COVID-19 lockdown contribution to spring surface solar irradiance record in Western Europe

Spring 2020 broke sun duration records across western Europe. The Netherlands recorded the highest surface irradiance since 1928, exceeding the previous extreme by 13 %, and the diffuse fraction of the total irradiance measured a record low percentage (38 %). The coinciding irradiance extreme and a reduction in anthropogenic pollution due to the COVID-19 measures triggered the hypothesis that cleaner-than-usual air contributed to the record. Based on analyses of ground-based and satellite observations and experiments with a radiative transfer model, we estimate a 1.3 % (2.3 W m$^{-2}$) increase in surface irradiance with respect to the 2010-2019 mean due to fewer aerosols in the atmosphere, and a 17.6 % (30.7 W m$^{-2}$) increase due to the exceptional dry and cloud-free weather conditions. The noticeable signal of cleaner air in the observed irradiance in western Europe suggests that larger COVID-19-related irradiance increases can be expected for more polluted regions.