Evolution of solar radiative effects of Mount Pinatubo at ground level

The field research reported here contributes to the documentation on the effect of stratospheric aerosols produced by the 1991 eruption of Mount Pinatubo. Using ground-based data obtained at 2 radiometric stations, located at very different latitudes (Estonia and Spain), we have studied the evolution of the Pinatubo eruption effects. Following the eruption of Mount Pinatubo there is a significant reduction in direct solar radiation, of about 10% in Almenria (Spain) and 7% in Tiirikoja (Estonia). The maximum reduction, of about 15% and 9% respectively, is measured during the 1991-92 winter. The aerosol optical depth in Almeria presents, aside from the seasonal dependence, evident influences due to the volcanic aerosol cloud. We have tried to isolate this last effect. As a result, we have found that the 1991/1992 winter presented the maximum volcanic effects, with a decay along 1992 and a recovery in the 1992/1993 winter. These results are in agreement with more sophisticated studies of the aerosol cloud effects. By the 1993-1994 winter, our analysis shows evidence of a recovery of pre-eruption conditions confirmed by the aerosol optical depth behaviour during 1994. The analysis of Tiirikoja data set using an atmospheric integral transparency coefficient leads to similar results. The volcanic aerosol effect shows an exponential decay in both locations estimated at 8.6+/-1.9 months for Almeria and 8.9 +/- 3.5 months for Tiirikoja.