The aerosol effect on direct normal irradiance in Europe under clear skies

• The effect of spatiotemporal variability of AOD on direct normal irradiance in Europe under clear skies is studied.
• The attenuation of DNI from aerosols can reach values up to 35% and 45%, which corresponds to 4 and 6 kWh/m2 per day.
• DNI has increased during the recent years due to the decreasing tendency of AOD over most areas of Europe.
• Day-to-day differences on DNI (due to AOD changes) up to ±20% from the monthly mean can occur.

The effect of spatial and temporal variability of aerosol optical depth (AOD) on direct normal irradiance (DNI) under clear skies is studied, with the synergetic use of satellite and ground-based data as well as calculations from a radiative transfer model. The area of interest is Europe; data from May to September during 13 years (2000–2012) are analyzed. The aerosol effect on DNI is high in areas influenced by desert dust intrusions and intense anthropogenic activities, such as the Mediterranean basin and the Po Valley in Italy. In May, the attenuation of DNI from aerosols, over these areas, can reach values up to 35% and 45% respectively, which corresponds to 4 and 6 kWh m−2 per day. In most areas, even for periods with lower values of AOD, the attenuation of DNI is found to be around 20%, which corresponds to about 2–3 kWh m−2 less received DNI per day, compared to the corresponding value on an aerosol clean day. However, the DNI has increased during the recent years, due to the decreasing tendency of AOD over most areas of Europe. The increase is around 6–12%, which corresponds to an amount of 0.5–1.25 more kWh m−2 received per day, compared to a clean day. The percentage differences of daily DNI from the corresponding monthly climatological value reveals that day-to-day differences (due to AOD changes) from the monthly mean, by ±20%, can occur. The significance of the aerosol changes in Europe reveals the necessity for near real-time measurements or forecasts of AOD when reliable estimations of DNI are required.