Characteristics of the Pinatubo aerosol cloud

Abstract

Optical depths at visible and infrared wavelengths obtained in Tucson, Arizona before and after the Pinatubo eruption in June 1991 have been used to investigate the characteristics of the stratospheric aerosols due to the Pinatubo eruption. The intrusion of the Pinatubo aerosols over Tucson first occurred on July 26, 1991 when the spectral optical depth values rose to two to four times their normal values. In general, there was a pattern of increase between June 1991 and April 1992, and a gradual decrease after April 1992. The stratospheric Pinatubo aerosol in April 1992 was characterized by a typical columnar total number density on the order of 8.78 x 106 in the size range of 0.2-0.7 μm. The total number density decreased to the order of 9.28 x 105 by April 1994. Simulations of the size distribution using a simple polydisperse coagulation and fallout model showed that both of the processes played a very important role in the evolution and transport of the particles in the interval from April 1992 to March 1993. A strong seasonal variation was observed in the aerosol optical depth data. The values are higher in the winter and spring and lower in the summer and fall. This variation is explained by more effective transport of particles from the tropics poleward in the winter and spring than in the summer and fall. We also observed that there was a reduction in stratospheric ozone associated with the Pinatubo aerosols, possibly because of the extra sites available for heterogeneous chemical reactions. The reduction was more noticeable in the spring and summer than in other seasons. The magnitude of the ozone reduction was in a good agreement with other studies.