High UV-B Exposures in the Continental USA: Towards Realistic Short-term Exposure Regimes for Plant-effects Research

Understanding the biological effects of acute ultraviolet-B (UV-B) exposure requires understanding the typical intensity and duration of such exposures. The occurrence of high hourly biologically effective UV-B (UV-B(BE)) exposures was evaluated using two response functions (1971 and 2003) for the 1997-2002 summer growing seasons (May-August) at five locations across the continental United States. The frequency of occurrence of the upper 5% of all seasonal UV-B(BE) hourly exposures of 1 h to 4 h duration in 1 day and repeating the same exposure over consecutive days was evaluated. High hourly UV-B(BE) exposures occurred most frequently during June and July. There was a 30% frequency of occurrence of a day during the growing season with 2 h of hourly exposure in the upper 5% of UV-B(BE) (1971) values across any of the five locations studied. The frequency of occurrence of 2 h of UV-B(BE) (2003) exposure in the uppermost 5% of all observed hourly values was 14%. An approach and specific experimental square-wave enhancement exposure regimes that are consistent with the range of actual exposures and total ozone column (TOC) during the May through August period are provided. A 2 day high UV-B event with 2 h of high UV-B(BE) occurred at least 10% of all days in the growing season, representing a reasonable short-term high-exposure regime. Different exposure statistics and resulting enhancement regimes would likely result if only June and July were included in the analysis.