Simulations of the Observed Seasonal Variability of Biomass Burning Aerosol Absorption Over the Southeast Atlantic Ocean

Absorption remains an important but under-constrained aerosol property, determined by the size, shape, composition, and mixing state of the aerosol population. The presence of absorbing aerosols in the atmospheric column impacts its radiative and thermodynamic profiles, which in turn affect relative amounts of surface cooling and atmospheric heating, and so also the properties and distributions of clouds. An important source of absorbing aerosol in the atmosphere is from biomass burning, with significant and seasonally varying sources prominent over southern Africa. Biomass burning aerosols from southern Africa are transported over the southeast Atlantic during the peak burning months of August, September, and October. The significance of biomass burning in this region motivated the three-year NASA ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES) airborne field campaign. Each of the three ORACLES deployments focused on a different month of the burning season (September 2016, August 2017, and October 2018), and in situ and remote sensing measurements collected during ORACLES highlight seasonal variability in the loading, composition, and absorbing properties of these aerosols. Notably, the aerosols sampled toward the end of the burning season (i.e., October 2018 deployment) were found to be less absorbing than earlier in the season. We investigate this feature of the observations using the NASA Goddard Earth Observing System (GEOS) Earth system model, presenting simulations over each of the ORACLES deployment periods. Previous work focused on the ORACLES September 2016 campaign revealed that the model needed adjustments to its assumed properties and lifetimes of biomass burning organic aerosols to reach agreement with ORACLES observations. Here we expand our simulations to include the other two ORACLES deployment years and test the effectiveness of our previous model modifications in representing the observed seasonal variability. Satellite observations from the absorption-sensitive Ozone Monitoring Instrument (OMI) on the NASA Aura spacecraft are also included to provide a broader regional context and an independent assessment of seasonal variability in aerosol distributions and properties. Satellite and airborne measurements are linked through a novel observation simulator by making use of the GEOS model output to simulate not only the observed airborne measurements but also the satellite radiance measurements. We discuss results of our comparisons and present paths forward for future model improvement.