Impact of typical days on the optical and microphysical parameters of aerosols in urban zone of Burkina Faso, West Africa

Abstract

Our work is based on in situ solar photometer measurements, the aim of which is to show the impact of typical days on the optical, microphysical and radiative properties of aerosols at the Ouagadougou site. To this end, a classification according to the atmospheric aerosol load defined by the aerosol optical depth (AOD) and the Angstrom coefficient was used to identify four types of day: clear, standard, mixed and dusty. These days are characterized by considerable interannual variability, with frequency varying according to the period. Furthermore, a climatological analysis indicates that aerosols are at their highest on typical days during the harmattan period, mainly in winter and spring. In addition, a correlation between AOD and the water vapour column shows the impact of the rainy season on the aerosol cycle, with aerosols being eliminated by sedimentation, given their hydrophilic nature, and by absorption by raindrops. These particles are strongly dominated by desert dust, due to their diffusive nature associated with the cooling observed at the surface and at the top of the atmosphere. An analysis of the impact of typical days on the aerosol volume size distribution highlights the appearance of two main modes with an intermediate mode whose appearance and intensity depend on the state of the atmosphere defined by the typical day. This testifies to the mixed state of the aerosol population at the Ouagadougou site, characterized by fine combustion particles and coarse particles associated with mineral dusts that evolve according to the harmattan and monsoon flow regimes.