v3) vibrational
temperatures and hence to larger retrieved O3 VMR. The fact that the non-LTE deviation of O3(v3)
vibrational temperatures from the kinetic temperature is larger in daytime than at night could explain the larger
daytime bias.
MIPAS differences with other instruments are larger above 70 km than below. These differences are of 10–20 % with
SMILES and GOMOS (nighttime). When compared with ACE-FTS, MIPAS O3 is significantly smaller (<-50 %) near
80 km (see Fig. ). This large relative difference is in part caused by the very small values of
O3. The difference in absolute values is smaller than 0.1 ppmv. Also, the difference could be due to
different solar illuminations along the LOS of both instruments.
In the polar winter around 70 km, where the tertiary maximum develops (see Sect. ), MIPAS agreement with
GOMOS is excellent (3 %) (see Fig. , winter at 70–90∘). MIPAS and GOMOS lie between the
rest of the other instruments, being on average 10 % smaller than SABER (night), 20 % larger than MLS (during
night), within ∼ 30 % of ACE-FTS (terminator), and 50 % larger (although with a very few coincidences) than
SMILES (night). reported that O3 from SABER version 1.07 did not exhibit a tertiary maximum, which
contrasts with version 2.0 in which SABER shows the largest ozone tertiary maximum of all the instruments considered.
At altitudes above 85–90 km during nighttime the differences increase with altitude and are generally larger. In the
case of MIPAS and SABER the differences are partially due to a vertical shift of 1–2 km (not shown).
At the altitude of the secondary maximum, the agreement between all instruments for daytime conditions is very good,
except ACE-FTS in autumn and winter. MIPAS daytime ozone differences with SABER and SMILES are within 5 and 10 %,
respectively, except for low latitudes in the solstices (10 %) and summer high latitudes (20 %) for SABER and
autumn high latitudes and winter low latitudes (20 %) for SMILES. MIPAS ozone is generally larger than that of ACE (by
a mean of ∼ 20–30 %) at this altitude. This difference can be partially due to the different illumination
conditions along the LOS of both instruments. We performed a test, restringing MIPAS data to
SZA < 85∘, and the difference was reduced to just 10 % (0.2–0.3 ppmv).
During nighttime, however, MIPAS ozone is between 5 and 25 % larger than the other instruments around the secondary
maximum. The differences somehow vary with season, being largest in spring and autumn. MIPAS ozone is 10–20 % larger
than SABER and SMILES and 10–25 % larger than GOMOS.
At altitudes above around 93 km, this larger MIPAS nighttime ozone is most likely caused by the large atomic oxygen in
WACCM. If using the MSIS atomic oxygen in this region those differences would be reduced by 10–15 % (see
Figs. and ).
Overall, focusing on Figs. and , MIPAS O3 has an accuracy
better than 5 % at and below 50 km, with a positive bias of only a few percent. In the 50–75 km region, MIPAS
O3 has a positive bias of approximately 10 %, possibly caused by spectroscopic errors. Between 75 and 90 km,
nighttime MIPAS O3 is within 10 % of SABER and SMILES O3 but has a positive bias of about 10 %
with respect to GOMOS. In this region, during daytime, the relative differences are larger, with a positive difference of
10–20 % with SMILES and negative difference of 10–50 % with ACE and SABER. Given that ACE measures at the
terminator and SABER daytime O3 has a known positive bias, we think that MIPAS O3 in this region is
accurate to within 10–20 %. Above 90 km, MIPAS O3 in daytime is in agreement with other instruments by
10 %. At night, however, it shows a positive bias increasing from 10 % at 90 km to 20 % at 95–100 km,
which is attributed to the large atomic oxygen of WACCM.
Climatology
The MIPAS middle atmosphere O3, with a global (pole-to-pole) latitude coverage and day- and nighttime measurements
taken at two fixed local times, spanning the altitude range from 20 to 100 km, represents a very important dataset
for studying the middle atmosphere. In this section we present some ozone distributions, represented against altitude,
latitude and time, showing the major O3 features in the middle and upper mesosphere.
Composite monthly zonal mean of MIPAS data taken in the MA mode for
the 2007–2012 period for daytime (local time of 10:00). White areas
denote regions where MIPAS has no sensitivity to measure the very low ozone
values. Contours are 0.1, 0.5, 1, 1.5, 2, 4, 6, 8, 10, and 12 ppmv.
Composite monthly zonal mean of MIPAS data for the 2017–2012 period for nighttime. Contours are 0.5, 1, 1.5, 2, 4, 6, 8, 10, 15, and 20 ppmv.
Figures and show composite monthly zonal means of MIPAS O3 data for
the 2007–2012 period for day- and nighttime, respectively. Here we consider “daytime” the measurements with a SZA smaller than the SZA of the terminator,
SZAter(z)= 180 -arcsin(R⊕/(R⊕+z))180/π (R⊕ is the Earth's radius
and z is altitude), decreased in 3∘, and taken at 10:00 LT. The decrease in 3∘ was done to avoid scans
partially in dark conditions. “Nighttime” is taken when the observed altitude is in dark conditions,
SZA(z)> SZAter(z), and the measurements are taken at a local time of 22:00 LT. Those figures show the
typical primary, secondary, and tertiary maxima and their seasonal evolution.
The latitudinal–seasonal distribution of daytime O3 in the secondary maximum, near 90–95 km, shows maxima near
the polar winters (November–February in the Northern Hemisphere (NH) and May–August in the Southern Hemisphere (SH)) (Fig. ).
In general we observe a minimum in the daytime O3 secondary maximum near the tropics (except perhaps for October)
which is attributed to tidal effects see e.g.. The diurnal tertiary maximum, taking
place usually around 60–75 km (Fig. ), occurs during the winter seasons polewards of
60–70∘.
The ozone secondary maximum at night presents larger values near the polar winters, more precisely in the early polar
winters (e.g. November in the NH and May in the SH). This polar winter maximum decreases as the season progresses in both
hemispheres and starts recovering near the end of the winter (February in the NH and August in the SH). This is in
agreement with the results reported by . These investigators have shown that this O3 enhancement
is caused by the relatively weak meridional circulation at those times/regions, which leads to low temperatures and low H
concentrations, both favouring the production of O3.
Thus, during solstice, the O3 secondary maximum shows a clear latitudinal gradient, growing from summer to
winter. During the equinox months, the nighttime secondary maximum exhibits large O3 values across all latitudes,
reaching the highest values in April and October. In the equinox months of March, April, September, and October, the
signature of the diurnal migrating tide is apparent near the Equator. Larger values are found near the Equator than at
adjacent latitudes near 75–80 km, then smaller around 85–87 km, and larger again near 95 km. The signature is more
clearly seen in April and October when the largest values of the ozone secondary maximum are observed at the Equator near
95 km, reaching up to about 20 ppmv. The nighttime tertiary maximum has larger values than during daytime (see
Fig. ) and it is usually shifted equatorwards. These values are in good agreement with the
measurements of the nighttime tertiary maximum measured by GOMOS . At latitudes poleward of the
terminator it is usually displaced to lower altitudes. Although not clearly noticeable (because of the scale) in
Fig. , the nighttime tertiary maximum is slightly larger in the NH than in the SH, as reported by
.
Composite seasonal zonal mean of O3 diurnal differences
(10:00–22:00 LT in percentage of 22:00 LT measurements) of MIPAS data
taken in the MA mode for the 2007–2012 period. DJF stands for December,
January, and February; MAM for March, April, and May; JJA for June, July, and
August; and SON for September, October, and November. Contours are -80,
-50, -30, -20, -10, -5, -2, 2, 5, 10, and 20 %.
For completeness we also show the composite seasonal zonal mean of the MIPAS O3 diurnal differences
(10:00–22:00 LT in percentage of 22:00 LT measurements) in
Fig. . The day–night differences below about 50 km are very small, generally between
± 2 %. These differences are within the range of other measurements and model predictions, see, for
example,. This overall good agreement in the MIPAS
O3 diurnal variation with other measurements supports the good quality of MIPAS data in this LTE region. Above
about 45–50 km the effects of photodissociation start becoming important, showing a decline in the daytime O3
with respect to the nighttime at most latitudes (more pronounced near the tropics) and seasons except near the polar
regions. The diurnal differences increase with altitude. At ∼ 80 km the day–night differences are mainly ruled by
the nighttime O3 variations, which are largely controlled by the semi-annual oscillation (SAO) (see below) and the
tidal motions changing the atomic oxygen and hence O3. In the polar summer terminator near 80 km, daytime
O3 at 10:00 LT exceeds O3 at 22:00 LT. Above 90 km, nighttime O3 is significantly larger.
Seasonal evolution vs. latitude of O3 VMR at different altitudes for daytime (a) and nighttime (b). Note the different scales used in the different panels.
Annual variability
Figure shows the annual variability as latitude–month cross sections of O3 at
different altitudes for daytime (left column) and nighttime (right column).
At 40 km MIPAS observes larger O3 values in the tropics and mid-latitudes compared to the polar regions,
as well as
lower values in the SH polar winter compared to the NH winter. Around the Equator we observe slightly larger values during
solstice conditions (January and July) than at equinox (April and October). Maximum values also occur at mid-latitudes
(30–40∘) during May–June and September in the SH and during August and September in the NH. The day–night
differences are small, with ozone slightly larger during daytime, more noticeable in the NH.
At 60 km the day–night differences are very marked, with smaller daytime ozone values due to losses by
photodissociation. This pattern, due to illumination conditions, is very clear in the second panel of the first column.
At 70 km, the structure of the O3 tertiary maximum (second column, third panel) in the SH polar winter is very
obvious, exhibiting a “ring” shape following the terminator. The tertiary maximum has larger values in the early winter,
then decreases and increases again at the end of the winter. Note also the higher values of the tertiary maximum in the NH
polar winter (as discussed above). These results are consistent with the recent analysis of the tertiary maximum carried
out by .
The O3 latitude–month distribution near 80 km shows during equinox conditions significant increases at
mid-latitudes during daytime and in the tropics at night, which seem to be caused by tides. Near the polar regions, the
nighttime O3 at 80 km shows similar features to the tertiary maximum described above, with larger values early in
the winter in both hemispheres, and is larger in the NH.
At 90 km, the nighttime O3 in the polar winter shows a more marked seasonal evolution than at lower altitudes,
with much larger values in the early winter as has been shown by . The SAO is
also evident in the tropics and at mid-latitudes see e.g.. Similar features have also been
observed by GOMOS . Even though O3 has a short lifetime, and hence it is not significantly
advected, its distribution responds to changes in temperature, atomic oxygen, and other species associated with the
SAO. As a result of both effects ozone shows a kind of “U-shape” distribution at this altitude.
Altitude-resolved series for latitudes near the Equator
(10∘ S–10∘ N) (a, b), the southern polar region
(70–90∘ S) (c, d), and the northern polar region
(70–90∘ N) (e, f), for daytime (left column) and nighttime
(right column).
Cross section of latitude–time MIPAS O3 at 50, 70 and 90 km
for daytime (a) and nighttime (b). Note the different colour
scales in some panels.
Altitude-resolved time series
In order to study the inter-annual variability We show in this section altitude-resolved time series of O3 at the
tropical and polar latitudes (Fig. ) and as latitude–time cross sections at given altitudes
(Fig. ). We see clearly the SAO above around 75 km in both day- and nighttime in
the upper panels of Fig. . We also observe in these panels that the O3 secondary maximum is
located at higher altitudes in nighttime than during daytime. In addition, there is a hint that the daytime O3
secondary maximum shows the lowest concentrations close to the solar cycle minimum in 2009/2010, which is also visible in
the bottom/left panel of Fig. .
Focusing on the polar regions (middle and bottom panels of Fig. ), we observe that the stratospheric
maximum (25–40 km) is slightly larger in the NH polar region than in the SH polar region. As discussed above, the
nighttime tertiary maximum is slightly larger in the NH than in the SH polar region. In the SH it shows a double peak in
each winter (early and late in the winter) while in the NH it is not so pronounced. Moving to higher altitudes, the
nighttime O3 secondary maximum shows a clear winter variability, with larger values early in the winter. The
double peak structure appearing each winter is very evident in the SH, as it is in the tertiary maximum, but
not so clear in the NH.
Figure shows that the diurnal variation is clearly seen already at 50 km at the tropics and
mid-latitudes, with slightly smaller values in daytime due to losses by photodissociation. This diurnal variation
increases with altitude and at 70 km it is already quite large.
The presence of the tertiary maximum near 70 km at the polar regions leads to larger O3 VMR values at these
latitudes than at tropical and mid-latitudes, opposite to the latitudinal gradient shown at 50 km. In the middle/right
panel of Fig. we also observe that the nighttime tertiary maximum near 70 km is larger in the NH than
in the SH, as discussed above.
As described above, the bottom/left panel shows that the tropical daytime O3 at 90 km shows lowest concentrations
close to the solar cycle minimum in 2009/2010. This behaviour is in concordance with model simulations and
is explained by decreased odd oxygen production via O2 photolysis at low solar activity conditions. The opposite
occurs, however, at 70 km (middle/left panel), with a tendency to decrease towards 2012. Observational evidence for
a negative solar ozone response at these altitudes has been provided by the analysis of Solar Mesosphere Explorer (SME)
data on solar rotation timescales . Our observations suggest a long-term decline rather than a solar
cycle variation since no O3 increase before the solar minimum in 2009 can be identified. However, a clear
attribution is not possible due to the relatively short observation period of MIPAS.
Summary and conclusions
In this paper we describe the stratospheric and mesospheric ozone distributions (version V5r_O3_m22) retrieved from
MIPAS observations in the three middle atmosphere modes (MA, NLC, and UA) taken with an unapodized spectral resolution of
0.0625 cm-1 from 2005 until April 2012. The non-LTE modelling of O3 is described in detail with
emphasis on the unknown atmospheric and model parameters required to retrieve ozone from limb emission measurements near
10 µm. In particular we discuss the role of atomic oxygen, which can be obtained during daytime by assuming it
is in photochemical equilibrium with the retrieved O3, but it is a source of uncertainty at night.
We succinctly described the retrieval method and update the O3 non-LTE model parameters and the new microwindows.
Regarding the quality of the retrieved MIPAS O3, for daytime it has an average vertical resolution of 3–4 km
below 70 km, 6–8 km at 70–80 km, 8–10 km at 80–90, and 5–7 km at the secondary maximum (90–100 km). For
nighttime conditions the vertical resolution is similar below 70 km, and it is better in the upper mesosphere and lower
thermosphere. It is 4–6 km at 70–100 km (except a narrow region near 80 km where it is coarser), 4–5 km at the
secondary maximum, and 6–8 km at 100–105 km. We recommend using MIPAS O3 only when the absolute value of the
diagonal (or the mean diagonal when averaging) of the averaging kernel is larger than 0.03.
The ozone noise error for daytime is typically smaller than 2 % below 50 km, 2–10 % between 50 and 70 km,
10–20 % at 70–90 km, and ∼ 30 % above 95 km. For nighttime, the noise errors are very similar below
around 70 km but significantly smaller above, being 10–20 % at 75–95 km, 20–30 % at 95–100 km, and larger
than 30 % above 100 km.
The major O3 parameter errors are the spectroscopic data uncertainties below 50 km (10–12 %) and the non-LTE
and temperature errors above about 70 km. The validation analysis points to differences vs. other datasets that are well
within the estimated systematic uncertainties.
The non-LTE error (including the uncertainty of atomic oxygen at night) is significant only above ∼ 85 km with
values of 15–20 %. The temperature error varies from ∼ 3 % near 80 km to 15–20 % near 100 km.
Between 50 and 70 km, the pointing and the spectroscopic errors are the dominant uncertainties.
The ozone of this version shows, compared with the previous V4O_O3_m02 version (V4O_502 in some papers), an increase of
2–3 % (0.2–0.5 ppmv) below around 40 km (except in the polar winter where it is smaller), a decrease of
∼ 5–10 % (∼ 0.4–0.5 ppmv) between 40 and 50 km (due to the use of MWs of channel A only), and an
increase of about 2–5 % (0.1–0.2 ppmv) at 50–80 km. In the upper mesosphere, there is a general decrease
of ∼ 0.5–1 ppmv at night, principally caused by neglecting the removal of O3(v3) by
chemical reaction with atomic oxygen. Above around 95 km, the O3 in the new version is larger in about
5–10 % due to the use of the larger atomic oxygen from the WACCM model.
The validation performed in comparisons with SABER, GOMOS, MLS, SMILES, and ACE-FTS shows that MIPAS O3 has an
accuracy better than 5 % at and below 50 km, with a positive bias of a few percent. In that region, MIPAS systematic
errors, mainly caused by the O3 air-broadened half-widths of the v2 band, seem to be overestimated. In the
50–75 km region MIPAS O3 has a positive bias of approximately 10 %, which is possibly caused in part by
O3 spectroscopic errors in the 10 µm region. Between 75 and 90 km, MIPAS nighttime O3 is in
agreement with other instruments by 10 %. In daytime, in this region, because of the low O3 values, the error
can be ∼ 10–20 %. Above 90 km, MIPAS O3 in daytime is in agreement with other instruments by
10 %. At night, however, it shows a positive bias increasing from 10 % at 90 km to 20 % at 95–100 km,
which is attributed to the large abundances of atomic oxygen of the WACCM model.
The systematically larger O3 measured by SABER below 50 km when compared with all instruments considered here,
including MIPAS, which in this region uses the v2 band near 14.8 µm but not the 10 µm bands
used by SABER, suggests that there might be a problem in the spectroscopic data of the O3 10 µm bands
(or another unknown problem).
The global latitude coverage together with measurements at two fixed local times makes MIPAS O3 very useful for
studying the seasonal changes and partially its diurnal variation, globally in both hemispheres. The major features are
summarised here. The latitudinal–seasonal distribution of daytime O3 in the secondary maximum, near 90–95 km,
shows maxima near the polar winters where the SZA is rather large and losses by photodissociation smaller. Near the
tropics it exhibits a minimum which is attributed to tidal effects see e.g..
MIPAS O3 data also show the typical tertiary maximum, taking place around 60–75 km in the winter seasons at
latitudes polewards of 60–70∘. Its extension and magnitude are larger in the middle of the winters and exhibit
a hemispheric asymmetry with larger values in the NH. Furthermore, it has larger values during nighttime when it is
usually shifted equatorwards. These features are in concordance with the results reported by .
Note that O3 is higher in these regions in the late autumn to early winter (November–December in the NH and
May–June in the SH), similar to the results reported by . These investigators have shown that this
O3 enhancement is caused by the relatively weak meridional circulation at those times and regions, which leads to low
temperatures and low H concentrations, both favouring the production of O3.
The ozone nighttime secondary maximum presents the largest values in the early polar winters, decreases as the season
progresses, and recovers near the end of the winter. This is in concordance with the results reported by
. Thus, during solstice, the O3 nighttime secondary maximum shows a latitudinal gradient growing
from summer to winter. During equinox, the nighttime secondary maximum exhibits an apparent signature of the diurnal
migrating tide, reaching the highest VMRs (up to about 20 ppmv) at the Equator near 95 km.
MIPAS O3 upper-mesospheric data also exhibit the effects of the SAO in the tropics and
mid-latitudes.
The tropical daytime O3 at 90 km shows the lowest concentrations close to the solar cycle minimum in 2009/2010 in
concordance with model simulations and explained by decreased odd oxygen production via O2
photolysis at low solar activity conditions. At 70 km, however, one observes an opposite effect, with a tendency for
ozone decreases towards 2012.
Observational evidence for a negative solar ozone response near 70 km has been provided by the Solar Mesosphere Explorer
. MIPAS data suggest, however, more of a long-term decline than a solar cycle variation, although
a clear attribution is not possible due to the relatively short observation period of MIPAS.
The MIPAS datasets used in this paper can be accessed at
https://www.imk-asf.kit.edu/english/308.php.
The authors declare that they have no conflict of
interest.
Acknowledgements
The IAA team was supported by the Spanish MICINN under the project ESP2014-54362-P and EC FEDER funds. The IAA and IMK teams were partially
supported by ESA O3-CCI and MesosphEO projects. Maya García-Comas was financially supported by MINECO through
its “Ramón y Cajal” subprogram. Funding for the Atmospheric Chemistry Experiment comes primarily from the Canadian
Space Agency. Work at the Jet Propulsion Laboratory was performed under contract with the National Aeronautics and Space
Administration.
Edited by: William Ward
Reviewed by: two anonymous referees
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