Increasing solar UV radiation in Dortmund, Germany: data and trend analyses and comparison to Uccle, Belgium

Increasing solar ultraviolet radiation (UVR) can raise human exposure to UVR and adversely affect the environment. Precise measurements of ground-level solar UVR and long-term data series are crucial for evaluating time trends in UVR. This study focuses on spectrally resolved data from a UVR measuring station in Dortmund, Germany (51.5° N, 7.5° E, 130 m a.s.l.). After a strict quality assessment, UV data, such as the daily maximum UV Index (UVI_max) and daily erythemal radiant exposure (H_er,day) values, were analyzed concerning monthly and annual distribution, frequency, occurrence of highest values and their influencing factors. An advanced linear trend model with a flexible covariance matrix was utilized and applied to monthly mean values. Missing values were estimated by a validated imputation method. Findings were compared to those from a station in Uccle, Belgium (50.8° N, 4.3° E, 100 m a.s.l.). Parameters possibly influencing trends in both UVR and global radiation, such as ozone and sunshine duration, were additionally evaluated. The 1997-2022 trend results show a statistically significant increase in monthly mean of H_er,day (4.9% p. decade) and UVI_max (3.2% p. decade) in Dortmund and H_er,day (7.5% p. decade) and UVI_max (5.8% p. decade) in Uccle. Total column ozone shows a slight decrease in the summer months. Global radiation increases similarly to the UV data, and sunshine duration in Dortmund increases about twice as much as global radiation, suggesting a strong influence of change in cloud cover. To address health-related consequences effectively, future adaptation and prevention strategies to climate change must consider the observed trends.