Absorbing aerosols play a role in cloud and radiation processes, but these interactions are complex and have a high degree of uncertainty. The interactions of Biomass Burning (BB) aerosols with the semipermanent subtropical stratocumulus cloud deck over the Southeast Atlantic Ocean are reflected by large General Circulation Model (GCM) residuals in the region. Nearly two decades (1995-2013) of AERONET (AErosol RObotic NETwork) sun photometer measurements have documented an intense BB aerosol emission season from August to October, with subseasonal variations in aerosol scattering/absorption occurring in the source region of Southern Africa. During 2016-2018, NASA conducted the ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) airborne campaign to study BB aerosol-cloud-radiation interactions and BB aerosol transport above, within, and below the stratocumulus cloud deck over the Southeast Atlantic. ORACLES employed a 4STAR (Spectrometer for Sky-Scanning, Sun-Tracking Atmospheric Research) hyperspectral sun-sky spectrophotometer to measure solar radiation, including direct beam irradiances and sky radiances. The subseasonal variability of aerosol radiative properties retrieved from 4STAR during ORACLES is examined in the context of the extended AERONET record.

4STAR-retrieved aerosol radiative properties include Single Scattering Albedo (SSA), Aerosol Optical Depth (AOD), Aerosol Absorption Optical Depth (AAOD), and Absorption Ångström Exponent (AAE). SSA shows subseasonal variations in aerosol scattering and absorption, which is echoed by an analogous trend in AAOD. The subseasonal variation in SSA is large enough (0.04 - 0.06) that it can affect aerosol-induced radiative flux. The median AAE values (nominally 1) and small subseasonal variations indicate that aerosol type is dominated by spectrally uniform Black Carbon (BC), and that the subseasonal change in aerosol scattering/absorption is likely due to changes in BB aerosol composition. The subseasonal variations in SSA and AOD found during ORACLES over the SE Atlantic is in remarkable agreement with that of the extended AERONET record in Southern Africa, speaking to 4STAR’s utility as a mobile, airborne AERONET-like platform.