Aircraft and ground measurements of dust aerosols over the west African coast in summer 2015 during ICE-D and AER-D
Authors: Liu Dantong , Taylor Jonathan W. , Crosier Jonathan , Marsden Nicholas , Bower Keith N. , Lloyd Gary , Ryder Claire L. , Brooke Jennifer K. , Cotton Richard , Marenco Franco , Blyth Alan , Cui Zhiqiang , Estelles Victor , Gallagher Martin , Coe Hugh , Choularton Tom W. 
Published: March 2018

Abstract. During the summertime, dust from the Sahara can be efficiently transported
westwards within the Saharan air layer (SAL). This can lead to high aerosol
loadings being observed above a relatively clean marine boundary layer (MBL)
in the tropical Atlantic Ocean. These dust layers can impart significant
radiative effects through strong visible and IR light absorption and
scattering, and can also have indirect impacts by altering cloud properties.
The processing of the dust aerosol can result in changes in both direct and
indirect radiative effects, leading to significant uncertainty in climate
prediction in this region. During August 2015, measurements of aerosol and
cloud properties were conducted off the coast of west Africa as part of the
Ice in Cloud Experiment – Dust (ICE-D) and AERosol properties – Dust
(AER-D) campaigns. Observations were obtained over a 4-week period using the
UK Facility for Atmospheric Airborne Measurements (FAAM) BAe 146 aircraft
based on Santiago Island, Cabo Verde. Ground-based observations were
collected from Praia (14∘57′ N, 23∘29′ W;
100 m a.s.l.), also located on Santiago Island. The dust in the SAL was
mostly sampled in situ at altitudes of 2–4 km, and the potential dust age
was estimated by backward trajectory analysis. The particle mass
concentration (at diameter d = 0.1–20 µm) decreased with
transport time. Mean effective diameter (Deff) for supermicron
SAL dust (d = 1–20 µm) was found to be 5–6 µm
regardless of dust age, whereas submicron Deff
(d = 0.1–1 µm) showed a decreasing trend with longer
transport. For the first time, an airborne laser-induced incandescence instrument (the
single particle soot photometer – SP2) was deployed to measure the hematite
content of dust. For the Sahel-influenced dust in the SAL, the observed
hematite mass fraction of dust (FHm) was found to be
anti-correlated with the single scattering albedo (SSA,
λ = 550 nm, for particles d < 2.5 µm); as
potential dust age increased from 2 to 7 days, FHm increased from
2.5 to 4.5 %, SSA decreased from 0.97 to 0.93 and the derived imaginary
part (k) of the refractive index at 550 nm increased from 0.0015 to
0.0035. However, the optical properties of Sahara-influenced plumes (not
influenced by the Sahel) were independent of dust age and hematite content
with SSA ∼ 0.95 and k ∼ 0.0028. This indicates that the
absorbing component of dust may be source dependent, or that gravitational
settling of larger particles may lead to a higher fraction of more absorbing
clay–iron aggregates at smaller sizes. Mie calculation using the measured
size distribution and size-resolved refractive indices of the absorbing
components (black carbon and hematite) reproduces the measured SSA to within
±0.02 for SAL dust by assuming a goethite ∕ hematite mass ratio of 2.
Overall, hematite and goethite constituted 40–80 % of the absorption for
particles d < 2.5 µm, and black carbon (BC) contributed
10–37 %. This highlights the importance of size-dependent composition in
determining the optical properties of dust and also the contribution from BC
within dust plumes.