Chemical and aerosol characterisation of the troposphere over West Africa during the monsoon period as part of AMMA

Abstract

During June, July and August 2006 five aircraft took part in a campaign over West Africa to observe the aerosol content and chemical composition of the troposphere and lower stratosphere as part of the African Monsoon Multidisciplinary Analysis (AMMA) project. These are the first such measurements in this region during the monsoon period. In addition to providing an overview of the tropospheric composition, this paper provides a description of the measurement strategy (flights performed, instrumental payloads, wing-tip to wing-tip comparisons) and points to some of the important findings discussed in more detailed in other papers in this special issue. The ozone data exhibits an "S" shaped vertical profile which appears to result from significant losses in the lower troposphere due to rapid deposition to forested areas and photochemical destruction in the moist monsoon air, and convective uplift of O3-poor air to the upper troposphere. This profile is disturbed, particularly in the south of the region, by the intrusions in the lower and middle troposphere of air from the Southern Hemisphere impacted by biomass burning. Comparisons with longer term data sets suggest the impact of these intrusions on West Africa in 2006 was greater than in other recent wet seasons. There is evidence for net photochemical production of ozone in these biomass burning plumes as well as in urban plumes, in particular that from Lagos, convective outflow in the upper troposphere and in boundary layer air affected by nitrogen oxide emissions from recently wetted soils. This latter effect, along with enhanced deposition to the forested areas, contributes to a latitudinal gradient of ozone in the lower troposphere. Biogenic volatile organic compounds are also important in defining the composition both for the boundary layer and upper tropospheric convective outflow. Mineral dust was found to be the most abundant and ubiquitous aerosol type in the atmosphere over Western Africa. Data collected within AMMA indicate that injection of dust to altitudes favourable for long-range transport (i.e. in the upper Sahelian planetary boundary layer) can occur behind the leading edge of mesoscale convective system (MCS) cold-pools. Research within AMMA also provides the first estimates of secondary organic aerosols (SOA) across the West African Sahel and have shown that organic mass loadings vary between 0 and 2 μg m−3 with a median concentration of 1.07 μg m−3. The vertical distribution of nucleation mode particle concentrations reveals that significant and fairly strong particle formation events did occur for a considerable fraction of measurement time above 8 km (and only there). Very low aerosol concentrations were observed in general in the fresh outflow of active MCSs, likely as the result of efficient wet removal of aerosol particles due to heavy precipitation inside the convective cells of the MCSs. This wet removal initially affects all particle size ranges as clearly shown by all measurements in the vicinity of MCSs.

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Atmos. Chem. Phys., 10, 7575–7601, 2010
www.atmos-chem-phys.net/10/7575/2010/
doi:10.5194/acp-10-7575-2010
© Author(s) 2010. CC Attribution 3.0 License.
Atmospheric
Chemistry
and Physics
Chemical and aerosol characterisation of the troposphere over West
Africa during the monsoon period as part of AMMA
C. E. Reeves1, P. Formenti2, C. Afif2,3, G. Ancellet4, J.-L. Atti´ e5,6, J. Bechara2, A. Borbon2, F. Cairo7, H. Coe8,
S. Crumeyrolle6,9, F. Fierli7, C. Flamant7, L. Gomes6, T. Hamburger10, C. Jambert5, K. S. Law4, C. Mari5,
R. L. Jones11, A. Matsuki9,12, M. I. Mead11, J. Methven13, G. P. Mills1, A. Minikin10, J. G. Murphy1,*, J. K. Nielsen14,
D. E. Oram1, D. J. Parker15, A. Richter16, H. Schlager10, A. Schwarzenboeck9, and V. Thouret5
1School of Environmental Sciences, University of East Anglia, Norwich, UK
2LISA, UMR CNRS 7583, Universit´ e Paris Est Cr´ eteil et Universit´ e Paris Diderot, Institut Pierre Simon Laplace,
Cr´ eteil, France
3Department fo Chemistry, Faculty of Sciences, Saint Joseph University, Beirut, Lebanon
4LATMOS, Universit´ e Paris VI, Universit´ e Versailles-St-Quentin, CNRS, Paris, France
5Laboratoire d’A´ erologie, Universit´ e de Toulouse, CNRS, UMR, Toulouse, France
6Centre National de Recherches Meteorologiques, Meteo-France, Toulouse, France
7Istituto di Scienze dell’Atmosfera e del Clima, Consiglio Nazionale delle Ricerche, Italy
8School of Earth, Atmospheric & Environmental Sciences, University of Manchester, Manchester, UK
9Laboratoire de M´ et´ eorologie Physique, Universit´ e Blaise Pascal, Clermont-Ferrand, France
10Deutsches Zentrum f¨ ur Luft- und Raumfahrt (DLR), Institut f¨ ur Physik der Atmosph¨ are,
Oberpfaffenhofen Wessling, Germany
11Department of Chemistry, Univeristy of Cambridge, Cambridge, UK
12Frontier Science Organization, Kanazawa University, Japan
13Department of Meteorlogy, University of Reading, Reading, UK
14Danish Meteorological Institute, Research and Development Division, Copenhagen, Denmark
15School of Earth and Environment, University of Leeds, Leeds, UK
16Institute of Environmental Physics, University of Bremen, Bremen, Germany
*now at: Department of Chemistry, University of Toronto, Toronto, Canada
Received: 14 January 2010 – Published in Atmos. Chem. Phys. Discuss.: 16 March 2010
Revised: 20 July 2010 – Accepted: 26 July 2010 – Published: 16 August 2010
Abstract. During June, July and August 2006 five aircraft
took part in a campaign over West Africa to observe the
aerosol content and chemical composition of the troposphere
and lower stratosphere as part of the African Monsoon Mul-
tidisciplinary Analysis (AMMA) project. These are the first
such measurements in this region during the monsoon pe-
riod. In addition to providing an overview of the tropospheric
composition, this paper provides a description of the mea-
surement strategy (flights performed, instrumental payloads,
wing-tip to wing-tip comparisons) and points to some of the
important findings discussed in more detail in other papers in
this special issue.
Correspondence to: C. E. Reeves
(c.reeves@uea.ac.uk)
The ozone data exhibits an “S” shaped vertical profile
which appears to result from significant losses in the lower
troposphere due to rapid deposition to forested areas and
photochemical destruction in the moist monsoon air, and
convective uplift of ozone-poor air to the upper troposphere.
This profile is disturbed, particularly in the south of the re-
gion, bytheintrusionsinthelowerandmiddletroposphereof
air from the southern hemisphere impacted by biomass burn-
ing. Comparisons with longer term data sets suggest the im-
pact of these intrusions on West Africa in 2006 was greater
than in other recent wet seasons. There is evidence for net
photochemical production of ozone in these biomass burning
plumes as well as in urban plumes, in particular that from
Lagos, convective outflow in the upper troposphere and in
boundary layer air affected by nitrogen oxide emissions from
recently wetted soils. This latter effect, along with enhanced
Published by Copernicus Publications on behalf of the European Geosciences Union.

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7576C. E. Reeves et al.: Chemical and aerosol characterisation of the troposphere over West Africa
deposition to the forested areas, contributes to a latitudinal
gradient of ozone in the lower troposphere. Biogenic volatile
organic compounds are also important in defining the com-
position both for the boundary layer and upper tropospheric
convective outflow.
Mineral dust was found to be the most abundant and
ubiquitous aerosol type in the atmosphere over Western
Africa. Data collected within AMMA indicate that injec-
tion of dust to altitudes favourable for long-range transport
(i.e. in the upper Sahelian planetary boundary layer) can oc-
cur behind the leading edge of mesoscale convective sys-
tem (MCS) cold-pools. Research within AMMA also pro-
vides the first estimates of secondary organic aerosols across
the West African Sahel and have shown that organic mass
loadings vary between 0 and 2µgm−3with a median con-
centration of 1.07µgm−3. The vertical distribution of nu-
cleation mode particle concentrations reveals that significant
and fairly strong particle formation events did occur for a
considerable fraction of measurement time above 8km (and
only there). Very low concentrations were observed in gen-
eral in the fresh outflow of active MCSs, likely as the re-
sult of efficient wet removal of aerosol particles due to heavy
precipitation inside the convective cells of the MCSs. This
wet removal initially affects all particle size ranges as clearly
shown by all measurements in the vicinity of MCSs.
1Introduction
The African tropical regions are critical for climate because
the high solar irradiance and humidity make this region im-
portant for determining the global oxidative capacity and
hence the lifetime of some greenhouse gases. The tropics are
also an important source of gases and aerosols from biogenic
emissions (Guenther et al., 1995), biomass burning (Hao and
Liu, 1994), industrial and urban areas (Aghedo et al., 2007;
van Aardenne et al., 2001), and aeolian erosion (Goudie,
1992; N’Tchayi Mbourou, G., 1997; N’Tchayi Mbourou,
1994).
TheWestAfricanMonsoon(WAM)leadstoastrongzonal
gradient in precipitation and consequently vegetation. This
is characterised by forest along the Guinea coast, shrub and
grasslands in the Sahel, with bare soil and desert in the north
of the region. The onset of the monsoon is typically in June,
with the wet season associated with the northernmost posi-
tion of the Inter-Tropical Convergence Zone (ITCZ). In the
lower troposphere (LT) the wind flow is characterised by the
south-westerly, moist and relatively cool monsoon flow and
the north-easterly, dry, warm Harmattan wind which meet at
the Inter-Tropical Discontinuity (ITD). Above the monsoon
flow is the African Easterly Jet (AEJ) (Thorncroft and Black-
burn, 1999; Parker et al., 2005; Thorncroft et al., 2003) at
around 600hPa, whilst in the upper troposphere (UT) there
is the Tropical Easterly Jet (TEJ) (Peyrille et al., 2007). Deep
convection occurs in organised systems known as Mesoscale
Convective Systems (MCS) (Mathon and Laurent, 2001).
Deep convection in the tropics associated with the ITCZ
can lead to the rapid uplift and large-scale redistribution of
gaseous pollutants and aerosols.
Model studies suggest biogenic emissions of volatile or-
ganic compounds (VOCs) from West Africa to be important
for tropospheric ozone (Aghedo et al., 2007; Pfister et al.,
2008). However measurements of biogenic VOCs (BVOCs)
from African vegetation have previously been focussed in
southern (Otter et al., 2003; Greenberg et al., 2003; Otter
et al., 2002; Guenther et al., 1996) and central Africa (Serca
et al., 2001; Guenther et al., 1999; Greenberg et al., 1999;
Klinger et al., 1998). BVOCs are also important in forma-
tion of secondary organic aerosols (SOA) (Kavouras et al.,
1998). The vegetation can also act as a rapid sink for ozone
via dry deposition (Cros et al., 2000).
The West African region can also be a source of biogenic
nitrogen oxides (NOx) both from soils (Jaegle et al., 2004)
and from lightning (Schumann and Huntrieser, 2007). Note
that both these sources are linked to the meteorology, with
the emissions from Sahel soils triggered by the wetting of
soils by precipitation, particularly at the onset of the wet sea-
son, and the lightning often associated with the MCSs.
Biomass burning in West Africa is linked to agricultural
practice at latitudes south of 10N. Its pattern follows a well
determined annual cycle related to the seasonal shift in the
ITCZ. In the wet season, the peak emissions of anthro-
pogenic biomass burning aerosol and trace gases occur in
the southern Hemisphere, outside of W. Africa, between the
equator and 10◦S (Hao and Liu, 1994).
cal profiles of ozone observed above Lagos (Sauvage et al.,
2005) and elsewhere in Africa (Nganga et al., 1996) sug-
gest biomass burning in the southern hemisphere can have
a widespread affect, even into the northern hemisphere and
over West Africa.
Aerosols are known to significantly affect the solar and
terrestrial radiation budget and the cloud properties of the
African region at the regional scale thereby modifying the
planetary albedo and the outgoing long-wave radiation, re-
ducing the radiation flux available to the surface, heating the
atmosphere and impacting the dynamics of the synoptic flow,
interacting with the ITCZ and the monsoon cycles (Forster et
al, 2007; Denman et al, 2007). Intense source regions as
well as horizontal transport processes near such sources can
be identified by radiometry from satellite (Generoso et al.,
2003, 2008; Legrand, 1997), but one of the main difficulties
remains to precisely identify the characteristics of particles
and their vertical extent, which is critical for their dispersion
in the atmosphere and the estimation of their radiative im-
pact. To date, little is known about aerosol emissions and
properties in West Africa and their spatial and temporal vari-
ability (Echalar et al., 1995; Formenti et al., 2003; Haywood
et al., 2003; Yoshioka et al., 2005). Amongst the major areas
of uncertainties remain: (i) the contribution of soil erosion by
However verti-
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C. E. Reeves et al.: Chemical and aerosol characterisation of the troposphere over West Africa7577
mesoscale convective systems in the Sahel to the global bud-
get of mineral dust (Tegen and Fung, 1995; Yoshioka et al.,
2005); (ii) the processes leading to the emission of organic
aerosols from vegetation and biomass burning, and the par-
titioning between primary and secondary formation are far
from being elucidated (Volkamer et al., 2006; Kanakidou et
al., 2005).
During June, July and August 2006 a multi-aircraft cam-
paign took place over West Africa to observe the aerosol
content and chemical composition of the troposphere and
lower stratosphere as part of the African Monsoon Multi-
disciplinary Analysis (AMMA) project (Redelsperger et al.,
2006; Lebel et al., 2009). These measurements on board 5
research aircraft provide the first detailed, in-situ characteri-
sation of the aerosol and trace gas composition of the tropo-
sphere in this region. The aircraft were equipped with instru-
ments to make measurements of ozone (O3), many of its pre-
cursor species (e.g. carbon monoxide (CO), NOxand VOCs),
as well as photochemical products (e.g. radical species and
oxygenated VOCs (OVOCs)) and properties and composi-
tion of atmospheric aerosols. Details of the meteorological
situation over West Africa during the summer of 2006 are
given in Janicot et al. (2008).
The aim of this paper is to provide an overview of the
flights made by the 5 research aircraft and to provide the first
comprehensive characterisation of aerosols and trace gases
over West Africa during the monsoon period. The scientific
questions that the aircraft campaigns addressed are given be-
low. This paper focuses on the average patterns observed,
and in particular those gained by looking at the data across
several measurement platforms, and how these address the
scientific questions. More detailed or case studies which ad-
dress individual aspects of the scientific questions are dealt
with by other papers, as indicated below. In this paper the
aircraft data are also compared to other data sets that are
available for this region from ozone sondes and satellites.
One of the main questions was the role of natural versus
anthropogenic emissions from W. Africa on the oxidizing ca-
pacity of the atmosphere. What evidence was there for ex-
tensive emissions of BVOCs and to what degree were they
transported throughout the troposphere (Bechara et al., 2009;
Garcia-Carreras et al., 2010; Murphy et al., 2010)? Were the
observed BVOC distributions consistent with that expected
from emission models (Ferreira et al., 2010)? Could emis-
sions of NOxfrom recently wetted soils be detected in the
boundary layer and what was the impact on ozone (Delon et
al., 2008; Stewart et al., 2008)? Were major coastal cities
such as Lagos a large source of ozone precursors (Hopkins et
al., 2009; Minga et al., 2010)?
Another issue was the impact of biomass burning emis-
sions from the southern hemisphere on the tropospheric com-
position over W. Africa. Could the impact implied by ozone
soundings be confirmed by observations of biomass burning
tracers (Murphy et al., 2010)? How widespread was the ef-
fect and what were the routes by which these pollutants were
being transported into W. Africa (Mari et al., 2008; Real et
al., 2010; Thouret et al., 2009; Williams, 2010; Fiedler et al.,
2010)? What are the physico-chemical properties of biomass
burning aerosols, in particular their composition (Matsuki et
al., 2010a)?
A key area of study was the role of convection on redis-
tributing pollutants and its impact on oxidants and aerosols
in the upper troposphere and lower stratosphere. What are
the roles of convective physical processes, vertical transport
and mixing on the budget of major oxidants and aerosols in
the free troposphere over West Africa (Ancellet et al., 2009;
Bechara et al., 2009; Fierli et al., 2010; Law et al., 2010;
Homan et al., 2010)? How do deep convective processes in-
fluence the distributions of chemical constituents in the trop-
ical tropopause layer (TTL) compared to other transport pro-
cesses (Barret et al., 2008; Barret et al., 2010; Fierli et al.,
2010; Homan et al., 2010; Law et al., 2010; Mari et al., 2008;
Real et al., 2010; Williams, 2010; Schiller et al., 2009; Voigt
et al., 2008; Palazzi et al., 2009)? What is the composition of
the lowermost stratosphere and to what extent is it influenced
by local convection (Fierli et al., 2010; Khaykin et al., 2009;
Borrmann et al., 2010; Liu et al., 2010)?
Overall, what are the impacts of all these emission sources
on the major sources and sinks of the oxidants over W. Africa
(Saunois et al., 2009; Stone et al., 2010; Andr´ es-Hern´ andez
et al., 2010; Commane et al., 2010)? What are the relative
roles of the anthropogenic and natural emissions on the tro-
pospheric oxidant loading (Saunois et al., 2009; Williams,
2010; Williams et al., 2009)? What is the impact of the pro-
duction of NOxfrom lightning within the convective systems
on ozone formation (Andres-Hernandez et al., 2009; Barret
et al., 2010; Williams et al., 2009)?
Regarding aerosols, a key issue was the potential for sec-
ondary organic particle formation from biogenic and anthro-
pogenic gas-phase precursors (Capes et al., 2009) and for
aerosol nucleation in the free troposphere. Attention was
given to investigating the emission processes and properties
of mineral dust. It was considered important to better un-
derstand the processes leading to dust emissions and the ver-
tical redistribution of dust after emission, in particular with
respect to the role of convective systems and of vegetation
heterogeneities (Bou Karam et al., 2008; Bou Karam et al.,
2009; Crumeyrolle et al., 2008, 2010; Flamant et al., 2007;
Flamant et al., 2009a, b; Marsham et al., 2008). An outstand-
ing issue was the physico-chemical properties driving the cli-
matic impacts of mineral dust, in particular with respect to
theirvariabilitybetweenSaharanandSaheliansources, emis-
sion versus transport conditions, and modifications of hygro-
scopic properties induced by cloud-processing (Crumeyrolle
et al., 2008; Formenti et al., 2010; Matsuki et al., 2010a, b).
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7578C. E. Reeves et al.: Chemical and aerosol characterisation of the troposphere over West Africa
Table 1. Aircraft detachment periods for each SOPs 1 & 2. The names of the aircraft
campaigns that took place within the wider AMMA SOPs were given the suffixes “aN”
where “a” signifies “aircraft” and “N” is the number of the aircraft campaign within that
SOP. Operational bases are colour-coded (red = Niamey, blue = Dakar, yellow =
Ouagadougou)
Table 1. Aircraft detachment periods for each SOPs 1 & 2. The names of the aircraft campaigns that took place within the wider AMMA
SOPs were given the suffixes “aN” where “a” signifies “aircraft” and “N” is the number of the aircraft campaign within that SOP. Operational
bases are colour-coded (red=Niamey, blue=Dakar, yellow=Ouagadougou).
1600
52
Tables
1595


Week
SOP #
Dates
Aircraft
Dates
BAe146
29/5 5/6 12/6
SOP1
19/6 26/6 3/7 10/7 17/7 24/7 31/7 7/8 14/8 21/8 28/8 4/9 11/9
SOP2
1 Jun – 30 Jun
SOP 1a
1–15 Jun
1 Jul–15 Sep
SOP 2a2
17 Jul–25 Aug
17 Jul–21 Aug
Chemistry, aerosol and
atmospheric structure
SOP 2a1
1–15 Jul
SOP 2a3
1–15 Sep
22–28 Aug
Aerosols and
atmospheric
structure


ATR-42 1–15 June
Aerosols,
Atmospheric
turbulence
1–15 Jun
Aerosols and
water vapour
1–15 July
Aerosols,
Atmospheric
turbulence
1–15 Jul
Aerosols and
water vapour
25 Jul–21 Aug
Aerosol and chemistry,
atmospheric turbulence

F-F20 10–21 Aug
Chemistry
and
atmospheric
structure
1–16 Aug
Chemistry and
aerosols
1–13 Aug
Chemistry and
aerosols
6–15 Sep
RALI and
dropsondes
D-F20 1–14 Jul
Wind 3-D fields

M55
2 Field campaign
2.1Flying programme
The AMMA field measurement programme was divided into
several different types of observational periods (International
Science Plan, http://science.amma-international.org/science/
docs/AMMA ISP May2005.pdf) (Table 1). The measure-
ments presented here are from the Special Observational Pe-
riods 1 and 2 (SOP 1 and SOP 2). SOP1 targeted the pre-
onset period of the Monsoon development in June. SOP2
then followed and targeted the monsoon onset and maximum
in July and August 2006 with the aircraft instrumented for
chemical measurements.
During June, July and August 2006 five research aircraft
(three based in Niamey, Niger: FAAM BAe-146, SAFIRE
Falcon (F-F20) and ATR-42; and two in Ouagadougou,
Burkina Faso: DLR Falcon (D-F20) and Geophysica (M55)
(Table2)madecomprehensivemeasurementsofaerosolsand
trace gases from the boundary layer to the lower stratosphere
(around 50hPa), from 2◦N to 21◦N, and between 10◦W
and 7◦E. The horizontal and vertical ranges covered by
each aircraft are illustrated in Fig. 1. The ATR-42 focussed
on the lower troposphere, the BAe-146 on the lower and
mid-troposphere, the two Falcons on the upper troposphere
and the M55 on the upper troposphere/lower stratosphere
(UTLS). In combination this provides coverage throughout
the full depth of the troposphere.
As part of the coordinated flight planning several differ-
ent flight strategies were designed to address different sci-
entific questions. Each of these flight strategies was called
an Intensive Observational Period (IOP). The types of IOPs
flown during SOP1 and SOP2 are given in Table 3. IOP1.1
was aimed at exploring of the inter-tropical front (ITF) and
surveying of the spatial and temporal evolution of the at-
mosphere in the coupled monsoon-harmattan-AEJ system.
IOP1.2 focussed on the description of the role of mesoscale
convective systems on the emission budget of mineral dust
from the Sahel. IOPs 1.4, 1.5 and 1.6 were designed to in-
vestigate the impact of local (W. African) emissions on the
chemical composition of the PBL. In particular, IOPs 1.4 and
1.5 aimed at investigating the impact of biogenic emissions
from soils of different moisture characteristics from different
vegetation types, while IOP1.6 targeted anthropogenic emis-
sions from urban areas. IOP2 was aimed at investigating the
impact ofMCSs on thetransport and transformationof pollu-
tants in air as it was convectively uplifted. This included co-
ordinated flights with some aircraft probing the PBL prior to
uplift and others sampling the UT in regions of detrainment.
IOP3targetedairmassesundergoinglongrangetransport, ei-
ther into the W. African region (e.g. biomass burning plumes
from the southern hemisphere) or those in the UT following
convective uplift some days previously. In addition to these
targeted studies, data were also collected throughout the dif-
ferent flights to build up a large scale picture of the chemical
composition and processing of air over W. Africa.
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C. E. Reeves et al.: Chemical and aerosol characterisation of the troposphere over West Africa 7579
Fig. 1. Flight tracks of the 5 research aircraft during SOPs 1a1 (top), 2a1 (middle) and 2a2 (bottom).
Table 2. Aircraft capabilities.
AircraftF-F20ATR-42BAe-146D-F20M55
Operating altitude (km)
Payload (kg)
Range (km)
Staffing
Duration (hrs)
Ground speed (m/s)
0.15–13
1200
3200
3 crew, 2 scientists
4
170–200
0.15–8
2500
3000
3 crew, 7 scientists
4
95
0.015–11
4000
3700
3 crew, 18 scientists
5.5
120–180
0.015–12
1200
3200
3 crew, 3 scientists
3–4
170–240
15–22
1500
3500
1 crew
5.5
210
Table 1 of the Supplementary Information shows when
each aircraft flew and which IOP was addressed by each
flight, whilst Tables 2a-e of the Supplementary Information
give details about each flight aircraft by aircraft. In all a total
of 107 science flights were made involving over 389 hours of
flying time.
2.2Aircraft payloads
The aircraft were fitted with a range of instrumentation for
measuring chemical species (Tables 3a-e of the Supplemen-
tary Information).
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7580 C. E. Reeves et al.: Chemical and aerosol characterisation of the troposphere over West Africa
Table 3. Intensive Observational Periods (IOPs). The IOP number-
ing system is based on that widely used in the implementation of the
AMMA programme. Flights relating to the IOPs listed were flown
during the campaigns when the aircraft were fitted with instrumen-
tation for chemical and aerosol measurements.
IOPDescription
1.1Surface-atmosphere: Inter-tropical front and heat low
surveys
Surface-atmosphere-aerosol:
aerosol emissions surveys
Surface-atmosphere:North-South
atmosphere interactions surveys
Surface-atmosphere: Land-atmosphere interactions
Vegetation and soil emission surveys
Urban surveys
Aerosol mixing and hygroscopicity
Turbulence
Dynamics and chemistry of Mesoscale Convective
Systems (MCS)
Long range transport surveys
Intercomparison flights
1.2Squall-line related
1.3land-ocean-
1.4
1.5
1.6
1.7
1.8
2
3
6
2.3Flight planning
Flights were planned using a number of tools depending
on the IOP to be flown.Meteorological forecast reports
were provided by a team of local forecasters at ACMAD
(African Centre of Meteorological Application for Develop-
ment). These indicated the predicted positions of such fea-
tures as the AEJ, the TEJ, the ITD, African Easterly Waves
(AEW) and organised convection at 06:00 and 18:00UTC.
Meteosat infra-red images were used for now-casting the lo-
cation ofMCSs, which wasparticularly usefulfor IOP2. Tra-
jectories calculated from ECMWF data were used for plan-
ning IOP3 (long range transport). Flight planning for IOP1.4
used near real-time Land Surface Temperature Anomaly data
derived from LandSAF (http://landsaf.meteo.pt/) to identify
regions of recent precipitation and for IOP1.5 MODIS tree
cover data from Land Processes Distributed Active Archive
Center (http://lpdaac.usgs.gov) was used.
One Chemical Transport Model (LMDz-INCA) and one
regional model (Meso-NH) driven by ECMWF meteorologi-
cal products delivered chemical, biogenic and lightning trac-
ers forecast continuously during the mission. A Lagrangian
model, FLEXPART-GIRAFE, was used to track the biomass
burning plumes from the southern hemisphere (Mari et al.,
2008). MOPITT data were also available in near real-time
over West Africa.
3Intercomparison of aircraft data
The airborne observation dataset from AMMA was collected
from several different aircraft operated in different locations
and at different times. The true value of the complete dataset
can only be realised if the measurements are integrated to-
gether in a consistent manner. The assimilation of the data
must take account of the uncertainty in the measurements
associated with systematic and random errors estimated for
each instrument. It is essential to estimate these errors in an
operating environment, which for aircraft entails wingtip-to-
wingtip comparison flights along straight and level runs at
various altitudes.
On 16 August 2006 four of the aircraft participated in a
comparison flight to the west of Niamey. The BAe-146 flew
alongside three aircraft in turn at a variety of altitudes: the
D-F20, F-F20 and ATR42. Therefore, differences between
measurements are all estimated relative to the BAe-146 mea-
surements. For details of the flights and how the data were
processed see Appendix A.
Table 4 shows the mean, standard deviation and rank cor-
relation between the time series from all comparable instru-
ments on the different aircraft during the formation flight
segments. Longitude and latitude measurements have an in-
significant bias with an average difference of approximately
40m, consistent with the aircraft separation. Measurements
of static pressure do indicate small biases, the largest being
between the ATR-42 and BAe-146 (3.6hPa). However on
this comparison the BAe146 was flying slightly higher than
the ATR-42 as a precaution of the turbulence experienced at
0.9km altitude. The turbulence also accounts for the lower
pressure correlation between these two aircraft.
From the initial comparison an error was identified in the
processing to obtain wind components for the BAe-146, ac-
counting for aircraft motion. This was corrected and the
comparison re-performed as shown in Table 4. Mean and
standard deviation of the differences are less than 1ms−1.
Wind data from the F-F20 was not submitted for compari-
son.
Temperature differences were approximately 1K and rel-
ative humidity (RH) differences were a few percent. It is
worth noting that on the comparison legs RH varied between
40 and 85% with rapid fluctuations at FL190. On the profiles
very thin cirrus cloud layers were crossed and these fluctua-
tions are likely to be associated with old cirrus layers. Rela-
tive humidity with respect to ice was measured on the highest
legs (FL190) and with respect to liquid water below this.
Differences in ozone measurements were lower than
during comparison flights from previous campaigns (e.g.,
ICARTT; Fehsenfeld et al., 2006) with means and standard
deviations of only 1ppbv. Figure 2 compares the ozone time
series from all 4 aircraft during the comparison legs. Note
that the time stamp from the F-F20 instrument had not been
synchronised and suffered an obvious lag of 60s which has
been corrected for in Table 4 and Fig. 2.
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C. E. Reeves et al.: Chemical and aerosol characterisation of the troposphere over West Africa 7581
Table 4. Results from comparison flight legs in formation with BAe-146. For each aircraft, the difference from the BAe-146 measurement
is summarised by its mean, standard deviation and rank correlation (R). A running median filter with a window width of 10s was applied to
all data before putting on common time-base.
Variable UnitsD-F20F-F20 ATR42
mean
0.00016
−0.00022 0.00037
−0.93
0.17
-0.23
−0.09
2.4
0.1
0.1
sdev
0.00102
R
1.000
1.000
0.978
0.989
0.982
0.999
0.973
0.936
0.906
mean
0.00025
0.00015
−1.18
sdev
0.00102
0.00077
0.18
R
1.000
1.000
0.949
mean
0.00015
0.00012
3.61
0.15
−0.21
−0.83
−3.4
1.2
18.1
sdev
0.00044
0.00069
0.32
0.65
0.42
0.09
1.7
1.4
7.1
R
1.000
0.994
0.663
0.957
0.890
0.976
0.914
0.763
0.746
Longitude
Latitude
Pressure
U
V
Temperature
RH
Ozone
CO
deg
deg
hPa
ms−1
ms−1
K
%
ppbv
ppbv
0.29
0.33
0.40
0.13
3.3
0.9
4.3
1.11
−1.6
−2.4
4.7
0.22
2.6
1.0
4.6
0.996
0.962
0.980
0.715
Fig. 2. Comparison of ozone measured on board the BAe-146
(black), D-F20 (blue), F-F20 (brown) and ATR42 (red) during
wing-tip to wing-tip flights.
The initial carbon monoxide comparison was much
worse than had been observed in previous campaigns (e.g.,
ICARTT). This motivated a comparison of the CO-standards
used during AMMA against two NOAA standards. The per-
centage differences of the measured concentration of each
AMMAstandardfromitsquotedconcentrationwere: −0.3%
for D-F20, +6.9% for BAe-146 and −7.2% for the F-F20.
The comparison of CO mixing ratios is shown in Table 4
after multiplying the time series by the appropriate scaling
factors: D-F20 (0.997), BAe-146 (1.069) and F-F20 (0.928).
The correction eliminates the bias between the D-F20 and
BAe-146measurements, buttheF-F20measurementsremain
high relative to the BAe-146 CO, although comparable with
the standard deviation in the case of the F-F20. These scaling
factors have been applied to the data before integrating them
Table 5. Results from comparison flight legs between BAe-146
and D-F20 in formation. The mean and standard deviation of BAe-
146 measurements is shown. The difference between D-F20 and
BAe-146 is summarised by its mean, standard deviation and rank
correlation (R). A running median filter with a window width of
10s was applied to all data before putting on common time-base.
VariableUnitsBAe-146D-F20
mean sdev
373.6 1.4
276
372
mean sdev
1.1
32
13
R
0.756
0.828
0.374
CO2
NOy
HCHO
ppmv
pptv
pptv
0.8
97
42
92
51
together to create average plots. The ATR-42 standard was
not involved in the comparison, so CO data from the ATR-42
are not included in this analysis.
Additional measurements were available for comparison
between the BAe-146 and D-F20 (Table 5). The mean and
standard deviation between CO2measurements is approxi-
mately 1ppmv and therefore of the same order as the stan-
dard deviation in the time series itself since CO2has a small
dynamicrange. Neverthelessthecorrelationisalmostashigh
as for CO.
Averaging across the entire comparison window the mean
difference between NOy measurements is approximately
12%. The correlation is also high. However, the BAe-146
measurementshowedlessdynamicrange, reportingoftheor-
der100pptvhigherat697hPaand100pptvlowerat485hPa.
The reasons for this have not been resolved and the compar-
ison was not as good as when the UK instrument was in-
stalled on the Met Research Flight C130 aircraft (Brough et
al., 2003).
Formaldehyde (HCHO) mixing ratios have only been
comparedontheprofileascentandhigherflightleg(485hPa)
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7582 C. E. Reeves et al.: Chemical and aerosol characterisation of the troposphere over West Africa
because the BAe-146 data showed an unrealistic oscillation
on the lower leg.
A comparison has also been made of peroxy radicals as
measured on board the BAe-146 using the FAGE (Laser in-
duced fluorescence) (Heard, 2006) (HO2only) and by PER-
CAs (Peroxy Radical Chemical Amplifier) (RO2+ HO2) on
board both the BAe-146 (Green et al., 2006) and the D-F20
Falcon (Andres-Hernandez et al., 2009). The results are re-
ported in Andres-Hernandez et al. (2010).
The intercomparison exercise for aerosols concerned only
three aircraft, the BAe-146, the D-F20 and the ATR-42 (the
F-F20 did not carry any in situ aerosol instrumentation). In-
tercomparing aerosol measurements onboard aircraft is chal-
lenging for a number of reasons (see Appendix A) and as
a consequence, the different aerosol datasets have not been
merged to provide mean patterns but, whenever appropriate,
they have instead been used individually to answer specific
science questions.
4 Overview of results
4.1Large scale patterns of trace gases
Figure 3a shows average O3concentrations from the 5 air-
craft plotted as a function of latitude and altitude. The av-
erages are created by taking 1-min data for each flight from
each aircraft, separating them into bins of 100hPa of atmo-
spheric pressure and 1 degree of latitude and then calculating
the mean. Figure 3b is the same as 3a except longitude bins
are used instead of latitude bins. A number of features evi-
dent in the O3distribution can be related to the dynamics of
the region.
There is a general vertical gradient with concentrations de-
clining towards the surface. A sharp gradient is observed
around 100hPa, with concentrations up to 1700ppv above
in the lower stratosphere, whilst concentrations in the tropo-
sphere rarely exceed 100ppbv. This vertical gradient largely
reflects an anti-correlation with water vapour, with high con-
centrations of ozone in the drier stratosphere and lower con-
centrations in the humid monsoon layer air in the lower tro-
posphere. The vertical gradient will be considered in more
detail in Sect. 4.2.
Note that there is no obvious pattern of ozone concen-
tration with longitude, at any altitude. However there are
a couple of features that can be identified when looking at
the latitude versus pressure plots. At low latitudes, around
650hPa, air was sampled that contained higher concentra-
tions of ozone than at other latitudes at similar pressures.
This air also tended to be drier. It was influenced by biomass
burning and had been advected into the region (see Sect. 4.3).
A second feature is that at the lowest altitudes bins (pressure
>800hPa) there is a tendency for concentrations of ozone
to be lower in the moister air to the south and higher in the
drier air to the north. Not only is the latitudinal gradient af-
Fig. 3. Average ozone concentrations in bins of 100hPa of atmo-
spheric pressure and (a) 1 degree of latitude and (b) 1 degree of
longitude based on 1-min data from the BAe-146, the DLR Falcon,
the French Falcon, French ATR and the M55 between 20 July and
21 August. Note that the ozone concentrations in the top layer are
averages between of 100 and 50hPa and are off the colour scale,
extending up to 1700ppbv.
Fig. 4. Average CO concentrations in bins of 100hPa of atmo-
spheric pressure and (a) 1 degree of latitude and (b) 1 degree of
longitude based on 1-minute data from the BAe-146, DLR Falcon
and French Falcon between 20 July and 21 August. Note that the
concentrations have been scaled according to the comparisons of
standards.
fected by the dynamics, but also by the land surface which
transitions from ocean in the south, via mosaic forest and
savanna to desert in the north. The impact of this on the
chemical composition in the lower atmosphere is discussed
in Sect. 4.5.
Figure 4 shows latitudinal and longitudinal average pro-
files of CO from the D-F20, F-F20 and BAe-146 scaled ac-
cording to the results of the comparison of standards. There
is no strong spatial pattern longitudinally; however there
are a number of interesting features in the latitudinal pro-
file. There is a tendency for higher concentrations in the
south at all altitudes, except over the ocean in the bound-
ary layer. In the lower troposphere the highest concentra-
tions were found between 5◦and 13◦N over the vegetated re-
gions and urban regions. The influence of landing at Cotonou
and a low level circuit of Lagos can be seen at 7◦N and of
landings at the aircraft bases of Ouagadougou and Niamey
at 13◦N. The influence of the biomass burning plumes can
be seen in the southerly region of the mid-troposphere. The
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C. E. Reeves et al.: Chemical and aerosol characterisation of the troposphere over West Africa 7583
Fig. 5. Average vertical profile of CO concentrations in bins of
100hPa of atmospheric pressure based on 1-min data from the BAe-
146, the DLR Falcon and the French Falcon between 20 July and 21
August. The black marker is the median, the red box the range of
the 25 to 75 percentiles, the blue whiskers the range of the 10 to 90
percentiles and the green points outliers less than the 1 percentile
and greater than the 99 percentile. Note that the concentrations have
been scaled according to the comparisons of standards.
average vertical profile of CO (Fig. 5), exhibits a “C” shape
with highest median concentrations in the lower and upper
troposphere. However the impact of biomass burning in the
mid-troposphere is evident in the high values of the 90th per-
centiles and the outliers (>99th percentiles).
To put the aircraft data into the wider context, both spa-
tially and temporally, and to help explain the features ob-
served, they are compared to satellite measurements of CO
from MOPITT (Emmons et al., 2009). Figure 6 shows the
aircraft and satellite data in layers centred on 700hPa and
250hPa. The MOPITT data are for August 2006, the month
during which most of the aircraft flights were made.
At 700hPa the elevated CO seen by the aircraft is part of a
large scale feature originating over the western part of south-
ern Africa (centred around Angola and Zaire) and extending
north westward into the Gulf of Guinea. The aircraft tended
to observe higher CO in the southern legs of the flights than
measured by satellite. Much of this is due to some flights tar-
geting these southern hemispheric plumes. At 250hPa this
feature, although much weaker, is still evident. It is not clear
how much of this CO at 250hPa results from convective up-
lift of CO-rich air from W. Africa, or how much is due to
long-range upper tropospheric transport into the region fol-
lowing up-lift elsewhere. Either way, the satellite data sug-
gests that emissions in southern Africa contributed to ele-
vated CO throughout much of the troposphere along the Gulf
of Guinea coast.
At 250hPa the aircraft tended to observe higher CO in the
region 4–10◦N than measured by satellite, largely because
many of the flights at this altitude where aimed at sampling
outflow from MCSs. There is sparse data coverage from the
satellite in this zone due to the frequent occurrence of cloud,
however there is still some suggestion of elevated CO in this
region, certainly compared to further north. The source of
Fig. 6. Top panel: average CO concentrations based on 1-min data
from the BAe-146, DLR Falcon and French Falcon between 20 July
and 21 August for the layers 650–750hPa (left) and 200–300hPa
(right). Note that the concentrations have been scaled according
to the comparisons of standards. Middle panel: average CO con-
centrations for August 2006 from MOPITT for the levels 700hPa
(left) and 250hPa (right). Lower panel: MOPITT CO anomalies
for August 2006 compared to the average for the years 2000–2008
(excluding 2001) for the levels 700hPa (left) and 250hPa (right).
this CO is discussed in several papers (Barret et al., 2008;
Ancellet et al., 2009; Law et al., 2010).
At 700hPa the aircraft data suggest slightly higher CO
concentrations in the north-west of the region. This is in
agreement with the satellite data which shows extensive re-
gions of CO above 100ppbv across northern Africa, with a
hot spot over Sudan (not shown).
Comparing the MOPITT data for August 2006 with Au-
gusts of previous years (2000–2008, excluding 2001 for
which there is insufficient data) (Fig. 6, bottom panel), it can
be seen that at 700hPa, CO was considerably higher than in
other years. As an average for the region shown in the figure
it was 10ppbv above the mean. This anomaly is highest in
the south of the region with values up of 60ppbv. It suggests
that the influence of biomass burning in the southern hemi-
sphere on CO in the lower troposphere over the West African
region was greater in 2006 then is typical of recent years.
Note that the CO in the north of the region at this altitude
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7584 C. E. Reeves et al.: Chemical and aerosol characterisation of the troposphere over West Africa
Fig. 7. Ozone and CO correlations coloured by a) water vapour and
b) acetonitrile. Data from the BAe-146 only.
in 2006 was also higher than the mean. Unlike 700hPa, CO
at 250hPa in August 2006 was close to the mean of recent
years.
Figure 7 shows the relationship between ozone and CO
(for the BAe-146 data only).
into a population where there is a negative correlation be-
tween these two chemical compounds. This correlation is
related to water vapour in that the air with higher ozone
and lower CO concentrations tends to be drier, whilst the
air with lower ozone and higher CO concentrations tends to
be moister. This illustrates the stratospheric source of tro-
pospheric ozone and the relatively short photochemical life-
time of ozone in the moist, monsoon layer air into which
CO is emitted. A second population of data shows a positive
relationship between ozone and CO. It occurs at a range of
water vapour concentrations and typically displays a ratio of
1 ozone molecule to 4 CO molecules. This population can
be subdivided into 2 groups: 1 that is influenced by biomass
burning as indicated by the elevated concentrations of ace-
tonitrile (Murphy et al., 2010) (Fig. 7b) and is in relatively
dry air and 1 that does not show increased acetonitrile but
The bulk of the data falls
Fig. 8. Average tropospheric column densities of NO2(top) and
HCHO (bottom) for July and August 2006 from SCIAMACHY.
is due to anthropogenic (non-biomass burning) emissions as
observed in a flight in the moist boundary layer around La-
gos, Nigeria. Figure 7 suggests that with the exception of a
fewpollutedairmasseswheretherewereclearenhancements
ofozonewithincreasedlevelsofCO(e.g.theCotonou/Lagos
city plume as discussed in Ancellet et al., 2009), most of the
air sampled over W. Africa shows a tendency for net photo-
chemical loss of ozone.
Measurements from the satellite instrument SCIA-
MACHY can be used to get a picture of the overall distri-
bution of NO2(Richter et al., 2005) and HCHO (Wittrock
et al., 2006). In contrast to the aircraft measurements, these
data give the integrated tropospheric column without vertical
profile information and have a spatial resolution of the order
of 30 x 60km2for individual pixels. In Fig. 8, the tropo-
spheric NO2and HCHO fields from SCIAMACHY averaged
over all data from July and August 2006 are shown.
The NO2distribution is characterised by high values over
regions of intense biomass burning and anthropogenic emis-
sions and low values over desert and ocean. In the Sahel re-
gion, NO2is slightly elevated probably as result of soil NOx
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C. E. Reeves et al.: Chemical and aerosol characterisation of the troposphere over West Africa7585
emissions (also observed by the aircraft; Stewart et al., 2008)
with no sharp gradient with latitude before about 18◦N. In-
terestingly, NO2values over the ocean close to the coast are
also elevated, and backward trajectories indicate that this is
a result of transport of biomass burning affected airmasses
from central Africa. No vertical information is available
from the measurements but as the distances involved are
rather large, transport must have taken place at higher alti-
tudes where the NOxlifetime is larger.
Formaldehyde as an indicator of VOC chemistry is shown
in the lower panel of Fig. 8. Its main sources are biogenic
isoprene and biomass burning. Consequently, high values
are observed over the region with fires and also over the
densely vegetated part of the AMMA region. Some indica-
tion of transport from the biomass burning region can also be
seen in the HCHO map, but to a much smaller degree than
in the NO2. In spite of the general similarities between the
NO2and HCHO distributions, several important differences
exist. Firstly, no anthropogenic (non-biomass burning) sig-
nals can be discerned in the region shown. Secondly, the
HCHO maximum attributed to biomass burning is shifted to
the north, indicating secondary production and contributions
from biogenic emissions. And lastly, as no soil emissions
are expected for HCHO, the drop off in the AMMA region is
more to the south than for NO2with generally larger values
and some hot spots south of 10◦N and low values north of
15◦N.
As satellite measurements are all taken in relatively cloud
free situations, the averages shown have a clear sky bias.
Also, for comparison with the in-situ data discussed in the
following sections, the integrated nature of the remote sens-
ing has to be taken into account as well as the fact that NO2
is measured, not NOx.
4.2Tropospheric profiles of ozone
Figure 9 shows the average vertical profile of ozone concen-
trations measured on all aircraft. Rather than simply exhibit-
ing an increase from the surface to the stratosphere, the pro-
file is “S” shaped with a maximum in the median at around
400hPa and a minimum at around 250hPa. This is typical
of that seen elsewhere in the tropics where the minimum
at 250hPa can be explained by the detrainment of ozone
poor air that has been convectively lifted from lower alti-
tudes (Folkins et al., 2002). Note that the maximum in the
outlier concentrations (>99 percentiles) at 650hPa is asso-
ciated with the biomass burning plumes in the south of the
region (see below and Fig. 10a).
Figure 10 illustrates how the vertical profile changes with
latitude. At 6◦N the “S” shaped profile is exaggerated with a
maximum and large variability at 650hPa due to some air
masses having been influenced by biomass burning emis-
sions. Although there is a minimum in the median at 350hPa
it is not clear if this is generated by the enhanced ozone con-
centrations below resulting from the biomass burning or due
Fig. 9. Average vertical profile of ozone concentrations in bins of
100 hPa of atmospheric pressure based on 1-minute data from the
BAe-146, the DLR Falcon, the French Falcon, French ATR and the
M55 between 20 July and 21 August 2006. The black marker is the
median, the red box the range of the 25 to 75 percentiles, the blue
whiskers the range of the 10 to 90 percentiles and the green points
outliers less than the 1 percentile and greater than the 99 percentile.
Note that the x- axis is limited to 200ppbv to show the variation of
ozone in the troposphere. Ozone concentrations in the top layer are
limited to data between 100 and 50hPa bin and exceed 200ppbv.
to low concentrations of ozone convectively lifted to this alti-
tude. At 10◦N there is again no obvious sign of detrainment
of convectively lifted ozone-poor air at 250hPa. Instead of
an “S” shape profile, the ozone concentrations increase from
the surface to 550hPa where they remain reasonably con-
stant up to 250hPa, before increasing into the stratosphere.
At 13◦N the “S” shape profile is apparent with clear minima
at the surface and at 250hPa. At 16◦N there is also an “S”
shape profile with the minimum at 250hPa, although the sur-
face concentrations are greater than further to the south such
that up to 750hPa the ozone concentrations are fairly similar,
if anything decreasing with altitude.
This change in vertical profile with latitude suggests that
the maximum in the convective uplift of ozone poor air is
around 12–14◦N. This is slightly further north than the loca-
tion of the inter-tropical convergence zone (ITCZ) which was
centred around 10–12◦N as indicated by the outgoing long-
wave radiation (OLR) for July and August 2006 (Janicot et
al., 2008). It should be noted that some flights were designed
specifically to target high altitude outflow from MCSs. This
may therefore have biased the average profiles and latitudinal
location of the maximum effect of this convective uplift.
Figure 11a shows the general profile behaviour of tro-
pospheric ozone concentrations recorded by ozone sondes
launched from Cotonou (6◦N) between 21 July and 22 Au-
gust. Concentrations from 9 individual soundings have been
averaged into bins 100hPa thick as for the aircraft data pre-
sented in Fig. 10. Further details on these soundings data can
be found in Thouret et al. (2009). In general, the compos-
ite profile from aircraft data at 6◦N (Fig. 10a) and the av-
eraged profile from the soundings highlight the same tropo-
spheric behaviour described above as an ”S” shape (i.e. low
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7586C. E. Reeves et al.: Chemical and aerosol characterisation of the troposphere over West Africa
Fig. 10. Average vertical profiles of ozone concentrations in bins of 100hPa of atmospheric pressure at (a) 6◦N, (b) 10◦N, (c) 13◦N, and
(d) 16◦N based on 1-min data from the BAe-146, the DLR Falcon, the French Falcon and the M55 between 20 July and 21 August. The
black marker is the median, the red box the range of the 25 to 75 percentiles, the blue whiskers the range of the 10 to 90 percentiles and the
green points outliers less than the 1 percentile and greater than the 99 percentile. Ozone concentrations in the top layer are limited to data
between 100 and 50hPa bin and exceed 200ppbv.
surfaceconcentrations, highozonebetween800and400hPa,
lower ozone around 250hPa and higher ozone in the UT by
100hPa).
Both data sets exhibit high to very high ozone concentra-
tions in layers between 800 and 400hPa. This highlights the
seasonal characteristic called “intrusions of biomass burning
products from the southern hemisphere”. As firstly revealed
by the MOZAIC data recorded over Lagos and Abidjan
(Sauvage et al., 2007; Sauvage et al., 2005), these high ozone
concentrations go along with high CO values as well as other
biomass burning tracers like acetonitrile (Fig. 7b). Consis-
tent with the MOPITT CO data discussed above, Thouret
et al. (2009) found by comparing the ozone sondes from
Cotonou during AMMA with vertical profiles obtained by
MOZAIC in other years, that these southern intrusions of
biomass burning were particularly frequent in 2006 com-
pared to other wet seasons.
In the upper troposphere, above 400hPa, data from the
soundings show higher concentrations with a median value
close to 70ppbv between 400 and 200hPa and close to
85ppbv between 100 and 200hPa while the aircraft data
exhibit around 20ppbv less. This may reflect the differ-
ences in the measurement techniques (Thouret et al., 2009)
and in sampling strategies. The soundings were launched
on predefined days irrespective of the conditions, but some
flights were designed specifically to target high altitude out-
flow from MCSs. This may therefore have biased the average
profiles and latitudinal location of the maximum effect of this
convective uplift. For example a siginifcant ozone concentra-
tion difference (20–30ppbv) is seen at 250hPa for the F-F20
flights around MCS and during the dry spells (Ancellet et al.,
2009).
Figure 11b shows the average profile for 29 ozone son-
des that were released from Niamey (13N) between 26 July
and 25 August. Although the concentrations are similar to
those observed by the aircraft near the surface (Fig. 10c),
like the Cotonou sondes the Niamey ozone sonde data gives
higher concentrations in the UT than the aircraft data. This
might be the different measurement technique and sampling
strategy. For Cotonou the ozone sondes were launched on
prescribed days regardless of conditions and for Niamey the
strategy was to smoothly cover the campaign with ozone
soundings, whereas the aircraft flights were for specific mis-
sions and many of those in the UT were sampling around
MCSs. No remarkable difference can be seen in the ozone
data from ozone sondes that were launched to target convec-
tive systems and in fact one profile which diverges dramati-
cally from the rest does not have any noticeably different his-
tory. It was a clear sky observation on 1 August 2006 early
afternoon.
The M55 observing strategy was directed partly to sample
the direct effects of MCSs on the UTLS and partly to char-
acterize the UTLS in as nearly an unperturbed state as possi-
ble, i.e. far from MCS events (Cairo et al., 2010). The flight
hours were nearly evenly distributed across these two goals.
Following an analysis of air mass origins in the TTL (Law
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C. E. Reeves et al.: Chemical and aerosol characterisation of the troposphere over West Africa7587
Fig. 11. Average ozone profiles based on (top) 9 soundings from
Cotonou (6◦N, 2◦E) launched at 10:00 or 11:00a.m. local time
between 21 July and 22 August 2006 and (bottom) 29 soundings
from Niamey (13◦N 2◦E) between 26 July and 25 August 2006.
Theblackmarkeristhemedian, theredboxtherangeofthe25to75
percentiles, the blue whiskers the range of the 10 to 90 percentiles
and the green markers are less than the 1 percentile and greater than
the 99 percentile.
et al., 2010), the M55 flights appear to be reasonably repre-
sentative of air masses impacting these altitudes over West
Africa. Model aided analysis confirms the presence of direct
injection up to the tropical tropopause by intense convective
systems and indicates that composition is dependent on the
residence time in the TTL after convective uplift (Fierli et al.,
2010).
Observations of coupled variations of total peroxy radi-
cals with NO and non-methane hydrocarbons (NMHC), in-
cluding isoprene, made on board the D-F20 and F-F20 in
some of regions of convective outflow indicate uplift of rad-
ical and O3precursors coincident with either uplift of NO
or an emission of NO from lightning (Andres-Hernandez et
al., 2009; Bechara et al., 2009). Significant net O3produc-
tion rates of around 1ppbv/h have been calculated from mea-
surements made in this outflow. Backward trajectories have
also been overlaid on Meteosat Second Generation images
to identify convectively influenced flight segments followed
by forward photochemical modelling, initialised with data, to
estimate net O3production in these air masses. This shows
that not only is O3poor air lifted from the lower troposphere
to the UT in these convective systems, but that this is coun-
teracted to some degree by the uplift and emission of O3pre-
cursors. In some cases the precursors observed in the UT
originated from biomass burning in the southern hemisphere
over Central Africa and have been calculated to contribute to
continued net O3production for several days as the air was
transported out over the ocean (Real et al., 2010), possibly
contributing to the O3maxima previously identified over the
southernAtlantic(Welleretal., 1996; Thompsonetal., 1996;
Jenkins and Ryu, 2004a, b). The role of convection and NOx
from lightning on the composition of the UT over W. Africa
has been further examined in a study using 4 global chemi-
cal transport models (Barret et al., 2010), which showed that
important differences between the UT CO and ozone distri-
butions simulated by each of the models could be explained
by differences in the convective transport parameterizations
and, more particularly, the altitude reached by convective up-
drafts. ModelsensitivitystudiesclearlyindicatedthattheCO
maxima and the elevated ozone concentrations south of the
equatorareduetoconvectiveupliftofairmassesimpactedby
Southern African biomass burning, in agreement with previ-
ous studies. Moreover, during the West African Monsoon,
NOxfrom lightning over W. Africa is calculated to be re-
sponsible for 10–20ppbv enhancements in UT ozone over
the tropical Atlantic.
4.3 Biomass burning
As already mentioned above,
(>100ppbv) of ozone were observed in the southern mid-
troposphere of the region sampled by the aircraft dur-
ing AMMA. Ozone vertical profiles collected through the
MOZAIC program between 1997 and 2003 over Lagos and
Abidjan, Ivory Coast, during July and August, a similar time
of year to the AMMA flights, also exhibit elevated concen-
trations around 650hPa (Sauvage et al., 2005). This feature
in the MOZAIC data was not systematic, but rather a variable
one. Similarly the range in the percentiles at 750 and 650hPa
at 6◦N (Fig. 10) illustrate that this was a variable feature in
the AMMA aircraft data.
June to August is the wet season in West Africa when
biomassburningisataminimum. Sauvageetal(2005), how-
ever, attributedtheenhancedozoneobservedintheMOZAIC
data to biomass burning in the southern hemisphere by using
trajectory analysis and fire count data. The acetonitrile data,
collected during AMMA on the BAe-146 aircraft (Murphy et
al., 2010), unequivocally confirms that biomass burning had
influenced these air masses. During SOP2, observations of
the size-resolved chemical composition of individual aerosol
particles by transmission and scanning electron microscopy
coupledwithenergydispersiveX-rayanalysis(Fig. 12)show
the presence of submicron biomass burning aerosol particles
even in the area around Niamey. Biomass burning particles
have been identified as K (in addition to S) enriched particles
composed mainly of C (and O), whereas particles showing a
fractal or chain like aggregate structure and giving only the
X-ray peak of C have been classified as elemental carbon. A
elevated concentrations
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7588 C. E. Reeves et al.: Chemical and aerosol characterisation of the troposphere over West Africa
0% 50% 100%
4…
4.5
4…
4
3…
3.5
3…
3
2…
2.5
2…
2
1…
1.5
1…
1
0…
0.5
0…
0
[km]
5
Dp< 1µm
Silicate
Biomass Burn.
Sulfate
Elem. Carbon
Sea salt
Others
0%50%100%
4…
4.5
4…
3…
3.5
3…
2…
2.5
2…
1…
1.5
1…
0…
0.5
0…
[km]
5
Dp> 1µm
Quartz
Feldspar
Clay, Mica
Amph., Garn.
Calcite
Dolomite
SeaSalt + Dust
SeaSalt (fresh)
SeaSalt (aged)
Sulfate
Biomass Burn.
Biogenic
Unknown
4
3
2
1
0
Fig. 12. Various types of aerosol particles identified by individ-
ual particle analysis in the submicron and supermicron sizes, and
their relative abundance averaged over all ATR-42 flights during
SOP2a2.
detailed discussion of the particle identification procedure is
presented in Matsuki et al. (2010a).
Further, using a particle dispersion model coupled with
daily active fire products from MODIS, Mari et al. (2008)
identifies these air masses to have been impacted by biomass
burning which took place in southern Africa and subse-
quently transported into West Africa in the easterly flow as-
sociated with the southern AEJ (S-AEJ). The variability in
the ozone concentrations reflects the variability in the occur-
rence of such transport events which is dependent on whether
the S-AEJ is active or not. Real et al. (2010) also examined in
further detail a particular event sampled by the M55 at 13km
over southern West Africa. It appears that northerly transport
of biomass burning emissions into central Africa followed by
convective uplift and westward transport by the TEJ was the
main transport mechanism. Williams et al. (2010) show that
the ability of chemistry transport models to simulate these
biomass burning transport events is highly dependent on the
meteorological analysis data that is used to drive the model.
The modification of the aerosol vertical profile induced
by such events is illustrated in Fig. 13 by the comparison
of the vertical profiles of the aerosol scattering coefficient
measured by the AVIRAD aerosol sampling system onboard
the ATR-42 during two different flights over the Gulf of
Guinea in the 3–4◦N area (longitude 2.5◦E), the first con-
ducted during SOP 1a1 and the second during SOP 2a1 (For-
menti et al., 2010). The spectral dependence of scattering
indicates that the aerosol size distribution is dominated by
particles in the accumulation mode. Flight V024 (14 June)
represents background marine conditions in the area, with
low scattering values in the boundary layer (∼50Mm−1;
1Mm−1=10−6m−1), decreasing to zero above. Transport
on biomass burning on 4 July (flight V030 of the F ATR-
42) results in the appearance of an elevated layer extending
up to 3km and showing values up to 100Mm−1at 450nm,
whereas the aerosol vertical distribution in the boundary
Fig. 13. Vertical profiles of the aerosol scattering coefficient mea-
sured during flights V024 of SOP 1a1 and V030 of SOP 2a1 at 450
(blue line), 550 (green line), and 700nm (red line) by the ATR-42.
layer remains unperturbed. This additional layer contributes
0.15 to the column scattering optical depth (obtained by in-
tegrating the scattering profile over the depth of the layer),
which is comparable to the annually-averaged median total
aerosolextinctionopticaldepthestimatedbytheAEROCOM
modelling exercise (Textor et al., 2006), especially when
considering that the absorption optical depth (not accounted
for by the aircraft data) should be significant. Further, the
aircraft sounding was topped at 3km, so it is not possible to
conclude anything about the presence of a secondary layer
in the upper troposphere, as evident in some of the ozone
profiles analysed by Thouret et al. (2009), which could also
contribute significantly to the columnar light extinction.
Although intrusions of such air during the wet season had
been observed before (Sauvage et al., 2005), it was largely
expected that biogenic emissions would dominate at this time
of year. Despite this, one of the main features of the AMMA
campaign was the large impact of biomass burning in the
southern hemisphere on the composition of the troposphere
over West Africa. However analysis of several years of verti-
calozoneprofiles(Thouretetal., 2009)andtheCOdatafrom
MOPPIT (see above) suggests that the impact of the biomass
burning was in fact greater in the wet season of 2006 than in
other recent years.
4.4Land surface impact
For ozone, the most noticeable gradient in the lower tropo-
sphere is a latitudinal one (Fig. 3). Focussing on data at pres-
sures greater than 900hPa, average ozone concentrations de-
cline from 30 to 20ppbv from over the ocean (5◦N) inland
to around 11◦N (Fig. 14a). There is then a sharp increase
to over 40ppbv at 15◦N, before declining again further to
the north. The ITD was located closer to 20◦N (Janicot et
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C. E. Reeves et al.: Chemical and aerosol characterisation of the troposphere over West Africa7589
Fig. 14. Average latitudinal profile at altitudes below 900hPa atmospheric pressure between 20 July and 21 August. The black marker is the
median, the red box the range of the 25 to 75 percentiles, the blue whiskers the range of the 10 to 90 percentiles and the green points outliers
less than the 1 percentile and greater than the 99 percentile. (a) ozone (all aircraft), (b) CO (D-F20, F-F20, BAe only), (c) benzene (BAe
only), (d) acetonitrile (BAe only), (e) isoprene (BAe only), (f) NOx(BAe only), (g) HCHO (BAe only), and (h) acetone (BAe only).
al., 2008) so the reason for the pattern in the O3is unlikely
to simply be a function of the south westerly monsoon air
being lower in O3than the north easterly Harmattan. There
is, however, a strong vegetation gradient around 12–13◦N
(Janicot et al., 2008), with substantial tree cover to the south
and scrubland and bare soil to the north. A similar pattern
for O3has been observed at surface IDAF (IGAC-DEBITS-
AFRICA) sites during the wet season, which shows mixing
ratios to be around 10ppbv higher over dry savannas in the
north compared to wet savannas to the south (Adon et al.,
2010).
The changing vegetation and land-use will not only af-
fect emissions of relevant trace gases but also deposition.
The lower O3concentrations between 6 and 12◦N are likely
to be at least partly due to rapid deposition to trees (Cros
et al., 2000). The fact that ozone decreases in concentra-
tion from the coast inland over the trees, in the direction
of the predominant monsoon flow, suggests that the sink
(chemical and depositional) is greater than the source. CO
concentrations (Fig. 14b) are generally anticorrelated with
ozone, the highest values being between 9 and 11◦N. The
exceptions are at 7◦N, which is largely due to emissions
from the coastal cities such as Lagos (Hopkins et al., 2009)
and Cotonou, and at 13◦N, which coincides with the air-
craft bases of Niamey and Ouagadougou. Interestingly the
predominantly anthropogenic tracer, benzene, shows a sim-
ilar pattern to CO. Acetonitrile, the biomass burning tracer,
showsnolatitudinalgradientatthesealtitudes(Fig.14d)sug-
gesting little contribution from anthropogenic sources. Iso-
prene (Fig. 14e), a biogenic species with a lifetime of only
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