Chlorine stratospheric chemistry

Figure 2. Schematic representation of the more important reactions and compounds in the stratospheric chemistry of chlorine at low and mid-latitudes.

An additional area of concern with respect to stratospheric ozone is possible direct emissions of NOj into the stratosphere by high-flying supersonic aircraft. This issue has come up repeatedly over the past 20 years, as air travel and pressure from commercial airlines has increased. However, despite substantial research effort to understand stratospheric chemistry, the question is complicated by the changing levels of stratospheric chlorine, first due to a rapid accumulation of tropospheric CFCs, followed by a rapid decline in CFC emissions due to the Montreal Protocol. To quote from the from the 1994 WMO/UN Scientific assessment of ozone depletion, executive summary (WMO 1995) ... 

Haas, B.-M., K. C. Crellin, K. T. Kuwata, and M. Okumura, Reaction of Chloride Ions with Chlorine Nitrate and Its Implications for Stratospheric Chemistry, J. Phys. Chem., 98, 6740-6745 (1994). 

We point the reader to numerous sources to find information on using molecular mechanics and ab initio calculations in the physical chemistry curriculum. First, Warren Hehre, who presented a talk at the symposium but did not author a chapter for this book, has written a comprehensive description of molecular mechanics and ab initio calculations. (9) An example of using computational chemistry to understand the role of chlorine oxides in stratospheric chemistry can be found in the Journal of Chemical Education. (10) Also, several workbooks are available with computational chemistry exercises for students to carry out. (11-13)... 

F. S. Rowland, Chlorofluorocarbons and the depletion of stratospheric ozone Am. Sci. 77, 36-45 (1989) T.-L. Tso, L. T. Molina, and F. C.-Y. Wang, Antarctic stratospheric chemistry of chlorine nitrate, hydrogen chloride and ice release of active chlorine. Science 238, 1253-1260 (1987) J. G- Anderson, D. W. Toohey, and W. H. Brune, Free radicals within the Antarctic vortex the role of CFCs in Antarctic ozone loss. Science 251, 39-46 (1991) P. S. Zurer, Complexities of ozone loss continue to challenge scientists. Chem. Eng. News June 12, 20-23 (1995). 

Rowland F.S. and Molina M.J., Stratospheric chemistry of chlorine compounds. Abstracts of 169th Meeting of the American Chemical Society, Philadelphia, Penna., April 6-11, Physical Chemistry Abstract No. 70 (1975b). 

The present results are based on a one-dimensional model. They should be extended to consider variations of stratospheric chemistry with latitude and season. Extension of the model to more than one dimension may be particularly important in studying the impact of high concentrations of chlorine since results should depend sensitively on details of the calculated latitudinal distributions of chlorine and NO. ... 

Molina, M. J., T-L. Tso, L.T. Mohna, and F.C.-Y. Wang, Antarctic stratospheric chemistry of chlorine nitrate, hydrogen chloride, and ice Release of active chlorine,... 

Estimates have been made of the annual emissions of HCl, HF, and SO2 from volcanic eruptions to the tropo- and strato-spheres. The results indicate that man-made chlorofluorocarbons are potentially more important in stratospheric chemistry than halides of volcanic origin. Lovelock has listed the concentration and concentration profiles of halocarbons in the troposphere and the lower stratosphere over the U.K. and the mid-Atlantic he has also estimated the total quantity of chloro-species transferred to the stratosphere by halocarbon sources. The interest in such materials, and especially in chlorofluorocarbons, e.g. CFCI3 and CF2CI2, used as propellants and refrigerants, is that they are photolysed to give chlorine atoms in the stratosphere the Cl then destroys ozone by reactions (part of the CIO cycle) such as (1). 

While gas phase chemistry leads to much higher levels of active bromine compared to chlorine, this is even more the case for iodine. Solomon et al. suggested that iodine might be of importance in ozone depletion. At present there is no information on the amounts of iodine in the stratosphere, but heterogeneous reactions will probably not play a significant role. Conversely, fluorine is almost completely in its deactivated form HF, and also heterogeneous reactions have been found to be immeasurably slow. Hence, fluorine species are not expected to influence stratospheric chemistry. 

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. 

Chlorine nitrate and bromine nitrate are recognized as key species in the chemistry of the stratosphere. In... 

Zhang, R., M.-T. Leu, and L. F. Keyser, Heterogeneous Chemistry of HONO on Liquid Sulfuric Acid A New Mechanism of Chlorine Activation on Stratospheric Sulfate Aerosols, . /. Phys. Chem., 100, 339-345 (1996). 

Although there has been some controversy over whether there is indeed a true ozone deficit problem (e.g., Crutzen et al., 1995), a combination of measured concentrations of OH, HOz, and CIO with photochemical modeling seems to indicate that it may, indeed, exist (Osterman et al., 1997 Crtuzen, 1997), although the source of the discrepancy remains unclear. Measurements of CIO in the upper stratosphere have found concentrations that are much smaller (by a factor of 2) than predicted by the models (e.g., Dessler et al., 1996 Michelsen et al., 1996). Because of the chlorine chemistry discussed later, model overestimates of CIO will also result in larger predicted losses of 03 and hence smaller concentrations. 

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