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

Abstract

Biomass Burning Aerosols (BBA) are a source of uncertainty in climate simulations over the Southeast Atlantic. Following emission in Southern Africa, the BBA smoke plume overlies a stratocumulus cloud deck, resulting in aerosol-cloud-radiation interactions that are difficult to observe. During 2016–2018, NASA conducted the ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) airborne campaigns to constrain BBA uncertainty, employing 4STAR (Spectrometer for Sky-Scanning, Sun-Tracking Atmospheric Research) to retrieve aerosol properties. We use all three months of ORACLES 4STAR sky-scans to investigate the subseasonal variability of BBA radiative properties. Changes in Single Scattering Albedo (SSA) indicate increased scattering relative to extinction as the BBA emission season progresses. We attribute this aerosol brightening to changes in composition, rather than aerosol type, as our Absorption Ångström Exponent analysis provides no evidence for Brown Carbon contributions. SSA from our 4STAR sky-scans is compared against a 30 year climatology from 31 Southern African AERONET (AErosol RObotic NETwork) stations, showing consistent changes in aerosol scattering across land and sea. ORACLES in situ SSA is also examined spatially. Our latitudinal analysis indicates that the September decrease in SSA from 4STAR is affected by the southerly sampling of ORACLES 2016 compared to the other campaigns. Westward gradual increases and sharper decreases in SSA are attributed to late-transport aging processes identified by recent studies. These processes start further eastward in October, in conjunction with the southeastward shift in fires. Understanding these subseasonal and spatial changes in BBA radiative properties has implications for aerosol modelling and validation of satellite products.