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We propose to support the algorithm development of the Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) mission with the following objectives:

I. Maintenance and further development of the radiative transfer packages that we developed for the PACE instruments.

The first objective of this proposal is to support and maintain the software that we developed for the PACE mission, and develop new capabilities, which include the implementation of land surface reflectance models (snow and vegetation, for example), plastic and bubble particles in ocean waters, broader spectral coverage to calculate short wave radiative forcing.

II. Improve the aerosol and ocean bio-optical representations in the Fast Multi-Angular Polarimetric Ocean coLor (FastMAPOL) algorithm to include coastal waters.

FastMAPOL is a joint inversion algorithm which can retrieve aerosol optical depth (AOD), single scattering albedo (SSA), size distribution, and remote sensing reflectance (Rrs) from Multi-Angle Polarimeter (MAP) measurements over open ocean waters. FastMAPOL uses a neural network (NN) forward model to function in lieu of the RT model to achieve efficiency and maintain accuracy so that it can operationally process the PACE data. In the second objective we aim to further expand FastMAPOL to include flexible aerosol component models and coastal water bio-optical models, and use the spectral bands from both Spectro-polarimeter for Planetary Exploration (SPEXone) and Hyper-Angular Rainbow Polarimeter 2 (HARP-2) for better information content.

III. Provide an Instantaneous Photosynthetically Available Radiation (IPAR) data product derived from polarimeter measurements.

We have developed a NN algorithm which can calculate IPAR and its vertical profile in ocean waters based on the solar zenith angle, ozone amount in Dobson unit (DU), AOD, SSA, Ångström exponent, ocean chlorophyll a concentration (Chla), absorption coefficient of CDOM, and backscattering coefficient of oceanic particles. Test with independent datasets shows that the algorithm is highly efficient and accurate. In the third goal of this proposal, we will apply this algorithm to the PACE MAP data to generate the IPAR profile.

We propose to contribute to the Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) Science and Applications Team (SAT) with the expertise in the theories of radiative transfer in coupled atmosphere and ocean systems, aerosol and ocean color remote sensing, and climatology of aerosol properties from space lidar missions. Specifically, we proposed to achieve the following objectives for a three-year performance period: Objective I. Develop a radiative transfer simulator for the PACE instruments.

We will develop a radiative transfer package that can simulate synthetic datasets for the PACE instruments with flexible atmospheric and oceanic conditions. All major light- matter interaction mechanisms will be accounted for, including the polarization nature of light, atmosphere-ocean coupling, gas absorption, fluorescence of phytoplankton, Fluorescence of Dissolved Organic Matter (FDOM), Raman scattering of pure ocean water, and spherical shell effects. All three instruments onboard the PACE satellite, i.e., the Ocean Color Instrument (OCI) and two Multi-Angle Polarimeters (MAP): Hyper Angular Research Polarimeter (HARP-2) and Spectro-polarimeter for Planetary Exploration (SPEXone), will be emulated by considering their spectral coverages, instrument line shape functions, and viewing geometries. Sensors can be placed at arbitrary locations. This work is important for validating the PACE's Level 2 science algorithms and testing the streamlines of PACE's data production systems.

Objective II. Joint retrieval of the aerosol and ocean color properties using the MAP data We have developed a joint retrieval algorithm for aerosol and ocean color properties using the MAP data for both open and coastal ocean waters. The algorithm can handle both absorbing and non-absorbing aerosols with help from the rich information content of the MAP measurements. The retrieved ocean parameters are the spectral water leaving radiances. The retrieved aerosol properties include the particle size distribution, the optical depth, single scattering albedo, and phase matrices. The joint retrieval algorithm has been demonstrated by both the radiative transfer synthetic dataset and the Research Scanning Polarimeter (RSP) measurements. We propose to further extend the algorithm's capabilities to process both the HARP-2 and SPEXone dataset, which have different spectral and spatial coverages rendering different information content for aerosol and ocean color properties. The sensitivity of aerosol and ocean color properties to the different characteristic of the two MAPs will be studied and the retrieval parameters will be adjusted accordingly. We will validate the algorithm for a large variety of ocean scenes.

Objective III. Atmospheric correction for OCI with the aerosol information retrieved from the MAP data.

Atmospheric correction for scenes involving coastal waters and absorbing aerosols is a challenging task for single-viewing spectrometers. We will use the retrieved aerosol properties from the MAP data to aid the atmospheric correction for the OCI data. The PACE platform will provide a plethora of co-located OCI and MAP data for this research. The resultant water leaving radiance from the atmospheric correction algorithm will be evaluated by in-situ measurements and/or co-located Aerosol Robotic Network (AERONET) data products.

Four important research areas are listed in the PACE SAT solicitation, covering theoretical and analytical studies using the precursors to OCI, HARP-2, and SPEXone. Our proposed research objectives address the first three out of the four areas outlined. The radiative transfer simulator and retrieval algorithm in this effort will be delivered to the Ocean Biology Process Group (OBPG) for implementing in their production system.