Bromine stratospheric chemistry

Lee, T. J., S. Parthiban, and M. Head-Gordon, Accurate Calculations on Excited States New Theories Applied to the -X, -XO, and -X02 (X = Cl and Br) Chromophores and Implications for Stratospheric Bromine Chemistry, Spectrochim. Acta A, 55, 561-574 (1999b). 

An important aspect of stratospheric bromine chemistry is the possibility of synergistic interactions between bromine and chlorine cycles via the following reaction,... 

Stratospheric Bromine Chemistry Insights from Computational Studies (S. Guha J. S. Francisco)... 

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. 

FIGURE 12.45 Schematic of gas-phase and heterogeneous bromine chemistry in the stratosphere. The heavier dark lines show the heterogeneous (het) chemistry. 

Erie, F., A. Grendel, D. Perner, U. Platt, and K. Pfeilsticker, Evidence of Heterogeneous Bromine Chemistry on Cold Stratospheric Sulphate Aerosols, Geophys. Res. Lett., 25, 4329-4332 (1998). 

These data also demonstrate the impact of bromine chemistry on the stratosphere (see Chapter 12.D). The initial ODP for methyl bromide is 15, due primarily to the large a factor associated with bromine chemistry. However, since it is removed by reaction with OH in the troposphere as well as by other processes such as hydrolysis in the oceans and uptake by soils and foliage (see Chapter 12.D), it has a short atmospheric lifetime of 1.3 years and hence the ODP decreases rapidly with time, toward a long-term steady-state value. 

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]. 

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

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. 

Figure 12.45 summarizes the most important chemistry of bromine in the stratosphere, both gas phase and heterogeneous (shown as the darker lines). Once the bromine-containing organics reach the stratosphere, they absorb light and photolyze in a manner analogous to the chlorofluorocarbons. As seen in... 

In summary, there are a variety of paths by which bromine can contribute to stratospheric chemistry. Excellent reviews of the gas-phase chemistry and of the heterogeneous chemistry of bromine relevant to the stratosphere are found in Lary (1996) and Lary et al. (1996), and of the thermochemistry of bromine oxides in Chase (1996). 

Hanson, D. R., and A. R. Ravishankara, Heterogeneous Chemistry of Bromine Species in Sulfuric Acid under Stratospheric Conditions, Geophys. Res. Lett., 22, 385-388 (1995). 

Hanson, D.R., and Ravishankara, A.R. (1995) Heterogeneous chemistry of bromine species in sulphuric acid under stratospheric conditions, Geophys. Res. Lett. 22,385-388. 

The atmospheric chemistry of bromine can be regarded as similar to that of chlorine. As far as HF is concerned, it does not react with hydroxyl since it persists, it limits the concentration of the atom F and its oxide FO. Hence, HF is the sink for fluorine in the stratosphere, before it disappears in the troposphere. 

Bureau H, Keppler H, Metrich N (2000) Volcanic Degassing of Bromine and Iodine Experimental Fluid/Melt Partitioning Data and Applications to Stratospheric Chemistry. Earth Planet Sci Lett 183 51... 

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). 

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