Climatology of aerosol properties and clear-sky shortwave radiative effects using Lidar and Sun photometer observations in the Dakar site

摘要

This paper presents the analysis of nearly a decade of continuous aerosol observations performed at theMbour site (Senegal) with Sun photometer, Lidar, and Tapered Electromagnetic OscillatingMicrobalance. This site is influenced all year-round by desert dust and sporadically, in wintertime, by biomass burning particles. Different patterns are revealed for winter and summer, seasons associated to air masses of different origin. The summer (wet season) is characterized by a high aerosol loading (optical thickness, AOT, around 0.57 at 532 nm) composed of large and weakly absorbing particles (Angstrom exponent, α, of 0.23 and single-scattering albedo, ?_0, of 0.94 at 532 nm). A lower aerosol loading (AOT = 0.32) is observed during winter (dry season) for finer and absorbing particles (α=0.48 and ?_0 = 0.87) revealing the presence of biomass burning aerosols and a greater proportion of local emissions. This latter anthropogenic contribution is visible at weekly and daily scales through AOT cycles. A decrease of about 30% in AOT has been featured in autumn since 2003. The derivation of the extinction profiles highlights a dust transport close to the ground during winter and in an aloft layer (up to 5 km) during summer. Accurate calculations of the daily aerosol radiative effect in clear-sky conditions are finally addressed. From spring to winter, seasonal shortwave radiative forcing averages of 14.15, 11.15, 8.92, and 12.06Wm~(-2) have been found respectively. Up to 38% of the solar clear-sky atmospheric heating can be attributed to the aerosols in this site.