Observations of an Extended Mesospheric Tertiary
Ozone Peak
D.A Degenstein1, R.L. Gattinger1, N.D. Lloyd1, A.E. Bourassa1, J.T. Wiensz1,
and E.J. Llewellyn1
[1] ISAS, Department of Physics and Engineering Physics, University of Saskatchewan,
Saskatoon, SK, S7N 5E2, Canada.
Correspondence to: Doug Degenstein
email:
(doug.degenstein@usask.ca)
Fax:
1 (306) 966-6447
Abstract
The OSIRIS instrument onboard the Odin spacecraft has recently made observations of
the Oxygen InfraRed Atmospheric band and OH Meinel band volume emission rate
profiles in the mesosphere. Features similar to the tertiary ozone peak reported earlier by
other investigators are clearly present in the observed emission profiles. This peak,
caused by a photochemical imbalance between the destruction of odd-oxygen by oddhydrogen and the production of odd-oxygen, was previously believed to be confined to
the winter hemisphere just equatorward of the polar terminator. However, northern
hemisphere measurements made by OSIRIS in the spring of 2002 indicate that the
tertiary peak is also present in the afternoon and early evening throughout the springtime,
with the altitude of the peak rising steadily from around 70 km in the polar regions near
local noon to about 80 km near the equator at sunset. These new observations of the
extended tertiary peak are presented and it is shown that they are directly related to the
previously observed tertiary ozone peak and are consistent with a simple one-dimensional
time-dependent photochemical model.
Keywords: Mesospheric Ozone, Tertiary Peak, OH Meinel Band, O2 Singlet Delta and
OSIRIS
1
Introduction
Ever since Chapman first proposed a chemical scheme that could describe the distribution
of ozone in the terrestrial atmosphere [Chapman, 1930] there have been efforts to move
beyond the simple chemistry of the stratospheric ozone layer that Chapman predicted. It
has been necessary to include other reactions, although a re-examination of Chapman’s
work suggests that he really used transfer functions that emulated the multiple reaction
paths we now know exist. The secondary ozone maximum that was first seen in the
daytime by Evans et al. [1968] is not predicted by an oxygen only atmosphere. Rather it
is necessary to include catalytic cycles involving odd hydrogen [Bates and Nicolet,
1950].
Nighttime measurements with the CRISTA instrument on Shuttle [Offermann et al.,
1999; Riese et al., 1999] and MLS on UARS [Froidevaux et al., 1996] have indicated a
tertiary peak at 72 km in the vertical volume mixing ratio profile of ozone, but only at
high latitudes in the winter hemisphere. Recent modeling work [Marsh et al., 2001] in
support of these observations has suggested that in the high latitude winter, when the
solar zenith angle is close to 90o, the large optical depth at the Lyman- wavelengths
prevents the production of odd-hydrogen through photodissociation of water while the
relatively smaller optical depth at wavelengths above 185 nm still allows the
photodissociation of molecular oxygen. Marsh et al. [2001, 2003] suggested that if there
is a shortage of odd-hydrogen for the catalytic destruction of odd-oxygen, and there is no
decrease in the production of odd-oxygen, then there must be an increase in odd-oxygen
and coincidentally the ozone volume mixing ratio. In effect the tertiary peak is not an
enhancement in the vertical ozone volume mixing ratio profile but it is an altitude
dependent decrease in the mechanisms that are responsible for ozone destruction.
The InfraRed Imager (IRI) subsection of the Canadian Optical Spectrograph and
InfraRed Imager System (OSIRIS) onboard the Swedish built Odin spacecraft makes
simultaneous measurements of the Oxygen InfraRed Atmospheric (OIRA) band at
1.27 m and the Meinel OH (3-1) band around 1.53 m. During the daytime the main


production mechanism for the O2 a1 g state that emits the OIRA band comes from the
photodissociation of ozone in the Hartley-Huggins bands,


O3  hv  O2 a1 g  O ,
(1)
where the wavelength of the incident radiation is less than 310 nm for the spin allowed
channel and may extend to 611 nm for the spin forbidden channel (www.iupackinetic.ch.cam.ac.uk). The OH Meinel bands that are formed by the reaction
H  O3  OH   O2 ,
(2)
are also dependent on ozone. In addition to emission from the higher OH* vibrational
levels formed in the photochemical reaction given in equation 2, there is emission from
low v' bands that are populated through radiative and collisional cascade from higher
vibrational levels. Thus the emission rate for the 3-1 band is proportional to the ozone
concentration although the time integrated emission is a measure of the odd oxygen loss.
As the common source for both measured emissions is ozone, enhancements seen
simultaneously in both daytime volume emission rate profiles result from localized
enhancements in the ozone number density profile.
In this work we present some new simultaneous daytime observations of the OIRA and
OH Meinel (3-1) bands that indicate the mechanism responsible for the reported tertiary
ozone maximum is not just restricted to high latitude winter, near the polar terminator,
but is also seen during the springtime months extending down to lower latitudes in the
evening twilight sector of the orbit. We show results from a simple one-dimensional
time-dependent photochemical model that qualitatively predicts the observed OIRA
volume emission rates without the addition of any dynamical effects. We also show that
the phenomenon is present in the northern hemisphere throughout April and May and
shifts to earlier local time at high latitudes.
2
The measurements
The Odin satellite carries instrumentation that was developed in Canada, Finland, France
and Sweden. Although the satellite mission is dual purposed [Murtagh et al., 2002] in
that both aeronomy and astronomy measurements are made, the present work uses only
the mesospheric volume emission rate data collected by the IRI. This instrument is a set
of three vertical imagers that each simultaneously measures along 100 lines of sight
separated by approximately 1 km tangent altitude at the limb of the Earth. Because the
imagers look straight ahead, all of the measurements or images are taken of the volume
emission rate contained within the orbit plane. Two of the imagers measure the OIRA
band volume emission rates over 10 nm wide filters centred at 1.26 m and 1.27 m,
while the third imager measures the OH Meinel band volume emission rate with a 40 nm
wide filter centred at 1.53 m. For a more detailed description of the IRI see Llewellyn et
al. [2004].
The presented two-dimensional volume emission rate profiles contained within the orbit
plane are retrieved from sets of the IRI images using the Multiplicative Algebraic
Reconstruction Technique developed for the IRI by Degenstein and co-workers
[Degenstein et al., 2003 and 2004; McDade and Llewellyn, 1991 and 1993; McDade et
al., 1991]. The actual retrieval grid has cells delimited by angle along the satellite track
and distance from the centre of the Earth. The final representation of the results is on a
grid delimited by altitude and angle along the satellite track. The grid cell size used for all
of the retrievals is 0.2° by 1000 m in angle along the satellite track and altitude
respectively. As for all retrieval techniques it is very important that the grid cell size not
be confused with spatial resolution. Degenstein et al. [2003] have shown that the retrieval
technique used with the OSIRIS instrument is capable of resolving structures with
horizontal and vertical scale sizes on the order of 100 km and 3 km respectively.
It is difficult to assess the accuracy of the retrievals. Degenstein et al. [2004] showed that
for the implemented technique the reconstructed observations fit very well with the actual
observations. The reconstructed measurements were smoother and showed no ill effects
as a result of observational noise. For the regions of the mesosphere addressed here the
signal to noise levels are high and the large scale features that are important to this work
are reliable; it is estimated the effect of the random noise in the observed signals is less
than 2% of the retrieved OIRA noon signatures at 75 km.
Odin is in a dawn-dusk sun-synchronous near circular orbit at a height of approximately
600 km above the surface of the Earth. The solar zenith angles at points along the sunlit
portion of the orbit track throughout the springtime are given in Figure 1. The important
thing to note from this figure is for this time of year the sun is the highest in the sky 90
along the orbit track or at the highest northern latitudes. Also, the solar zenith angle is
very near 90 at the ascending node crossing, or at the beginning, of the orbit. The
latitudes at angles covering the first 180° along the orbit track, from the ascending to the
descending node, are included as axes in Figure 2. The latitude and local time at a given
angular distance along the satellite track remain constant for the sun-synchronous orbit
but the solar zenith angles vary throughout the year. The latitude within the orbit plane is
never higher than 82°, due to the 98° inclination of the orbit, and the local times of the
observations vary rapidly with angle along the satellite track at high latitudes and slowly
for latitudes below 70°. As seen in Figure 2 the highest latitudes are sampled around local
noon and the ascending node equator crossing at 0 along the orbit track occurs around
18:00 local time.
The measured two-dimensional grids shown in Figure 2 are the averages taken from all of
the orbits of data collected on March 7th, 2002. These averages, as opposed to single orbit
data sets, are used to remove the smaller scale dynamical effects in the OIRA volume
emission rate profile and to enhance the results from the daytime Meinel OH
measurements. Migrating tidal effects are not removed because of the local time
constraint of these satellite observations, but it is shown later that they do not influence
the interpretation. The IRI imagers were not designed to accurately retrieve the Meinel
OH bands in the daytime, as the very weak OH volume emission rate is almost
overwhelmed by out of field scattered light. The main source of this scattered light is the
bright Earth that is just outside the instrument field of view. Nevertheless, a very
effective scattered light removal technique [Bourassa, 2003] has made it possible to
reliably detect the OH emissions during the daytime part of the orbit. In late February and
early March, when the sun is low on the horizon for the entire orbit, the Earth scattered
light levels are low enough for a coarse retrieval of daytime Meinel OH. It should also be
noted that additional smoothing of the retrieval over 10° of orbit track was done to further
enhance the retrieval of the very weak OH Meinel (3-1) band signal; this smoothing was
not done for the OIRA band data. The limitations associated with the retrieval of the OH
Meinel band volume emission rate profile are not important for the analysis. For the
purpose of this work it is sufficient to state the large scale enhancement clearly visible in
Figure 2b is indeed real and not an artifact of the data processing.
3
The data
The retrieved OIRA band volume emission rate profile covering the daytime portion of
the orbit (Figure 2a) clearly demonstrates on March 7, 2002 there is a peak in the vertical
profile that changes altitude from about 68 km at 82° N (noon) to around 81 km at the
equator (sunset). The same general feature is seen in Figure 2b, the illustration of the
daytime Meinel OH band volume emission rate coincident with the OIRA emissions
shown in Figure 2a. As mentioned above the results from March 7 were chosen because it
was possible to remove effectively the scattered light in the OH measurements so a
retrieval of the large scale features could be reliably performed. The simultaneous
enhancement in the two volume emission rate profiles seen in Figures 2a and 2b is the
result of an enhancement in the ozone number density.
Vertical slices from Figures 2a and 2b at different angles along the satellite track are
shown in Figure 3. These plots compare the scaled OH Meinel (3-1) band emission with
the actual OIRA band volume emission rates. The scaling was done to equalize the
volume emission rate profiles where the enhancement is seen. Figure 3c is a cross section
from 80° along the orbit track, 77.26° N latitude. The solar zenith angle for this data set
was 87.11°. Although it is only moderate between the dashed lines there is an
enhancement clearly visible at 68 km in both the OIRA and the OH Meinel bands. Figure
3b indicates that the altitude of this enhancement has risen in both volume emission rate
profiles to 77 km at 44.47° N latitude where the solar zenith angle is 86.47°. The first
panel in Figure 3 shows the comparison between the OH Meinel band and the OIRA
band vertical volume emission rate profiles at 14.75° N where the solar zenith angle is
86.95° and it is seen the enhancement has risen to 81 km. Inspection of Figures 3a
through 3c reveals the peaks rise in a continuous fashion from the pole to the equator. In
addition the plots in Figure 3 show the volume emission rate profiles of both the OH
Meinel and the OIRA bands are enhanced at the same time and place.
Figure 4 indicates the temporal evolution of the extended tertiary layer through all of
April and May. This figure illustrates samples of the daily averages of the retrieved
OIRA volume emission rate profiles at selected times throughout the two month period
and, as mentioned above, the finger-like enhancement in the two-dimensional volume
emission rate profile that starts at high latitude and rises in altitude to the equator
represents a layer in the vertical ozone number density profile. No coincident OH
measurements are available for this time period due to the scattered light issue that has
been addressed. In Figure 4 time moves forward from the top panel to the lowest panel
and these panels are not evenly spaced in time because of data availability. It is very clear
for all of April and early May the mechanism responsible for the tertiary layer still exists,
even though the high latitude enhancement has moved into the morning side of the orbit.
Throughout the middle of May and into June it is apparent that the effects of the
mechanism weaken and the high latitude portion moves even further into the morning
side and consequently to lower latitude. By the beginning of June there is no tertiary peak
apparent in the data set.
4
Discussion
The general features of the diurnal variation of the OIRA band height profile in Figures 2,
3, and 4 yield valuable insights into the photochemical interactions between odd oxygen
and odd hydrogen. The general behaviour of the observed diurnal variation of the OIRA
airglow is in good agreement with previous measurements [Evans et al. 1968, 1969,
1988]; what makes the present results different is they yield global structures with much
better resolution. Marsh et al. [2001] document the existence of a tertiary ozone
maximum in the high-latitude middle mesosphere, similar to the noon through dusk
enhancements seen in the present OIRA emissions from high to low latitudes. Their
model results (their Figure 4a) indicate an enhancement at 72 km that is dominant in the
post-noon sector at high latitudes and moves into the pre-noon sector at lower latitudes.
Subsequently, Marsh et al. [2003] presented an analysis of the interactions between
ozone and water vapour at low latitudes; their Figure 9 summarizes the local time
dependence of the maximum of the ozone tertiary layer as a function of altitude. It also
suggests a post-noon enhancement similar to the one reported here. An example of the
simulation of these general features with a time-dependent one-dimensional
photochemical model is shown Figure 2c. The model background atmospheric density
profiles, including atomic hydrogen and atomic oxygen, are initialized with MSIS values
[Picone et al., 2002], while water vapor mixing ratio altitude profiles as functions of
latitude and season are taken from the HALOE archives (haloedata.larc.nasa.gov).
Building on the interpretation of Marsh et al. [2001, 2003] and noting again the orbital
scans begin at dusk, progress through noon and on to dawn, the very different morning
and afternoon patterns give the first hint of explanation. The increase in brightness from
dawn through noon that extends from 65 km to 85 km is the clear signature of the buildup
of odd oxygen from the Herzberg continuum, replenishing the significant decay during
the previous night. By the post-noon period the concurrent buildup of odd hydrogen
species attains a sufficient concentration to generate considerable odd oxygen losses in
the 70 km to 80 km range through the well-known catalytic processes (see Figure 5). The
major odd hydrogen sources, in order of decreasing altitude dominance, are
photodissociation of water vapor by Lyman-, followed by photodissociation of water in
the Schumann Runge band region and finally by O(1D) on water. The altitude of the
resultant tertiary layer, formed in the afternoon, is determined by a combination of the
time history of the solar zenith angle and the altitude profile of the water vapor mixing
ratio. The observed tertiary layer altitude change from above 80 km at the equator to
below 70 km at late winter high latitudes is satisfactorily duplicated in Figure 2c with the
predicted layer altitudes differing by less than two kilometers from the observed altitudes.
Figure 6 gives vertical profiles of the ozone volume mixing ratio produced by the same
model used for Figure 2c. It is clear that the simple time-dependent photochemical model
predicts an enhanced ozone layer that rises in altitude towards the equator in the
afternoon.
It should also be stressed this model reproduces the large scale features of the
measurement set without including dynamical effects. However, vertical transport effects
of atomic oxygen must be included to extend the comparisons above 85 km. As pointed
out by Kaufmann et al. [2003], photochemical timescales are much shorter than vertical
transport timescales at tertiary ozone layer altitudes. Nevertheless, the impact of
migrating tides has been investigated by introducing into the model representative tidal
temperature and pressure profile variations (timed.hao.ucar.edu/cedar/satut/) as functions
of latitude, altitude and local time. Changes in the background atmosphere, similar to
those quantified by Khattatova et al. (1997) and Ward et al. (1999), are also simulated.
The calculated effect of the migrating tides, a maximum of 20% in ozone density
variations observed at low latitudes in the upper mesosphere, is small when compared
with the observed vertical profile variations present in the OIRA observations. Nonmigrating tidal effects are minimal since the observations are zonally averaged.
Although it is not shown the model also satisfactorily duplicates the general features of
the OIRA profiles shown in Figure 4, including the shift of the overall patterns to earlier
local times. The model predictions for the diurnal variation of the OH emission altitude
profiles are also in agreement with the observations in Figure 2b below approximately 85
km. The agreement between the model results and both the OIRA and OH observations
implies reliable mesospheric ozone profiles can be inferred from the IRI retrievals.
5
Summary
The data set collected by the InfraRed Imager component of the OSIRIS instrument
onboard Odin will be invaluable in the diagnosis of the mechanisms responsible for the
tertiary peak seen in the mesospheric ozone measurements and models. Through
simultaneous measurements of the OH Meinel (3-1) band and the Oxygen InfraRed
Atmospheric band volume emission rates the ozone enhancement has been shown to
extend from the winter hemisphere nighttime to the springtime sunlit hours. It has been
demonstrated this phenomena is not confined to high latitudes just equatorward of the
polar terminator. The extended tertiary ozone peak, which varies in altitude, is part of a
larger process that extends all the way to the equatorial region.
Acknowledgements
This work was supported by the Canadian Space Agency and the Natural Sciences and
Engineering Research Council (Canada). Odin is a Swedish-led satellite project funded
jointly by Sweden (SNSB), Canada (CSA), France (CNES) and Finland (Tekes). The
authors wish to thank Maura Hagan for use of the tidal model data referenced at
timed.hao.ucar.edu/cedar/satut/.
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Figure Captions
Figure 1. The solar zenith angles for angle along the satellite track during the springtime
of 2002. The sun is always near the horizon at the equatorial crossings and is highest in
the sky at the highest latitudes.
Figure 2. The retrieved Oxygen InfraRed Atmospheric band (a) and OH Meinel (3-1)
band (b) volume emission rates (photons/cm3/sec) for the daytime portion of the orbit.
The data, an average for all of the orbits collected on March 07, 2002, clearly indicate the
extended ozone enhancement from the equator to well past 72 along the orbit track.
Panel c) illustrates a model result for the same space and time as the measurements
shown in panel a).
Figure 3. Vertical profile comparisons between the retrieved Oxygen InfraRed
Atmospheric band and the OH Meinel (3-1) band volume emission rates for the daytime
portion of the orbits on March 07, 2002. The plots clearly indicate coincident peaks. The
OH profile has been scaled to fit on the same plot. The tertiary layer is indicated by the
region delimited by the dashed lines.
Figure 4. The retrieved Oxygen InfraRed Atmospheric band volume emission rate
(photons/cm3/sec) for the daytime portion of the orbit for data collected throughout April,
May and the beginning of June, 2002. Time moves forward from the top panel to the
bottom.
Figure 5. Modelled equatorial ozone and atomic hydrogen profiles at an altitude of 74
km. The ozone (and atomic oxygen) buildup after sunrise is arrested and reduced by the
slower buildup of hydrogen (plus OH and HO2). At higher latitudes the odd H production
rate is much slower (decreased H2O mixing ratio) leading to slower ozone removal.
Figure 6. Modelled ozone volume mixing ratio profiles at selected locations along the
OSIRIS March 7, 2002 orbit. The altitude shift, with latitude and local time, of the
tertiary ozone peak is generally consistent with the observations.