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. Mineral dust is an important component of the climate system, affecting theradiation balance, cloud properties, biogeochemical cycles, regionalcirculation and precipitation, as well as having negative effects onaviation, solar energy generation and human health. Dust size and compositionhas an impact on all these processes. However, changes in dust sizedistribution and composition during transport, particularly for coarseparticles, are poorly understood and poorly represented in climate models.Here we present new in situ airborne observations of dust in the Saharan AirLayer (SAL) and the marine boundary layer (MBL) at the beginning of itstransatlantic transport pathway, from the AERosolProperties – Dust (AER-D) fieldwork in August 2015,within the peak season of North African dust export. This study focuses oncoarse-mode dust properties, including size distribution, mass loading,shape, composition, refractive indices and optical properties. Sizedistributions from 0.1 to 100  µ m diameter ( d ) are presented, fullyincorporating the coarse and giant modes of dust. Within the MBL, meaneffective diameter ( deff ) and volume median diameter (VMD) were 4.6 and 6.0  µ m respectively, giant particles with a mode at20–30  µ m were observed, and composition was dominated by quartz andalumino-silicates at d  > 1  µ m. Within the SAL, particleslarger than 20  µ m diameter were always present up to 5 km altitude, inconcentrations over 10 −5  cm −3 , constituting up to 40 % oftotal dust mass. Mean deff and VMD were 4.0 and 5.5  µ mrespectively. Larger particles were detected in the SAL than can be explainedby sedimentation theory alone. Coarse-mode composition was dominated byquartz and alumino-silicates; the accumulation mode showed a strongcontribution from sulfate-rich and sea salt particles. In the SAL, measuredsingle scattering albedos (SSAs) at 550 nm representing d µ m were0.93 to 0.98 (mean 0.97). Optical properties calculated for the fullsize distribution (0.1  d µ m) resulted in lowerSSAs of 0.91–0.98 (mean 0.95) and mass extinction coefficients of 0.27–0.35 m 2  g −1 (mean 0.32 m 2  g −1 ). Variability in SSA was mainlycontrolled by variability in dust composition (principally iron) rather thanby variations in the size distribution, in contrast with previous observationsover the Sahara where size is the dominant influence. It is important thatmodels are able to capture the variability and evolution of both dustcomposition and size distribution with transport in order to accuratelyrepresent the impacts of dust on climate. These results provide a new SALdust dataset, fully representing coarse and giant particles, to aid modelvalidation and development.

Coarse-mode mineral dust size distributions, composition and optical properties from AER-D aircraft measurements over the tropical eastern Atlantic