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Discussion papers
https://doi.org/10.5194/acp-2019-791
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-791
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 05 Sep 2019

Submitted as: research article | 05 Sep 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

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

Mary Kacarab1, K. Lee Thornhill2, Amie Dobracki3, Steven G. Howell3, Joseph R. O'Brien4, Steffen Freitag3, Michael R. Poellot4, Robert Wood5, Paquita Zuidema6, Jens Redemann7, and Athanasios Nenes1,8,9 Mary Kacarab et al.
  • 1School of Earth & Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, 8 30332, USA
  • 2NASA Langley Research Center, Hampton, VA, 23666, USA
  • 3Department of Oceanography, University of Hawaii, Honolulu, HI, 96822, USA
  • 4Atmospheric Sciences Department, University of North Dakota, Grand Forks, ND, 58202, USA
  • 5Atmospheric Sciences, University of Washington, Seattle, WA, 98195, USA
  • 6Department of Atmospheric Sciences, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL, 33149, USA
  • 7School of Meteorology, University of Oklahoma, Norman, OK, 73072, USA
  • 8Institue for Chemical Engineering Sciences, Foundation for Research and Technology Hellas, Patras, 26504, Greece
  • 9Laboratory of Atmospheric Processes and their Impacts, School of Architecture, Civil and Environmental Engineering, Ecole Polytechnique Federale de Lausanne, Lausanne, 21 1015, Switzerland

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 vertical updraft velocity 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 mid-campaign to concentrations between 500–800 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, vertical updraft velocity, 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 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 for clean and polluted conditions. Higher concentrations increase the sensitivity to vertical velocity by more than ten-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 primarily driven by updraft velocity and not aerosol concentration is key for interpreting aerosol indirect effects, especially with remote sensing. Droplet number responds proportionally to changes in characteristic velocity, offering the possibility of remote sensing of w* under velocity-limited conditions.

Mary Kacarab et al.
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