Transport Pathways of African Biomass Burning Aerosols in the Southeast Atlantic Marine Boundary Layer and their Impacts on Cloud Properties

The transport of African biomass burning aerosols (BBAs) and their impacts on cloud formation and properties over the southeast Atlantic remain one of the largest sources of uncertainty in understanding climate effects across this region. In this study, vertical structures of thermodynamics, aerosol properties and cloud microphysics were characterized around Ascension Island during the CLARIFY-2017 (CLoud-Aerosol-Radiation Interactions and Forcing for Year 2017; August–September 2017) aircraft campaign, providing insights into the relationship between transported African BBAs and clouds in the marine boundary layer (MBL) over the remote southeast Atlantic.

The BB-impacted MBL exhibited substantially enhanced aerosol number concentration (N_a) compared to the clean MBL around Ascension Island, leading to increased cloud droplet number concentration (N_d) but smaller cloud effective radius (R_e). The N_d and R_e values were observed to be strongly correlated with aerosols below the cloud (sub-N_a), but were more weakly associated with aerosols immediately above the cloud. The increase in the sub-N_a is identified to be entrained BBAs from above-cloud to below-cloud regions following long-range transport. We also explored possible simplifications to establish relationships between N_d and sub-N_a or R_e from in-situ measurements. Similar droplet activation fractions were observed in the clean and moderately BB-impacted (sub-N_a < 700 cm^–3) clouds, while a greater bias was noted in more polluted clouds. The relationship between N_d and R_e remains consistent regardless of BB influence.

Backward simulations were conducted using UK Met Office’s Numerical Atmospheric Modelling Environment (NAME), to track the sources and pathways of air parcels reaching Ascension Island. NAME results indicate that air parcels arriving in the Ascension Island MBL can originate from both the BL and free troposphere (FT) along the long-range transport pathway, following air parcel entrainment from the FT to the MBL over the southeast Atlantic. BB pollution in the Ascension Island MBL could occur, when FT air parcels, primarily originating from the African continent (22°S – 0°N), carry BB smoke. By coupling NAME air parcel analysis to SEVIRI (Spinning Enhanced Visible and Infrared Imager) satellite-retrieved aerosol fields, it is inferred that efficient entrainment of elevated air parcels transported from Africa occurs over multiple days in the southeast Atlantic before reaching the Ascension Island MBL, mainly in the region to the west of 0°E for the cases examined. We also examined the response of cloud properties to entrained BBAs in prior to and after these regions of efficient mixing by tracking SEVIRI satellite-retrieved cloud properties (N_d and R_e) along simulated MBL air parcel footprints.

This study provides important aerosol and cloud characterisations that can be used to challenge climate models since the data challenge model representation of transport pathways and effects on clouds, and ultimately their radiative forcing over the southeast Atlantic. Additionally, the identified BBAs entrainment region provides constraints for refining the vertical transport of African BBAs in models, thereby improving the representation of the vertical structures of African BBAs. More case studies should be conducted to examine the entrainment patterns across various months and years, to characterize a climatological area for entrainment over the southeast Atlantic.