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Figure 1.8 Estimate of the mean trend (1980-96) using four instrument systems (SAGE, Umkehr, SBUV, and sondes) at northern mid-latitudes (thick line). The thin lines are the combined 1- and 2-ct uncertainties. (After SPARC [21], Figure 3.54.)

1.3.3 The eruption of Mount Pinatubo

The eruption of Mount Pinatubo, a volcano in the Philippines (15°N, 120°E), on June 15, 1991 injected -20 x 10': g (20 Mt) of SO, into the stratosphere [22,23]. This SO, is oxidized to sulfate, arid this in turn leads to the formation and growth of small liquid droplets comprised primarily of H,0 and HiSO,. These droplets, which are known as stratospheric sulfate aerosols, can perturb the chemical composition of the stratosphere two ways. First, they provide surfaces on which chemical reactions can take place. Second, they intercept and scatter solar and infrared radiation, which changes the heating of the stratosphere and the rates of photolytic reactions. The net result of the increase in aerosols after the eruption of Mount Pinatubo was a decline in column O, over most of the globe.

Figure 1.9 shows globally averaged column O, measurements between 1991 and 1993, the period 6 months before to about 2Vi years alter the eruption. Also shown are average, maximum, and minimum monthly averages for the period 1979-90. Column O, was very close to the average in 1991. Starting in early 1992, about 6 months after the eruption, column O, begins a noticeable decline. By late 1992, column O, was about 3% lower than pre-Pinatubo average column (),, a difference

1991 1992 1993

Figure 1.9 Column O, (DU) averaged between 65°N and 65°S versus month of year. Monthly averaged data for 1991, 1992, and 1993 are shown as the heavy solid line. The heavy dashed line is the monthly average for 1979—1990. The light dashed lines are the maximum and minimum monthly averages between 1979 and 1990. Data are a combination of version 7 Nimbus 7/TOMS and Meteoi/TOMS.

1991 1992 1993

Figure 1.9 Column O, (DU) averaged between 65°N and 65°S versus month of year. Monthly averaged data for 1991, 1992, and 1993 are shown as the heavy solid line. The heavy dashed line is the monthly average for 1979—1990. The light dashed lines are the maximum and minimum monthly averages between 1979 and 1990. Data are a combination of version 7 Nimbus 7/TOMS and Meteoi/TOMS.

that falls below the 1979-90 variability. By 1994 (not shown), column O, began recovering to its pre-eruption values. See also [24-26].

There is now a consensus that the low values of column O, in 1992-93 are causally related to the injection of SO, into the stratosphere by the 1991 eruption of Mount Pinatubo. We will discuss this further in Chapter 6.

We hope that this brief introduction has sparked an interest in stratospheric Ov In the rest of this book, our goal is to describe the chemical and physical processes that determine the distribution and abundance of O, in the stratosphere, as well as the chemical and physical processes that cause the Antarctic ozone hole, the mid-lali-tude trends, and the decline of Ot after the eruption of Mount Pinatubo. In so doing, we hope to give the reader a sense of the beauty and elegance of stratospheric O,.

Problems

1. Show how the constant 7.25 x 101S in Equation (1.3) is derived. What would the constant be if one expressed pressure in torr and wanted the density in mol cm "■'? And in g cm '?

2. Unit-mania. Determine the density in the form requested, given the pressure and temperature:

Form of density

Temperature

Pressure

molecules cm"'

-80°C

75 torr

kg m

0 0

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