29 Aug 2022
29 Aug 2022
Status: this preprint is currently under review for the journal ACP.

An attribution of the low single-scattering albedo of biomass-burning aerosol over the southeast Atlantic

Amie Dobracki1, Paquita Zuidema1, Steven Howell2, Pablo Saide3, Steffen Freitag2, Allison C. Aiken4, Sharon P. Burton5, Arthur J. Sedlacek III6, Jens Redemann7, and Robert Wood8 Amie Dobracki et al.
  • 1Department of Atmospheric Sciences, Rosenstiel School, University of Miami, Miami, Florida, USA
  • 2University of Hawai‘i at Manoa, Honolulu, Hawaii, USA
  • 3University of California Los Angeles, Los Angeles, California, USA
  • 4Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, New Mexico, USA
  • 5NASA Langley Research Center, Hampton, VA, USA
  • 6Brookhaven National Laboratory, Upton, New York, USA
  • 7University of Oklahoma, Norman, Oklahoma, USA
  • 8University of Washington, Seattle, WA, USA

Abstract. Aerosol over the remote southeast Atlantic is some of the most sunlight-absorbing aerosol on the planet: the in-situ free-tropospheric single-scattering albedo at the 530 nm wavelength (SSA530nm) ranges from 0.83 to 0.89 within ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) aircraft flights from late August–September. Here we seek to explain the low SSA. The SSA depends strongly on the black carbon (BC) number fraction, which ranges from 0.15 to 0.4. Organic aerosol (OA) to BC mass ratios of 8–14 and modified combustion efficiency values > 0.975 point indirectly to the dry, flame-efficient combustion of primarily grass fuels, with back trajectories ending in the miombo woodlands of Angola. The youngest aerosol plume, aged 4–5 days since emission and sampled directly west of Angola, broadly consisted of two plumes, with the higher, thicker plume transported more quickly off of the continent by stronger winds. The particle size and fraction of BC-containing particles increased with chemical age, consistent with vapor condensation and coagulation. The particle volume and OA : BC mass ratio reduced simultaneously, attributed primarily to evaporation through photochemistry rather than dilution or thermodynamics. The CLARIFY (CLoud-Aerosol-Radiation Interaction and Forcing: Year-2017) aircraft campaign held near the more remote Ascension Island in August–September 2017 report higher BC number fractions, lower OA : BC mass ratios, lower SSA yet larger mass absorption coefficients compared to this study's. Values from the one analyzed ORACLES-2017 flight, held midway to Ascension Island, are intermediate, confirming the long-range changes. Inorganic ammonium nitrate, thought responsible for the vertical structure in SSA at Ascension Island through thermodynamic gas-particle partitioning, increases from ~20 % of the total nitrate in the ORACLES September flights, to 50 % for the August 2017 ORACLES flight midway to Ascension. Overall the data are consistent with continuing oxidation through fragmentation releasing aerosols that subsequently enter the gas phase, reducing the OA mass, rather than evaporation through dilution or thermodynamics. The data support the following best-fit: SSA530nm=0.801+0055*(OA : BC) (r = 0.84). The fires of southern Africa emit approximately one-third of the world's carbon; the emitted aerosols are distinct from other regional BBAs and their aerosol composition also needs to be represented appropriately to realistically depict regional aerosol radiative effects.

Amie Dobracki et al.

Status: open (until 10 Oct 2022)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

Amie Dobracki et al.

Amie Dobracki et al.


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Short summary
Southern Africa produces approximately one-third of the world’s carbon from fires. The thick smoke layer this produces overlying the southeast Atlantic cloud deck is capable of altering regional circulation patterns and reducing rain over parts of Africa. We thereby seek to better characterize how the aerosol absorption can be represented in models as a function of the aerosol composition using aircraft data.