Multi-wavelength radiometry of aerosols designed for more accurate night sky brightness predictions

Abstract

Scattering by aerosols and gases cause a certain fraction of artificial light emitted upwards is redirected to the ground. Of all atmospheric constituents just the aerosols are most important modulators of night-sky brightness under cloudless conditions. Unlike most of the previous we highlight a crucial role of solar radiometry for determining the atmospheric optical depth before night-time observation is to be made. Aerosol optical depth at visible wavelengths extracted from the data measured provides then the information on size distribution or mean refractive index of aerosol particles that in turn are both necessary to make night sky brightness prediction more accurate. Therefore, combining daytime and night-time radiometry we can achieve accuracy much higher than ever before. This is due to significantly reduced uncertainty in aerosol properties.The aerosol data are retrieved from a new portable multi-wavelength optical analyzer that operates Ocean Optics spectrometer. The equipment provides the radiance data from 350 nm to 1000 nm with spectral resolution of 1 nm. Due to high sun radiance levels we use a system of mirrors each reducing the signal to about 4%, while keeping the integration time short. The minimum integration time of 3 ms allows for detection of direct sunlight. The system developed is sensitive to small changes in the aerosol system, while showing a good detection limit even under low turbidity conditions. The system performance is demonstrated in field experiment conducted shortly after front passage when most of aerosol particles is effectively removed by rain.