A comparison of ground level solar radiative effects of recent volcanic eruptions

The aerosols injected into the lower stratosphere by the El Chichón and Mt. Pinatubo eruptions have increased the scattering of solar radiation back to space, thus, leading to a decrease in the solar radiation input to the Earth System. In this work we study the evolution in time of the volcanic effects of El Chichónʹs eruption using broadband solar radiation measured at two radiometric stations located in northern mid and high latitudes: Madrid (40.42°N, 2.75°S) and Tiirikoja (58.87°N, 26.97°E). Further, we compare the radiative effects produced by the Mt. Pinatubo eruption at Madrid with those detected at a close radiometric station located in Almería (36.83°N, 2.41°S) that has been a matter of a previous study. After the El Chichón eruption the aerosol optical depth presented a maximum relative anomaly about 11% in Madrid. The radiative effects of the Mt. Pinatubo are responsible of an increase in aerosol optical depth that reached 11% at Madrid and 12% at Almería. The analysis of the time series of relative anomalies in aerosol optical depth indicates that the anomaly produced by the El Chichón eruption decreased with an average 1/e-decay time around 7±1 months. This e-folding decay is faster than that deduced by the authors in the analysis of the Mt. Pinatubo effect at Almería (36.83°N, 2.41°S). Although the El Chichón and Mt. Pinatubo eruption produced similar radiative effects in the initial stages at different mid-latitude stations, the persistence of the radiative influence of Mt. Pinatubo is greater than that of El Chichón. Due to the increase in aerosol optical depth associated with the El Chichón eruption we find at Madrid a reduction in solar direct irradiance of about 10%, while solar diffuse irradiance presents an increase about 33%. Finally, the effect observed on the global solar radiation is a reduction about 3.6%, giving an indication of the radiative cooling provided by the volcanic aerosol.