4241LNOT07_S11_PP

4241LNOT07_S11_PP - Environmental Geochemistry, GLY...

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Environmental Geochemistry, GLY 4241/5243, © David Warburton, 2011 1 LECTURE 7 - OZONE LAYER: NATURAL DEPLETION AND CURRENT STATUS Note: Slide numbers refer to the PowerPoint presentation which accompanies the lecture. Natural ozone depletion, slide 1 here Natural processes contribute to ozone depletion in the stratosphere. Volcanic eruptions can emit large quantities of sulfur gases, principally SO 2 , and chlorine. Stratovolcanoes are particularly violent and may eject material through the troposphere into the stratosphere. Natural ozone depletion, slide 2 here 1997 Montserrat Eruption In 1982 the volcano El Chichón, in Mexico (17.3°N, 92.3°W), erupted 7 to 20 megatons of SO 2 (Brasseur and Granier, 1992) into the stratosphere. The temperature of the lower stratosphere warmed (Labitzke et al. 1983; Parker and Branscombe, 1983; and Pollack and Ackerman, 1983), a reduction in stratospheric ozone took place (Hofmann and Solomon, 1989), and a slight depression of surface temperatures probably occurred (Hansen et al. , 1992). The surface temperature depression was complicated by a contemporaneous El Niño event, which made resolution of the volcanic cloud effect from natural variability difficult to resolve. The magnitude of the ozone loss in the mid- latitudes was as great as 15%, compared with an ongoing decline of 5% per decade in recent times (Kerr, 1990). Although this decline was temporary, it points out the potential effects that volcanoes can have on stratospheric ozone levels. More recently Mount Pinatubo, in the Philippines (15.1°N, 120.4°E), injected between 10 and 30 million tons of SO 2 into the stratosphere in June 1991(Brasseur and Granier, 1992; Kerr, 1990). Since the latitude of the two volcanoes is similar, the atmospheric distribution should be similar. Natural ozone depletion, slide 3 here
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Environmental Geochemistry, GLY 4241/5243, © David Warburton, 2011 2 7-1 Sulfur dioxide is converted to sulfuric acid droplets within a month. Equation 7-1 summarizes the total reaction that occurs; it is almost certainly a series of reactions. Sulfuric acid condenses into small particles, called aerosols. Generally these range in size from 10 -3 to 100 : (Bricard, 1977, p.313). Particles of this size are expected to remain in the atmosphere one to three years. Natural ozone depletion, slide 4 here Satellite observations of aerosol optical depth clearly reveal the effects of both volcanic eruptions. Peak optical depths occurred during August 1991 at MLO (Mauna Loa, Hawaii) (19.5 / N), but not until April 1992 at BRW(Point Barrow, Alaska) (71.3 / N). Stone et al. (1994) suggest that the formation of the north polar vortex during autumn 1991 prevented the penetration of the Pinatubo aerosols into that region until its breakup the following spring. Natural ozone depletion, slide 5 here “Pinatubo aerosols were removed from the stratosphere at a slower rate in the polar regions than in the tropics. The estimated e-folding time is 14 months at Barrow, Alaska compared with 10 months at Mauna Loa, Hawaii. The decay appears to be especially slow in the Arctic when compared with estimates that range from 8 to 10 months for other post-volcanic periods. The prolonged
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4241LNOT07_S11_PP - Environmental Geochemistry, GLY...

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