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Long-range transport of aerosol in the Saharan Air Layer (SAL) across the Atlantic plays an important rolefor weather, climate, and ocean fertilization. However, processes occurringwithin the SAL and their effects on aerosol properties are still unclear. Inthis work we study particle settling and vertical mixing within the SAL basedon measured and modeled vertical aerosol profiles in the upper 1 km of thetransported SAL. We use ground-based lidar measurements and airborne particlecounter measurements over the western Atlantic, collected during the SALTRACEcampaign, as well as space-based CALIOP lidar measurements from Africa to thewestern Atlantic in the summer season. In our model we take account of theoptical properties and the Stokes gravitational settling of irregularlyshaped Saharan dust particles. We test two hypotheses about the occurrence of vertical mixing within the SALover the Atlantic to explain the aerosol profiles observed by the lidars andthe particle counter. Our first hypothesis (H1) assumes that no mixing occursin the SAL leading to a settling-induced separation of particle sizes. Thesecond hypothesis (H2) assumes that vertical mixing occurs in the SALallowing large super-micron dust particles to stay airborne longer thanwithout mixing. The uncertainties of the particle linear depolarization ratio(δl) profiles measured by the ground-based lidars arecomparable to the modeled differences between H1 and H2 and do not allow usto conclude which hypothesis fits better. The SALTRACE in situ dataon size-resolved particle number concentrationsshow a presence of large particles near the SAL top that is inconsistent withH1. The analysis of the CALIOP measurements also reveals that the averageδl profile over the western Atlantic is inconsistent withH1. Furthermore, it was found that the average δl profile inthe upper 1 km of the SAL does not change along its transport path over theAtlantic. These findings give evidence that vertical mixing within the SAL isa common phenomenon with significant consequences for the evolution of thesize distribution of super-micron dust particles during transport over theAtlantic. Further research is needed to precisely characterize the processesthat are relevant for this phenomenon

Particle settling and vertical mixing in the Saharan Air Layer as seen from an integrated model, lidar, and in situ perspective