Southern Hemisphere, ozone hole

British Antarctic Survey at Halley Bay discovers Southern Hemisphere ozone hole. 

The catalyzed decomposition of ozone is known to be responsible for the ozone hole (Figure A) that develops in Antarctica each year in September and October, at the end of winter in the Southern Hemisphere. No ozone is generated during the long, dark Antarctic winter. Meanwhile, a heterogeneous reaction occurring on clouds of ice... 

Ozone depletion is by no means restricted to the Southern Hemisphere. In the extremely cold winter of 1994-1995, a similar "ozone hole" was found in the Arctic. Beyond that, the concentration of ozone in the atmosphere over parts of Siberia dropped by 40%. 

The ozone (03) layer aver the southern hemisphere stratosphere in August 2007. The bar at the bottom indicates the color-coding used to indicate the thickness of the layer. The thickness is measured in Dobsons (= 0.01 mm thick). The normal ozone layer for the stratosphere is 360 Dobsons (color coded as green). The ozone "hole" shown in pink is 200-220 Dobsons. The "hole" will increase when another reading is taken in September. 

Mario Molina and Sherwood Rowland used Crutzen s work and other data in 1974 to build a model of the stratosphere that explained how chlorofluorocarbons could threaten the ozone layer. In 1985, ozone levels over Antarctica were indeed found to be decreasing and had dropped to the lowest ever observed by the year 2000, the hole had reached Chile. These losses are now known to be global in extent and it has been postulated that they may be contributing to global warming in the Southern Hemisphere. 

FIGURE 20.29 This false color image shows total stratospheric ozone amounts over the southern hemisphere for September 24, 2006, as recorded by the Ozone Monitoring Instrument (OMI) mounted on the Aura spacecraft. The dramatic depletion of the ozone layer over Antarctica is revealed with the help of the false color scale at the bottom of the figure. Ozone amounts are commonly expressed in Dobson units 300 Dobson units is a typical global average over the course of a year. The size of the Antarctic ozone hole was near a record high and the levels of ozone near a record low on this date. 

A computer-generated image of part of the Southern Hemisphere on October 17, 1994, reveals the ozone hole" (black and purple areas) over Antarctica and the tip of South America. Relatively law ozone levels (blue and green areas) extend into much of South America as well as Central America. Normal ozone levels are shown in yellow, orange, and red. The ozone hole is not stationary but moves about as a result of air currents. (Courtesy NASA)... 

Based on measurements of the total column ozone content of the atmosphere from the ground as well as from satellites, a consistent picture of the current loss of stratospheric ozone can be derived. The most recent results are discussed in ref. [3]. Relative to the values in the 1970 s, the ozone loss at the end of the 1990 s is estimated to be about 50% in the Antarctic spring, where the ozone hole appears every year, and about 15% in the Arctic spring. In the mid-latitudes of the Southern hemisphere the loss is about 5% all the year round, while in the Northern hemisphere it is about 6% in winter/spring and about 3% in sum-mer/fall. No significant trend in ozone has been found in the Equatorial regions. In the second half of the 1990 s relatively little change in ozone has been observed in the mid-latitudes of both hemispheres. 

The amount of ozone depletion observed at both northern and southern mid-latitudes is considerably greater than that implied by a one-time end-of-winter dilution process (see e.g., Sze et al., 1989 Prather et al., 1990 Pitari et al., 1992). For the southern hemisphere such onetime dilution likely provides a contribution to the mid-latitude column ozone depletion of about 1-2%. Locally larger but transient dilution effects following the breakup of the Antarctic ozone hole in late spring... 

In recent years the amount of O3 in the stratosphere over the South Pole has decreased periodically, resulting in an ozone hole in the atmosphere. The decrease is most pronounced during the summer months of the Southern Hemisphere. If the amount of ozone continues to decrease, more UV hght will reach the surface of Earth, probably causing some skin damage and increasing the incidence of cancer. Chlorine atoms react with and destroy ozone ... 

In the Southern Hemisphere, chlorine activation leads to a remarkable springtime decrease in ozone that has come to be known as the Antarctic ozone hole on the other hand, polar ozone depletion is considerably smaller in the Arctic. The difference arises from the weaker wave driving of the stratospheric circulation of the Southern Hemisphere. As seen in Fig. 9, wave amplitudes are smaller in the the Southern Hemisphere, and so is wave driving, to the point that the southern polar jet persists much longer into the spring than its northern counterpart. The delayed breakdown of the southern polar vortex allows ozone loss to continue throughout the months of September and October. In the Northern Hemisphere, on the other hand, the conditions necessary for ozone loss usually disappear in mid- to late March, when the northern polar vortex breaks down. 

On a global scale, ground-based and satellite observations show significant decreases of total column ozone at middle latitudes in the northern hemisphere of 2.7% per decade in winter, 1.3% decade in summer, and 1.2% per decade in the fall. Similar decreases are apparent at middle latitudes in the southern hemisphere and at high latitudes, beneath the region of the Antarctic ozone hole, the decreases are 14% per year. The decreases have occurred primarily in the lower stratosphere. No trends in ozone concentrations have been observed in the tropics. 

A41. Cmtzen, P.J. Briihl, C., 1990 The Potential Role of HOx and ClOx Interactions in the Ozone Hole Photochemistry , in O Neil, A. (Ed.) Dynamics, Transport and Photochemistry in the Middle Atmosphere of the Southern Hemisphere (Dordrecht Kluwer) 203-212. 

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