Biologically effective daily radiant exposure for erythema appearance, previtamin D₃ synthesis and clearing of psoriatic lesions from erythema biometers at Belsk, Poland, for the period 1976–2023

Abstract. A long-term series of exposures to solar ultraviolet (UV) radiation is required to assess the risks and benefits of radiation on different human biological processes. However, homogenisation of the amount of biologically effective solar energy reaching the Earth's surface over long periods (i.e. energy weighted according to the sensitivity of the selected biological process to solar radiation) is challenging due to changes in measurement methods and instruments. This paper presents the world's longest homogenised time series of biologically effective daily radiant exposures (DRE) from regular monitoring with different erythemal biometers (EB) operated at the Central Geophysical Laboratory of the Institute of Geophysics, Polish Academy of Sciences (IG PAS), Belsk (20.79° E, 51.84° N) from 1 January 1976 to 31 December 2023. The following biological effects were considered: the appearance of erythema, cutaneous synthesis of previtamin D₃, and clearing of psoriatic lesions. The data for the latter two biological effects are estimated based on the proposed method of using EB measurements to calculate other non-erythemal DRE. The following broadband erythemal radiometers were used in the monitoring: Robertson-Berger (1975−1992), Solar Light model 501 (1993−1994 with #927, 1995−2013 with #2011) and Kipp-Zonen UV-AE-T #30616 from 5 August 2013 to the present. From 1976 to 2013, the homogenisation procedure consisted of comparing the measured erythemal DRE and UV index (erythemal irradiance at noon) with the corresponding synthetic values from simulations using a radiation transfer model. Between 2014 and 2023, the raw data were compared with data from a collocated reference instrument, the Brewer Mark II #64 spectrometer. Such comparisons resulted in a set of multipliers that were applied to the raw EB measurements. Two different versions of the homogenisation method were applied (for erythemal DRE and UV index with different selection of cloudless days), and three regression models were constructed for the erythemal data based on total column ozone, aerosol optical depth and global irradiance clearness index. Linear trends calculated from reevaluated and reconstructed time series (a total of seven time series were considered) showed a statistically significant increase in erythemal annual and summer (June to August) radiant exposures of about 6 % per decade over the period 1976−2005. Thereafter, no trend was observed. The same trend estimates were found for all biological effects considered. The raw and reevaluated data are made freely available via the following repository: https://doi.org/10.1594/PANGAEA.972139 (Krzyścin et al., 2024). An additional version of the reevaluated data, together with the corresponding clear sky and proxy data used in the UV data reconstruction, is archived in the IG PAS Data Portal: https://doi.org/10.25171/InstGeoph_PAS_IGData_Biologically_Effective_Solar_Radiation_Belsk_1976_2023 (Krzyścin, 2024).