Beschreibung:
<jats:p>Abstract. Mineral dust is an important component of the climate
system, interacting with radiation, clouds, and biogeochemical systems and
impacting atmospheric circulation, air quality, aviation, and solar energy
generation. These impacts are sensitive to dust particle size distribution
(PSD), yet models struggle or even fail to represent coarse (diameter (d)
>2.5 µm) and giant (d>20 µm) dust
particles and the evolution of the PSD with transport. Here we examine three
state-of-the-art airborne observational datasets, all of which measured the
full size range of dust (d=0.1 to >100 µm) at different
stages during transport with consistent instrumentation. We quantify the
presence and evolution of coarse and giant particles and their contribution
to optical properties using airborne observations over the Sahara (from the
Fennec field campaign) and in the Saharan Air Layer (SAL) over the tropical
eastern Atlantic (from the AER-D field campaign). Observations show significantly more abundant coarse and giant dust
particles over the Sahara compared to the SAL: effective diameters of up to
20 µm were observed over the Sahara compared to 4 µm in the
SAL. Excluding giant particles over the Sahara results in significant
underestimation of mass concentration (40 %), as well as underestimates of
both shortwave and longwave extinction (18 % and 26 %, respectively, from
scattering calculations), while the effects in the SAL are smaller but
non-negligible. The larger impact on longwave extinction compared to
shortwave implies a bias towards a radiative cooling effect in dust models,
which typically exclude giant particles and underestimate coarse-mode
concentrations. A compilation of the new and published effective diameters against dust age since uplift
time suggests that two regimes of dust transport exist. During the initial
1.5 d, both coarse and giant particles are rapidly deposited. During the
subsequent 1.5 to 10 d, PSD barely changes with transport, and the coarse
mode is retained to a much greater degree than expected from estimates of
gravitational sedimentation alone. The reasons for this are unclear and
warrant further investigation in order to improve dust transport schemes
and the associated radiative effects of coarse and giant particles in
models.
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