Re-examining the effect of particle phase functions on the remote-sensing reflectance

Even though it is well known that both the magnitude and detailed angular shape of scattering (phase function, PF), particularly in the backward angles, affect the color of the ocean, the current remote-sensing reflectance (R_rs) models typically account for the effect of its magnitude only through the backscattering coefficient (b_b). Using 116 volume scattering function (VSF) measurements previously collected in three coastal waters around the U.S. and in the water of the North Atlantic Ocean, we re-examined the effect of particle PF on R_rs in four scenarios. In each scenario, the magnitude of particle backscattering (i.e., b_bp) is known, but the knowledge on the angular shape of particle backscattering is assumed to increase from knowing nothing about the shape of particle PFs to partially knowing the particle backscattering ratio (B_p), the exact backscattering shape as defined by β˜_p(γ≥90°) (particle VSF normalized by the particle total scattering coefficient), and the exact backscattering shape as defined by the χ_p factor (particle VSF normalized by the particle backscattering coefficient). At sun zenith angle=30°, the nadir-viewed R_rs would vary up to 65%, 35%, 20%, and 10%, respectively, as the constraints on the shape of particle backscattering become increasingly stringent from scenarios 1 to 4. In all four scenarios, the R_rs variations increase with both viewing and sun angles and are most prominent in the direction opposite the sun. Our results show a greater impact of the measured particle PFs on R_rs than previously found, mainly because our VSF data show a much greater variability in B_p, β˜_p(γ≥90°), and χ_p than previously known. Among the uncertainties in R_rs due to the particle PFs, about 97% can be explained by χ_p, 90% by β˜_p(γ≥90°), and 27% by B_p. The results indicate that the uncertainty in ocean color remote sensing can be significantly constrained by accounting for χ_p of the VSFs.