Evolution of the Aerosol Surface Area from 1985 to 1997

As discussed in Chapter 2, a volcanic eruption can significantly increase the stratospheric aerosol surface area density (SAD) of the stratosphere. Figure 6.1 shows two views of the evolution of the lower stratospheric SAD between 1985 and 1997. Between 1985 and mid-1991 the lower-stratospheric SAD gradually decreased from ~3 to ~1 pm' cm as the aerosol from the eruptions of El Chichon in 1982 and Nevado del Ruiz in November 1985 was flushed from the stratosphere. The eruption of Nevado del Ruiz can be seen in both parts of Figure 6.1 as the increase in the SAD in late 1985 and early 1986 over the equator. In mid-1991, Mount Pinatubo erupted, and the effects on the stratosphere were dramatic [196]. The aerosol SAD

Figure 6.1 (a) Aerosol SAD (¬°Jirr cm ') at 19.5 km. The black region in late 1991 and early 1992 indicates regions where aerosols were so thick that there were no measurements, (b) Time series of the aerosol SAD at 19.5 kni for three different latitude bands. The data in both top and bottom plots are zonally and seasonally averaged 1194 ]. The tick marks in the plots are January 1 of each year.

Figure 6.1 (a) Aerosol SAD (¬°Jirr cm ') at 19.5 km. The black region in late 1991 and early 1992 indicates regions where aerosols were so thick that there were no measurements, (b) Time series of the aerosol SAD at 19.5 kni for three different latitude bands. The data in both top and bottom plots are zonally and seasonally averaged 1194 ]. The tick marks in the plots are January 1 of each year.

increased within months to about 30 times its abundance prior to the eruption [45]. In the years since the eruption, the aerosol SAD has decreased, and, by 1999, the aerosol SAD was nearing the background.

The maximum enhancement of the SAD from a volcanic eruption generally occurs in the lower stratosphere (e.g. see Figure 2.3) because the vast majority of the volcanic effluent is only energetic enough to reach this level of the stratosphere.

In addition to the changes in the aerosol SAD, the size distribution and the related properties of the particles (e.g. effective radius, optical depth) making up the aerosol cloud also varied after the eruption of Mount Pinatubo [48,197]. No other species of importance to stratospheric chemistry, in particular HQ, appear to have been injected into the stratosphere by the eruption [198,199],

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