Biomass Burning Aerosol as a Modulator of Droplet Number in the Southeast...

Kacarab, M., L. Thornhill, A. Dobracki, S. Howell, J. O'Brien, S. Freitag, M. Poellot, R. Wood, P. Zuidema, J. Redemann, and A. Nenes (2020), Biomass Burning Aerosol as a Modulator of Droplet Number in the Southeast Atlantic Region, Atmos. Chem. Phys., 20, 3029-3040, doi:10.5194/acp-20-3029-2020.

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, , 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 () 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.

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Research Program: 
Atmospheric Composition
Radiation Science Program (RSP)