  Spring 2020 broke sunshine duration records across western Europe. The
Netherlands recorded the highest surface irradiance since 1928, exceeding the
previous extreme of 2011 by 13 %, and the diffuse fraction of the irradiance
measured a record low percentage (38 %). The coinciding irradiance extreme and
a reduction in anthropogenic pollution due to 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 a low median
aerosol optical depth, and a 17.6 % (30.7 W m$^{-2}$) increase due to several
exceptionally dry days and a very low cloud fraction overall. Our analyses show
that the reduced aerosols and contrails due to the COVID-19 measures are far
less important in the irradiance record than the dry and particularly
cloud-free weather.

Atmospheric and Oceanic Physics