Coarse-mode mineral dust size distributions, composition and optical properties from AER-D aircraft measurements over the tropical eastern Atlantic
Authors: Ryder Claire L. , Marenco Franco , Brooke Jennifer K. , Estelles Victor , Cotton Richard , Formenti Paola , McQuaid James B. , Price Hannah C. , Liu Dantong , Ausset Patrick , Rosenberg Phil D. , Taylor Jonathan W. , Choularton Tom , Bower Keith , Coe Hugh , Gallagher Martin , Crosier Jonathan , Lloyd Gary , Highwood Eleanor J. , Murray Benjamin J. 
Published: December 2018

Abstract. Mineral dust is an important component of the climate system, affecting the
radiation balance, cloud properties, biogeochemical cycles, regional
circulation and precipitation, as well as having negative effects on
aviation, solar energy generation and human health. Dust size and composition
has an impact on all these processes. However, changes in dust size
distribution and composition during transport, particularly for coarse
particles, are poorly understood and poorly represented in climate models.
Here we present new in situ airborne observations of dust in the Saharan Air
Layer (SAL) and the marine boundary layer (MBL) at the beginning of its
transatlantic transport pathway, from the AERosol
Properties – Dust (AER-D) fieldwork in August 2015,
within the peak season of North African dust export. This study focuses on
coarse-mode dust properties, including size distribution, mass loading,
shape, composition, refractive indices and optical properties. Size
distributions from 0.1 to 100 µm diameter (d) are presented, fully
incorporating the coarse and giant modes of dust. Within the MBL, mean
effective diameter (deff) and volume median diameter (VMD) were 4.6 and 6.0 µm respectively, giant particles with a mode at
20–30 µm were observed, and composition was dominated by quartz and
alumino-silicates at d > 1 µm. Within the SAL, particles
larger than 20 µm diameter were always present up to 5 km altitude, in
concentrations over 10−5 cm−3, constituting up to 40 % of
total dust mass. Mean deff and VMD were 4.0 and 5.5 µm
respectively. Larger particles were detected in the SAL than can be explained
by sedimentation theory alone. Coarse-mode composition was dominated by
quartz and alumino-silicates; the accumulation mode showed a strong
contribution from sulfate-rich and sea salt particles. In the SAL, measured
single scattering albedos (SSAs) at 550 nm representing d < 2.5 µm were
0.93 to 0.98 (mean 0.97). Optical properties calculated for the full
size distribution (0.1 < d < 100 µm) resulted in lower
SSAs of 0.91–0.98 (mean 0.95) and mass extinction coefficients of 0.27–0.35 m2 g−1 (mean 0.32 m2 g−1). Variability in SSA was mainly
controlled by variability in dust composition (principally iron) rather than
by variations in the size distribution, in contrast with previous observations
over the Sahara where size is the dominant influence. It is important that
models are able to capture the variability and evolution of both dust
composition and size distribution with transport in order to accurately
represent the impacts of dust on climate. These results provide a new SAL
dust dataset, fully representing coarse and giant particles, to aid model
validation and development.