Unified Aerosol Algorithm: New insights into the global aerosol system from the Ocean Color Instrument on NASA's PACE mission

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Abstract. The launch of NASA's Plankton, Aerosols, Clouds, ocean Ecosystem (PACE) mission in 2024 with the Ocean Color Instrument (OCI) on board created new opportunity for characterizing aerosol properties from space. OCI observes Earth and its atmosphere across a broad spectral range in the reflective spectrum from the ultraviolet (UV) to the Short Wave InfraRed (SWIR). Heritage aerosol retrieval algorithms developed to accommodate radiometers measuring in the visible to SWIR wavelengths such as the MODerate resolution Imaging Spectroradiometer (MODIS) have been successful in characterizing aerosol optical depth (AOD) and indications of aerosol particle size but are less sensitive to aerosol particle absorption or aerosol layer height. Other heritage sensors measuring in the UV part of the spectrum such as the Total Ozone Mapping Spectrometer (TOMS) or the Ozone Monitoring Instrument (OMI) are sensitive to absorption and layer height but have insufficient information to constrain AOD in an aerosol retrieval. With OCI encompassing the entire spectral range of interest, the Dark Target and Deep Blue algorithms from the MODIS tradition and Near UV algorithm from the TOMS/OMI tradition are brought together, adapted for OCI and unified for retrievals of AOD, particle size parameter, aerosol absorption and aerosol layer height. Adaptations for OCI include adjustment for OCI sensor characteristics, modification of cloud and snow masking routines, production of the traditional UV Aerosol Index at unprecedented 1 km resolution, merging of the Dark Target and Deep Blue retrievals over land, extrapolation of spectral AOD retrieved in the visible range into the UV range, use of Oxygen-B bands for aerosol layer height, and retrieval of AOD above clouds that provides visualization of All Sky aerosol loading, Six months of retrievals have been compared with collocated AERONET AOD and single scattering albedo (SSA) resulting in a preliminary validation. AOD is biased high for shorter wavelengths, especially over ocean leading to asymmetrical error bounds for AOD. No similar bias is seen in the SSA retrievals which are exhibiting error bounds of ±0.04, at this stage. The results of the OCI Unified Aerosol Algorithm are quantitative characterization of the global aerosol system over ocean, vegetated and barren surfaces, in clear skies and above clouds.

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