Ozone depletion reactions involved

The ozone-depleting reaction involves a rather complicated series of reactions, all of which occur in the gas phase. Equation (8.14) describes the rate-determining step ... 

The chlorine-containing product species (HCl, CIONO2, HOCl) are "inert reservoirs" because they are not directly involved in ozone depletion however, they eventually break down by absorbing solar radiation or by reaction with other free radicals, returning chlorine to its catalytically active form. Ozone is formed fastest in the upper stratosphere at tropical latitudes (by reactions 1 and 2), and in those regions a few percent of the chlorine is in its active "free radical" form the rest is in the "inert reservoir" form (see Figure 3). 

An overview of the reactions involving trihalomethanes (haloforms) CHXYZ, where X, Y, and Z are halogen atoms, has been given in the context of ozone depletion (Hayman and Derwent 1997). Interest in the formation of trichloroacetaldehyde formed from trichloroethane and tetrachloroethene is heightened by the phytotoxicity of trichloroacetic acid (Frank et al. 1994), and by its occurrence in rainwater that seems to be a major source of this contaminant (Muller et al. 1996). The situation in Japan seems, however, to underscore the possible significance of other sources including chlorinated wastewater (Hashimoto et al. 1998). Whereas there is no doubt about the occurrence of trichloroacetic acid in rainwater (Stidson et al. 2004), its major source is unresolved since questions remain on the rate of hydrolysis of trichloroacetaldehyde (Jordan et al. 1999). 

Thus we see that environmental modeling involves solving transient mass-balance equations with appropriate flow patterns and kinetics to predict the concentrations of various species versus time for specific emission patterns. The reaction chemistry and flow patterns of these systems are sufficiently complex that we must use approximate methods and use several models to try to bound the possible range of observed responses. For example, the chemical reactions consist of many homogeneous and catalytic reactions, photoassisted reactions, and adsorption and desorption on surfaces of hquids and sohds. Is global warming real [Minnesotans hope so.] How much of smog and ozone depletion are manmade [There is considerable debate on this issue.]... 

Abstract Heterogeneous chemical reactions at the surface of ice and other stratospheric aerosols are now appreciated to play a critical role in atmospheric ozone depletion. A brief summary of our theoretical work on the reaction of chlorine nitrate and hydrogen chloride on ice is given to highlight the characteristics of such heterogeneous mechanisms and to emphasize the special challenges involved in the realistic theoretical treatment of such reactions. 

This is probably the key heterogeneous reaction for Antarctic stratospheric ozone depletion, and serves as a useful focus for the discussion of the theoretical challenges that must be addressed in dealing with fairly complex chemistry in a complex environment, challenges enlivened — as will be seen below — by the evident chemical involvement of the ice surface environment. 

BrO] show a pulse in the first two hours after dawn ascribed to the photolysis of inorganic bromine compounds produced either by the bromine explosion mechanism s or the photolysis of mixed bromo/ iodo-organohalogens S built up overnight. Using measured concentrations of BrO, 10 and HO2, the data in Table 7 show the ozone depletion cycle (Cycle type I) involving the BrO and lO cross-reaction is the most important with an O3 depletion rate of 0.3 ppbv h ... 

In the upper atmosphere the presence of nitric oxide has the opposite effect—the depletion of ozone. The series of reactions involved is... 

In this paper I will describe some aspects of important environmental problems from the point of view of the chemical reactions that occur in the atmosphere. An overview of the processes involved in stratospheric ozone depletion is given in the papers by Professor Rowland and Professor Anderson, in acid precipitation by Dr. Phillips, and in tropospheric photochemistry by Professor Chameides. It is not practical for me to discuss the details of all of these complicated systems, so I will concentrate on a few issues which are of current interest to me and which I believe touch on some of the key uncertainties in our understanding of the environmental problems. I will also limit my discussion to gas phase reactions, although we know that many liquid phase or heterogeneous reactions are taking place, especially in the troposphere. 

Interest in the chemistry of atmospheric halogens took a steep upward surge after it was postulated that the release of industrially produced halocarbons, in particular the chlorofluorocarbons (CFCs), CFCI3, and CF2CI2, could cause severe depletions in stratospheric ozone (Molina and Rowland, 1974) by the reactions involving the CFC photolytic product radicals. Cl and CIO, as catalysts. The first stratospheric measurements of CIO did indeed show its presence in significant quantities in the stratosphere so that by the end of the 1970s USA, Canada, and the Scandinavian... 

Ozone is a gas that occurs naturally in relatively large concentrations in the upper-atmospheric layer known as the stratosphere. The stratosphere is between 5-10.6 mi (8-17 km) to about 31 mi (50 km) above the earth s surface. Stratospheric ozone is very important to life on the surface of Earth because it absorbs much of the incoming solar ultraviolet radiation, and thereby shields organisms from its deleterious effects. Since the mid-1980s, there has been evidence that concentrations of stratospheric ozone are diminishing as a result of complex photochemical reactions involving chloroflno-rocarbons (CFCs). These persistent chemicals are synthesized by humans and then emitted to the lower atmosphere, from where they eventually reach the stratosphere and deplete ozone. 

Arrange your models to represent the three reactions involved in ozone depletion. 

Fluorine chemistry in the stratosphere has also been considered and attention has been drawn to the atmospheric chemistry of the FOx radicals. The compounds with O-F bonds have gained interest in connection with the ozone depletion problem. It has been suggested that FO and F02 radicals formed in the atmospheric degradation of hydrofluorocarbons (HFCs) could destroy ozone in chain reaction processes. Experimental studies of this hypothesis led to the conclusion that catalytic cycles involving F, FO, and F02 are irrelevant with respect to the chlorine cycle.8 However, kinetic investigations of the reactions of fluorine atoms with 02 and NOx provide useful information on the fluorine chemistry in the polluted atmosphere. 

Many investigations made by Romanian scientists are dedicated to the interrelations between chemical reactions in the atmosphere and specific meteorological conditions. The National Institute of Meteorology and Hydrology, Bucharest is closely involved in the study of the effects of increasing the CO2 concentration on the extreme temperatures and drought episodes in Europe, as well as ozone depletion related studies. 

There has been a good deal of study of the polyhalogenated methanes in hydrogen atom abstraction reactions toward hydroxyl (HO ) and chlorine radicals. These reactions are involved in both the atmospheric destruction of such compounds as well as their involvement in ozone depletion. Information is needed about these reactions to model the environmental impact of the compounds. 

The existence of reservoir species is central to the ozone depletion cycles. In every cycle a reactive free radical can be temporarily sequestered as a relatively unreactive reservoir species. In fact, HCI and CIONO2 together store as much as 99% of the active chlorine. Thus only a small change in the abundance of reservoir species can have a profound effect on the catalytic efficiency of a cycle. The importance of relative concentrations in determining the predominance of different reactions in the ozone depletion cycles can be illustrated in the case of CIO, cycles. Above 20 km, CIO, Cycle 1 is a dominant contributor to ozone loss. At lower altitudes where atomic oxygen levels are significantly lower, other cycles, which involve coupling with HO, and NO, become important ... 

---