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

• Published in: Atmospheric Chemistry and Physics
• Main Authors: Wu, Huihui, Taylor, J. W., Szpek, Kate, Langridge, Justin M., Williams, Paul, Flynn, Michael, Allan, James, Abel, Steven J., Pitt, Joseph, Cotterell, Michael I., Fox, Cathryn, Davies, Nicholas W., Haywood, Jim, Coe, Hugh
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2020
• Subjects: South Atlantic Ocean
• Online Access: https://research.manchester.ac.uk/en/publications/6fbb24aa-7cb5-48ec-8a84-abd88d427367; https://doi.org/10.5194/acp-20-12697-2020; https://pure.manchester.ac.uk/ws/files/185527165/acp_20_12697_2020.pdf

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, using airborne in situ measurements made during the CLoud-Aerosol-Radiation Interactions and Forcing for Year 2017 (CLARIFY-2017) campaign. BB aerosols emitted from flaming-controlled fires 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 ageing process during > 7 d transit from source, as indicated by the highly oxidized organic aerosol. The highly aged BB aerosols in the far-field CLARIFY region were also especially rich in black carbon (BC), with relatively low singlescattering albedos (SSAs), compared with those from other BB transported regions. The column-weighted dry SSAs during CLARIFY were observed to be 0.85, 0.84 and 0.83 at 405, 550 and 658 nm respectively. We also found significant vertical variation in the dry SSA, as a function of relative chemical composition and size. The lowest SSA in the column was generally in the low FT layer around 2000 m altitude (averages: 0.82, 0.81 and 0.79 at 405, 550 and 658 nm). This finding is important since it means that BB aerosols across the southeast Atlantic region are more absorbing than currently represented in climate models, implying that the radiative forcing from BB may be more strongly positive than previously thought. Furthermore, in the FT, average SSAs at 405, 550 and 658 nm increased to 0.87, 0.86 and 0.85 with altitude up to 5 km. This was associated with an enhanced inorganic nitrate mass fraction and aerosol size, likely resulting from increased partitioning of ammonium nitrate to the existing particles at higher altitude with lower temperature and ...