An overview of two years of ozone radio soundings over Cotonou as part of AMMA

Abstract

As part of the African Monsoon Multidisciplinary Analysis (AMMA) program, a total of 98 ozone vertical profiles over Cotonou, Benin, have been measured during a 26 month period (December 2004–January 2007). These regular measurements broadly document the seasonal and interannual variability of ozone in both the troposphere and the lower stratosphere over West Africa for the first time. This data set is complementary to the MOZAIC observations made from Lagos between 0 and 12 km during the period 1998–2004. Both data sets highlight the unique way in which West Africa is impacted by two biomass burning seasons: in December–February (dry season) due to burning in the Sahelian band and in June-August (wet season) due to burning in southern Africa. High interannual variabilities between Cotonou and Lagos data sets and within each data set are observed and are found to be a major characteristic of this region. In particular, the dry and wet seasons are discussed in order to set the data of the Special Observing Periods (SOPs) into a climatological context. Compared to other dry and wet seasons, the 2006 dry and wet season campaigns took place in rather high ozone environments. During the sampled wet seasons, southern intrusions of biomass burning were particularly frequent with concentrations up to 120 ppbv of ozone in the lower troposphere. An insight into the ozone distribution in the upper troposphere and the lower stratosphere (up to 26 km) is given. The first tropospheric columns of ozone based on in-situ data over West Africa are assessed. They compare well with satellite products on seasonal and interannual time-scales, provided that the layer below 850 hPa where the remote instrument is less sensitive to ozone, is removed.

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Atmos. Chem. Phys., 9, 6157–6174, 2009
www.atmos-chem-phys.net/9/6157/2009/
© Author(s) 2009. This work is distributed under
the Creative Commons Attribution 3.0 License.
Atmospheric
Chemistry
and Physics
An overview of two years of ozone radio soundings over Cotonou as
part of AMMA
V. Thouret1,2, M. Saunois1,2, A. Minga3, A. Mariscal1,2,*, B. Sauvage1,2, A. Solete4, D. Agbangla4, P. N´ ed´ elec1,2,
C. Mari1,2, C. E. Reeves5, and H. Schlager6
1Universit´ e de Toulouse, UPS, LA (Laboratoire d’A´ erologie), 14 avenue Edouard Belin, 31400, Toulouse, France
2CNRS, LA (Laboratoire d’A´ erologie), 31400 Toulouse, France
3Facult´ e des Sciences, Universit´ e Marien NGouabi, BP 2702 Brazzaville, Congo
4Agence pour la SECurit´ e de la Navigation A´ erienne en Afrique et ` a Madagascar (ASECNA), BP 96, Cotonou, Benin
5School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, UK
6Deutsches Zentrum fuer Luft-und Raumfahrt (DLR), Institut fuer Physik der Atmosphaere, Oberpfaffenhofen,
82234 Wessling, Germany
*now at: LGIT (Laboratoire de G´ eophysique Interne et Technophysique), BP 53, 38 041 Grenoble, Cedex 09, France
Received: 8 April 2009 – Published in Atmos. Chem. Phys. Discuss.: 5 May 2009
Revised: 22 July 2009 – Accepted: 6 August 2009 – Published: 28 August 2009
Abstract. As part of the African Monsoon Multidisciplinary
Analysis (AMMA) program, a total of 98 ozone vertical pro-
files over Cotonou, Benin, have been measured during a 26
month period (December 2004–January 2007). These reg-
ular measurements broadly document the seasonal and in-
terannual variability of ozone in both the troposphere and
the lower stratosphere over West Africa for the first time.
This data set is complementary to the MOZAIC observations
made from Lagos between 0 and 12km during the period
1998–2004. Both datasets highlight the unique way inwhich
West Africa is impacted by two biomass burning seasons: in
December–February (dry season) due to burning in the Sa-
helian band and in June-August (wet season) due to burning
in southern Africa. High interannual variabilities between
Cotonou and Lagos data sets and within each data set are ob-
served and are found to be a major characteristic of this re-
gion. In particular, the dry and wet seasons are discussed in
order to set the data of the Special Observing Periods (SOPs)
into a climatological context. Compared to other dry and wet
seasons, the 2006 dry and wet season campaigns took place
in rather high ozone environments. During the sampled wet
seasons, southern intrusions of biomass burning were partic-
ularly frequent with concentrations up to 120ppbv of ozone
in the lower troposphere. An insight into the ozone distribu-
tion in the upper troposphere and the lower stratosphere (up
to 26km) is given. The first tropospheric columns of ozone
Correspondence to: V. Thouret
(valerie.thouret@aero.obs-mip.fr)
based on in-situ data over West Africa are assessed. They
compare well with satellite products on seasonal and inter-
annual time-scales, provided that the layer below 850hPa
where the remote instrument is less sensitive to ozone, is re-
moved.
1Introduction
Tropospheric ozone is an important trace gas in particular
via its role in the oxidative capacity of the global atmosphere
and its climate effect. Two main reasons make the tropical
regions of interest regarding tropospheric ozone. Firstly pho-
tochemistry and OH formation are more active in the tropics
due to high UV radiation and humidity. Secondly, the trop-
ics are important source regions of ozone precursors espe-
cially through biomass burning (Andreae and Merlet, 2001;
van der Werf et al., 2006), biogenic (Guenther et al., 1995;
Serc ¸a et al., 1998; Jaegl´ e et al., 2004, 2005; Guenther et al.,
2006; Aghedo et al., 2007) and lightning emissions (Moxim
and Levy, 2000; Sauvage et al., 2007b; H¨ oller et al., 2009).
The previous African campaigns (from TROPOZ 1987 to
TRACE-A and SAFARI 2000) have effectively highlighted
the importance of the tropical region, and particularly Africa,
as a source region of ozone precursors (e.g., Cros et al., 1992,
2000; Delmas et al., 1999; Jonqui` ere et al., 1998; Jacob et al.,
1996; Pickering et al., 1996; Swap et al., 2003). Most of the
data obtained during those campaigns come from low spatial
or temporal sampling, meaning that regional and/or seasonal,
annual or interannual variabilities are not well documented.
Published by Copernicus Publications on behalf of the European Geosciences Union.

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6158V. Thouret et al.: Overview of Cotonou ozone soundings
Cotonou
Fig. 1. Map of West Africa. The experimental site of Cotonou,
Benin is represented by the red dot, 110km west of Lagos, Nigeria.
A map of the SHADOZ sites as visible on the web site http://croc.
gsfc.nasa.gov/shadoz/ is presented as well showing that Cotonou
fills an empty place within the SHADOZ network.
More recently, the only data set providing regular ozone pro-
files over West Africa has come from the MOZAIC pro-
gram (Marenco et al. (1998) and http://mozaic.aero.obs-mip.
fr/web/). This program recorded ozone concentrations over
Abidjan, Ivory Coast and Lagos, Nigeria from 1998 to the
beginning of 2004, with most of the data in 2000 and 2003.
Thus the first climatology of tropospheric ozone over West
Africa up to 200hPa (12km) was established by Sauvage
et al. (2005). In particular, they showed that the ozone distri-
bution over West Africa is influenced by two biomass burn-
ing seasons, in December to February (DJF) due to fires in
the Sahelian area and in June to August (JJA) due to fires
in the northern part of the southern hemisphere. Further de-
tails can be found in the following studies: Sauvage et al.
(2006, 2007b,c). Previously, the complexity of the ozone dis-
tribution in this region (equatorial Atlantic and adjacent con-
tinents) was highlighted by the analysis of Thompson et al.
(2000); Martin et al. (2002); Edwards et al. (2003); Jenkins
and Ryu (2004) based on satellite data or in-situ data during
a cruise between the East coast of the US and South Africa.
As part of the African Monsoon Multidisciplinary Anal-
ysis (AMMA) program, which aims in particular to docu-
ment the chemical composition of the West African atmo-
sphere and its variations linked to dynamics and climate,
an ozone sounding station was set up in Cotonou, Benin
(6.21◦N, 2.23◦E) (Fig. 1) thanks to the IRD (Institut de
Recherche et D´ eveloppement) and the SMN/ASECNA (Ser-
vice M´ et´ eorologique National/Agence de securit´ e et de nav-
igation a´ erienne) people located there. These regular sound-
ings were performed during the Enhanced Observation Pe-
riod (EOP) in 2005–2006 and aimed to better assess the sea-
sonal and interannual variability of ozone vertical profiles in
Equatorial Africa.
Fig. 2. Vertical profiles of ozone up to 6km from the RS data
set: August monthly mean (black solid line) and standard deviation
(black dots), average of the two soundings on 10 and 14/08 (red
solid line), sounding on 14/08 (grey solid line); from BAe-146 in
the region between 5.5◦N and 7◦N on the 08 and 13/08 (dark blue
solid line); and from D-F20 in the region 4–5.5◦N on the 13/08
(green solid line).
This new data set supplies complementary and additional
knowledge regarding the ozone vertical distribution in the
tropics and in particular in West Africa where ozonesondes
were launched for the first time. The uses of such a data
set range from ozone distribution studies (its main features
and variations related to dynamics and climate), remote sens-
ing satellite validation (in particular the tropospheric ozone
column) to validation of chemistry-transport models used to
interpret observations from various platforms (satellite, air-
borne, in-situ). The objectives of this paper are the follow-
ing:
1. present the entire data set from December 2004 to
January 2007, including the evaluation against aircraft
measurements during the campaign in summer 2006
and against the MOZAIC climatology (Sect. 2)
2. detail the measurements performed during the Special
Observing Periods (SOPs) during the dry and wet sea-
sons of 2006 (December–February and June–August re-
spectively) and set these data into a climatological con-
text (Section 3).
3. highlight new features identified in this data set. In par-
ticular, we will characterize the UTLS (Upper Tropo-
sphere/LowerStratosphere-above200hPa)thathasnot
been sampled previously (Sect. 4). In Sect. 5, we will
give a first assessment of the tropospheric columns over
West Africa based on the in-situ data.
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V. Thouret et al.: Overview of Cotonou ozone soundings6159
Table 1. Number of ozone soundings recorded over Cotonou during AMMA.
JanuaryFebruaryMarchAprilMay June July AugustSeptemberOctober NovemberDecember
2004
2005
2006
2007
2
3
1
2
5
6
4
3
2
4
4
3
3
4
4
9
0
8
3
7
3
3
5
3
4
3
2 Data presentation and evaluation in the lower and
mid troposphere
2.1Data acquisition
Regular weekly measurements were performed from Decem-
ber 2004 to January 2007.
during the SOPs in January 2006 and July–August 2006 to
provide better statistics during the dry and wet seasons. A
total of 98 profiles are now archived and available in the
AMMA data base (http://database.amma-international.org/),
on the SHADOZ archive (Southern Hemisphere Additional
Ozonesondes; http://croc.gsfc.nasa.gov/shadoz/ and Thomp-
son et al. (2003a,b)) and in the WOUDC archive (World
Ozone Data Center sponsored by the World Meteorological
Organization (WMO); http://www.woudc.org) as well. Ta-
ble 1 gives the number of profiles for each month. Due to
serious technical problems in July 2005, January 2006 and
December 2006, we were not able to perform as many sound-
ings as scheduled.
The ozone measurements were made with balloon-borne
ECC (Electrochemical Concentration Cell) ozonesondes
coupled with a standard radiosonde including a sensor for
relative humidity and temperature. We used the Vaisala man-
ufactured RS80 radiosondes during the entire AMMA pe-
riod. The air for the ozone measurements was sampled with
a Science Pump 6A type. Briefly, the principle of an ECC
sensor is the following. A potential difference is set up be-
tween two cells (anode and cathode) with different strengths
of KI (potassium iodide) solution (Komhyr, 1967). Then,
the amount of ozone present in the sampled air is given by a
formulaincluding thecurrent developeddue toelectrochemi-
cal reactions, the current relative to a zero-ozone background
value, the temperature of the pump, the flow rate, and a cor-
rection factor accounting for the response time of the solu-
tion and the slowdown in the efficiency of the ozonesonde
pump at high altitude and low temperature. All these terms
are sources of uncertainties. It has long been recognized that
the pump efficiency correction is the greatest source of un-
certainty in the profile as a whole (Komhyr, 1986; DeBacker
et al., 1998). Similar to the SHADOZ data (Thompson et al.,
2003a), no correction factors from a co-located total ozone
instrument have been applied to these AMMA soundings
data. The overall uncertainties in the ozone soundings mea-
surements have been evaluated to be in the range 4–12%
This frequency was doubled
for the ECC sondes (Barnes et al., 1985; Beekmann et al.,
1994; Komhyr et al., 1995). As part of the SHADOZ pro-
gramme (tropical area), Thompson et al. (2003a) have shown
an ozonesonde precision of 5% when compared to indepen-
dent Dobson instruments. More recently, sonde intercom-
parisons experiments have been conducted, as a WMO spon-
sored work, in simulation chamber (Smit et al., 2007) or in
balloon flight (Deshler et al., 2008) for looking at data qual-
ity critically. They finally showed that standardization of
operating procedures for ECC-sondes yields a precision bet-
ter than ±(3–5)% and an accuracy of about ±(5–10)% up
to 30km altitude. Thompson et al. (2007) have also evalu-
ated the SHADOZ data with simulated flight profiles. Smit
et al. (2007) also showed that the best agreement with UV-
photometer was found with ECC-SP sondes types using a
1.0% buffered solution, which is that used in Cotonou. When
compared to UV analysers onboard commercial aircrafts,
Thouret et al. (1998) have shown that mean concentrations
derived from ozonesondes are about 3 to 13% higher than
those obtained by the MOZAIC program in the free tropo-
sphere in a similar geographic location.
2.2Comparison with aircraft data in August 2006
It is well recognized that ozone soundings are quite difficult
to operate. Many sources of errors can be attributed to the
sonde preparation 4 to 7 days in advance, to the storage of
the sondes and electrochemical solutions and to the balloon
preparation just before launching. These errors are particu-
larly sensitive to tropical conditions due to high temperature
and humidity. There is no easy way to check the data quality
and consistency. Each ozonesonde launched is a new instru-
ment. During the AMMA SOPs, ozone measurements were
also made onboard research aircraft using UV photometers.
Characteristics of the instruments (accuracies, detection lim-
its) are summarized in Reeves et al. (2009). Unfortunately
we did not have the opportunity to coordinate a sounding
with the take off or landing from Cotonou airport of one of
the aircraft measuring ozone. A good agreement between the
average vertical profiles of ozone from aircraft data around
6◦N and those from the radiosoundings over Cotonou dur-
ing July and August 2006 has been found by Reeves et al.
(2009). However, they note the soundings data are higher
than the aircraft data in the upper troposphere (between 550
and 150hPa) due to the difference in measurement technique
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6160 V. Thouret et al.: Overview of Cotonou ozone soundings
  
Fig. 3. Monthy mean vertical profiles of O3from the Cotonou RS
(December 2004 and January 2007: green, 2005: blue and 2006:
red) and from the MOZAIC climatology (black) over Lagos (1◦E
from Cotonou) from the surface to 12km.
but also, in part, to the strategy of operations. Balloons were
launched every 3 to 4 days regardless of meteorological con-
ditions while some aircraft flights in the upper troposphere
were dedicated to the analyses of the mesoscale convective
system outflows (explaining the relatively low ozone con-
centrations in the UT). The July–August 2006 mean pro-
files measured from both the ozonesondes and aircraft show
an ozone enhancement in the lower troposphere characteris-
tic of the biomass burning plumes from the southern hemi-
sphere as detailed in Sauvage et al. (2005, 2007a) and Mari
et al. (2008). Reeves et al. (2009) show that on average, the
ozone radiosoundings data exhibit lower concentrations than
the aircraft data in the lower troposphere (around 3–4km)
and suggest that a greater amount of air sampled by the air-
craft had been influenced by biomass burning.
A thorough comparison between soundings and aircraft
data is made in Fig. 2 for a shorter period (8–14 August
2006). This period is characterized by the so-called “ex-
treme event” recorded on 14 August 2006 when the high-
est ozone concentrations (120ppbv) were measured over
Cotonou (Fig. 2, grey line). Figure 2 presents the mean ver-
tical profiles between the surface and up to 6km recorded by
the soundings on the 10th and 14th, the BAe-146 aircraft on
the 8th and 13th between 5.5–7◦N and the D-F20 aircraft on
the 13th around 4–5.5◦N. They all show maxima from 80
to more than 130ppbv between 3 and 5km, well above the
seasonal average (black line). The mean ozone profile from
the BAe data (dark blue line) shows good agreement with the
ozonesonde data regarding the altitude and magnitude of the
enhancement. The two data sets also well agree in the lower
  
Fig. 4. Differences calculated between Cotonou RS and MOZAIC
climatology over Lagos between 2 and 8km for December 2004,
years 2005 and 2006 and January 2007.
troposphere. On the 13th the D-F20 (green line) sampled a
biomass burning pollution plume located below 5km with
highly enhanced concentrations of ozone and ozone precur-
sors (Andr` es-Hern´ andez et al., 2009). The ozone maximum
sampled by the D-F20 is higher and at lower altitude prob-
ably because it has been sampled further south and further
west. However, the top of the enhanced ozone layer is very
similar to that observed from soundings.
2.3Comparison with the MOZAIC climatology
An objective for this new data set is to be a reference for
the ozone distribution throughout the troposphere and the
lower stratosphere over West Africa. Therefore, it is nec-
essary to assess how well this AMMA data set agrees with
the MOZAIC climatology over Lagos presented by Sauvage
et al. (2005). Lagos is situated 110km east of Cotonou, also
by the Gulf of Guinea coast (Fig. 1) and we expect simi-
lar seasonal characteristics at least above the boundary layer.
Equipped MOZAIC aircraft started collecting data en-route
to Lagos in 1998, but unfortunately stopped their regular op-
eration there in March 2004 before the start of the AMMA
soundings (December 2004). No direct comparison is there-
fore possible to evaluate any bias between the two measure-
ment techniques.
Figure 3 presents the monthly mean vertical profiles
of ozone over Lagos for the MOZAIC climatology and
over Cotonou for the 26month period (both averaged ev-
ery 100m). As expected, both data sets show similar sea-
sonal variations. In particular, both are marked with ozone
enhancements around 2–3km in the Harmattan layer during
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V. Thouret et al.: Overview of Cotonou ozone soundings6161
Fig. 5. Time series of the ozone profiles from the surface up to 17km during December-January-February 2005/2006.
  
(a) (a)(b)(c)
Fig. 6. Monthly mean ozone profiles over Lagos and Cotonou in December (a), January (b) and February (c). Colour Code for Cotonou RS:
dry season 2004–2005 (red), 2005–2006 (black), 2006–2007 (pink).
the dry season (December-January-February) and around 3-
4km during the wet season (June-July-August) due to fires in
the Sahelian region and in the northern part of the Southern
Hemisphere, respectively. However, it seems that the sound-
ings data show higher concentrations than the MOZAIC data
in the middle and upper troposphere (e.g. see JJA, DJF, as
well as October). Indeed, Thouret et al. (1998) have shown
that there can be up to 13% difference between the ozoneson-
des and MOZAIC data. However, this overestimation does
not seem to be global or systematic.
soundings and aircraft data show only small differences.
For instance, September and November 2006 exhibit profiles
In April and May,
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6162 V. Thouret et al.: Overview of Cotonou ozone soundings
close to the MOZAIC average whereas large differences rela-
tive to the MOZAIC climatology appear in the mid and upper
troposphere in 2005. Conversely, the January profiles from
soundings are similar to each other and high compared to
MOZAIC. In December, all four profiles show very different
ozone levels above 6km.
To further quantify this variability, Fig. 4 gives the differ-
ence (%) between MOZAIC (taken as the reference) and the
sonde data for the layer 2–8km and for the 3 years of sam-
pling. This is the layer where the comparison is probably the
most relevant because the local surface effects (different ur-
ban pollution and time of sampling) do not have a significant
influence and the locations of the different measurements are
intherangeofonetotwohundredkm. Ingeneral, thehighest
differences relatively to MOZAIC are observed in JJA, DJF
and October. In JJA, the sounding profiles are similar while
the MOZAIC distributions were more variable (Figs. 3 and
9 which is detailed in Sect. 3.2 and presents MOZAIC and
AMMA monthly means for each sampled wet season). How-
ever, in December, the differences range from 7 to 60% as
also seen in Fig. 6 (which is detailed in Sect. 3.1 and presents
MOZAIC and AMMA monthly means for each sampled dry
season). This difference is noticeable throughout the tropo-
sphere. For the annual average, the difference is 20% (sondes
higher than MOZAIC) which is higher than the range of un-
certainties of the measurements. As there is no overlap in
the sampling periods, we cannot conclude on differences or
bias between instruments. Given that the strongest differ-
ences are seen in DJF and JJA when ozone maximize in the
lower troposphere, the interannual variabilities of the pro-
cesses leading to ozone enhancements (emission and trans-
port of biomass burning products) may explain most of these
20%.
3 Measurements during the Special Observation Peri-
ods
One of the objectives of the EOP measurements was to give
a more general and regional context for the data from the
SOPs. The following sub-sections focus on the dry and the
wet seasons for which SOPs were carried out in 2006.
3.1 Climatological context for the 2005-2006 dry season
DJF
The first SOP, SOP-0, took place in January 2006 (Hay-
wood et al., 2008). It mainly focused on the measurement
of aerosols originating from Saharan dust and biomass burn-
ing as detailed in Mallet et al. (2008) and Raut and Chazette
(2008). The individual soundings measured during the dry
season 2005–2006 are presented in Fig. 5 as a time series.
The highest values recorded during this period were on 20
and 22 December 2005 notably in the lower troposphere but
also for the entire tropospheric column. In addition to the
very high ozone concentrations measured above 2km, ex-
treme ozone values were recorded around 1km on 20 De-
cember (Fig. 5) (Minga et al., 2009). Biomass burning signa-
turesareclearlyobservedineachprofileinJanuaryat3–4km
in the Harmattan layer. High ozone concentrations were also
measured near 8–10km in January 2006. These ozone en-
hancedlayersprobablyoriginatefrombiomassburningprod-
ucts lifted by convection as previously described in the frame
of TRACE-A by Pickering et al. (1996). Within AMMA,
Mari et al. (2008) described such processes during the wet
season. Although these processes are similar, they are sym-
metrical in that they occur in opposing hemispheres and sea-
sons. Pollutants emitted by fires in the southern (northern)
hemisphere during the wet (dry) season are trapped over the
continentandadvectedtothenorth(south)bythetradewinds
(Harmattan flow) where they reach convective regions lo-
cated further north (south) of the fire regions and are injected
in the upper troposphere. A further study should be dedi-
cated to better analyse these signatures and draw symmetri-
cal schemes of the processes involved during the dry and the
wet seasons. In February, the profiles are more homogeneous
vertically with lower values in the lower troposphere.
Figure 6 compares the monthly mean of ozone vertical
profiles over Cotonou recorded during the months of Decem-
ber, January and February between 2004 and 2007, with the
MOZAIC measurements taken over Lagos in 2002, 2003 and
2004. In January, average concentrations in the Harmattan
layer (1–3km) almost reach 80ppbv. This is quite similar
to that observed over Lagos in 2002, 2003 and 2004. How-
ever, MOZAIC aircraft measured more ozone in the lower
part of the Harmattan layer in 2002 than the soundings in
2006. Concentrations over Lagos reach 84ppb at 2.2km and
are greater than 70ppb between 1 and 3.6km altitude this
month. On the other hand soundings in 2005 and 2007 show
average concentrations as high as 100ppb at 2km altitude.
Within the AMMA data set, overall 2006 shows the highest
values above 4km. This is consistent with December 2005
showing very high ozone concentrations from the ground to
the lower stratosphere compared to 2004 and 2006 (Fig. 6a).
Indeed, individual soundings from the beginning of January
(6, 9, 12) show higher concentrations than those from the
end of the month (16, 19) especially in the upper troposphere
(Fig. 5). In general, this dry season DJF 2005–2006 presents
higher ozone concentrations which is consistent with higher
concentrationsofblackcarbonfrombiomassburningin2005
compared to 2006 (C. Liousse, personal communication).
3.2Climatological context for the wet season JJA 2006
The third SOP, SOP-2, took place in July and August 2006
with various chemical objectives described by Janicot et al.
(2008) and a large aircraft operation between Niamey, Niger
and the Gulf of Guinea. The chemical characterization of the
troposphere over West Africa during this period is widely de-
scribed by Reeves et al. (2009). Other aircraft data regarding
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V. Thouret et al.: Overview of Cotonou ozone soundings6163
Fig. 7. Time series of the ozone profiles from the surface up to 17km during June–July-August 2006. Dates with O3enhancement are
circled.
(a)
JuneJuly August
(b)
JuneJulyAugust
(c)
C
AEJ­N
AEJ­S
AEJ­S
AEJ­N
AEJ­S
06/011020 3007/1020 3008/102030
(d)
C
AEJ­N
AEJ­S
AEJ­S
AEJ­N
AEJ­S
06/0110 203007/10 20 30 08/10 20 30
Fig. 8. Time series during the period June-July-August 2005 (a) and June-July-August 2006 (b) of the mean ozone concentration (blue)
and zonal wind speed (red) as measured by radiosounding and averaged between 600 and 800hPa. Date with O3enhancement are circled.
Bottom panel: time-latitude diagram of the mean zonal wind speed in ms−1between 0◦E and 10◦E from NCEP reanalysis at 700hPa for
June-July-August 2005 (c) and June-July-August 2006 (d). The dark line corresponds to the latitude of Cotonou.
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6164V. Thouret et al.: Overview of Cotonou ozone soundings
  
(a)(a)(b)(c)
Fig. 9. Monthly mean ozone profiles over Lagos (2000: dark blue and 2003: light blue) and Cotonou (2005: black and 2006: red) in June
(a), July (b) and August (c).
the influence of convection on the chemical composition are
presented by Ancellet et al. (2009) and Andr` es-Hern´ andez
et al. (2009).
Figure 7 presents the 24 vertical profiles
recorded in JJA 2006. In contrast to the dry season DJF,
the vertical gradient of ozone is more pronounced. This pe-
riodischaracterizedbyozoneenhancedlayersbetween3and
5km attributed to southern biomass burning products (see
Sect. 2.2) and profiles with such ozone layers correspond to
the dates circled. These southern intrusions were rare in June
(only the 30th) and July (2 on 8) whilst they were nearly
a daily occurrence during August 2006. Ozone concentra-
tions in the upper troposphere are higher in June and in the
beginning of July compared to August. We have character-
ized the SOP-2 by the “extreme event” (Sect. 2.2, Fig. 2)
when up to 120ppb were measured at 4km on 14 August.
Figure 8 has been designed to highlight the intra seasonal
variability and the relationship between the ozone concen-
trations and the meteorological conditions in the layer 600–
800hPa. For each sounding in JJA 2005 and 2006, the ozone
concentrations and the zonal wind speed have been averaged
between 600 and 800hPa (Fig. 8a and b). The ozone back-
ground value at this pressure is around 40–50ppbv. Days
without intrusions generally show strong easterly zonal wind
(<−5ms−1) attributed to the northern African Easterly Jet
(AEJ-N) while days with intrusions are characterized by
weak zonal wind speed, the AEJ-N being further north. Fig-
ure 8c and 8d give the temporal variation of the zonal wind
at the logitude of Cotonou during JJA 2005 and 2006 respec-
tively from NCEP reanalysis. A good agreement is found be-
tween in-situ observations and NCEP reanalysis zonal wind
speed, especially regarding temporal variations during the
JJA period. When Cotonou is under the influence of the AEJ-
N, the jet prevents biomass burning intrusions going further
north (“blocking effect”). Mari et al. (2008) have already
suggested that the intra seasonal variability of the biomass
burning intrusions is related to the activity of the AEJ-S (lo-
cated at ∼5◦S, latitude of the fires regions). Indeed, most of
days with intrusions have been preceded by days character-
ized by strong easterly winds at 2.5–5◦S (AEJ-S). This jet
allows an efficient transport of pollutants over the ocean and
the Gulf of Guinea. The same analysis has been done for
JJA 2005 with 5 out of 8 profiles showing biomass burning
signatures (Fig. 8b). The zonal wind is much more variable
during the 2005 wet season and the AEJ-S is not well pro-
nounced for long periods (as in August 2006 for instance).
As for 2006, days with intrusions show weak zonal wind.
However, it is difficult to see if all these days were preceded
by a strong transport by the AEJ-S over the ocean. Never-
theless, these two figures highlight the strong intra-seasonal
andinterannualvariabilityregardingwindpatternsandozone
concentrationsinthisregion. Theysuggestthatintrusionsare
favoured by an AEJ-N located north of Cotonou and possi-
bly by an active AEJ-S over the fires region in the southern
hemisphere. However, other processes are still to be investi-
gated to completely explain these southern intrusions. At the
monthlymeanscale, differentdynamicalsituationsinAugust
2005 and 2006 can lead to similar ozone vertical profiles as
discussed below.
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V. Thouret et al.: Overview of Cotonou ozone soundings6165
Dec 04
Dec 05
Dec 06
Feb 05
Apr 05
Jun 05
Aug 05
Oct 05
Feb 06
Apr 06
Jun 06
Aug 06
Oct 06
Fig. 10. 26 month timeseries of ozone (ppb) monthly means over Cotonou between 0 and 17km, averaged every 100m.
Figure 9 presents on overview of the interannual vari-
ability of the JJA profiles as seen in the AMMA program
and in the MOZAIC measurements over Lagos. In August,
the two mean profiles from soundings are similar while the
MOZAIC profiles in 2000 and 2003 show the two distinct
behaviours (with and without the southern intrusions, re-
spectively). Cotonou data exhibit thicker ozone layers with
higher concentrations compared to the aircraft data over La-
gos.The years 2005 and 2006 seem to have had more
favourable conditions for these southern intrusions. Within
the MOZAIC data set, 28% of the profiles in JJA 2003 pre-
sented such layers while the soundings time series (2005 and
2006) exhibited this phenomenon for 41% of the samples.
The monthly mean for 2006 in July is closer to the one ob-
served over Lagos in 2000, at least up to 5km altitude. The
mean vertical profiles in June do not present a well marked
layer in general. Little signature of the southern intrusion is
seen in 2005 and to a less extent in 2006 (only one profile
presents such layers out of the 9 recorded).
Similar to the dry season, the AMMA SOP-2 took place in
a rather “high ozone” environment with frequent enhanced
layers originating from the southern hemisphere. The SOP-2
has also been characterized by the “extreme event” on 14 Au-
gust 2006 when up to 120ppbv were measured in the lower
troposphere, a value similar to the highest measured during
the dry season.
4 Upper troposphere/lower stratosphere
MOZAIC vertical profiles are limited to 12km, the air-
craft cruise altitude, which is well below the tropical
tropopause. The soundings have the capability of sampling
ozone throughout the troposphere and the lower stratosphere
(balloons have reached altitudes of 26km). Consequently
they have provided the first data regarding the ozone distri-
bution in the UTLS over West Africa. The aim of this section
is to present the seasonal cycles of ozone and temperature in
the UTLS.
Figure 10 shows the monthly mean time series of ozone
concentrations averaged every 100m from the ground to
17km altitude between December 2004 and January 2007.
The UT over Cotonou presents ozone concentrations higher
than expected from the interpolation presented in Thompson
et al. (2003b) and from the study by Sauvage et al. (2006)
when they assumed the values of 70ppb recorded at 12km
to be valid up to the tropopause. This AMMA data set shows
that ozone concentrations between 12 and 17km are closer to
90–110ppbv on average, and up to 150ppbv in August and
September.
For further details of this UTLS region and for clarity of
colour scale, Fig. 11a and b give the monthly mean ozone
and temperature distributions between 12 and 22km aver-
aged every 100m. The white line on the figures gives an
indication of the tropopause altitude. This has been defined
as the lowest altitude where the temperature vertical gradient
reaches the value of −2K/km. As expected, the tropopause
altitude is around 17km (16.9±0.4 as standard deviation).
However, from June to October the tropopause is a little bit
lower and warmer than in the period from December to May.
The difference is up to 10K, in agreement with Reed and
Vicek (1969) and Reid and Gage (1981) and visible in both
2005 and 2006. The ozone seasonal cycle seems to be in
phase with the well-known annual cycle of temperature. For
better clarity we have averaged over the two years of sam-
pling and we present the mean seasonal cycle of ozone con-
centrations and temperature at 12–14, 14–16, 16–17 and 17–
18km altitude in Fig. 12. The lowest layer shows no sig-
nificant seasonal variation (70–80ppb throughout the year)
while a broad summer maximum of ozone is observed in the
three upper layers. Nevertheless, the amplitude below 16km
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6166 V. Thouret et al.: Overview of Cotonou ozone soundings
  
Dec 04
Dec 05
Dec 06
Feb 05
Apr 05
Jun 05
Aug 05
Oct 05
Feb 06
Apr 06
Jun 06
Aug 06
Oct 06
Dec 04
Dec 05
Dec 06
Feb 05
Apr 05
Jun 05
Aug 05
Oct 05
Feb 06
Apr 06
Jun 06
Aug 06
Oct 06
(a)
(b)
Fig. 11. 26 month timeseries of (a) ozone (ppb) and (b) temperature (K) monthly means over Cotonou between 12 and 22km, averaged
every 100m.
is very weak (less than 20ppb or 20%) while it increases
to 50ppb at 16–17km and up to around 150ppb or 100%
in the layer just above the tropopause, at 17–18km. Con-
centrations between 17 and 18km above Cotonou are in the
range of 140–290ppb, in agreement with that shown in Ran-
del et al. (2007) based on a study of the SHADOZ profiles
not including Cotonou. They linked this large annual cycle in
ozone above the tropical tropopause to the variability of the
Brewer-Dobson circulation. The amplitude of the tempera-
ture seasonal cycle is also higher above 17km (up to 10K)
compared to the tropospheric layers (less than 3K) and in
phase with the ozone cycle, as expected. Ozone concentra-
tions at 16-17km are attributed to the upper troposphere and
its seasonal cycle is very similar to the one just above the
tropopause (17–18km), but with a lower amplitude though.
A similar pattern is observed for the temperature cycle. This
may reflect a non-negligible influence of the stratosphere on
the ozone distribution in the UT over Cotonou during the
broad summer period (June to October).
This ozone data set is of particular interest for further in-
vestigation of the different origins of air masses recorded in
the TTL (Tropical Tropopause Layer, between 150hPa or
14.5km and 70hPa or 17km as defined by Fueglistaler et al.
(2009) and references therein) over this location throughout
the year. Indeed, Barret et al. (2008) have shown the influ-
ence of the Indian Monsoon and the Tropical Easterly Jet on
the composition of the TTL over Northern Africa. Moreover,
Law et al. (2009) systematically analyzed the high altitude
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V. Thouret et al.: Overview of Cotonou ozone soundings6167
Table 2. Monthly mean tropospheric ozone columns (from the ground to the tropopause as calculated in Sect. 4) in Dobson Unit (DU) based
on ozone soundings over Cotonou.
JanuaryFebruary MarchApril MayJuneJuly August September OctoberNovember December
2004
2005
2006
2007
39.4
67.2
45.3
49.0
49.5
48.3
43.4
43.9
32.5
44.7
41.8
43.3
37.7
38.4
43.8
45.9
–42.0
40.5
39.2
30.8
42.3
40.3
46.1
38.541.8
Fig. 12. Seasonal cycle of ozone (ppb) and temperature (K) aver-
aged over the two years of the RS data set for four different layers,
12–14km in red, 14–16km in blue, 16–17km in green and 17–
18km in black.
aircraft data recorded during the SOPs in JJA 2006. They
show that the region is mainly under the influence of African
convectionaround200hPa(12.5km), andoftheIndianMon-
soon around 150hPa (14.5km) while air masses sampled
around 100hPa (17km) are related to stratospheric origins.
5 Tropospheric columns of ozone
This present data set is actually the first one that allows a
thorough quantification of tropospheric columns of ozone
(TCO) over West Africa. The objectives of this section are
to present the seasonal variations of these TCO and to com-
pare them with satellite retrievals. Comparisons are based on
monthly means to evaluate the seasonal and interannual vari-
ations of the columns. The goal is not a thorough comparison
to the satellite data on a daily basis. Given that the satellite
data are the only available records representative of monthly
mean columns over this region, we aim to check whether our
data set matches them despite its low sampling frequency.
Déc. 2004 &Jan. 2007
Year 2005
Year 2006
Fig. 13. Monthly mean tropospheric columns of ozone in Dob-
son Unit (DU) over Cotonou between December 2004 and Jan-
uary 2007. Columns are divided into 5 layers from 1000hPa to the
tropopause (called Top and calculated as defined in Sect. 4). Each
year corresponds to one color: blue for 2005, red for 2006 and green
for December 2004 and January 2007.
Meanwhile, we specifically discuss the sensitivity of remote
instruments in the lower troposphere.
5.1Seasonal variations
Figure
ations
soundings
1DU=2.69×1016moleculescm−2).
from 32.5 to 67.2DU, with high annual and interannual
variabilities (up to 20DU). March, September and December
show the strongest interannual variability. To better assess
which atmospheric layer contributes to these variabilities,
the TCO were divided into 5 layers, related to air masses
origin.The first layer from 1000 to 900hPa is under
monsoon flux influence, the air masses coming from S-SW.
The second layer (900–700hPa) roughly corresponds to the
Harmattan layer during boreal winter (DJF) and to trade
winds during boreal summer (JJA). The third one from
700 to 500hPa generally relates to the African Easterly Jet
altitude, with large scale transport influence. The fourth and
13
of
and
monthly
and
Table2present
TCO
in
the
derived
Dobson
The
annual
from
Unit
TCO
vari-
the
(DU,
range
mean
calculated
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6168V. Thouret et al.: Overview of Cotonou ozone soundings
Fig. 14. Time evolution between December 2004 and January 2007
of the monthly mean tropospheric columns of ozone in DU from
OMI/MLS Ziemke et al. (2006) (green dashed line) and derived
from the ozone soundings from the surface to 100hPa (dark dashed
line),the surface to the tropopause (called Top and calculated as de-
fined in Sect. 4) (dark solid line), and 850hPa to the tropopause
(blue dashed line.
fifth ones represent for mid-upper and upper tropospheric
layers which can be both effected by large scale transport
and deep convection. The fifth one is also influenced by
the lower stratosphere as seen in Sect. 4. The tropospheric
ozone columns show little difference between 2005 and
2006 (2–3DU) except for the beginning of the dry season
(November and December) and the months of March and
September (Fig. 13).
The seasonal variations appear to be largely driven by the
first three layers partly due to surface emission changes be-
tween dry and wet seasons. A maximum of partial TCO is
clearly seen during the dry season (DJF) when emissions
from biomass burning maximize with around 15DU from
the surface up to 700hPa whilst rarely exceeding 10DU the
rest of the year. The mean ozone column amount averaged
over the DJF season show 48 DU over Cotonou which is
higher than that found over Lagos (41DU) by Sauvage et al.
(2006) using MOZAIC data. The difference in total ozone
amount can be partly attributed to the climatological values
of 70ppbv between 186 and 100hPa used by Sauvage et al.
(2006) to augment MOZAIC vertical profiles. Figures 11a
and 12 have shown that ozone values over Cotonou can reach
up to 130ppbv near the tropopause, higher than the assumed
value used in their study. Nevertheless interannual and ge-
ographical variabilities might be part of the difference too.
The ozone column below 700hPa contributes to 14.9DU of
the TOC in DJF which is in fairly good agreement with the
Sauvage et al. (2006) results over Lagos (17.3DU).
In the following, some observed interannual variabilities
are highlighted. However our goal is more to give a first as-
sessment of this tropospheric ozone variability and its prob-
able causes rather than a detailed explanation of them that
would require further studies. A 12DU difference, essen-
tially confined to the lower layers (below 500hPa), is cal-
culated between March 2005 and March 2006 showing a
lower (higher) amount of ozone in 2005 (2006). Such a dif-
ference is less pronounced in February and April. However
OMI/MLS does not capture this difference (Fig. 14 and next
subsection), suggesting that unfortunately sparse sampling (3
soundings each month) might be responsible of this differ-
ence. A 8.4DU difference is calculated between September
2005 and September 2006 showing a lower amount of UT
ozone in 2006 in agreement with the mean ozone profiles
in Fig. 3. Only three ozone soundings were performed for
each September month; however this difference is supported
by OMI/MLS TCO retrievals (Fig. 14 and next subsection).
NCEP Reanalysis show lower Outgoing Long wave Radia-
tion (OLR) above Western and Equatorial Africa in Septem-
ber 2006 compared to September 2005 (not shown). Thus
more convective events in 2006 decreasing UT ozone by up-
lifting poor ozone air masses from the boundary layer are
likely to be responsible for part of the variability between the
sampled profiles. More specific studies should be made to in-
vestigate these hypotheses. Lower ozone columns are found
during the dry season 2006 compared to 2005 with −7.6DU
(20%) and −21.9DU (48%) in November and December, re-
spectively. The monthly mean profiles for December (Figs. 3
and 6) show that the differences are essentially in the lower
troposphere under the AEJ influence (above the Harmattan
layer and below 5km or 500hPa) and in the upper tropo-
sphere (above 10km). Those two parts contribute equally
to the interannual difference. 61% and 48% of this differ-
ence is confined in the lower troposphere below 500hPa in
November and December respectively. The variability in the
lower part of the troposphere is probably due to enhanced
biomass burning emissions in 2005 in Africa (as discussed
previously in Sect. 3.1.2). Logan et al. (2008) also found
lowerozoneinthemiddletroposphereoverEquatorialAfrica
in December 2006 compared to 2005 using data from the
Tropospheric Emission Spectrometer (TES) and suggested
that these changes could be related to the late 2005 drought
in eastern Africa leading to enhanced biomass burning as
well as lower NOxproduction from lightning in 2006 com-
pared to 2005 leading to lower ozone production in the UT
in 2006. The 2005–2006 difference in the upper part of the
troposphere might be due to enhanced convection in Decem-
ber 2005 leading to more NOxin the UT and more biomass
burning pollutants up-lifted, which favour enhanced ozone
production.
A secondary maximum is seen in JJA related to biomass
burning plume intrusions from the Southern Hemisphere
as discussed in Sect. 3.2. Here we attempt to assess the
contribution of the Southern Hemisphere influence on the
tropospheric ozone total amount by selecting and compar-
ing two groups of ozone profiles (with and without ozone
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V. Thouret et al.: Overview of Cotonou ozone soundings6169
Table 3. Mean tropospheric ozone columns in DU over Cotonou during July and August for profiles with and without an ozone enhancement
around 700–500hPa. Standard deviation for each group appears in brackets.
1000–900hPa 900–700hPa 700–500hPa500–250hPa250hPa–Top1000hPa–Top
With O3layer
W/o O3layer
Diff (W-W/o)
2.45(±0.42)
2.40(±0.56)
6.95(±1.3)
5.42(±0.76)
11.20(±2.10)
7.74(±0.59)
11.41(±3.00)
13.07(±1.66)
9.50(±2.20)
9.52(±1.49)
41.40(±3.97)
37.94(±3.30)
0.051.533.46
−1.660.02 3.46
enhancement). For this comparison we use only July and
August profiles to avoid the variations of upper tropospheric
ozone levels which are clearly higher in June compared to
July and August. During July and August 2005 and 2006,
18 ozone soundings were performed and 10 of them present
an ozone layer located between 750hPa and 500hPa. When
processing these two groups of profiles separately, we found
a 3.5DU difference in the total tropospheric ozone amount
(41.4DU and 37.9DU for profiles with and without the
ozone layer, respectively, see Table 3). Ozone enhancements
fromsouthernbiomassburningpollutionarelocatedbetween
750hPa and 500hPa, consequently the biomass burning in-
fluence is seen mostly in the third level (700–500hPa) and
little in the second level (900–700hPa). Therefore, a differ-
ence of 5DU between 900 and 500hPa is computed. How-
ever this positive difference is balanced by a 1.7DU nega-
tive difference between 500 and 100hPa. These results are
consistent with those found by Mari et al. (2008) and sup-
port the hypotheses of a mid-troposphere enrichment of CO
and ozone from biomass burning during the active phase
of the southern AEJ (AEJ-S) whilst during its break phase,
pollutants are first confined to the continent where they can
reach a convective region further north and then be up-lifted
to the upper troposphere. The up-lifting of biomass burn-
ing pollutants from the southern hemisphere contribute to
the 1.66DU. However, the electrical activity associated with
the convective systems produces nitrogen oxides which may
also enhance ozone levels in the UT (e.g. Smyth et al., 1996;
Pickering et al., 1996; Moxim and Levy, 2000; Martin et al.,
2000; DeCaria et al., 2005; Sauvage et al., 2007d; Saunois
et al., 2008a). Overall the biomass burning pollution from
the Southern Hemisphere influences West Africa, at least the
region of the Guinean coast close to Cotonou, either in the
mid- or in the upper-troposphere depending on meteorologi-
cal conditions and leads to up to 5DU (12%) ozone enhance-
ment in July and August.
5.2Comparison
OMI/MLS tropospheric residuals
betweenozonesoundingsand
Since the launch of Aura on 15 July 2004, tropospheric
ozone columns or residuals can be derived by subtracting the
stratospheric column estimated by Aura MLS (Microwave
Limb Sounder) from the total ozone column measured by
Aura OMI (Ozone Monitoring Instrument). OMI/MLS tro-
pospheric ozone residuals (TOR) have been compared with
ozonesondes by Ziemke et al. (2006) (denoted as Z06 in
the following) in the tropics and outside the tropics us-
ing a pressure-averaged ozone volume mixing ratio (VMR).
The tropopause pressure used by Z06 was determined from
NCEP reanalysis using the 2K/km thermal vertical gradient
criterion and corresponds to an average of 100hPa (∼17km)
in the tropics with a range from 95 to 110hPa over the
two year period. Z06 did not find a substantial offset rel-
ative to ozonesondes using a pressure-averaged ozone mix-
ing ratio (VMR). However their highest differences corre-
sponded to tropical SHADOZ stations, Natal and Ascen-
sion with 5.9 and 3.1ppbv. They found an average offset
of 2DU between OMI/MLS and several WOUDC/SHADOZ
stations and pointed out that considering VMR reduces the
scatter. Schoeberl et al. (2007) (denoted as S07 in the fol-
lowing) estimate the 200-hPa-to-surface ozone column us-
ing a trajectory model to increase the horizontal resolution of
the stratospheric columns derived from MLS. Their product,
TTOR 200TSC, shows an improvement over Z06, especially
in the mid-latitudes. They bilinearly interpolate their TTOR
200TSC product to compare with ozonesonde points. They
found a 2.4DU offset in the tropics, with TTOR 200TSC
being lower and a 5DU standard deviation probably repre-
senting the total uncertainty of the measurements.
In Fig. 14, we compare the monthly means of OMI/MLS
TOR derived by Z06 and publicly available from the
NASA Goddard web page (http://hyperion.gsfc.nasa.gov/
Data services/cloud slice) with the monthly means of
ozonesonde columns (calculated from the ground to the al-
titude of the tropopause as defined in Sect. 4). The com-
parison shows a good agreement between ozonesondes and
OMI/MLS tropospheric columns. In particular they both
agree for smaller values in May 2005 and 2006 and Septem-
ber 2006. Although similar, amplitudes of the seasonal cy-
cle and interannual variability seem weaker in OMI/MLS
data set. During the 26 month period, the highest tropo-
spheric ozone amount over Cotonou was measured in De-
cember 2005 by the ozone soundings. This extreme value
is also reported by OMI/MLS, but to a lesser extent though.
The OMI/MLS tropospheric column reaches up to 41.4DU
in December 2005 whilst its mean value is 35.5DU through
the 26 months (i.e. 17% higher). Generally, OMI/MLS tro-
pospheric residuals from Z06 exhibit a significant negative
offset relative to ozonesondes: 11.8DU and 6.7DU in DJF
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6170V. Thouret et al.: Overview of Cotonou ozone soundings
Table 4. Mean tropospheric ozone columns in DU over Cotonou
during DJF and JJA for OMI/MLS and ozonesonde columns and
their absolute and relative differences.
columns from the surface up to the tropopause level and RS2 corre-
spond to the columns from 850hPa up to the tropopause.
RS1 correspond to the
Mean DJFMean JJA
OMI/MLS (DU)
RS1 (surf-Top) (DU)
RS2 (850hPa-Top) (DU)
RS1-sat (DU)
RS2-sat (DU)
36.4
48.2
41.9
36.1
42.8
38.9
RS1-sat (%)
RS2-sat (%)
11.8
5.5
24.5%
13.1%
6.7
2.8
15.7%
7.2%
and JJA, respectively (Table 4). Those results are greater
than those found by Z06 or S07. The different sampling fre-
quency of ozonesondes (3 to 9 days a month) and of satellite
data (almost everyday) used to compute monthly means as
well as the missing ozone variation within the pixel size of
OMI/MLS data (1◦×1.25◦) are two basic sources of discrep-
ancy for such a comparison. This could be the main rea-
son for large differences observed in December 2005 when 2
out of our 3 profiles were characteristics of an extreme small
scale event. Also MLS stratospheric columns are biased such
that they are slightly high by a few DU and assuming no bias
for the OMI column, the TTOR would also be low by a few
DU(S07). Z06explicitlyaddedozonebasedonarecenteval-
uation of OMI and MLS offset differences (S07). However
the MLS high bias does not explain our 7–12DU differences.
Compared to Cotonou, most of the SHADOZ stations are
subject to low amounts of pollution and pretty clean near
the surface. Considering that OMI is not very sensitive near
the surface due to strong Rayleigh scattering (S07), we have
computed ozonesonde TCO minus the lowest layers below
850hPa. These reduced columns compare better with the
OMI/MLS tropospheric residuals (Fig. 14) and diminish the
offsets to 5.5DU and 2.8DU for DJF and JJA, respectively
(Table 3). These results are consistent with the low sensi-
tivity of OMI in the boundary layer. Also using a model
comparison Z06 noticed that a band of +10DU difference
centered over Northern Africa is persistent and coincides
with mineral dust plumes from desert. This discrepancy was
not explained but they suggested that desert dust affects ei-
ther OMI measurements or the model. This assumption is
in agreement with our higher difference in DJF compared
to JJA; in DJF the Harmattan flux transporting mineral dust
from the Sahara as well as biomass burning pollution reaches
the Guinean coast, and so Cotonou. A sensitivity test to
the tropopause pressure of the column show that the ozone
amount is not significantly affected by the tropopause pres-
sure chosen (Fig. 14).
6Conclusions
In this paper, we have presented the first ozone data set ob-
tained through balloon-sondes launches in West Africa. Dur-
ing the AMMA campaign, a total of 98 vertical profiles over
Cotonou, Benin, were measured over a 26 month period.
These regular measurements document the seasonal and in-
terannual variability of ozone in both the troposphere and the
lower stratosphere over West Africa for the first time. This
data set complements the MOZAIC observations made from
Lagos between 0 and 12km. Moreover it provides a first as-
sessment of the ozone distribution in the UTLS and gives the
firsttroposphericcolumnsofozonebasedonin-situdataover
West Africa.
The comparison with aircraft data has shown that the
ozone concentrations from the soundings are in reasonable
agreement with the ones recorded by UV analyzers. As ex-
pected, Cotonou ozone soundings show the same seasonal
characteristics as those derived from the MOZAIC measure-
ments over Lagos in the lower and mid-troposphere. Both
data sets highlight the unique way in which West Africa is
impacted by two biomass burning seasons from two differ-
ent hemispheres. This leads to ozone enhanced layers in
the lower troposphere in DJF (due to burning in the Sahelian
band) and in JJA (due to burning in southern Africa) as pre-
viously described by Sauvage et al. (2005, 2007a) and Mari
et al. (2008). Even though a 20% average difference has been
calculated between the MOZAIC data and ozone soundings
measurements between 2 and 8km, no systematic difference
has been found between the two data sets. On the contrary,
high variabilities appear in each data set (examples of July
and August for MOZAIC, March, September and December
for the soundings). Thus, the rather high interannual variabil-
ity of the ozone profiles throughout the troposphere is found
to be a major characteristic of this region and has been high-
lighted in most of the sections. The example of the dry sea-
son 2005–2006 shows that the influences of biomass burn-
ing emission and dynamics changes on the ozone amount
are large. This study has also shown that the enhancement
of ozone originating from the biomass burning products in
the Southern Hemisphere was more frequent during the wet
seasons of 2005 and 2006 than previously established based
on the MOZAIC data set with 41% of the profiles show-
ing ozone enhancement compared to 28%. This major fea-
ture should be considered when analyzing this period. The
high interannual variability of ozone in this region seems to
be related to the multiplicity of the trace gas sources with
their own variabilities (biomass burning, lightning, megac-
ity of Lagos) and also to the particular atmospheric circu-
lation. This latter factor combines the characteristics of a
transition zone between the monsoon and Harmattan flows
and a region under monsoon and convective influences; the
three of them have significant temporal variations. Indeed,
suchvariabilitiesmightbetypicalofEquatorialAfrica. Since
December 2005, a MOZAIC aircraft has been operated by
Atmos. Chem. Phys., 9, 6157–6174, 2009 www.atmos-chem-phys.net/9/6157/2009/

Page 15

V. Thouret et al.: Overview of Cotonou ozone soundings6171
Air Namibia and sampled the upper troposphere between
Windhoek, Namibia and Europe. The mean ozone values
in the UT around 6◦N–7◦N along the flight tracks also show
high interannual variability with differences of up to 20–30%
from one year to the next especially in February, May, June,
October and December (J-P. Cammas, personal communica-
tion). As the flights go east of Cotonou and Lagos (∼10–
15◦E), a quantitative comparison would not have been rele-
vant here. However, the Air Namibia data set confirms the
high interannual variabilities of ozone concentrations in the
African upper troposphere.
The West African UTLS shows the same patterns regard-
ing the ozone and temperature annual cycle, as those found
over other tropical stations.
trations registered in the UTLS above Cotonou are higher
than expected from previous tropical studies. Ozone concen-
trations range around 90–110ppbv on average between 12
and 17km, and reach up to 150ppb in August and Septem-
ber. A first quantification of tropospheric ozone columns
based on in-situ data has been derived over Cotonou. They
maximize in DJF with 45DU on average.
is higher than the one calculated by Sauvage et al. (2006)
from the MOZAIC profiles over Lagos (41DU). This differ-
ence is mainly attributed to their underestimate of the ozone
distribution in the upper troposphere. These tropospheric
columns derived from the ozonesoundings have been com-
pared to satellite data and a qualitative good agreement has
been found with the OMI/MLS tropospheric column. Quan-
titatively, the difference is around 7–12DU and can be re-
duced to 3–6DU if we omit the lowest layers of the tropo-
sphere (below 850hPa) in the calculation from the sound-
ings, suggesting that much of the negative bias may be at-
tributed to the lack of sensitivity of OMI to the boundary
layer.
The AMMA program has allowed the acquisition of the
data set presented here that has already been used for various
studies such as satellite validations (Schoeberl et al., 2007;
Jiang et al., 2007), data analysis (Reeves et al., 2009) and
models studies (Mari et al., 2008; Williams et al., 2009b,a).
We believe its use will last long after the AMMA pro-
gramme. Quantifying the tropospheric ozone budget over
West Africa will require further analysis. For example it is
still a challenge for Chemical Transport Models to be able
to reproduce these seasonal characteristics (Saunois et al.,
2008b; Williams et al., 2009b,a).
However the ozone concen-
Such a value
Acknowledgements. Based on a French initiative, AMMA was
built by an international scientific group and is currently funded
by a large number of agencies, especially from France, the United
Kingdom, the United States, and Africa. It has been the beneficiary
of a major financial contribution from the European Community’s
Sixth Framework Research Programme. Detailed information on
scientific coordination and funding is available on the AMMA
International Web site at www.amma-international.org. Besides,
authors warmly thank all the people from IRD based in Cotonou
for their help before, during and after this sounding operation, as
well as Mr Francis Did´ e, from SMN/ASECNA. Finally, collabo-
rations with Mrs Aristide Akpo, Etienne Houngninou and Basile
Kounouhewa from the University of Abomey-Calavi have been
really appreciated.
Edited by: J. Williams
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