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Discussion papers
https://doi.org/10.5194/acp-2020-197
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-197
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 16 Mar 2020

Submitted as: research article | 16 Mar 2020

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This preprint is currently under review for the journal ACP.

Vertical variability of the properties of highly aged biomass burning aerosol transported over the southeast Atlantic during CLARIFY-2017

Huihui Wu1, Jonathan W. Taylor1, Kate Szpek2, Justin Langridge2, Paul I. Williams1,3, Michael Flynn1, James D. Allan1,3, Steven J. Abel2, Joseph Pitt1,a, Michael I. Cotterell2,4,b, Cathryn Fox2, Nicholas W. Davies2,4, Jim Haywood2,4, and Hugh Coe1 Huihui Wu et al.
  • 1Department of Earth and Environmental Sciences, University of Manchester, Manchester, UK
  • 2Met Office, Fitzroy Road, Exeter, EX1 3PB, UK
  • 3National Centre for Atmospheric Science, University of Manchester, Manchester, UK
  • 4College of Mathematics, Engineering and Physical Science, University of Exeter, UK
  • anow at: School of Marine and Atmospheric Science, Stony Brook University, Stony Brook, USA
  • bnow at: School of Chemistry, University of Bristol, Bristol, BS8 1TS, UK

Abstract. Seasonal biomass burning (BB) from June to October in central and southern Africa leads to absorbing aerosols being transported over the south Atlantic Ocean every year, and contributes significantly to the regional climate forcing. The vertical distribution of submicron aerosols and their properties were characterized over the remote southeast Atlantic for the first time, using airborne in-situ measurements made during the CLoud-Aerosol-Radiation Interactions and Forcing for Year 2017 (CLARIFY-2017) campaign. BB aerosols were intensively observed in the region surrounding Ascension Island, in the marine boundary layer (MBL) and free troposphere (FT) up to 5 km. We show that the aerosols had undergone a significant aging process during > 7 days transit from source, as indicated by highly oxidized organic aerosol and thickly coated black carbon (BC). The highly aged BB aerosols in the CLARIFY region were also especially rich in BC compared with those from other regions.

We also found significant vertical variation in the single scattering albedos (SSA) of these aerosols, as a function of relative chemical composition and size. The lowest SSA was generally in the low FT layer around 2000 m altitude (medians: 0.83 at 405 nm and 0.80 at 658 nm). This finding is important since it means that BB aerosols across the east Atlantic region are more absorbing than is currently represented in climate models. Furthermore, in the FT, we show that SSA increased with altitude and this was associated with an enhanced inorganic nitrate mass fraction and aerosol size. This likely results from increased partitioning to the existing particles at higher altitude with lower temperature and higher relative humidity. After entrainment into the BL, aerosols were generally smaller in size than were observed in the FT, and had a larger fraction of scattering material with resultant higher average dry SSA, mostly due to marine emissions and aerosol removal by drizzle. Our results provide unique observational constraints on aerosol parameterizations used in modelling regional radiation interactions over this important region. We recommend that future work should consider the impact of this vertical variability on climate models.

Huihui Wu et al.

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Short summary
This study presents the first airborne measurements of aerosol distribution and properties in the remote southeast Atlantic, which is affected by the long-range transport of southern African biomass burning every year and is important climatically. These observations demonstrate the persistence of strongly absorbing aerosols across wide regions of the South Atlantic and indicate significant vertical variation in aerosol properties, which provide unique parameters used in future climate models.
This study presents the first airborne measurements of aerosol distribution and properties in...
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