Coarse and giant particles are ubiquitous in Saharan dust export regions and are radiatively significant over the Sahara

Abstract

Abstract. Mineral dust is an important component of the climate\nsystem, interacting with radiation, clouds, and biogeochemical systems and\nimpacting atmospheric circulation, air quality, aviation, and solar energy\ngeneration. These impacts are sensitive to dust particle size distribution\n(PSD), yet models struggle or even fail to represent coarse (diameter ( d )\n >2.5 µ m) and giant ( d>20 µ m) dust\nparticles and the evolution of the PSD with transport. Here we examine three\nstate-of-the-art airborne observational datasets, all of which measured the\nfull size range of dust ( d=0.1 to >100 µ m) at different\nstages during transport with consistent instrumentation. We quantify the\npresence and evolution of coarse and giant particles and their contribution\nto optical properties using airborne observations over the Sahara (from the\nFennec field campaign) and in the Saharan Air Layer (SAL) over the tropical\neastern Atlantic (from the AER-D field campaign). Observations show significantly more abundant coarse and giant dust\nparticles over the Sahara compared to the SAL: effective diameters of up to\n20\u2009 µ m were observed over the Sahara compared to 4\u2009 µ m in the\nSAL. Excluding giant particles over the Sahara results in significant\nunderestimation of mass concentration (40\u2009%), as well as underestimates of\nboth shortwave and longwave extinction (18\u2009% and 26\u2009%, respectively, from\nscattering calculations), while the effects in the SAL are smaller but\nnon-negligible. The larger impact on longwave extinction compared to\nshortwave implies a bias towards a radiative cooling effect in dust models,\nwhich typically exclude giant particles and underestimate coarse-mode\nconcentrations. A compilation of the new and published effective diameters against dust age since uplift\ntime suggests that two regimes of dust transport exist. During the initial\n1.5\u2009d, both coarse and giant particles are rapidly deposited. During the\nsubsequent 1.5 to 10\u2009d, PSD barely changes with transport, and the coarse\nmode is retained to a much greater degree than expected from estimates of\ngravitational sedimentation alone. The reasons for this are unclear and\nwarrant further investigation in order to improve dust transport schemes\nand the associated radiative effects of coarse and giant particles in\nmodels.