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We propose to apply our automated and operational RSP-MAPP retrieval algorithm (Stamnes et al., 2018), which was developed for the airborne NASA GISS Research Scanning Polarimeter (RSP) (Cairns et al., 1999), to the PACE observing system to accurately retrieve aerosol and ocean products for PACE. A key feature of this proposed PACE-MAPP algorithm is that it is coupled; the atmosphere and ocean are solved together as one system from a radiative transfer perspective. Conservation of energy thus ensures that negative water-leaving radiances are impossible; and by simultaneously solving for the aerosol and ocean products, we can determine the optimal solution, together with a full accounting of the uncertainties of each parameter. A missing piece in the puzzle for spaceborne sensing of estuarine and coastal zones is provided by PACE with its introduction of multi-angle, multi-channel polarimeter measurements capable of accurately retrieving the complex aerosol scenes that arise in coastal areas, and which will now be available thanks to PACE. The coupled approach, using polarimeter measurements, is the only reliable and accurate way to retrieve aerosol and ocean products in the Earth's complex coastal zones. The PACE-MAPP algorithm will thus be capable of providing aerosol and ocean products for both the global ocean and coastal areas.

The emphasis of the PACE-MAPP algorithm products will be two-fold: i) accurate retrieval of aerosol microphysical properties including aerosol absorption (single- scattering albedo), aerosol location, aerosol effective radius, and the separation of aerosol optical depth into a fine mode and two coarse modes (sea salt and dust), and ii) simultaneous retrieval of ocean products using a polarized bio-optical model that parameterizes phytoplankton and nonalgal particles across multiple water types including the open ocean, phytoplankton blooms, and coastal zones. The PACE-MAPP retrieval algorithm will be tested using simulated datasets, and with airborne PACE-like data collected during the SABOR, NAAMES and ACEPOL campaigns, and the future ACTIVATE dataset.

All aerosol and ocean data products will include uncertainties produced by the PACE- MAPP optimal estimation retrieval, considering the instrument measurement error uncertainty and a priori information. Algorithm validation will be accomplished by performing retrievals upon simulated PACE SPEXone/HARP2/OCI-SWIR data for the full range of realistic aerosol and ocean parameters. Additionally, validation of the ocean retrieval products from airborne RSP/SPEXair/AirHARP will be accomplished using ship-based in situ measurements and collocated HSRL (High Spectral Resolution Lidar) ocean measurements. Validation of the aerosol retrieval products will be accomplished via HSRL aerosol measurements and AERONET station overpass comparisons.

The proposed PACE-MAPP retrieval algorithm will be robust so that it will have the capability to provide improved results for aerosol and ocean products data if either SPEXone or HARP2 is operational, by combining data from a single polarimeter with the OCI SWIR channels, or using only data from both the polarimeters without the OCI SWIR channels.