Reproduction of night sky brightness variations in urban areas of Japan caused by aerosols, artificial ground-based light, and surface reflectance

To quantitatively assess the increasingly severe light pollution in recent years, approaches capable of estimating night sky brightness with high spatial-temporal precision is crucial. However, Falchi et al. (2016) global model did not adequately represent variations in environmental conditions such as aerosols. Therefore, this study developed a new regional-scale night sky brightness model capable of accounting for local characteristics. This model inputs aerosol, ground-based artificial light, and surface reflectance, and performs radiative transfer calculations that consider multiple scattering in the atmosphere and multiple reflections at the surface. Furthermore, it considers a point spread function based on the Monte Carlo method and calculates the night sky brightness as a hemispherical mean radiance. The results enabled a better reproduction of the spatial distribution of brightness in urban areas and provided estimates closer to observed values in Japan compared to Falchi et al. (2016). Fig. 1 The areas and their distribution of night sky brightness, calculated for seven urban areas in Japan.Additionally, analysis of long-term variations in seven large cities in Japan using this model suggests that night sky brightness generally correlates with population size while also being influenced by urban structure. Although no significant increasing trend was observed between 2013 and 2023, brightness decreased in many cities during the COVID-19 pandemic period, with contributions from both ground-based artificial light and aerosol changes indicated. This study provides a new assessment methodology, applicable not only within Japan but also extendable to regions worldwide, for quantitatively understanding the current state and variation factors of light pollution.