Observing and simulating the polarization neutral point dependence on atmospheric turbidity

Atmospheric aerosols influence the polarization signature of the sky by scattering or absorbing skylight. Neutral points are features in the sky’s polarization where single and multiple scattering processes incoherently add, yielding a degree of polarization equal to zero. The zenith angle of these features is highly sensitive to atmospheric turbidity. DeLeon and Kupinski (2025) demonstrated a method for tracking the Babinet neutral point, located above the Sun, using the Ultraviolet Linear Stokes Imaging Polarimeter (ULTRASIP). This study furthers that work by analyzing the Babinet neutral point zenith changes across multiple days under varying atmospheric conditions, quantified by data from an Aerosol Robotic Network (AERONET) ground station. The AERONET data is used with the Generalized Retrieval of Atmosphere and Surface Properties (GRASP) open-source radiative transfer model to obtain simulated Babinet neutral point zenith angle changes. Comparisons of ULTRASIP field observations and AERONET-based GRASP simulations of the Babinet neutral point diurnal trajectories are presented to assess sensitivity to atmospheric turbidity. Both observations and simulations reproduce the expected diurnal trend of the Babinet neutral point moving farther from the Sun at lower solar zenith angles, although simulations systematically overestimate the zenith separation. Linear fits to the diurnal trajectories show moderate correlations between observed and simulated slopes (ρ = 0.522) and intercepts (ρ = 0.439), which increase when high aerosol loading days are excluded. Variations in slope and intercept correspond to changes in aerosol optical depth, single scattering albedo, and particle size distribution. These results demonstrate that Babinet neutral point tracking offers a quantifiable, sensitive approach for monitoring atmospheric turbidity and capturing day-to-day aerosol variability.