Changes in surface shortwave solar irradiance from 1993 to 2011 at Thessaloniki (Greece)

ABSTRACT The variability of shortwave downward solar irradiance (SDR) received at Earth's surface over Thessaloniki, Greece for the period 1993–2011 is investigated, focusing on the effects from the aerosols variability on the irradiance trends derived for different solar zenith angles (SZA). Linear trends have been calculated for the entire dataset, for cloud‐free cases, and for different SZAs, separately for each season. The global upward trend in SDR after 1990 (0.33% year−1) is reconfirmed and is found to depend strongly on SZA, ranging from ∼0.1 to +0.6% year−1. The long term changes in aerosols in conjunction with the local aerosol patterns result in differences of up to 0.1% year−1 in the derived trends in SDR between morning and afternoon hours. Finally, based on the analysis of the cumulative sums of the differences in monthly averages of SDR from the long term mean we report signs of a slowdown in the upward trend in SDR during the beginning of the 2000s.