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.

Impact of Sea Breeze Dynamics on Atmospheric Pollutants and Their Toxicity in Industrial and Urban Coastal Environments

“…In order to precisely deduce the structure of the SB system (SBF and GC top), we applied the transform covariance method on the slowness meridional component to each profile [22]. We also used the discrete Haar wavelet covariance transform method, which is well suited to identifying the transition zones.…”

“…They are widely used to estimate flow meteorological characteristics over sites influenced by local effects such as SB or slope breeze in complex terrain [49][50][51][52]. In our study, we proposed to deduce the recirculation factor R by varying the transport period from 15 min to 5 h at the observation site from ultrasonic anemometer measurements (see Crumeyrolle et al [22] for more details). The characteristics of the air flows within the surface layer, as well as the change in the turbulence quantities (turbulence kinetic energy dissipation rate, spectral scale) before and after the SBF penetration, were derived from the power spectrum of the wind velocity component by applying the inertial dissipation method to the ultrasonic anemometer measurements [53].…”

“…While many studies have been devoted to better understand the potential role of SB activity on distribution of pollutants [21][22][23][24][25] or the association of fine particle exposure with acute respiratory inflammation [26,27], only few studies have focused on fine spatiotemporal scale variations in aerosol and gas concentrations associated with SB front passage and related health impacts. Higashi et al [28] have evaluated the effect of the exposure to desert dust on daily cough occurrence by using lidar measurements coupled with clinical data of patients being treated for respiratory diseases in hospital over a period of six months.…”

Machine Learning and Deterministic Methods for Detection Meteorological Phenomena from Ground Measurements: Application for Low-Level Jet and Sea-Breeze Identification in Northern France

“…This phenomenon occurs due to an adverse atmospheric pressure gradient generated by the temperature difference between the land and sea during the daytime. SB evolution depends on a large range of geophysical factors including topography, Coriolis force, heat diffusion, solar radiation, synoptic forcing, and sea surface temperature [4][5][6].…”