9-03-2 Measured Ozone Depletion

9-03-2 Measured Ozone Depletion - Measured Ozone Depletion...

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Measured Ozone Depletion Global Ozone After carefully accounting for all of the known natural variations, a net decrease of about 3% per decade for the period 1978-1991 was found. This is a global average over latitudes from 66 degrees S to 66 degrees N (i.e. the arctic and Antarctic are excluded in calculating the average). The depletion increases with latitude, and is somewhat larger in the Southern Hemisphere. Over the US, Europe and Australia 4% per decade is typical; on the other hand there was no significant ozone loss in the tropics during this period. The depletion is larger in the winter months, smaller in the summer. The annual and long-term trends at Caribou, ME. Between 1991 and 1993 ozone depletion appeared to accelerate. Satellite and ground- based measurements showed a remarkable decline in stratospheric ozone for 1992 and early 1993, a full 4% below the average value for the preceding twelve years and 2- 3% below the lowest values observed in the earlier period. By February 1994 ozone over the United States had recovered to levels similar to 1991. Sulfate aerosols from the July 1991 eruption of Mt. Pinatubo are the most likely cause of the exceptionally low ozone in 1993; these aerosols can convert inactive "reservoir" chlorine into active ozone-destroying forms, and can also interfere with the production and transport of ozone by changing the solar radiation balance in the stratosphere. The rapid ozone loss in 1992 and 1993 was a transient phenomenon, superimposed upon the slower downward trend identified before 1991. Polar Ozone Polar regions reflect the greatest changes in ozone concentrations, especially the South Pole. The topography of Antarctica is such that a stagnant whirpool of extremely cold stratospheric air forms over the region during the long polar night. The air stays within this polar vortex all winter, becoming cold enough to allow the formation of polar stratospheric clouds. f The Antarctic Ozone Hole The springtime Antarctic Ozone Hole was first observed by ground-based measurements by the British Antarctic Survey from Halley Bay on the Antarctic coast, during the years 1980-84. (At about the same time, an ozone decline was seen at the Japanese Antarctic station of Syowa; this was less dramatic than those seen at Halley since Syowa is about 1000 km further north, and did not receive as much attention.) With hindsight, one can see the hole beginning to appear in the data around 1976, but it grew much more rapidly in the 1980's. For the past two decades or so, ozone levels over Antarctica have fallen to abnormally low values between August and late November. At the beginning of this period, ozone levels are already low, but instead of slowly increasing as the light comes back in the spring, they drop. In the lower stratosphere, between 9 and 12 miles (15 and 20 km) altitude about 95% of the ozone is destroyed. Above 15 miles (25 km) the decreases are small and the net result is a thinning of the ozone layer by about 50%. In the late
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9-03-2 Measured Ozone Depletion - Measured Ozone Depletion...

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