Polar stratosphere, atmospheric measurements

The discovery of ozone holes over Antarctica in the mid-1980s was strong observational evidence to support the Rowland and Molina hypothesis. The atmosphere over the south pole is complex because of the long periods of total darkness and sunlight and the presence of a polar vortex and polar stratospheric clouds. However, researchers have found evidence to support the role of CIO in the rapid depletion of stratospheric ozone over the south pole. Figure 11-3 shows the profile of ozone and CIO measured at an altitude of 18 km on an aircraft flight from southern Chile toward the south pole on September 21, 1987. One month earlier the ozone levels were fairly uniform around 2 ppm (vol). 

A detailed analysis of the atmospheric measurements over Antarctica by Anderson et al. (19) indicates that the cycle comprising reactions 17 -19 (the chlorine peroxide cycle) accounts for about 75% of the observed ozone depletion, and reactions 21 - 23 account for the rest. While a clear overall picture of polar ozone depletion is emerging, much remains to be learned. For example, the physical chemistry of the acid ices that constitute polar stratospheric clouds needs to be better understood before reliable predictions can be made of future ozone depletion, particularly at northern latitudes, where the chemical changes are more subtle and occur over a larger geographical area. 

After the discovery of the Antarctic ozone hole" in 1985, atmospheric chemist Susan Solomon led the first expedition in 1986 specifically intended to make chemical measurements of the Antarctic atmosphere by using balloons and ground-based spectroscopy. The expedition discovered that ozone depletion occurred after polar sunrise and that the concentration of chemically active chlorine in the stratosphere was 100 times greater than had been predicted from gas-phase chemistry. Solomon s group identified chlorine as the culprit in ozone destruction and polar stratospheric clouds as the catalytic surface for the release of so much chlorine. 

Vapour pressures for a number of atmospherically relevant condensed systems have been measured with mass spectrometry. These systems include hydrates of HC1, HjS04 and HNO, supercooled liquids and pure water-ice, as well as the interactions of HC1 vapour with die solids, ice and NAT [23,47,50-55]. Vapour pressure measurements over HNOj/HjO hydrates have also been made using infrared optical absorption with light originating from a tunable diode laser [29]. This technique allowed the identification of the metastable NAD in presence of the more stable NAT under temperature and vapour pressure conditions near to those found in the polar stratosphere. Vapour pressures of Up, HN03, HC1, HBr over supercooled aqueous mixtures with sulfuric acid have been calculated using an activity model [56]. It provides a parameterized model for vapour pressures over the stratospheric relevant temperatures (185-235 K). 

Brune WH, Anderson JG, Toohey DW, Fahey DW, Kawa SR, Jones RL, Mckenna DS, Poole LR (1991) The potential for ozone depletion in the arctic polar stratosphere. Science 252 1260-1266 Bunte SW, Rice BM, Chabalowski CF (1997) An ab initio QCISD study of the potential energy surface for the reaction HN0+N0 N20+0H. J Phys Chem A 101 9430-9438 Calvert JG, Chatfield RB, Delany AC, Martel EA (1985) Evidence for short SO2 lifetimes in the atmosphere—an in situ measurement of atmospheric SO2 lifetime using cosmic-ray produced s-38. Atmos Environ 19 1205-1206... 

Systematic smdies carried out over the past 25 years have left no doubt that chlorine atoms derived from human-made substances such as CFCs are largely responsible. In the extreme cold of the polar stratosphere in late winter and early spring, clouds containing ifitrogen oxides form that enhance the ozone-destroying effects of chlorine. Satellite measurements of CIO correlate directly with ozone depletion values. Furthermore, the observation of stratospheric HF, which has no other atmospheric source besides the light-induced breakdown of CFCs in the presence of hydrocarbons, strongly supports these conclusions. 

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. 

Brogniez, C J. Lenoble, R. Ramananaherisoa, K. H. Fricke, E. P. Shettle, K. W. Hoppel, R. M. Bevilacqua, J. S. Hornstein, J. Lumpe, M. D. Fromm, and S. S. Krigman, Second European Stratospheric Arctic and Midlatitude Experiment Campaign Correlative Measurements of Aerosol in the Northern Polar Atmosphere, J. Geophys. Res., 102, 1489-1494 (1997). 

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