Abstract. Mineral dust is an important component of the climate system, interacting with radiation, clouds and biogeochemical systems, and impacting atmospheric circulation, air quality, aviation and solar energy generation. These impacts are sensitive to dust particle size distribution (PSD), yet models struggle or even fail to represent coarse (diameter (d) > 2.5 µm) and giant (d>20 µm) dust particles and the evolution of the PSD with transport. Here we examine three state-of-the-art airborne observational datasets, all of which measured the full size range of dust (d = 0.1 to > 100 µm) at different stages during transport, with consistent instrumentation. We quantify the presence and evolution of coarse and giant particles and their contribution to optical properties. Observations are taken from the Fennec fieldwork over the Sahara and in the Saharan Air Layer (SAL) near the Canary Islands, and from the AER-D fieldwork in the vicinity of the Cape Verde Islands in the SAL. Observations show significantly more abundant coarse and giant dust particles over the Sahara compared to the SAL: effective diameters of up to 20 µm were observed over the Sahara, compared to 4 µm in the SAL. Mass profiles show that over the Sahara 40 % of dust mass was found in the giant mode, contrasting to 2 to 12 % in the SAL. Size-resolved optical property calculations show that in the shortwave (longwave) spectrum excluding the giant mode omits 18 % (26 %) of extinction over the Sahara, compared to 1–4 % (2–6 %) in the SAL. Excluding giant particles results in significant underestimation of both shortwave and longwave extinction over the Sahara, as well as of mass concentration, while the effects in the SAL are smaller but non-negligible. Omitting the giant mode results in a greater omission of dust longwave radiative effects compared to the shortwave, suggesting a bias towards a radiative cooling effect of dust when the giant mode is excluded and/or the coarse mode is underestimated. This will be important in dust models, which typically exclude giant particles and underestimate coarse mode concentrations. A compilation of effective diameters against dust age since uplift time suggests that two regimes of dust transport exist. During the initial 1.5 days, both coarse and giant particles are rapidly deposited. During the subsequent 1.5 to 10 days, PSD barely changes with transport, and the coarse mode is retained to a much greater degree than expected from estimates of gravitational sedimentation alone. The reasons for this are unclear, and warrant further investigation in order to improve dust transport schemes, and the associated radiative effects of coarse and giant particles in models.
Solar radiative effects of a Saharan dust plume observed during SAMUM assuming spheroidal model particles
