Chlorophyll fluorescence and the polarized underwater light field: comparison of vector radiative transfer simulations and multi-angular hyperspectral polarization field measurements

Abstract

Previous partial simulations and field measurements by us, had demonstrated the impact of the un-polarized nature of algal chlorophyll fluorescence to reduce the observed degree of polarization of the underwater light field in the spectral vicinity of fluorescence. (Polarization otherwise existing as a result of non-algal particulate (NAP) and molecular elastic scattering). The magnitude of this fluorescence driven dip in the observed degree of polarization was also seen to be theoretically related to the fluorescence magnitude. The recent availability to us of the RayXP vector radiative transfer code (VRTE) for the coupled atmosphere ocean system now permits us to make complete simulations of the underwater polarized light field, using measured inherent optical properties (IOPs) as inputs. Based on these simulations, a much more comprehensive analysis of the fluorescence impact is now possible. Combining the results of these new simulations with underwater field measurements in eutrophic waters using our hyperspectral multi angle polarimeter, we verified the theoretical relationship. In addition, comparisons of VRTE simulations and hyperspectral polarized field measurements for various coastal water conditions permit retrieval of fluorescence magnitudes. Comparisons of these polarization based fluorescence retrievals with retrievals obtained using fluorescence height over baseline or Hydrolight scalar simulations, together with total unpolarized radiance measurements, show good agreement.