Bromine-ozone chemistry, stratospheric

In Section C.3, we saw that gas-phase chlorine chemistry in the stratosphere is inextricably intertwined with bromine chemistry. Because of this close interrelationship, altering the concentrations of only one of the halogens (e.g., through controls) may not have the proportional quantitative result that might be initially expected. We explore in this section in more detail the role of brominated organics in stratospheric ozone destruction and the interrelationship with chlorine chemistry. 

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). 

The family method was applied within an atmospheric chemistry box model to NOy, HOx, Cly, Ox and Bry families in order to study the effect of increases in ground level bromine emissions on stratospheric ozone by Ramarosmi et al. (1992), and for simulations of lower stratospheric HCl in Douglass and Kawa (1999). The nonlinear features of tropospheric ozone production from nitrogen oxides and VOCs were reproduced using a numerical method based on family methods in Elliott et al. (1996). 

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. 

The chemistry involving NOx is closely intertwined with that of the halogens (CIO,. and BrOx) and of HO, so that the predicted effects of a given set of emissions from the HSCT depend on these species as well. Because halogen chemistry is treated in more detail in later sections, we shall focus here primarily on the reasons for the different effects of NO, emissions at different altitudes. How closely these chemistries are intertwined will be apparent in the treatment below of destruction of stratospheric ozone by chlorofluorocar-bons (CFCs) and brominated compounds. 

Bromine compounds, when diffused to and photolyzed in the stratosphere, can produce Br atoms that participate in catalytic ozone destruction that can reach efficiencies that are 40-50 times more effectively than Cl atoms [9,22-24]. The stratospheric chemistry of brominated organic compoimds is similar to that of... 

In 1995 the Nobel Prize for chemistry was awarded to F. Sherwood Rowland and Mario Molina, physical chemists from the University of California, Irvine. In then-published ozone depletion hypothesis [1], they proposed that chlorine atoms could form high in the stratosphere. As an offshoot of this work as well as some other work, the Montreal Protocol on Substances That Deplete the Ozone Layer was signed in 1987. The treaty took effect on January 1, 1989, and has since undergone several revisions. The treaty addresses ozone-depleting compounds that contain chlorine or bromine. Fluorine is not included in the treaty because it has not been shown to harm the ozone layer. 

Ozone in the lower stratosphere may in principle be affected by iodine chemistry (Solomon et al, 1994). The abundance of total iodine in the troposphere is believed to be of the order of pptv, but the fractional partitioning of iodine free radicals (I and IO) is much higher than in the case of other halogens (chlorine and even of bromine see Section 5.6.3). 

Section 5.6.3 discussed the chemistries of the halogens chlorine and bromine, and outlined their interactions with one another and with ozone. More than two decades after the pioneering prediction of possible ozone destruction through humankind s use of halogenated chemicals, upper stratospheric ozone observations began to reveal systematic depletion indicative of a changing chemical state (see SPARC, 1998, and references therein). 

If propagation reactions compete favorably with termination reactions, the formation of two chlorine radicals could result in the reaction of many molecules of methane. It should be noted that this effect is important in the loss of stratospheric ozone resulting from the production of chlorine or bromine radicals from freons. Free radicals are very important in biological processes involving oxygen and their production is involved in some toxicological mechanism. In organic chemistry, free radicals are a major factor in polymerization processes. 

The solubility of HBr in sulfuric acid has been studied as well [37]. Bromine radicals contribute to ozone destruction through a catalytic reaction cycle involving BrO and CIO. Thus heterogeneous chemistry of bromine containing species merits some attention, even though most of the stratospheric bromine is already present in active species. Table 1 shows these results in a manner analogous to the HCl and HNO3 results. 

Significant atmospheric ozone depletion is not restricted just to the Antarctic stratosphere. In addition to other locations in the atmosphere [1], there is significant ozone depletion in the springtime Arctic tropospheric boundary layer, where bromine is much more abimdant than elsewhere, and where heterogeneous bromine chemistry is implicated in extremely rapid ozone depletion near ground level [25-27]. 

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. 

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