Time evolution of the optical effects and aerosol characteristics of Mt Pinatubo origin from ground-based observations

The time evolution of the perturbations in aerosol spectral optical depths associated with the Mt. Pinatubo volcanic eruption of June 1991 is investigated using a ground-based multiwavelength solar radiometer at the tropical station, Trivandrum. The spectral optical depths of the volcanic aerosois deduced from the radiomeler data are inverted to retrieve the size distributions ofthe particles, from which the effective radius and columnar mass loading are estimated. Observations showed that during the initial phase (within one year after the eruption) the optical depths showed fluctuations with two peaks, one in November 1991 and another in February 1992 superposed over a very weak decreasing trend. During this period, the size distributions have been generally bimodal, with a secondary large particle mode occurring at similar to 0.75 mu m, and the columnar mass loading varied between about 170 and 110 mg m(-2). The optical depths and mass loading decreased with an e-folding rime of about 15 months. The effective radius is found to increase from similar to 0.35 mu m in September 1991 to similar to 0.60 mu m in October 1492 and then remain rather steady, while the mass loading decreased to reach near-background levels (within error limits) by early 1993. (C) 1996 Elsevier Science Ltd.