AUTHOR=Zhai Peng-Wang , Gao Meng , Franz Bryan A. , Werdell P. Jeremy , Ibrahim Amir , Hu Yongxiang , Chowdhary Jacek 
  
TITLE=A Radiative Transfer Simulator for PACE: Theory and Applications
  
JOURNAL=Frontiers in Remote Sensing
  
VOLUME=Volume 3 - 2022
  
YEAR=2022
  
URL=https://www.frontiersin.org/journals/remote-sensing/articles/10.3389/frsen.2022.840188
  
DOI=10.3389/frsen.2022.840188
  
ISSN=2673-6187
  
ABSTRACT=We report a radiative transfer simulator that computes the synthetic data of all three instruments onboard NASA’s Plankton Aerosol, Cloud, ocean Ecosystem (PACE) observatory, at the top of the atmosphere (TOA). The instrument suite includes the ocean color instrument (OCI), the Hyper-Angular Rainbow Polarimeter 2 (HARP2), and the Spectro-Polarimeter for Planetary Exploration 1 (SPEXone). The core of the PACE simulator is a monochromatic radiative transfer model based on the successive order of scattering (RTSOS), which accounts for atmosphere and ocean coupling, polarization, and gas absorption. Inelastic scattering, including Raman scattering from pure ocean water, fluorescence due to chlorophyll, and colored dissolved organic matter (CDOM), are also simulated. This PACE simulator can be used to explore the sensitivity of the hyperspectral and polarized reflectance of the Earth system with tunable atmosphere and ocean parameters, which include aerosol and cloud number concentration, refractive indices, and size distribution, ocean particle microphysical parameters, and solar and sensor-viewing geometry. We have used the PACE simulator to study two important case studies. One is the impact of the significant uncertainty in pure ocean water absorption coefficient to the radiance field in the ultraviolet (UV) spectral region, which can be as much as 6%. The other is the influence of different amounts of brown carbon aerosols and CDOM on the polarized radiance field at TOA. We found that the wavelength of 450 nm is an inflection point at which the reflectance of different volume fractions of brown carbon aerosols is roughly the same, given that total optical depth is kept constant.  Both case studies are important for aerosol and ocean color remote sensing and have not been previously reported in the literature.