Biomass burning aerosol as a modulator of the droplet number in the southeast Atlantic region

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 o...

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Published in:Atmospheric chemistry and physics 2020-03, Vol.20 (5), p.3029-3040
Main Authors: Kacarab, Mary, Thornhill, K. Lee, Dobracki, Amie, Howell, Steven G, O'Brien, Joseph R, Freitag, Steffen, Poellot, Michael R, Wood, Robert, Zuidema, Paquita, Redemann, Jens, Nenes, Athanasios
Format: Article
Language:English
Subjects:
Aerosol concentrations
Aerosol size distribution
Aerosol-cloud interactions
Aerosols
Aircraft
Altitude
Biomass
Biomass burning
Boundary layers
Burning
Cloud condensation nuclei
Cloud droplets
Clouds
Clouds (Meteorology)
Condensation nuclei
Decks
Deployment
Droplets
Hygroscopicity
Parameter sensitivity
Parameterization
Plumes
Pollution
Remote sensing
Sensitivity
Size distribution
Supersaturation
Time
Updraft
Velocity
Vertical distribution
Vertical velocities
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cited_by cdi_FETCH-LOGICAL-c480t-45ec6dd65abb05a1f74052aac9d66b3281a28c4fe1aa68a16e66cf5a0501122f3
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creator Kacarab, Mary
Thornhill, K. Lee
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Redemann, Jens
Nenes, Athanasios
description 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 (
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Lee ; Dobracki, Amie ; Howell, Steven G ; O'Brien, Joseph R ; Freitag, Steffen ; Poellot, Michael R ; Wood, Robert ; Zuidema, Paquita ; Redemann, Jens ; Nenes, Athanasios</creator><creatorcontrib>Kacarab, Mary ; Thornhill, K. Lee ; Dobracki, Amie ; Howell, Steven G ; O'Brien, Joseph R ; Freitag, Steffen ; Poellot, Michael R ; Wood, Robert ; Zuidema, Paquita ; Redemann, Jens ; Nenes, Athanasios</creatorcontrib><description>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 (&lt;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. 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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 (&lt;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. 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subjects Aerosol concentrations
Aerosol size distribution
Aerosol-cloud interactions
Aerosols
Aircraft
Altitude
Biomass
Biomass burning
Boundary layers
Burning
Cloud condensation nuclei
Cloud droplets
Clouds
Clouds (Meteorology)
Condensation nuclei
Decks
Deployment
Droplets
Hygroscopicity
Parameter sensitivity
Parameterization
Plumes
Pollution
Remote sensing
Sensitivity
Size distribution
Supersaturation
Time
Updraft
Velocity
Vertical distribution
Vertical velocities
title Biomass burning aerosol as a modulator of the droplet number in the southeast Atlantic region
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