Ozone polar chemistry

Lu J. Y., Schroeder W. H., Barrie L. A., Steffen A., Welch H. E., Martin K., Lockhart L., Hunt R. V., Boila G., and Richter A. (2001) Magnification of atmospheric mercury deposition to polar regions in springtime the link to tropospheric ozone depletion chemistry. Geophys. Res. Lett. 28, 3219-3222. 

Interaction of acidic gases such as HCl with ice particles of PSCs is a key step in polar ozone hole chemistry [3]. The uptake efficiency of HCl, y, on ice is defined by the fractional collision frequency that leads to the reactant loss on ice surface. Until recently, a single laboratory measurement y 0.4 has been performed at HCl vapor... 

Heterogeneous chemistry occurring on polar stratospheric cloud particles of ice and nitric acid trihydrate has been estabUshed as a dorninant factor in the aggravated seasonal depletion of o2one observed to occur over Antarctica. Preliminary attempts have been made to parameterize this chemistry and incorporate it in models to study ozone depletion over the poles (91) as well as the potential role of sulfate particles throughout the stratosphere (92). 

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 first reports of this phenomenon, major field campaigns were launched, which clearly established a relationship between ozone destruction and chlorine chemistry. For example, Fig. 1.8 shows simultaneous aircraft measurements of ozone and the free radical CIO as the plane flew toward the South Pole. As it entered the polar vortex, a relatively well-contained air mass over Antarctica, 03 dropped dramati-... 

Jaffe, D The Relationship between Anthropogenic Nitrogen Oxides and Ozone Trends in the Arctic Troposphere, in The Tropospheric Chemistry of Ozone in the Polar Regions (H. Niki and... 

Barnes, I., K. H. Becker, and R. D. Overath, Oxidation of Organic Sulfur Compounds, in The Tropospheric Chemistry of Ozone in the Polar Regions (H. Niki and K. H. Becker, Eds.), NATO ASI Series, Vol. 17, pp. 371-383, Springer-Verlag, Berlin, 1993. 

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 several reasons for the dramatic ozone destruction (see Fig. 2.17) low temperatures may have prolonged the presence of polar stratospheric clouds, which play a key role in the ozone destruction, the polar vortex was very stable, there were increased sulfate aerosols from the 1991 Mount Pinatubo volcanic eruption, which also contribute to heterogeneous chemistry, and chlorine levels had continued to increase. These issues are treated in more detail shortly. 

Evidence for the contribution of the CIO + BrO interaction is found in the detection and measurement of OCIO that is formed as a major product of this reaction, reaction (31a). This species has a very characteristic banded absorption structure in the UV and visible regions, which makes it an ideal candidate for measurement using differential optical absorption spectrometry (see Chapter 11). With this technique, enhanced levels of OCIO have been measured in both the Antarctic and the Arctic (e.g., Solomon et al., 1987, 1988 Wahner and Schiller, 1992 Sanders et al., 1993). From such measurements, it was estimated that about 20-30% of the total ozone loss observed at McMurdo during September 1987 and 1991 was due to the CIO + BrO cycle, with the remainder primarily due to the formation and photolysis of the CIO dimer (Sanders et al., 1993). The formation of OCIO from the CIO + BrO reaction has also been observed outside the polar vortex and attributed to enhanced contributions from bromine chemistry due to the heterogeneous activation of BrONOz on aerosol particles (e.g., Erie et al., 1998). 

In short, the overall features of the chemistry involved with the massive destruction of ozone and formation of the ozone hole are now reasonably well understood and include as a key component heterogeneous reactions on the surfaces of polar stratospheric clouds and aerosols. However, there remain a number of questions relating to the details of the chemistry, including the microphysics of dehydration and denitrification, the kinetics and photochemistry of some of the C10x and BrOx species, and the nature of PSCs under various conditions. PSCs and aerosols, and their role in halogen and NOx chemistry, are discussed in more detail in the following section. 

The finding that the heterogeneous chemistry that occurs on polar stratospheric clouds also occurs in and on liquid solutions in the form of liquid aerosol particles and droplets in the atmosphere provided a key link in understanding the effects of volcanic eruptions on stratospheric ozone in both the polar regions and midlatitudes. As discussed herein, the liquid particles formed from volcanic emissions are typically 60-80 wt% H2S04-H20, and hence the chemistry discussed in the previous section can also occur in these particles (Hofmann and Solomon, 1989). We discuss briefly in this section the contribution of volcanic emissions to the chemistry of the stratosphere and to ozone depletion on a global scale. For a brief review of this area, see McCormick et al. (1995). 

Of course, it is not just the chemistry but also the dynamics that determine the net effect on total column ozone in midlatitudes. Transport of air from the tropics to midlatitudes was discussed earlier in Section A.l. There is also evidence for the influence of high-latitude air on ozone at midlatitudes. It has been proposed, for example, that the Arctic polar vortex acts more like a flowing processor than an isolated air mass. In this scenario, air flows through the polar vortex and as it does, undergoes the chemistry described earlier for the... 

Anderson, J. G Polar Processes in Ozone Depletion, in Progress and Problems in Atmospheric Chemistry, Advanced Series in Physical Chemistry (J. R. Barker, Ed.), Vol. 3, pp. 744-770, World Scientific, Singapore, 1995. 

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. 

R. S. Stolarski, The Antarctic Ozone Hole, Scientific American, January 1988. The 1995 Nobel Prize in Chemistry was shared by Paul Crutzen, Mario Molina, and F. Sherwood Rowland for their work in atmospheric chemistry, particularly concerning the formation and decomposition of ozone. Their Nobel lectures can be found in P. J. Crutzen, My Life with 03, NO, and Other YZO Compounds, Angew. Chem. lnt. Ed. Engl. 1996,35, 1759 M. J. Molina, Polar Ozone Depletion, ibid., 1779 F. S. Rowland, Stratospheric Ozone Depletion by Chlorofluorocarbons, ibid., 1787. 

Figure 1. This graph shows the rapid variation of CIO and 03 as the edge of the chemically perturbed region in the Antarctic polar vortex is penetrated by the National Aeronautics and Space Administration (NASA) ER-2 high-altitude aircraft over the Palmer Peninsula of Antarctica on September 16, 1987 (5). It is one of a series of 12 snapshots, or individual flights, during the Airborne Antarctic Ozone Experiment (AAOE) that show the development of an anticorrelation between CIO and 03 that began as a correlation in mid-August. When these two measurements are combined with all the others from the ER-2 aircraft, the total data set provides a provocative picture of how such chemistry occurs and what it is capable of doing to ozone.

There is already one excellent example of our failure to make such a predictive leap—the Antarctic ozone hole. The reason for the failure to anticipate the rapid loss of ozone in the lower stratosphere was a failure to appreciate the potential role of the subtle photochemistry, in particular, the heterogeneous chemistry. Nor did researchers have a full appreciation for the consequences of the air parcels inside the polar vortices being relatively isolated from midlatitude air. Some of these same issues are important in the Arctic region in wintertime, but researchers lack the predictive capability to determine how ozone will ultimately be affected. 

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