Quantifying the effects of a low-ozone event and shallow stratocumulus clouds on ultraviolet erythemal radiation exposure.

Meteorological and dosimetric ultraviolet (UV) erythemal radiation (UVER) measurements were performed in Didcot, England, on 6 and 7 April 2017. Both days were characterized by clear-sky conditions in the morning and the afternoon with development of shallow stratocumulus clouds (SSC) around noon. In addition, a low-ozone event occurred on 7 April characterized by a 34 DU (Dobson Unit) drop in total stratospheric ozone content. Compared to 6 April, the ozone mini-hole caused UVER increases of 2.67 standard erythema dose (SED) for diffuse and 4.32 SED for global radiation characterized by radiation amplification factors (RAF) of 1.62 and 1.52, respectively. The total global UVER dose reductions due to SSC coverage amount to 2.33 SED (6 April) and 2.81 SED (7 April). As innovation the RAF is decomposed into two parts, named cloud ozone factor (COF) and radiation amplification factor based on measured data (RAFm), to quantify the low-ozone event's effect and the SSC influence in independently modifying the UVER doses. Hereby, the weight of each of these two effects acting during the same low-ozone event is expressed by the new COF. In this case, the COF values range between -0.13 and -0.11 for diffuse UVER and -0.03 to -0.07 for the global UV and UV-B parts. A positive COF value (0.18) results for the global UV-A range.