Impact of Natural and Anthropogenic Aerosols on Clouds and Radiative Properties over West Africa

Abstract

Using a high-resolution regional climate model COSMO-ART, the impact of natural and anthropogenic aerosols loads on radiative and cloud optical properties over West Africa is assessed through sensitivity experiments. Based on a monthly climatology, model simulations compare satisfactory with wind fields from reanalysis data, cloud observations, daily average particulate matter (PM), temperature, relative humidity and satellite retrieved CO mixing ratio. However, COSMO-ART shows a slight overestimation of spatial distribution of CO mixing ratio compared to satellite observation. Regarding the atmospheric composition, COSMO-ART shows good representation with the observation of aerosol mass concentration in upwind marine and urban outflow, whereas it underestimates the observed aerosols in the regional background. Evaluating two emission inventories datasets; simulation conducted with DACCIWA emissions performed better than with EDGAR emissions for the chemistry. However, aerosols compounds are mostly underestimated by COSMO-ART except for NO3 using EDGAR inventory. Focusing on a case study of 02 July 2016, using CO as an indicator for biomass burning plume, individual mixing events south of the coast of Côte d’Ivoire due to midlevel convective clouds injecting parts of the biomass burning plume into the boundary layer were identified. Idealized tracer experiments suggested that about 20% of the CO mass from the 2–4km layer are mixed below 1km within two days over the Gulf of Guinea. Quantifying the impact of biomass burning aerosols on cloud and radiation, it is found that 27% increase of cloud droplets number concentration (CDNC) over the Gulf of Guinea and 7.5% over the whole simulated domain spatially averaged. A reduction of 50 W m-2 of surface shortwave (SW) radiation and 20 W m-2 respectively over the Gulf of Guinea (3–4ºN; 9 ºW –1ºE) and over the study domain (3–11ºN; 9 ºW –1ºE) spatially averaged for all sky conditions. The maximum increase of surface concentration for 05- 06 July 2016 case study due to biomass burning aerosols is 154 μg m-3 for CO, 27 μg m-3 for O3 and 6–7 μg m-3 for PM which agrees with WRF-CHEM simulation. Same investigation was done to quantify the impact of local source anthropogenic aerosols on clouds and radiation. 90% of the CDNC are from anthropogenic aerosols and a shift of median clouds radius from 12 to 10 μm as result of anthropogenic aerosols. Regarding the surface SW, a decrease of up to 60 W m-2 is simulated over the study domain as a result of anthropogenic aerosol. A reduction of monsoon wind speed of about 1.2 m s-1 occurred as a result of anthropogenic aerosols. The impact of future anthropogenic emissions increase is examined under two RCP 2.6 8.5 scenarios. All pollutants are projected to increase more or less, over the whole West African region except for BC which shows some decrease in Nigeria part of Ghana and Niger. Looking at local scale, coastal cities (Abidjan and Accra) is predicted to exceed the World Health Organization (WHO) with respect to NOx and O3. In conclusion, the results of this study underscore the need to investigate the impact of aerosols (anthropogenic, biomass burning) on the cloud properties and radiative budget on a longer time scales and a need for air quality monitoring over the region especially over the coastal cities.