Diurnal cycle of coastal anthropogenic pollutant transport over southern West Africa during the DACCIWA campaign

Abstract

Abstract. During the monsoon season, pollutants emitted by large coastal cities and biomass burning plumes originating from central Africa have complex transport pathways over southern West Africa (SWA). The Dynamics–Aerosol–Chemistry–Cloud Interactions in West Africa (DACCIWA) field campaign has provided numerous dynamical and chemical measurements in and around the super-site of Savè in Benin (≈185 km away from the coast), which allows quantification of the relative contribution of advected pollutants. Through the combination of in situ ground measurements with aircraft, radio-sounding, satellite, and high-resolution chemistry-transport modeling with the CHIMERE model, the source attribution and transport pathways of pollutants inland (here, NOx and CO) are carefully analyzed for the 1–7 July 2016 period. The relative contributions of different sources (i.e., emissions from several large coastal cities) to the air quality in Savè are characterized. It is shown that a systematic diurnal cycle exists with high surface concentrations of pollutants from 18:00 to 22:00 UTC. This evening peak is attributed to pollution transport from the coastal city of Cotonou (Benin). Numerical model experiments indicate that the anthropogenic pollutants are accumulated during the day close to the coast and transported northward as soon as the daytime convection in the atmospheric boundary layer ceases after 16:00 UTC, reaching 8∘ N at 21:00 UTC. When significant biomass burning pollutants are transported into continental SWA, they are mixed with anthropogenic pollutants along the coast during the day, and this mixture is then transported northward. At night, most of the coastal anthropogenic plumes are transported within the planetary boundary layer (below about 500 m above ground level), whereas the biomass burning pollutants are mostly transported above it, thus generally not impacting ground level air quality.

The role of aerosol-cloud interactions in linking anthropogenic pollution over southern West Africa and dust emission over the Sahara

“…For the wind speed, and at all sites, it is shown that a local maximum is present at 500 m above ground level (AGL) with values between 8 and 12 m s −1 . These profiles present the same structure as described in (Deroubaix et al, 2019) with three 215 clearly defined vertical layers from the surface to 4 km AGL: (i) the monsoon layer in the first kilometer (with a maximum at 0.5 km AGL); (ii) a vertical wind shear layer from 1 to 2 km AGL in which wind speed decreases with altitude to a minimum around 2 km AGL and (iii) a layer above in which wind speed is increasing with altitude and is influenced by the presence of the African Easterly Jet (AEJ). The overall structure of the wind speed profile is well reproduced in the simulation.…”

“…They quantified the important impact of aerosol on shortwave radiation (a decrease of -20 W m −2 ) whereas the impact 70 on longwave radiation was found to be negligible. In the continuity of this work, Deetz et al (2018b) showed that the amount of aerosols over southern West Africa impacts the dynamics of the daytime coastal moist front generated along the Gulf of Guinea and the intensity of its inland propagation, this feature modulating the transport of anthropogenic aerosols emitted at the coast (Deroubaix et al, 2019).…”

“…Comparisons between observation and model output in the first 4 km of the troposphere are presented in already used in (Deroubaix et al, 2019) for similar vertical profiles. All information about the soundings and the experimental campaign are in (Flamant et al, 2018).…”

The role of aerosol–radiation–cloud interactions in linking anthropogenic pollution over southern west Africa and dust emission over the Sahara

“…Over desert (albedo > 0.3), dust will warm the atmospheric column. Using measurements, di Sarra et al (2011) showed a large dust effect on shortwave and longwave radiation when dust plumes pass over Lampedusa. Rémy et al (2015) quantified the feedbacks between free-troposphere dust layers and boundary layer meteorology and showed that maximum temperatures are reduced, increasing atmospheric stability, then decreasing 10 m wind speed during daytime.…”

Sensitivity of <i>CHIMERE</i> to changes in model resolution and chemistry over the northwestern Iberian Peninsula

“…In this context, most of the aforementioned conclusions for CHIMERE (the model applied here) have been drawn for the Île de France region, which is densely populated, relatively flat and not directly influenced by sea-salt emissions. The model has been applied for a number of other regions, but the map is still incomplete and sensitivity testing is not the main focus of the corresponding studies (Mazzeo et al, 2018;Menut et al, 2018;Monteiro et al, 2018;Brasseur et al, 2019;Deroubaix et al, 2019). This is where the present study comes into play: A series of sensitivity test has been run with CHIMERE over the northwestern Iberian Peninsula, a region characterized by a complex coastline, forested mountain terrain and the advection of sea-salt from the surrounding Atlantic Ocean.…”