The visible-to-short-wave-infrafred spectrum of skylight polarization
Skylight becomes partially polarized when sunlight is scattered in the atmosphere. The resulting degree of linear polarization (DoLP) depends on the optical wavelength, atmospheric properties (especially aerosol content), and surface reflectance. The degree of linear polarization for a clear sky was calculated previously for the visible-to-near-infrared (VNIR) spectral range using a successive-orders-of-scattering radiative transfer model and the calculations were validated through comparison with measurements from an all-sky polarization imager. Results from that study showed that VNIR skylight polarization in the visible to the near-infrared spectrum could trend upward, downward, or even have unusual spectral discontinuities that arose because of sharp features in the optical properties of underlying surface and atmospheric aerosols. However, the results were limited to wavelengths below 1 microns from a lack of data at longer wavelengths. This report describes skylight polarization calculations from 0.35 microns to 2.5 microns (visible to SWIR). Inputs to the model included spectral extrapolations of aerosol properties retrieved from a ground-based solar radiometer and measurements of spectral surface reflectance from a hand-held spectrometer. The simulations were run for different environments: a Rayleigh-scattering environment (no aerosol optical depth and no surface reflectance), varied aerosols over a constant-reflectance surface, spectrally constant aerosols over varied surfaces, and a set of more realistic environments that coupled different measured surface reflectance spectra with actual aerosol conditions. Results showed skylight polarization dependence on aerosols and surface reflectance when one element was added, changed, or taken out of an environment. The results were also compared against skylight polarization measurements taken with a SWIR-MWIR polarimeter. Polarization results in the SWIR were highly dependent on the aerosol size distribution and the resulting relationship between the aerosol and Rayleigh optical depths. Once the aerosol optical depth became greater than the Rayleigh optical depth, the predicted polarization deviated significantly from Rayleigh scattering theory. As aerosol optical depths increased, the degree of linear polarization spectrum generally decreased with wavelength at a rate dependent on the aerosol size distribution. Unique polarization features in the modeled results were attributed to the surface reflectance and the skylight DoLP generally decreased as surface reflectance increased.