Antarctic ozone depletion observations

Roscoe, H.K., Jones, A.E. and Lee, A.M., (1997) Midwinter Start to Antarctic Ozone Depletion Evidence from Observations and Models, Science, VOL. 278, 93-96... 

Observations of PSCs, low NO2 amounts in polar regions (Figure 6.12), enhanced polar HNO3 (Murcray et al, 1975 Williams et al, 1982) and the vertical profile of the ozone depletion based upon the Japanese measurements (Chubachi, 1984) were cited in support of heterogeneous chemistry as the primary process initiating Antarctic ozone depletion. Such a mechanism would be most effective in the Antarctic due to colder temperatures and greater PSC frequencies there than in the corresponding seasons in the Arctic (McCormick et al, 1982), a point discussed further below. 

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... 

Roscoe, H.K., A.E. Jones, and A.M. Lee, Midwinter start to Antarctic ozone depletion Evidence from observations and models. Science 278, 93, 1997. 

Rosenfield, J. E M. R. Schoeberl, and P. A. Newman, Antarctic Springtime Ozone Depletion Computed from Temperature Observations, J. Geophys. Res., 93, 3833-3849 (3988). 

The continued depletion of Antarctic ozone and the appearance of spring ozone depletion over the Northern hemisphere between the latitudes of 40 and 55 N. detectable by the improved analysis of older data and the development of better ozone observational methods, resulted in a tightening of the CFC phaseout provisions during the 1990 and 1992 reviews,10 held in London and Copenhagen, respectively, and the extension of the restrictions to other, potentially ozone-depleting substances. Table 2 summarizes the current position. 

Wessel S., Aoki S., Winkler P., Weller R., Herber A., Gernandt H., and Schrems O. (1998) Tropospheric ozone depletion in polar regions. A comparison of observations in the Arctic and Antarctic. Tellus 50B, 34-50. 

In 1984, a remarkable and totally unpredicted phenomenon was discovered by the British Antarctic Survey, the so-called ozone hole. The discovery of ozone depletion over Antarctica during the spring period provided the first observational support for the possible effect of CFCs on the stratospheric ozone. However, the observation of ozone loss did not indicate its cause. From 1984 to 1988, several theories were postulated from CFC chemistry to atmospheric dynamics or even to cosmic electron fluxes. It was not until 1988, with the results of the 1987 Airborne Antarctic Ozone Expedition, that a probable link with CFCs was established. This prompted the Natural Resources Defense Council, an American pressure group, to sue the EPA to fulfill its 1980 promise to seek legislation to further control the manufacture and use of CFCs in the United States of America. 

The Antarctic "ozone hole" is one of the most dramatic indications of anthropogenic environmental change. Depletion of stratospheric ozone via the catalytic mechanisms described in Chapters 5, 10, and 12 was first detected in a surprising fashion by British observers in Antarctica (Farman et al., 1985). They measured increases of springtime solar UV radiation penetrating the atmosphere at wavelengths that normally are absorbed by O3. The results showed almost a factor of 2 depletion... 

Fig. 16-2 Depletion of Antarctic ozone during October between 1956 and 1985. Adapted from Stolarski (1988). Dobson data are from ground-based observations with the permission of John Wiley and Sons, Inc.

Numerical models have been used to predict the potential ozone depletion in response to the emission of halocarbons, based on different plausible scenarios. All of these models indicate that the time required for the middle atmosphere to respond to surface emissions of these halocarbons is very long (several decades). In particular, even with the measures taken to reduce or phase-out the emissions of the CFCs and other halocarbons, it is expected that the Antarctic ozone hole will be observed each spring (September-October) at least until the year 2040. It should also be noted that the halocarbons are active in the infrared, and contribute to the greenhouse effect. 

While not a true hole in the sense that some column ozone remains even in the most extreme depletions observed in the mid 1990s (when October ozone minima were near 100 Dobson Units over the South Pole, or depletion of about two-thirds of the historical levels, see Hofmann et al, 1997), the descriptor captures the fact that the peak depletion is sharply limited to Antarctic latitudes. Dobson (1968 and references therein) noted that there is less ozone naturally present over Antarctica than over the Arctic in winter and much of the spring, but this climatological difference between the natural ozone levels over the poles of the two hemispheres should not be confused with the abrupt decline that began near the mid-1970s as depicted in Figure 6.9. Newman (1994) discusses these and other historical measurements of total ozone and shows that the Antarctic ozone hole began only in the last few decades. 

Figure 6.11 shows measurements of the seasonal cycle of ozone at Halley in historical and recent data, which show that the depletion occurs only over a limited portion of the year. These observations demonstrate that contemporary observations of ozone at Halley in late August (end of austral winter) are near historical levels, while the bulk of the ozone loss there occurs rapidly during the month of September. In recent years, measurable ozone depletion is also observed in Antarctic summer — in part the result of dilution of the extreme losses in spring. 

It is important to note that denitrification was observed but was rather limited in degree in the Arctic springs of 1993, 1996, and 1997 (Santee et al., 1998 1999), so that the observations of ozone depletion of the order of 60-120 DU in each of these years are not associated with extensive denitrification. Rather, as in the Antarctic and consistent with current understanding of liquid aerosol chemistry, the evidence suggests that heterogeneous reactions in the sunlit atmosphere are... 

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