Study and characterization of light pollution in Catalonia

Abstract

The term light pollution refers to different harmful effects produced by artificial light sources. This phenomena affects observational astronomy, the flora and fauna of the species adapted to the natural cycles of light and it also contributes to climate change. With this thesis, we aim to contribute to the understanding of the physics behind light pollution and to provide strategies to improve and ease its assessments. We established two different lines of action: measuring and modelling. The objectives of the measuring branch are to characterise the quality of the night sky in large areas within Catalonia and to study in detail the sky over key locations from the territory. The first objective was addressed using dynamic Sky Quality Meter (SQM) measurements of the zenith sky brightness. With this methodology we covered more than 3500 km in a total of 26 nights distributed in four campaigns. We created a processing software to deal with the resulting data and to ease and homogenize the process for obtaining reliable zenith sky brightness maps. The resulting maps provided reliable information about the quality of the sky in vast areas of Catalonia. For the second objective, we used Digital Single Lens Reflex images processed with Sky Quality Camera software. The resulting all-sky maps are useful to point out the most polluted directions and as a consequence the most probable sources of light pollution. Eight different locations have been assessed using this methodology. Light pollution models estimate the sky brightness by reproducing the interactions between the electromagnetic radiation and the constituents of the atmosphere. They have been validated as a helpful tool for assessing the night sky brightness and to understand light pollution at a physics level. In particular we worked with the Illumina light pollution numerical model. The objectives of the modelling branch are to derive a tool to point out the principal sources of light pollution affecting a particular location, to find a methodology to reproduce measurements that can be used as a prediction tool and to define a process to assess very large areas. For each objective we created a different simulation product: contribution maps, all-sky brightness maps and zenith sky brightness maps of large areas. The spectral range studied goes from 350 nm to 830 nm covering the Johnson-Cousins B, V and R bands. Contribution maps show the percentage to the artificial radiance received that was emitted by each source. They are a useful tool to sort the light pollution sources that are affecting the sky over a particular location. They are also useful for deciding which sources to include in all-sky brightness simulations. All-sky brightness maps derived with Illumina are a reliable tool to reproduce all-sky brightness measurements. This kind of maps can be used for predicting the changes in the sky that would be produced by changes in the lighting systems. We have used Illumina to create a zenith sky brightness map of Catalonia in B and V bands with a sampling grid of 5 km. It provides an accurate picture of the light pollution current situation of the country. The maps reveal a complex pattern, in agreement with the obtained in the measurements campaign, linked to the heterogeneity of the territory regarding population distribution and orography. The principal sources of light pollution are Barcelona and Tarragona metropolitan areas. The coastal line, in general, is very polluted. The darkest regions of the country are the Pyrenees, especially in the west, the Ports de Beseit natural park in the south west of the country, and the area around Montsec mountain range in the west.

El terme contaminació lumínica fa referencia als diferents efectes perjudicials derivats de les fonts de llum artificial. Aquest fenomen afecta l’astronomia observacional, la salut de les espècies que estan adaptades als cicles naturals de llum i a més a més contribueix al canvi climàtic. En aquesta tesi, l’objectiu és contribuir en l’estudi de la contaminació lumínica a nivell físic i proporcionar estratègies per millorar i facilitar la seva avaluació. Per assolir aquest objectiu hem establert dues línies de treball diferents: mesures i modelització. A la branca de mesures hem estudiat el zenit en grans territoris utilitzant mesures dinàmiques amb un fotòmetre SQM. Aquesta metodologia ens ha permès recórrer més de 3500 km en un total de 26 nits distribuïdes en quatre campanyes anuals. També hem estudiat tot el cel en vuit localitzacions prenent imatges amb càmeres comercials (DLSR) i processant-les amb el software Sky Quality Camera . Els models de contaminació lumínica reprodueixen les interaccions entre la radiació electromagnètica i els components de l’atmosfera per estimar la brillantor de cel. En particular hem treballat amb el model numèric de contaminació lumínica Illumina. L’hem utilitzat per crear tres tipus de productes diferents: mapes de contribució que mostren el percentatge de la llum artificial total que prové de cada font; mapes de brillantor de tot el cel i mapes de brillantor zenital de grans àrees. Catalunya té un patró de brillantor del cel complex, degut a la heterogeneïtat en la distribució de la població i en l’orografia del territori. Les principals fonts de llum són les àrees metropolitanes de Barcelona i Tarragona. La línia de costa està molt contaminada en general. Les regions més fosques del territori són el Pirineu, especialment a l’oest, el parc natural dels Ports de Beseit al sud-oest i l’àrea al voltant de la serra del Montsec.