Aerosol-cloud interaction and light pollution on selected astronomical observatories

The increasing availability of satellite remote sensing data has transformed site testing and monitoring, leading to extended satellite surveys to evaluate atmospheric conditions for existing and potential telescope sites. This thesis explores the application of satellite-based observations for assessing and monitoring site suitability, focusing on parameters such as aerosol optical depth (AOD), cloud cover fraction (CF), land surface temperature, and night sky brightness (NSB). One of the aims of this work is the evaluation of site stability and long-term trends of the considered parameters for 6 astronomical sites: Cima Ekar (Asiago Observatory, Italy), Cerro Armazones and La Silla (Chile), Mount Graham (Arizona, USA), Xinglong Station (China, site of the Large Sky Area Multi-Object Fiber Spectroscopic Telescope) and Khomas Highland (Namibia, site of the High Energy Stereoscopic System). The analyzed time series are mainly derived from satellite data (e.g. AOD, CF and NSB) retrievals for the period 2003-2024 over spatial grids of 1° x 1° centered on the selected sites. Firstly we have analyzed AOD and cloud fraction trends, seasonality and correlation. Long-term trends of AOD show either a marked reduction (at contaminated sites) or a constant trend. A strong positive correlation between AOD and cloud fraction is found for all the sites. The calculated Pearson coefficients are equal to 0.57 (Asiago), 0.68 (Armazones), 0.71 (H.E.S.S. site), 0.54 (La Silla), 0.82 (Xinglong), 0.69 (Mt. Graham). The seasonal difference in the daytime and nighttime cloud fraction and their trends are also studied. Concerning temperature, we found a clear increase at all the selected astronomical sites. A negative correlation between temperature and cloud fraction is found for 4 out of 6 sites. The calculated Pearson coefficients are equal to -0.64 (Asiago), -0.69 (Armazones), -0.86 (La Silla), -0.67 (Mt. Graham). Finally, NSB radiance measurements are analyzed to detect a possible dependence on aerosols. The dependence in terms of long-term linear trend is evident: a decrease in AOD leads to an increase of NSB radiance values measured by satellite. The lack of correlation between AOD and radiance monthly data suggests further analysis, also including ground measurements of the NSB. This work aims to underscore the complex role of aerosols in atmospheric processes and their impact on astronomical observations. Aerosols represent the variable part of atmospheric extinction; they affect solar irradiance and alter the propagation of nightlights. The work therefore focuses on the aerosol cloud-temperature interaction mainly in relation to site testing but also to possible multidisciplinary applications such as light pollution and climate change