COMPARATIVE LIDAR STUDY OF THE OPTICAL, GEOMETRICAL, AND DYNAMICAL PROPERTIES OF STRATOSPHERIC POST-VOLCANIC AEROSOLS, FOLLOWING THE ERUPTIONS OF EL-CHICHON AND MOUNT-PINATUBO

The spatiotemporal evolution of aerosols formed from precursors injected into the stratosphere by major volcanic eruptions, such as those of Fl Chichon in 1982 and Mount Pinatubo in 1998, has been studied using a ground-based lidar system located at the Observatoire de Haute-Provence (OHP) in southern France (44 degrees N, 5 degrees E). From the inversion of the lidar signals the optical, geometrical and dynamical properties of the particles have been determined as a function of time after the eruption. In immediate post-volcanic conditions, when the optical thickness of particles in the stratosphere is largely enhanced, an estimate of the aerosols backscatter phase function has been evaluated directly from the lidar measurements, using a size-distribution model adjusted to in situ balloon measurements. The precision of this determination lies in the +/-15% range. Values of the mean radius of the particles, of their integrated content, surface areas, and sedimentation velocities are then derived from the systematic lidar measurements performed at OHP. These values are compared for the two major volcanic eruptions which have occurred over the last decade. Although the injection of sulphur dioxide was twice as large for the Mount Pinatubo eruption as compared to the El Chichon case, the diffusion of the cloud in the two hemispheres due to the interaction of the particular phase of the quasi-biennal oscillation with several other dynamical processes at the time of the eruption, led to the observation of similar values for the aerosol content over the Observatoire de Haute-Provence in the months just following the two events. However, the residence time of the particles in atmospheric layers below 20 km are 4 months longer after the Mount Pinatubo eruption, caused by the observed difference in the initial vertical distribution of the aerosol cloud.