Aerosol radiative effects under clear skies over Europe and their changes in the period of 2001-2012

In the study, the clear-sky aerosol radiative effects have been quantified at 52 European sites using empirical and radiative transfer modelling approaches. Furthermore, the trends of the aerosol radiative effects are also determined for the period of 2001-2012. The aerosol radiative effects are simulated using the Mesoscale Atmospheric Global Irradiance Code radiation code with aerosol and water vapour climatology. The annual mean of the surface aerosol radiative effects in clear-sky situations over Europe is -7.1 +/- 2.9Wm(-2). The trends of the aerosol radiative effects are derived assigning radiative effects to monthly Moderate Resolution Imaging Spectroradiometer aerosol optical depth time series applying linear fitting. In the indirect approach, the clear-sky situations are selected from daily surface solar radiation observations from the Word Radiation Data Centre, and the trends of the aerosol radiative effects are delimitated in these clear-sky radiation time series. The two approaches give good fit, based on the direct approach the annual trend of the aerosol radiative effects on clear-sky solar surface radiation is -4.41Wm(-2) per decade for the period of 2001-2013, while in the case of the indirect approach, this trend is found to be -4.46Wm(-2) per decade. The seasonal trends of the aerosol radiative effects vary between -7.57 and -0.17Wm(-2) per decade indicating stronger decreases in summertime. The results are presented for four sub-regions of Europe detecting negative changes in all cases with a stronger decrease in the central, north-eastern and southern part of the continent. The trends of the aerosol radiative effect have the same magnitude as the trends detected in clear-sky solar surface radiation, and are an order of magnitude higher than the radiative effects of water vapour (0.20Wm(-2) per decade).