Abstract. The southeastern Atlantic (SEA) and its associated cloud deck, off
the west coast of central Africa, is an area where aerosol–cloud
interactions can have a strong radiative impact. Seasonally,
extensive biomass burning (BB) aerosol plumes from southern Africa
reach this area. The NASA ObseRvations of Aerosols above CLouds and
their intEractionS (ORACLES) study focused on quantitatively
understanding these interactions and their importance. Here we
present measurements of cloud condensation nuclei (CCN)
concentration, aerosol size distribution, and characteristic
vertical updraft velocity (w∗) in and around the marine boundary
layer (MBL) collected by the NASA P-3B aircraft during the August
2017 ORACLES deployment. BB aerosol levels vary considerably but
systematically with time; high aerosol concentrations were observed
in the MBL (800–1000 cm−3) early on, decreasing
midcampaign to concentrations between
500 and 800 cm−3. By late August and early September,
relatively clean MBL conditions were sampled (<500 cm−3). These data then drive a state-of-the-art
droplet formation parameterization from which the predicted cloud
droplet number and its sensitivity to aerosol and dynamical
parameters are derived. Droplet closure was achieved to within
20 %. Droplet formation sensitivity to aerosol concentration,
w∗, and the hygroscopicity parameter, κ, vary and
contribute to the total droplet response in the MBL clouds. When
aerosol concentrations exceed ∼900 cm−3 and maximum
supersaturation approaches 0.1 %, droplet formation in the MBL
enters a velocity-limited droplet activation regime, where the cloud
droplet number responds weakly to CCN concentration increases. Below
∼500 cm−3, in a clean MBL, droplet formation is
much more sensitive to changes in aerosol concentration than to
changes in vertical updraft. In the competitive regime, where
the MBL has intermediate pollution (500–800 cm−3),
droplet formation becomes much more sensitive to hygroscopicity
(κ) variations than it does in clean and polluted
conditions. Higher concentrations increase the sensitivity to
vertical velocity by more than 10-fold. We also find that
characteristic vertical velocity plays a very important role in
driving droplet formation in a more polluted MBL regime, in which
even a small shift in w∗ may make a significant difference in
droplet concentrations. Identifying regimes where droplet number
variability is driven primarily by updraft velocity and not by aerosol
concentration is key for interpreting aerosol indirect effects,
especially with remote sensing. The droplet number responds
proportionally to changes in characteristic velocity, offering the
possibility of remote sensing of w∗ under velocity-limited
conditions.
How efficient is cloud droplet formation of organic aerosols?