Ambient Gamma Radiation as an Atmospheric Indicator in a Remote Oceanic Island Environment: Long-Term Variability and Meteorological Controls

Ambient gamma radiation is a key component of environmental radiation monitoring and is strongly modulated by atmospheric and meteorological processes. This study presents a long-term analysis of near-surface gamma radiation measured in Ponta Delgada (São Miguel Island, Azores), integrating continuous observations from the Portuguese National Alert Network for Environmental Radioactivity (RADNET) with meteorological data. The dataset spans more than a decade and includes a documented instrumental upgrade in 2020, which introduced enhanced sensitivity and radionuclide identification capability. Results reveal pronounced variability across daily, seasonal, and interannual timescales. Stepwise level shifts are identified in 2016 and 2020, associated with operational and instrumental modifications, respectively, rather than with changes in environmental radioactivity. Seasonal analysis shows higher gamma radiation values during autumn and winter and lower values in late spring and summer, consistent with precipitation-driven washout and boundary-layer dynamics. Generalized Additive Models (GAMs) highlight precipitation, wind speed, and relative humidity as dominant meteorological drivers acting through non-linear relationships. Overall, the results support the use of ambient gamma radiation as an atmospheric indicator of boundary-layer processes and meteorological modulation in remote maritime environments, extending its role beyond routine environmental surveillance.