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 (UVImax) and daily erythemal radiant exposure (Her,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 Her,day (4.9% p. decade) and UVImax (3.2% p. decade) in Dortmund and Her,day (7.5% p. decade) and UVImax (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.