Aerosol variability induced by atmospheric dynamics in a coastal area of Senegal, North-Western Africa

Abstract During the first Intensive Observation Period (IOP1) of the SHADOW (SaHAran Dust Over West Africa) campaign carried out during March–June 2015, the Mbour site (Senegal) was reinforced with a large setup of in-situ and remote sensing instruments to measure the aerosol optical, physical and chemical properties as well as the atmospheric dynamical features over the site. Here, we use this dataset to study the impact of Sea Breeze (SB) on the aerosol properties. Two case studies were selected: (i) one representative of SB events observed during typical Harmattan conditions, (ii) and one representative of SB events observed during a Harmattan desert dust event. Both cases show a strong influence of the SB period on the dry Mass Scattering Efficiency (MSE), which increases by a factor of up to 1.4. Given the fact that SB occurred every other day at this coastal site during IOP1 (dry season) and that the relative humidity may reach 96%, the effect of such event on the radiative forcing may not be negligible. In order to evaluate the effects of different flows on aerosol transport, recirculation and stagnation factors were retrieved from wind components. The air mass recirculation factor implies that aerosols previously transported from the land over the sea are then brought back inland. In both cases, high recirculation factors were calculated (between 0.84 and 0.98) and air mass stagnation was observed right before the SB establishment. However, this stagnation is observed over the sea during the first case while it is located near the coast during the second case, which has a significant impact on the aerosol evolution. Indeed, a stagnation of air masses over the sea is associated with an increase in fine aerosol mass concentrations (PM1, PM2.5) due to a higher contribution of marine and anthropogenic sources notably for organic aerosols, and a higher contribution of more-oxidized organics. This contribution is lower when the air mass stagnation occurs over the shoreline during the desert dust event.

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