Mount Pinatubo

The 1991 plinian eruption of Mount Pinatubo (15N, 120E) produced 5-10 km of dacitic pumice fall and ignimbrite ( Scott et al., in press) and is arguably the second largest eruption of this century ( Self et al., in press). Most of the magma was erupted in a 9-hr climactic phase on June 15, culminating in the formation of a 2.5 km diameter caldera. Eruption columns exceeded an altitude of 35 km ( Lynch and Stevens, in press) and formed a giant umbrella cloud that injected 17x10 kg of SO into the stratosphere ( Bluth et al., 1992; Read et al., 1993), about twice the amount produced by the 1982 El Chichón eruption. This SO is believed to have come from a water-rich magmatic fluid that exsolved from the magma during pre-eruption evolution ( Westrich and Gerlach, 1992), rather than directly from the erupted melt. The SO formed about 25-30x10 kg of sulfate (HSO) aerosols, which produced the largest perturbation to the stratospheric aerosol layer since the eruption of Krakatau in 1883 ( McCormick and Veiga, 1992; Sato et al., 1993).

The aerosol cloud spread rapidly around the Earth in about three weeks and attained global coverage after about 1 year. The dense aerosol cloud caused dramatic decreases in the amount of net radiation reaching the earth's surface, producing a climate forcing two times stronger than the aerosols of El Chichón ( Angell and Korshover, 1983). Resultant climate effects included a 0.5-0.6C surface cooling in the northern hemisphere, equivalent to a hemispheric-wide reduction in net radiation of 4 Wm ( Dutton and Christy, 1992; Russell et al., 1993; Minnis et al., 1993) and a cooling of perhaps as much as -0.4C over large parts of the earth in 1992-1993 ( Hansen et al., 1992, 1993). The Pinatubo climate forcing was stronger than the opposing, warming effects of either the El Niño event or anthropogenic greenhouse gases in the period 1991-1993.

As a result of the high stratospheric aerosol loading, mid-latitude ozone concentrations reached their lowest recorded levels during 1992-1993. Startling decreases in ozone abundance and in the rates of ozone destruction were also observed over Antarctica in 1991 and 1992. Depletion of southern hemisphere ozone may have been due in part to the presence of Pinatubo aerosols but also to the extra aerosols generated by the Mount Hudson eruption in Chile during August 1991 ( Doiron et al., 1991; Barton et al., 1992). A sharp decrease in ozone at 9-11 km altitude (approximately at the tropopause) in the austral spring of 1991 was noted at the time of arrival of the Pinatubo and Mount Hudson aerosols ( Deshler et al., 1992; Schoeberl et al., 1993a). The southern hemisphere ``ozone hole'' increased in 1992 to an unprecedented 27x10 km in size, and depletion rates were observed to be faster than ever before recorded ( Brasseur, 1992; Hofmann et al., 1992, 1994), raising concern about the amount of biologically destructive ultraviolet radiation reaching the earth's surface ( Vogelmann et al., 1992; Smith et al., 1992).

Analysis of the June 1991 eruption of Mt. Pinatubo will provide a wealth of new information on the dynamics of large eruption plumes and on the emplacement of pyroclastic flow deposits. A new post-eruption hazard, secondary pyroclastic flows spawned from previously emplaced June 1991 ignimbrite during rapid erosion events, was documented ( Torres et al., in press). Heralding future trends, much information about the Pinatubo eruptions was obtained remotely from satellite-borne sensors. These studies include an analysis of plume structure ( Woods and Self, 1992; Koyaguchi and Tokuno, 1993) and the long-term trends of global optical depth ( Stowe et al., 1992), measurements of SO using the total ozone mapping spectrometer (TOMS) ( Bluth et al., 1992), other ultra-violet measurements ( McPeters, 1993) and the microwave limb sounder (MLS) ( Read et al., 1993), and an investigation of the dispersal of stratospheric aerosols via the stratospheric aerosol and gas experiment (SAGE) II ( Trepte et al., 1993) and the improved stratospheric and mesospheric sounder (ISAMS) ( Grainger et al., 1993).