Dimming in Iran since the 2000s and the potential underlying causes

This study investigates the spatio-temporal variations of global solar radiation and its relationships with cloud cover (CC) during 1998–2015 in Iran, where no previous information about the variations of solar radiation based on direct measurements exists. The variations in the mentioned variables were detected using a modified Mann–Kendall trend test and the magnitudes of trends were estimated by using the Theil–Sen's slope estimator method. A widespread dimming with a magnitude of about −4.3 and −1.7% per decade is found under all- and clear-sky conditions in Iran since the 2000s (median over nine observation sites). A significant increase in CC was observed at most of the stations studied, at 20.0% per decade (median over nine sites). The decrease in the amount of all-sky global solar radiation seems to have occurred as a combined effect of aerosol and CC increase over Iran. However, in the south of Iran and at the station Tehran dimming appears to be mostly due to an increase in the amount of aerosol loadings.