Antarctic spring

The development of the ozone hole over Antarctica is accelerated by heterogeneous catalysis on microciystals of ice. These microcrystals form in abundance in the Antarctic spring, which is when the ozone hole appears. Ice microciystals are less common in the Arctic atmosphere, so ozone depletion has not been as extensive in the Northern Hemisphere. 

In 1985, Farman et al. reported that the total column ozone at Halley Bay in the Antarctic had decreased substantially at polar sunrise each year for about 5-10 years. Figure 12.16 shows the Farman et al. data supplemented by measurements taken since then (Jones and Shanklin, 1995). Clearly a major drop in column ozone has been occurring since the mid to late 1970s. The extent of this change, and the rapidity with which it occurred, were unprecedented and focused the atmospheric chemistry community s attention on the reasons for this massive destruction of stratospheric ozone in the Antarctic spring. 

There are also important differences in the gas-phase chemistry of the Antarctic ozone hole compared to the chemistry at midlatitudes. One is the formation and photolysis of the CIO dimer. In the Antarctic spring, recycling of CIO back to chlorine atoms via reaction (27) with oxygen atoms does not play a major role because of the relatively small oxygen atom concentrations at the low UV levels at that time. Molina and Molina (1987) proposed that the formation of a dimer of CIO could, however, lead to regeneration of atomic chlorine through the following reactions ... 

Mickley, L. J., J. P. D. Abbatt, J. E. Frederick, and J. M. Russell III, Evolution of Chlorine and Nitrogen Species in the Lower Stratosphere during Antarctic Spring Use of Tracers to Determine Chemical Change, J. Geophys. Res., 102, 21479-21491 (1997b). 

Shindell, D. T., and R. L. de Zafra, Limits on Heterogeneous Processing in the Antarctic Spring Vortex from a Comparison of Measured and Modeled Chlorine, J. Geophys. Res., 102, 1441-1449(1997). 

As discussed in Chapter 12, trends in stratospheric ozone in the Antarctic spring during formation of the ozone hole are clear. However, as treated in detail in... 

The importance of heterogeneous processes in the chemistry of stratospheric ozone has been dramatically illustrated by the annual appearance of the "Ozone Hole" during the Antarctic spring [1-4]. Heterogeneous reactions on particle surfaces in the polar... 

DeZafra, R.L., M. Jaramillo, A. Parrish, P. Solomon, R Connor and J. Barret (1987) High concentrations of chlorine monoxide at low altitudes in die Antarctic spring stratosphere diuraal variation. Nature 328 408-411. 

Figure 11.5 This diagram comes from NASA s ozone monitoring programme TOMS (Total Ozone Mapping Spectrometer). The ozone hole over the Antarctic (shown in purple and pink on the diagram) is largest in the Antarctic spring. Note Dobson Units are a measure of the total amount of ozone in a vertical column from the ground to the top of the atmosphere.

FIGURE 20.30 Worsening ozone depletion over Halley Bay, Antarctica. These measurements were all taken in the Antarctic spring (October) when depletion is at its worst. 

Antarctic spring to summer" Antarctic summer to autumn" Arctic late summer" Antarctic spring to summer Antarctic summer to autumn Arctic late summer... 

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. 

CFC-12 (CCI2F2) is the main known catalyst currently acting in the so-called "ozone hole" in the Antarctic spring. 

Gardiner, B.C., Comparative morphology of the vertical ozone profile in the Antarctic spring. Geophys Res Lett 15, 901, 1988. 

Massive annual ozone decreases during the Antarctic spring (September to November) have now been extensively documented since 1985 (Jones and Shanklin 1995). By the early 1990s, total column ozone decreases outside the Antarctic were established. Both ground-based and satellite data have confirmed a downward trend in the total column amount of ozone over midlatitude areas of the Northern Hemisphere in all seasons. 

In the Antarctic spring, PSCs evaporate and set free active halogens (CI2 + Cl + CIO + CI2O2 Fig. 5.21). Most NO is stored as HNO3 in solid PSCs and contributes only little to O3 depletion. After ending the ozone hole period (at the end of November), PSCs almost evaporate and remain only as liquid sulfuric acid particles. Meanwhile, the normal gas phase cycles of ozone depletion control the steady-state concentration. 

Chlorine gas is released into the gas phase. The CI2 bond is then broken by sunlight when the Sun rises in the Antarctic spring, producing chlorine atoms. These chlorine atoms then deplete ozone according to the preceding reactions. In the Antarctic summer, the polar vortex breaks down, along with the PSCs, and the... 

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