Sensitivity of Simulated Ozone Monitoring Instrument (OMI) Absorbing Aerosol Index (AAI) to Assumptions of Aerosol Optical Properties as Informed by Data from the NASA ORACLES Campaign

The Ozone Monitoring Instrument (OMI) flying onboard the NASA Aura spacecraft makes wide-swath, multi-spectral observations of the top-of-atmosphere (TOA) reflectance in the near-UV and visible part of the spectrum. An intermediary product of the standard OMI aerosol retrieval algorithms is the so-called Absorbing Aerosol Index (AAI), which relates the observed spectral contrast of the 354 and 388 nm channels with what is expected in a purely Rayleigh scattering atmosphere. The AI is especially sensitive to the vertical distribution of aerosols, their vertical position with respect to clouds, and their absorption characteristics. We have previously exploited this sensitivity to infer the spectral absorption properties of aerosols in dusty and biomass burning polluted environments. Here we extend the capabilities of an OMI forward simulator used to compute the OMI AI from simulated aerosol profiles provided by the NASA Goddard Earth Observing System (GEOS) global Earth system model. The simulator makes use of an improved representation of surface topography and includes now brown carbon and organic carbon aerosol species. GEOS is run at high spatial resolution for the observing periods of the 2016, 2017, and 2018 NASA ORACLES airborne campaigns that focused on the transport and properties of biomass burning aerosols traveling west of southern Africa during the burning season (mainly August-September). The GEOS model aerosol optical properties and distributions have been constrained by the ORACLES observations of the particle microphysical properties. Here we test various assumptions of the aerosol optical properties, including particle size, absorption, and hygroscopicity on our forward simulation of the OMI TOA reflectances to explore consistency between the ORACLES in situ and remote sensing observations and the OMI AI and aerosol optical depth and single scattering albedo retrievals.