Impact of biomass burning on pollutant surface concentrations in megacities of the Gulf of Guinea

Abstract

Abstract. In the framework of the Dynamics-AerosolChemistry-Cloud Interactions in West Africa (DACCIWA) project, the tropospheric chemical composition in large cities along the Gulf of Guinea is studied using the Weather and Research Forecast and CHIMERE regional models. Simulations are performed for the May-July 2014 period, without and with biomass burning emissions. Model results are compared to satellite data and surface measurements. Using numerical tracer release experiments, it is shown that the biomass burning emissions in Central Africa are impacting the surface aerosol and gaseous species concentrations in Gulf of Guinea cities such as Lagos (Nigeria) and Abidjan (Ivory Coast). Depending on the altitude of the injection of these emissions, the pollutants follow different pathways: directly along the coast or over land towards the Sahel before being vertically mixed in the convective boundary layer and transported to the south-west and over the cities. In July 2014, the maximum increase in surface concentrations due to fires in Central Africa is ≈ 150 µg m −3 for CO, ≈ 10 to 20 µg m −3 for O 3 and ≈ 5 µg m −3 for PM 10 . The analysis of the PM 10 chemical composition shows that this increase is mainly related to an increase in particulate primary and organic matter.

Aerosol influences on low-level clouds in the West African monsoon

“…where n(R) is the number concentration of drop with radius R. 20 For R Eff , average vertical profiles between 0 and 100 km inland showed the cloud probes agreed withiñ 1 µm, which is within the uncertainties of the instruments.…”

“…Using satellite measurements allows us to view the cloud field over a large region. We used the optimal cloud analysis (OCA, Watts et al, 2011) product taken from the Meteosat Spinning Enhanced Visible and 20 InfraRed Imager (SEVIRI) spectrometer. This product provides cloud top pressure (CTP) and cloud optical thickness (COT) for the top two cloud layers (looking from above), for scans every 15 minutes.…”

“…We have also only considered a limited number of aerosol species, and other components such as black carbon, sea salt, and mineral dust will have been present. This is not intended to be an exact 20 simulation, but a sensitivity analysis using physically reasonable approximations. A detailed study of aerosol activation is beyond the scope of this analysis and is non-trivial (e.g.…”

“…Haslett et al (submitted, 2019) recently described the aerosol properties measured during the DACCIWA aircraft campaign, and showed that a large background of transported biomass burning pollution from the southern hemisphere was ubiquitous in the West African boundary layer. Although city 20 emissions resulted in aerosol load increase directly downwind of urban conglomerates , the injection of aerosols prone to activate as cloud nuclei (> 0.1 µm) are thought to have limited impact on an already elevated background. This transported biomass burning background was reproducible in a modelling study by Menut et al (2018).…”

“…Although city 20 emissions resulted in aerosol load increase directly downwind of urban conglomerates , the injection of aerosols prone to activate as cloud nuclei (> 0.1 µm) are thought to have limited impact on an already elevated background. This transported biomass burning background was reproducible in a modelling study by Menut et al (2018). Painemal et al (2014) showed that cloud-aerosol interactions with biomass burning smoke were the main factor governing cloud microphysical properties in clouds north of 5 • S over 25 the South Atlantic, but so far no study has measured the microphysical properties of clouds over SWA to quantify the relative influences of transported pollution and local anthropogenic aerosol emissions.…”