Subseasonal and spatial variability of biomass burning aerosol radiative properties observed over the Southeast Atlantic during ORACLES 2016–2018

Abstract. During 2016–2018, NASA conducted the ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) airborne field campaigns to study aerosol-cloud-radiation interactions with the stratocumulus cloud deck over the Southeast Atlantic. ORACLES employed 4STAR (Spectrometer for Sky-Scanning, Sun-Tracking Atmospheric Research) to measure direct solar irradiances and diffuse sky radiances of free-tropospheric Biomass Burning Aerosols (BBA). Aerosol radiative properties, including Single Scattering Albedo (SSA), Aerosol Optical Depth (AOD), Aerosol Absorption Optical Depth (AAOD), Extinction Ångström Exponent (EAE), Absorption Ångström Exponent (AAE), and complex refractive indices are retrieved via an adapted AERONET inversion code. Changes in SSA indicate increased scattering as the biomass burning season progresses, which we attribute to an aerosol brightening from compositional changes, rather than a change in aerosol type, with an apparent lack of Brown Carbon throughout the season. A collection of 31 AERONET (AErosol RObotic NETwork) sun/sky photometer stations have operated in Southern Africa for over thirty years (1995–2025), creating a complete aerosol climatology for the first time, which can be compared with SSA and AOD from ORACLES observations. The spatial distributions of in situ SSA are also investigated by latitude, longitude, and altitude. Westward gradual increases and sharper decreases in SSA are attributed to late-transport aging processes identified by previous studies. These processes start further eastward in October, in conjunction with the southeastward shift in source fires. Collectively, ORACLES 4STAR retrievals and in situ measurements have identified subseasonal and spatial trends in SSA over the Southeast Atlantic that complement the Southern African AERONET climatology.