Interactions of atmospheric gases and aerosols with the monsoon dynamics over the Sudano-Guinean region during AMMA

Deroubaix, Adrien; Flamant, Cyrille; Menut, Laurent; Siour, Guillaume; Mailler, Sylvain; Turquety, Solène; Briant, Régis; Khvorostyanov, Dmitry; Crumeyrolle, S.

doi:10.5194/acp-18-445-2018

Cited by 12 publications

(11 citation statements)

References 67 publications

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“…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 north-ward 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).…”

Section: Discussionsupporting

confidence: 61%

“…For the two horizontal resolutions, the same physical parameterizations are used and are those described in Deroubaix et al (2018). The ABL scheme is the one proposed by Yonsei University , the microphysics is calculated using the Single Moment-6 class scheme (Hong and Lim, 2006), the radiation scheme is RRTMG (Mlawer et al, 1997), the cumulus parameterization is the Grell-Dévényi scheme, and the surface fluxes are calculated using the Noah scheme (Ek et al, 2003). The 10 km WRF simulation uses National Centers for Environmental Prediction (NCEP) Final global analyses as initial and boundary conditions.…”

Section: Wrf and Chimere Simulationsmentioning

confidence: 99%

“…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.…”

Section: Introductionmentioning

confidence: 99%

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Aerosol distribution in the northern Gulf of Guinea: local anthropogenic sources, long-range transport, and the role of coastal shallow circulations

et al. 2018

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Abstract. The complex vertical distribution of aerosols over coastal southern West Africa (SWA) is investigated using airborne observations and numerical simulations. Observations were gathered on 2 July 2016 offshore of Ghana and Togo, during the field phase of the Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa project. This was the only flight conducted over the ocean during which a downward-looking lidar was operational. The aerosol loading in the lower troposphere includes emissions from coastal cities (Accra, Lomé, Cotonou, and Lagos) as well as biomass burning aerosol and dust associated with long-range transport from central Africa and the Sahara, respectively. Our results indicate that the aerosol distribution on this day is impacted by subsidence associated with zonal and meridional regional-scale overturning circulations associated with the land–sea surface temperature contrast and orography over Ghana and Togo, as typically observed on hot, cloud-free summer days such as 2 July 2016. Furthermore, we show that the zonal circulation evidenced on 2 July is a persistent feature over the Gulf of Guinea during July 2016. Numerical tracer release experiments highlight the dominance of aged emissions from Accra on the observed pollution plume loadings over the ocean, in the area of aircraft operation. The contribution of aged emission from Lomé and Cotonou is also evident above the marine boundary layer. Given the general direction of the monsoon flow, the tracer experiments indicate no contribution from Lagos emissions to the atmospheric composition of the area west of Cotonou, where our airborne observations were gathered. The tracer plume does not extend very far south over the ocean (i.e. less than 100 km from Accra), mostly because emissions are transported northeastward near the surface over land and westward above the marine atmospheric boundary layer. The latter is possible due to interactions between the monsoon flow, complex terrain, and land–sea breeze systems, which support the vertical mixing of the urban pollution. This work sheds light on the complex – and to date undocumented – mechanisms by which coastal shallow circulations can distribute atmospheric pollutants over the densely populated SWA region.

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Section: Discussionsupporting

confidence: 61%

Section: Wrf and Chimere Simulationsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Aerosol distribution in the northern Gulf of Guinea: local anthropogenic sources, long-range transport, and the role of coastal shallow circulations

et al. 2018

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“…DACCIWA studies confirmed the importance of an import of aerosol from fires in Central Africa in addition to local pollution sources Haslett et al, 2019a). The strong monsoon flow during the LDS causes a fast spread of pollutants from the main sources in coastal cities inland (Deroubaix et al, 2019). Given the overall high concentration of aerosol particles and predominantly stratiform clouds, relatively little susceptibility of cloud microphysics to aerosol effects was found (Deetz et al, 2018b;Taylor et al, 2019).…”

Section: Introductionmentioning

confidence: 78%

The potential of increasing man-made air pollution to reduce rainfall over southern West Africa

et al. 2021

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Abstract. Southern West Africa has one of the fastest-growing populations worldwide. This has led to a higher water demand and lower air quality. Over the last 3 decades, most of the region has experienced decreasing rainfall during the little dry season (LDS; mid-July to end of August) and more recently also during the second rainy season (SRS; September–October), while trends during the first rainy season (FRS; mid-May to mid-July) are insignificant. Here we analyse spatio-temporal variations in precipitation, aerosol, radiation, cloud, and visibility observations from surface stations and from space to find indications for a potential contribution of anthropogenic air pollution to these rainfall trends. The proposed mechanism is that the dimming of incoming solar radiation by aerosol extinction contributes to reducing vertical instability and thus convective precipitation. To separate a potential aerosol influence from large-scale climatic drivers, a multilinear-regression model based on sea-surface temperature (SST) indices is used. During both LDS and SRS, weakly statistically significant but accelerating negative rainfall trends unrelated to known climatic factors are found. These are accompanied by a strong increase in pollution over the upstream tropical Atlantic caused by fire aerosol from Central Africa, particularly during the LDS. Over southern West Africa, no long-term aerosol records are available, inhibiting a direct quantification of the local man-made effect. However, significant decreases in horizontal visibility and incoming surface solar radiation are strong indicators for an increasing aerosol burden, in line with the hypothesized pollution impact on rainfall. The radiation trend is further enhanced by an increase in low-level cloudiness. The large spatial extent of potentially aerosol-related trends during the LDS is consistent with the stronger monsoon flow and less wet deposition during this season. Negligible aerosol impacts during the FRS are likely due to the high degree of convective organization, which makes rainfall less sensitive to surface radiation. The overall coherent picture and the accelerating trends – some of which are concealed by SST effects – should alarm policymakers in West Africa to prevent a further increase in air pollution as this could endanger water supply and food and energy production for a large and growing population.

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“…Submicron aerosol chemical composition was measured using Aerodyne compact time-of-flight aerosol mass spectrometers (AMSs) (Drewnick et al, 2005;Canagaratna et al, 2007), mounted on board all three aircraft. The instrument samples submicron particles from ambient air through an aerodynamic lens, which focuses the particles in the vacuum chamber into a narrow beam.…”

Section: Airborne Observationsmentioning

confidence: 99%

Remote biomass burning dominates southern West African air pollution during the monsoon

et al. 2019

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Abstract. Vast stretches of agricultural land in southern and central Africa are burnt between June and September each year, which releases large quantities of aerosol into the atmosphere. The resulting smoke plumes are carried west over the Atlantic Ocean at altitudes between 2 and 4 km. As only limited observational data in West Africa have existed until now, whether this pollution has an impact at lower altitudes has remained unclear. The Dynamics-aerosol-chemistry-cloud interactions in West Africa (DACCIWA) aircraft campaign took place in southern West Africa during June and July 2016, with the aim of observing gas and aerosol properties in the region in order to assess anthropogenic and other influences on the atmosphere. Results presented here show that a significant mass of aged accumulation mode aerosol was present in the southern West African monsoon layer, over both the ocean and the continent. A median dry aerosol concentration of 6.2 µg m−3 (standard temperature and pressure, STP) was observed over the Atlantic Ocean upwind of the major cities, with an interquartile range from 5.3 to 8.0 µg m−3. This concentration increased to a median of 11.1 µg m−3 (8.6 to 15.7 µg m−3) in the immediate outflow from cities. In the continental air mass away from the cities, the median aerosol loading was 7.5 µg m−3 (5.9 to 10.5 µg m−3). The accumulation mode aerosol population over land displayed similar chemical properties to the upstream population, which implies that upstream aerosol is a significant source of aerosol pollution over the continent. The upstream aerosol is found to have most likely originated from central and southern African biomass burning. This demonstrates that biomass burning plumes are being advected northwards, after being entrained into the monsoon layer over the eastern tropical Atlantic Ocean. It is shown observationally for the first time that they contribute up to 80 % to the regional aerosol loading in the monsoon layer over southern West Africa. Results from the COSMO-ART (Consortium for Small-scale Modeling – Aerosol and Reactive Trace gases) and GEOS-Chem models support this conclusion, showing that observed aerosol concentrations over the northern Atlantic Ocean can only be reproduced when the contribution of transported biomass burning aerosol is taken into account. As a result, the large and growing emissions from the coastal cities are overlaid on an already substantial aerosol background. Simulations using COSMO-ART show that cloud droplet number concentrations can increase by up to 27 % as a result of transported biomass burning aerosol. On a regional scale this renders cloud properties and precipitation less sensitive to future increases in anthropogenic emissions. In addition, such high background loadings will lead to greater pollution exposure for the large and growing population in southern West Africa. These results emphasise the importance of including aerosol from across country borders in the development of air pollution policies and interventions in regions such as West Africa.

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Interactions of atmospheric gases and aerosols with the monsoon dynamics over the Sudano-Guinean region during AMMA

Cited by 12 publications

References 67 publications

Aerosol distribution in the northern Gulf of Guinea: local anthropogenic sources, long-range transport, and the role of coastal shallow circulations

Aerosol distribution in the northern Gulf of Guinea: local anthropogenic sources, long-range transport, and the role of coastal shallow circulations

The potential of increasing man-made air pollution to reduce rainfall over southern West Africa

Remote biomass burning dominates southern West African air pollution during the monsoon

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