Chlorine ozone chemistry

It is also clear that during periods of low surface ozone, chlorine atoms are a major reactant for hydrocarbons (e.g., Jobson et al., 1994 Solberg et al., 1996 Ariya et al., 1998). Figure 6.39, for example, shows the measured ratios of isobutane, n-butane, and propane during an ozone depletion event (Jobson et al., 1994). These particular pairs of hydrocarbons were chosen to differentiate chlorine atom chemistry from OH reactions. Thus isobutane and propane have similar rate constants for reaction with Cl but different rate constants for reaction with OH. If chlorine atoms are responsible for the loss of these organics, their ratio should remain relatively constant in the air mass, as indicated by the line marked Cl. Similarly, isobutane and n-butane have similar rate constants for removal by OH but different rate constants for reactions with... 

The situation regarding the importance of chlorine in polar springtime surface ozone chemistry remains rather confusing. Contrary to most information, newer measurements of BrCl volume mixing ratios by up to 35pmolmoP, comparable to those of Br2 (=27 pmol moP ),... 

J. Masschelein, Chlorine Dioxide Chemistry and Environmental Impact of Oxychlorine Compounds, Ann Arbor Science I blishers, Ann Arbor, 1979, 190 pp. J. Katz (ed.). Ozone and Chlorine Dioxide Technology for Disinfection of Drinking Water, Noyes Data Corp., Park Ridge, New Jersey, 1980, 659 pp. 

Shortly thereafter, the effect on stratospheric ozone of chlorine released from human-made (anthropogenic) chlorofluorocarbons was predicted by Mario Molina and F. Sherwood Rowland. For their pioneering studies of atmospheric ozone chemistry, Crutzen, Molina, and Rowland were awarded the 1995 Nobel Prize in Chemistry. It was not until 1985, with the discovery of the Antarctic ozone hole by a team led by the British scientist Joseph Farman, that definitive evidence of the depletion of the stratospheric ozone layer emerged. 

It turns out that the HO NO, and CIO, cycles are all coupled to one another, and their interrelationships strongly govern stratospheric ozone chemistry. The NO, and CIO, cycles are coupled by reactions 4.34 and 4.35. For example, increased emissions of N2O would lead to increased stratospheric concentrations of NO and hence increased ozone depletion by the NO, catalytic cycle. Likewise, increasing CFC levels will lead to increased ozone depletion by the CIO, cycle. However, increased NO, will lead to an increased level of the CIONO2 reservoir and a mitigation of the chlorine cycle. Thus the net effect on ozone of si-... 

Halogenated organic substances are a potential risk to the stratospheric ozone, provided their residence times in the atmosphere are long enough for them to reach the stratosphere. The impact on the ozone chemistry increases with atomic number, i.e., bromine is more aggressive than chlorine. The atmospheric residence times of the most stable compounds are of the order of a hundred years, while others break down within a few days. Residence times are longer in seawater, except in anoxic waters Ballister and Lee, 1995 Tanhua et al., 1996). 

In view of this, it has been proposed that hydrated electrons generated on the surface of stratospheric ice crystals, via cosmic rays, could contribute to Cl formation via DEA of adsorbed CFCs. " Photodetachment of the chloride ions might then provide a mechanism to generate the Cl radicals that lead to ozone destruction. However, attempts to link these laboratory observations directly to stratospheric ozone chemistry have been strongly criticized, " although modeling does leave open the possibility that, at the very least, HCl destruction on ice crystals might be important for stratospheric chlorine chemistry. More work is evidently needed to resolve this controversy. 

Ozone, in turn, can be destroyed by interaction with another photon that breaks it into an oxygen molecule (O2) and an oxygen atom (O). Stratospheric ozone also can be destroyed by reaction with other species, such as nitric oxide (NO), as shown in Eq. (4.42), and halogen atoms, such as chlorine and bromine. Chlorine and bromine atoms are released into the stratosphere from the photodegradation of haloalkanes, often called halons. Classes of haloalkanes that impact ozone chemistry include CFCs and hydrochlorofluorocarbons (HCFCs). The net concentration of ozone in the stratosphere is established by the rates of both the production and the destruction reactions. 

Compute the enthalpy change for the destruction of ozone by atomic chlorine by subtracting the dissociation energies of O2 and CIO from the dissociation energy for ozone. What model chemistry is required for accurate modeling of each phase of this process The experimental values are given below (in kcal-moT ) ... 

You ll need to run five calculations at each model chemistry oxygen atom, chlorine atom, O2, CIO and ozone (but don t forget that you can obtain lower-level energies from a higher-level calculation). Use the experimental geometries for the various molecules and the following scaled zero-point energy corrections ... 

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. 

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) ... 

In homogeneous catalysis, both the catalyst and the reactants are in the same phase, i.e. all are molecules in the gas phase, or, more commonly, in the liquid phase. One of the simplest examples is found in atmospheric chemistry. Ozone in the atmosphere decomposes, among other routes, via a reaction with chlorine atoms ... 

Typically, intense chemiluminescence in the UV/Vis spectral region requires highly exothermic reactions such as atomic or radical recombinations (e.g., S + S + M - S2 + M) or reactions of reduced species such as hydrogen atoms, olefins, and certain sulfur and phosphorus compounds with strong oxidants such as ozone, fluorine, and chlorine dioxide. Here we review the chemistry and applications of some of the most intense chemiluminescent reactions having either demonstrated or anticipated analytical utility. 

Subsequent chemistry leads to release of additional chlorine, and for purposes of discussion, it is here assumed that all of the available chlorine is eventually liberated in the form of compounds such as HC1, CIO, C102, and Cl2. The catalytic chain for ozone that develops is... 

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-... 

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. 

Fluorine chemistry in the stratosphere was also considered and it was concluded that ozone depletion by chlorine was > 104 more efficient than that by fluorine (Rowland and Molina, 1975 Stolarksi and Rundel, 1975). Since then, the kinetics of reaction of F atoms with 02 to form the F02 radical and its thermal decomposition have been measured (e.g., see Pagsberg et al., 1987 Lyman and Holland, 1988 Ellerman et al., 1994 and review in DeMore et al., 1997). The equilibrium constant for the F-F02 system... 

In short, the net effect of fluorine atom chemistry on ozone destruction is very small, lCL-lO4 times smaller than the effect of chlorine on a per-atom basis (Sehested et al., 1994 Ravishankara et al., 1994 Li et al., 1995b Lary, 1997). 

Analogous to chlorine chemistry, the formation of bromine nitrate represents the major short circuit in its ozone destruction cycle ... 

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. 

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