Light pollution offshore: zenithal sky glow measurements in the Mediterranean coastal waters

Abstract

Light pollution is a worldwide phenomenon whose consequences for the natural environment and the human health are being intensively studied nowadays. Most published studies address issues related with light pollution inland. Coastal waters, however, are spaces of high environmental interest, due to their biodiversity richness and their economical significance. The elevated population density in coastal regions is accompanied by correspondingly large emissions of artificial light at night, whose role as an environmental stressor is increasingly being recognized. Characterizing the light pollution levels in coastal waters is a necessary step for protecting these areas. At the same time, the marine surface environment provides a stage free from obstacles for measuring the dependence of the skyglow on the distance to the light polluting sources, and validating (or rejecting) atmospheric light propagation models. We present a proof-of-concept of a gimbal measurement system that can be used for zenithal skyglow measurements on board of small boats and large vessels under actual navigation conditions. We report the results obtained in the summer of 2016 along two routes in the Mediterranean waters offshore Barcelona, travelling 9 and 31.7 km into the sea. The first route was carried out under atmospheric conditions comparable to the ones used by recently published models of anthropogenic sky brightness, and its results are highly compatible with the theoretical predictions. The results obtained in the second route, conducted under a clearer atmosphere, showed systematic differences that can be traced back to two expected phenomena, consequence of the smaller aerosol content: the reduction of the anthropogenic sky glow at short distances from the sources, and the slower rate of brightness decay with distance, which gives rise to an excess of brightness at large distances from the coastline.