Abstract
Abstract. Between June and September large amounts of biomass burning aerosol are released into the atmosphere from agricultural fires in central and southern Africa. Recent studies have suggested that this plume is carried westward over the Atlantic Ocean at altitudes between 2 and 4 km and then northward with the monsoon flow at low levels to increase the atmospheric aerosol load over coastal cities in southern West Africa (SWA), thereby exacerbating air pollution problems. However, the processes by which these fire emissions are transported into the planetary boundary layer (PBL) are still unclear. One potential factor is the large-scale subsidence related to the southern branch of the monsoon Hadley cell over the tropical Atlantic. Here we use convection-permitting model simulations with COSMO-ART to investigate for the first time the contribution of downward mixing induced by clouds, a process we refer to as downward cloud venting in contrast to the more common process of upward transport from a polluted PBL. Based on a monthly climatology, model simulations compare satisfactory with wind fields from reanalysis data, cloud observations, and satellite-retrieved carbon monoxide (CO) mixing ratio. For a case study on 2 July 2016, modelled clouds and rainfall show overall good agreement with Spinning Enhanced Visible and InfraRed Imager (SEVIRI) cloud products and Global Precipitation Measurement Integrated Multi-satellitE Retrievals (GPM-IMERG) rainfall estimates. However, there is a tendency for the model to produce too much clouds and rainfall over the Gulf of Guinea. Using the CO dispersion as an indicator for the biomass burning plume, we identify 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 PBL. Idealized tracer experiments suggest that around 15 % of the CO mass from the 2–4 km layer is mixed below 1 km within 2 d over the Gulf of Guinea and that the magnitude of the cloud venting is modulated by the underlying sea surface temperatures. There is even stronger vertical mixing when the biomass burning plume reaches land due to daytime heating and a deeper PBL. In that case, the long-range-transported biomass burning plume is mixed with local anthropogenic emissions. Future work should provide more robust statistics on the downward cloud venting effect over the Gulf of Guinea and include aspects of aerosol deposition.
Highlights
The interest in air pollution over southern West Africa (SWA) has grown substantially in recent years (Knippertz et al, 2015)
The following data from space-borne platforms and reanalysis are used for this study: 1. The Moderate Resolution Imaging Spectroradiometer (MODIS) is a key instrument on board the Earth Observing System (EOS) Terra satellites
Analysing the simulated diurnal cycle, we found that over the ocean clouds appear after 07:00 UTC and are persistent throughout the day, while carbon monoxide (CO) becomes increasingly visible in the planetary boundary layer (PBL) and eventually reaches the surface
Summary
The interest in air pollution over southern West Africa (SWA) has grown substantially in recent years (Knippertz et al, 2015). In addition to the local pollution, long-range transport of dust from the Sahel and the Sahara as well as biomass burning aerosol from central and southern Africa affects the atmospheric composition. Gerken et al (2016) even found evidence for transport of ozone-rich air from the midtroposphere to the surface, enhancing the volume mixing ratio in the boundary layer by as much as 25 ppbv on the regional scale, while Wang et al (2016) demonstrated the injection of high concentrations of small aerosol particles into the PBL by strong convective downdraughts. This study uses simulations with COSMO (COnsortium for Small-scale MOdelling) (Baldauf et al, 2011) coupled online with Aerosol and Reactive Trace gases (ART; Vogel et al, 2009) to investigate cloud-induced transport of biomass burning aerosols from midlevel tropospheric layers into the PBL over the Gulf of Guinea and SWA. The last section presents a summary of the results and conclusions
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