Aerosol–cloud–climate interactions play a fundamental role in radiative balance and energy redistribution in the tropics. Aerosol particles from natural and anthropogenic origins can serve as cloud condensation nuclei (Haywood and Boucher, 2000; Carslaw et al., 2010) and interact with solar and terrestrial radiation through absorption and scattering.

The atmosphere over southern West Africa (SWA) is a complex mix of local emissions (vegetation, traffic, domestic and waste fires, power plants, oil and gas rigs, ships) and remote sources (dust from the north and wild-fire related biomass burning aerosols from central Africa) (Knippertz et al., 2015a; Brito et al., 2018). In order to enhance our understanding of aerosol–cloud–climate interactions in SWA, it is of paramount importance to better characterize the composition and vertical distribution of the aerosol load over the eastern tropical Atlantic. This is particularly vital, since SWA is currently experiencing major economic and population growths (Liousse et al., 2014), and is projected to host several megacities (cities with over 10 million inhabitants) by the middle of the 21st century (World Urbanization Prospect, 2015). This will likely boost anthropogenic emissions to unprecedented levels and imply profound impacts on population health (Lelieveld et al., 2015), on the radiative budget over SWA, and also on the West African Monsoon (WAM) system (Knippertz et al., 2015b). This will also add to the dust and biomass burning aerosol related perturbations already evidenced for the precipitation in the area (e.g. Huang et al., 2009). Likewise, urban pollution may also affect surface–atmosphere interactions and associated lower tropospheric dynamics over SWA such as for instance dust over the tropical Atlantic (e.g. Evan et al., 2009) or biomass burning aerosols over Amazonia (Zhang et al., 2008, 2009).

One of the aims of EU-funded project Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa (DACCIWA, Knippertz et al., 2015b) is to understand the influence of atmospheric dynamics on the spatial distribution of both anthropogenic and natural aerosols over SWA after emission. One particularly important aspect is the fate of anthropogenic aerosols emitted at the coast as they are being transported away from the source. In addition, DACCIWA aims at assessing the impact of this complex atmospheric composition on the health of humans and ecosystems.

Urban aerosols are mostly transported with the southwesterly monsoon flow below 700 hPa (e.g. Deroubaix et al., 2018). They may also reach the nearby ocean as the result of complex dynamical interactions between the monsoon flow, the northeasterly flow from the Sahel above, and the interactions with the atmospheric boundary layer (ABL) over the continent coupling the two layers when it is fully developed during daytime. This is because, as opposed to the marine ABL, the continental ABL exhibits a strong diurnal cycle (e.g. Parker et al., 2005; Lothon et al., 2008; Kalthoff et al., 2018). On hot, cloud-free summer days, land–sea breeze systems can develop at the coast (in conditions of moderate background monsoon flow, Parker et al., 2017), which contribute to the transport of pollutants emitted along the urbanized coastal strip of SWA.

The main objective of the present study is to understand how shallow overturning circulations developing in the lower troposphere over SWA on hot, cloud-free days can shape the urban pollution plumes emitted from coastal cities such as Accra, Lomé, Cotonou, and Lagos, both over the Gulf of Guinea and inland. Here, we take advantage of the airborne measurements acquired during the DACCIWA field campaign (June–July 2016, Flamant et al., 2018) as part of the European Facility for Airborne Research (EUFAR) funded Observing the Low-level Atmospheric Circulation in the Tropical Atlantic (OLACTA) project to assess the characteristics of different aerosol layers observed over the Gulf of Guinea. To study the role of atmospheric dynamics in aerosol spatial distribution, we use a unique combination of airborne observations from 2 July 2016, space-borne observations, and finally high-resolution simulations performed using the Weather and Research Forecast (WRF) and CHIMERE models. We show that the aerosol distribution on this day is impacted by subsidence associated with zonal and meridional regional-scale overturning circulations linked with land–sea surface temperature contrast and orography over Ghana and Togo, and that the zonal circulation evidenced on 2 July is a persistent feature over the Gulf of Guinea during July 2016. The flight made on the afternoon of 2 July is unique in the sense that it is the only flight conducted over the ocean during which a downward-looking lidar was operational. The combination of remote sensing to monitor the aerosol landscape over the Gulf of Guinea and in situ measurements to assess the nature of the observed aerosols was only possible on that day. Therefore, one should keep in mind that we are detailing a few mechanisms possibly responsible for shaping the aerosol composition over the Gulf of Guinea, and caution should be exercised when drawing more general conclusions regarding the role of the observed circulation in the aerosol redistribution in this region.

The airborne and space-borne data used in this paper are presented in Sect. 2, whereas the simulations are detailed in Sect. 3. Section 4 presents the synoptic situation and airborne operations over SWA on 2 July 2016. Atmospheric composition over the Gulf of Guinea as observed from aircraft in situ and remote sensing data is discussed in Sect. 5. Insights into the distribution of anthropogenic aerosols from tracer experiments are presented in Sect. 6 and long-range transport of aerosols related to regional-scale dynamics is described in Sect. 7. The influence of lower-tropospheric overturning circulations induced by the land–sea surface temperature gradient on the vertical distribution of aerosols over SWA is discussed in Sect. 8. In Sect. 9, we summarize and conclude.

The entire DACCIWA aircraft campaign took place during WAM post-onset conditions (Knippertz et al., 2017), i.e. after the migration of the climatological precipitation maximum from the coast to the Sahel, with the monsoon flow being well established over SWA. The campaign took place after the onset of the Atlantic Cold Tongue as evident in Fig. 3 of Knippertz et al. (2017), which also highlights that the coastal upwelling started progressively building up around 27 June 2016.

In the period spanning from 29 June to 5 July 2016, the major weather disturbances over SWA are associated with African easterly waves travelling along the well-organized African easterly jet (AEJ). A cyclonic centre propagating to the south of the AEJ (identified from ECMWF 850 hPa streamline charts, not shown) originated from eastern Nigeria on 29 June, sweeping through SWA during the following days.

On 2 July 2016, the cyclonic centre is located at the coast of Sierra Leone (see the disturbance labelled “F” in Fig. 14 of Knippertz et al., 2017). The monsoonal winds are almost southerly over the Gulf of Guinea (south of 4∘ N) and progressively veer to southwesterly farther north and over the continent (Fig. S3a). In the mid-troposphere, SWA is under the influence of easterly flow conditions (Fig. S3b). West of 5∘ E, the AEJ is located over the Sahel and is intensified along its northern boundary by a strong Saharan high located over Libya. The AEJ maximum is seen off the coast of Senegal.

The region of interest experiences high insolation on 1 July with temperatures in the 30s of ∘C across SWA and widespread low-level clouds dissolving rapidly in the course of the morning. On 2 July, there is a clear indication of land–sea breeze clouds in the high-resolution SEVIRI image at 12:00 UTC (Fig. 2a) with relatively cloud-free conditions over the ocean, where the ATR 42 flew later on. The land–sea breeze front is seen inland to follow the coastline from western Ghana to western Nigeria. The front is observed to move farther inland until 15:00 UTC (Fig. 2b) with shallow convective cells forming along it. Farther south the area is free of low-level clouds (both over land and ocean). Oceanic convection occurred offshore on the previous day and mesoscale convective systems were present over northern–central Nigeria on the morning of 2 July. Satellite images show both oceanic and inland convection to be decaying by midday (Fig. 2a).

On 2 July, the ATR 42 aircraft took off from Lomé at 14:45 UTC (NB: UTC equals local time in July in Togo) and headed towards the ocean, flying almost parallel to the Ghana coastline (Fig. 1a) at low level (in the marine ABL). Before reaching Cape Three Points (close to the border between Ghana and Ivory Coast), the ATR 42 changed direction and headed south. Upon reaching its southernmost position (∼3∘ N), the ATR 42 turned around, climbed to 3200 m a.m.s.l., and finally headed back to Lomé at that level. On the way back, the aircraft changed, heading around 16:53 UTC to fly along the coast prior to landing. The ATR 42 passed the longitude of Accra at 17:29 UTC and landed in Lomé at 18:07 UTC. The high-level flight back allowed mapping out of the vertical distribution of aerosols and clouds using the ULICE lidar. In situ aerosol- and gas-phase chemistry measurements will be used in the following to characterize the composition of aerosols and related air masses sampled with the lidar, particularly during the ascent over the ocean (between 16:33 and 16:47 UTC), the elevated levelled run, and the descent towards Lomé airport (between 17:53 and 18:07 UTC).

Figure 3 shows ULICE-derived ASR and VDR cross sections acquired between 16:40 and 18:00 UTC, including data gathered during the aircraft ascent over the ocean and descent in the vicinity of the coast. It is worth noting that most of the lidar data shown in Fig. 3 were acquired while the aircraft was flying along the coastline (from 16:53 UTC on). Wind measurements from the Abidjan, Accra, and Cotonou soundings as well as from the ATR 42 sounding over the ocean clearly show that above 1.2 km a.m.s.l. the flow is easterly over the region of aircraft operation (Fig. 4). Given that the heading of the aircraft along this elevated leg is 65∘, the lidar “curtains” above 1.2 km a.m.s.l. in Fig. 3 map out aerosol layers that are transported westward (with the ATR 42 flying against the mean flow).

Several outstanding features are highlighted in Fig. 3. Generally few clouds were encountered along the flight track (they appear in dark red colours). Exceptions are the low-level clouds at the top of the marine ABL with a base around 500 m a.m.s.l. to the west of the track between 16:55 and 17:02 UTC (Fig. 3a). The vertical extension and the number of the cumulus clouds topping the marine ABL decrease towards the east. This shoaling of the marine ABL is likely ascribed to the increasing trajectory length of near-surface parcels over the cold coastal waters (as the aircraft flies over the coastal upwelling region). Near Lomé, the top of the marine ABL can only be identified from the higher ASR values, reflecting the impact of high relative humidity on the scattering properties of the marine aerosols (Fig. 3a). An isolated deeper convective cloud is observed before 16:48 UTC between 2 and 2.5 km, which is also sampled in situ by the ATR 42 cloud probes. The top of the cloud is likely connected to a temperature inversion observed during the aircraft ascent over the ocean (not shown). High lidar-derived ASRs are observed near the marine ABL top and to some extent in the mixed layer (Fig. 3a). The ASR-enhanced layers do not show in the VDR plot, possibly because they are related to the presence of sea-salt aerosols, which are spherical particles that do not depolarize the backscattered lidar signal. However, the high ASR values could also be related to the advection of biomass burning aerosols from the south in the marine ABL (e.g. Menut et al., 2018) as suggested by the relatively high CO and extinction coefficient values observed in the ABL over the ocean (110 ppb and 50 Mm-1, respectively) in Fig. 5c and e. Biomass burning aerosols are also generally associated with low VDR values.

In addition to clouds and marine ABL aerosols, several distinct aerosol features in the free troposphere stand out from the lidar plot.

Features A and B correspond to plumes with high values of ASR (larger than 1.2) and VDR (larger than 0.8 %) observed near the coast between the surface and 0.5 km a.m.s.l. and between 0.5 and 1.5 km a.m.s.l., respectively, during the aircraft descent towards Lomé. According to the aircraft in situ observations, feature B is located in a strong wind shear environment at the top (∼600 m) of the ABL (Fig. 4) with its upper part being located in the easterly flow, while feature A is associated with a south-southwesterly flow. This sheared environment likely explains the slanted structure of the aerosol plume associated with feature B.

ATR 42 observations associated with feature A (below 0.5 km a.m.s.l.) show increases in NOx, CO, and PM1 aerosol concentrations (Fig. 5a, c, f, respectively) as well as an extinction coefficient (Fig. 5e), together with an O3 concentration reduction (Fig. 5b). Plume A is related to fresh anthropogenic emissions from Lomé, including NOx. The addition of a large quantity of NOx into the atmosphere can lead to a significant shift in the ozone chemical equilibrium, which can effectively result in near-source consumption, as observed here. No CPC-derived aerosol concentrations are available below 0.5 km a.m.s.l. The few PSAP measurements made around 0.5 km a.m.s.l. during the descent yield an AAE value around ∼1 (Fig. 5g). These are solid indications that the ATR 42 sampled a fresh urban anthropogenic plume near Lomé (Brito et al., 2018), advected with the south-southwesterly monsoon flow (the ATR 42 being downstream of Lomé then).

ATR 42 observations associated with feature B (between 0.5 and 1.5 km a.m.s.l.) show increases in concentrations for all variables under scrutiny, including O3. The latter (Fig. 5b) is the most significant difference between the characteristics of features B and A. Other differences include the much smaller increases in CO concentration and OPC aerosol (NPM1, NPM2.5, and NPM10) concentrations as well as extinction coefficients observed in feature B (Fig. 5c, e, f, respectively). The O3/CO ratio (an indicator of air mass aging, e.g. Jaffe and Wigder, 2012, and Kim et al., 2013) observed to be associated with feature B increases with respect to feature A (0.25 vs. 0.15, i.e. a 65 % increase), which is compatible with a further processed urban plume, as also corroborated by wind measurements. These observations, together with wind measurements, suggest that feature B corresponds to a more aged urban plume. This could be an indication that the ATR 42 sampled more than just the Lomé plume. This will be investigated using tracer experiments in Sect. 6. Above 2 km a.m.s.l., the AAE increases to larger values (>1.5), evidencing a change in aerosol nature, i.e. a transition from local urban emissions to elevated background pollution (Fig. 5g), possibly resulting from a mixture of long-lived anthropogenic pollution and long-range transport of dust and biomass burning aerosols from previous days.

Regarding feature C, the in situ measurements do not allow characterization of the nature of the aerosols. The origin of this layer will also be investigated using tracer experiments (see Sect. 6).

The in situ measurements along the elevated ATR 42 track reveal significant differences in aerosol- or gas-phase concentrations and properties between the western part (where feature E is observed with the lidar) and the eastern part (where feature D is observed) of the ATR 42 leg (Fig. 6). In the western part, ATR 42 measurements highlight enhanced O3 and CO concentrations (>60 ppbv and >200 ppvb, respectively, Fig. 6a, b) together with AAE values of ∼1.5 (Fig. 6f), suggesting the presence of biomass burning aerosol. Furthermore, aerosol number concentrations NPM1 and N10 show enhanced values for small particles (100 and ∼1000 no. cm-3, respectively, Fig. 6c, d). The observed O3, CO, and N10 concentrations are larger than the background values measured during the ascent over the ocean (∼40 ppbv, 150 ppbv, and 500 no. cm-3, respectively, Fig. 5b, c, f). Large extinction values are also observed (100 Mm-1), largely exceeding the background value of 30 Mm-1 (compare Figs. 6e and 5e).

In the eastern part of the leg, AAE values of ∼1.5 also suggest that biomass burning aerosols are sampled. O3, CO, NPM1, and N10 concentrations diminish approximately halfway through the leg to their background values (from 17:16 UTC on, Fig. 6a, b, c, d), as does the extinction coefficient. However, NPM2.5 and NPM10 concentrations increase significantly, as opposed to NPM1, which combined with enhanced lidar-derived VDR suggest mixing with larger particles, possibly dust. Further insight into the origin of these aerosols, observed as a result of long-range transport, will be investigated in Sect. 7.

Finally, in Sect. 8 we will investigate the cause of the slanting of the elevated aerosol layers from west to east along the flight track, which also possibly leads, in addition to the colder SSTs, to a thinning of the marine ABL and the suppression of clouds at its top in the vicinity of Lomé (Fig. 3).

The objectives of the tracer experiments are 3-fold: (i) understand how the lower tropospheric circulation shapes the structure of the urban pollution plume emitted from coastal cities and observed with the ULICE lidar (marked A and B in Fig. 3), (ii) assess which cities contribute to the plume observed with ULICE and whether it results from Lomé emissions only, and (iii) provide insight into the origin of the intermediate aerosol layer (marked B in Fig. 3). For this we have analysed along the ATR 42 aircraft flight track the tracer simulations introduced in Sect. 3.

As an ancillary objective, we also aim to assess how far over the ocean the urban pollution aerosols can be transported by the complex low-level circulation over SWA. For this, we have analysed the tracer simulations along four 0.5∘ wide north–south transects spanning the longitudinal range of the ATR 42 flight (centered at 0.75∘ W, 0.25∘ W, 0.25∘ E and 0.75∘ E; cf. Fig. 1b).

To gain insights into the origin of the aerosol layers sampled by the ATR along the elevated leg and observed by lidar (features D and E in Fig. 3), 10-day back-trajectories ending at 2500 m a.m.s.l. at 17:00 UTC on 2 July are computed using CHIMERE. The backplume associated with feature D is shown in Fig. 9a (the one associated with feature E is nearly identical and will not be discussed). The back-trajectories suggest that feature D originates from a broad area including Gabon, Congo, and the Democratic Republic of Congo. Most of the back-trajectories then travel over the Gulf of Guinea towards SWA in the free troposphere (Fig. 9b). Daily mean AOD derived from MODIS and SEVERI observations on 2 July (Fig. 10a) shows large values offshore of Gabon and Congo known to be biomass burning aerosol emission hotspots at this time of year (e.g. Menut et al., 2018). This is corroborated by the CAMS biomass burning aerosol forecast at 12:00 UTC (Fig. S4a).

The afternoon CALIOP observations acquired to the east of the ATR 42 flight track across the enhanced AOD feature (see the track in Fig. 10a) indeed classify the aerosols over the ocean as elevated smoke, transported between 1.5 and 4 km a.m.s.l. (Fig. 10b). The altitude of transport is consistent with that derived from the CHIMERE backplume (Fig. 9b) as also shown by Menut et al. (2018). Along this transect, dust is observed to almost reach the SWA coastline from the north (Fig. 10b), consistent with the moderate AOD values observed over Togo and Benin (Fig. 10a). Furthermore, the morning ATR 42 flight conducted on 2 July in the region of Savè (Benin, ∼8∘ N) highlighted the presence of dust over northern Benin (Flamant et al., 2018). Interestingly, at the coast (∼6∘ N), CALIOP shows evidence of polluted dust, possibly resulting from the mixing of dust with anthropogenic emissions from coastal cities. However, the CAMS forecast does not show dust reaching the SWA coast (Fig. S4b).

The backplume and regional-scale dynamics analyses indicate that the upper-level aerosol features D and E (as observed by lidar) are related to biomass burning over central Africa. In the case of feature D, closer to Lomé, MODIS, SEVIRI, and CALIOP observations suggest the possibility of mixing with dust, which is consistent with the ATR in situ and lidar-related observations.

IFS vertical velocity computed between 850 and 600 hPa (i.e. above the monsoon flow) shows that most of the northern Gulf of Guinea is under the influence of subsiding motion on 2 July at 18:00 UTC (Fig. 11b). Stronger subsidence is seen to the east of the region of operation of the ATR 42 at that time. Strong subsidence is also seen over the eastern part of the ATR 42 flight track at 12:00 UTC (Fig. 11a). However, at 12:00 UTC, the eastern part of the northern Gulf of Guinea is characterized by upward motion, possibly in relationship with the SST gradient (cold water to the west linked with the coastal upwelling and warmer waters to the east in the Niger Delta region). The signature of the sea breeze is also visible inland in the IFS analysis at 12:00 UTC (Fig. 11a) in the form of a line of strong ascendance running parallel to the coastline.

At the regional scale, IFS analyses show the existence of a marked surface temperature difference between the ocean and the continent at 12:00 UTC (Fig. S5d) because of the high insolation across SWA as noted in Sect. 2. The surface temperature gradient across the coast creates shallow overturning circulations as evidenced by IFS analyses at 18:00 UTC (Fig. 12). A well-defined closed zonal cell can be identified below 600 hPa around 5∘ N and between 0 and 8∘ E (Fig. 12a), while a well-defined meridional cell is seen around 0∘ E between 3 and 8∘ N (Fig. 12c). It is worth noting that the overturning circulations are most intense and better defined at 18:00 UTC than at 12:00 UTC (compare Fig. 12a with Fig. S5c for the zonal cell), even though the surface temperature difference across the coast is weaker (compare Fig. 12b with Fig. S5d). The overturning circulation exhibits a strong diurnal cycle (Fig. S5), which is driven by the surface temperatures over land. The quality of IFS skin temperature during the day was verified against observed land-surface temperature observations (the so-called Copernicus product; see Fig. S6). In spite of a systematic bias of the order of 2 ∘C over land, IFS skin temperature analyses are seen to be consistent (in terms of spatio-temporal distribution) with the Copernicus product (Fig. S6). This gives us confidence that the overturning circulations exist and contribute to enhancing subsidence over the Gulf of Guinea. Furthermore, we have conducted an analysis of the correlation between the land–sea skin temperature gradients associated with both the zonal and meridian cells and the vertical velocity over the Gulf of Guinea at different times of day for the whole of July 2016, based on IFS data (Table 2). The analysis shows that the zonal land–sea skin temperature gradient at 12:00 and 18:00 UTC is significantly correlated with vertical velocity at 18:00 UTC, with values around 0.5. Hence, the overturning cells evidenced on 2 July appear to be persistent features over the Gulf of Guinea, at least in post-monsoon onset conditions. On the other hand, the meridional land–sea skin temperature gradient at 12:00 UTC is correlated (0.34) with vertical velocity at 12:00 UTC, possibly due to the presence of orography as discussed in the following. The meridional gradient of skin temperature between the sea and the land is an indicator of the pressure difference and thus drives the intensity of the southerly flow associated with the land–sea breeze. When the southerly flow impinges on the low terrain over SWA, as it progresses over the continent, enhanced vertical motion is generated.

In addition to the subsidence generated at the regional scale by the land–sea temperature gradient, the interaction of the monsoon flow with the orography over Ghana and Togo is responsible for more local coastal circulations. This interaction is reflected in the vertical velocity anomaly simulated with WRF along the westernmost and easternmost transects in Fig. 1b (transects I and IV, respectively). The anomalies are computed with respect to the average vertical velocity between 1∘ W and 1∘ E. Figure 13 shows that in the region where orography is more pronounced (i.e. to the west), the vertical velocity anomaly is positive, while it is negative to the east where orography is less marked (compare Fig. 13a and b). As a result, the eastern region of ATR 42 operation on 2 July is under the influence of a strong subsiding motion. This subsiding motion suppresses low-level cloudiness near Lomé and is key to the interpretation of the ATR 42 lidar observations along the track regarding the slanting of the elevated aerosol layers and, possibly, the thinning of the marine ABL towards the eastern end of the aircraft track, together with an additional effect of colder SSTs.

MODIS observations show the existence of an SST dipole across the northern part of the Gulf of Guinea (Figs. S7 and 11), between the coastal upwelling offshore of Lomé and Accra (SSTs of the order of 26 ∘C) and the warmer SST to the east in the Bight of Bonny (offshore Nigeria, where SSTs of the order of 28 ∘C are generally observed). Even though this SST dipole may also generate a secondary circulation over the Gulf of Guinea (e.g. around 900–800 hPa and between 0 and 1∘ E in Fig. S5c), it is very likely that the lower tropospheric dynamics in the region of operation of the aircraft are dominated by the monsoon dynamics to the first order and by the sea–land surface temperature gradient at the regional scale.

In this study, detailed aircraft observations on 2 July 2016 and accompanying model simulations were used to analyse the distribution of aerosols over the Gulf of Guinea and its meteorological causes. We show that land–sea surface temperature gradients between the northern part of the Gulf of Guinea and the continent as well as orography over Ghana and Togo play important roles for the distribution of aerosols and gases over coastal SWA on that day. The former creates large-scale subsidence conditions over the northern part of the Gulf of Guinea through the generation of zonal and meridional overturning circulations below 600 hPa, with the downward branch of the circulation around 0∘ E over the ocean. The latter generates enhanced subsidence over the eastern part of the ATR 42 operation area, near Lomé and Accra. Together this leads to a west–east tilting of the aerosol layers (that can be considered passive tracers of the dynamics) along the flight track. The ATR 42 sampled remotely and in situ the complex aerosol layering occurring between 2.5 and 3.2 km a.m.s.l. over the Gulf of Guinea as a result of long-range transport of dust (from the northeast) and biomass burning aerosol from the south (feature E in Fig. 3) and the mixing between these (feature D).

The orography-forced circulation also has an influence on the structure of the urban pollution plumes from Accra, Lomé, and Cotonou as assessed from airborne lidar measurements on 2 July and numerical passive tracer experiments using the WRF and CHIMERE models. When accounting for the relative size of the emitting cities along the coast (∼2 times more emissions in Accra than in Lomé), we find that the tracer experiment designed to include emissions from 1 and 2 July is the most realistic in reproducing the lidar observations. The analysis shows that (a) the large pollution plumes observed at the coast up to 1.5 km (features A and B) are essentially related to emissions in the Lomé area from both 1 and 2 July, with a moderate contribution from Accra and Cotonou, (b) the elevated plume over the northern part of the Gulf of Guinea (feature C) is related to emissions from Accra exclusively from the day before the ATR 42 flight (i.e. 1 July) and these clearly dominate the composition of the tracer plume in the region covered by the flight track on 2 July, (c) given the general direction of the monsoon flow, Lagos emissions (taken to be 13 times that of Cotonou) do not appear to have affected the atmospheric composition west of Cotonou, where our airborne observations were gathered, as also shown by Deroubaix et al. (2018) in post-monsoon onset conditions, and (d) the tracer plumes do not extend very far over the ocean during the short period under scrutiny, mostly because they are transported northward within the marine ABL and westward above it so that their extent is controlled by the equatorward component in the mostly easterly flow as modulated by synoptic-scale disturbances (Knippertz et al., 2017).

The unique combination of in situ and remote sensing observations acquired over the Gulf of Guinea during the 2 July 2016 OLACTA flight together with global and regional model simulations revealed in detail the impact of the complex atmospheric circulation at the coast on the aerosol composition and distribution over the northern Gulf of Guinea. We show that on hot, cloud-free summer days such as 2 July, the western Gulf of Benin is a place favourable for subsidence in the afternoon due to three factors, namely cool SSTs, zonal overturning connected with the Niger Delta region, and meridional overturning connected with the main West African landmass, anchored geographically at the Mampong and Akwapim–Togo ranges. We also show that the overturning cells are robust features of the atmospheric circulation over the Gulf of Guinea in July 2016. To the best of the authors' knowledge such features have not been documented in the literature to date. Still, one should keep in mind that the mechanisms described in detail are based on a unique dataset. Even though we have highlighted the fact that some of the key dynamical features are persistent during July 2016, and hence not just representative of 2 July, caution should be exercised when drawing more general conclusions regarding the role of observed circulation in aerosol redistribution in this region.

Further research will be dedicated to enhancing our understanding of the complex interactions between the monsoon flow and the orography north of major coastal cities as well as the land–sea and land–lake breezes, and their impact on the dispersion of pollution emissions from major coastal cities in SWA. Future research will also be conducted to assess the long-term impact of the land–sea surface temperature gradient (and related shallow overturning circulation) on the distribution of aerosols over the northern Gulf of Guinea.