Bromine in the stratosphere

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... 

One issue of interest with respect to reaction (59) is whether there is a second channel that produces the reservoir species HBr, rather than the reactive HOBr, which would have a major effect on the partitioning of reactive bromine in the stratosphere (Lary, 1996). Larichev et al. (1995) place an upper limit of 1.5% on this potential path over the temperature range 233-298 K, which is consistent with indirect estimates of an upper limit of 0.01% (Mellouki et al., 1994) as well as with measurements of HBr and upper limits for the HOBr concentration in the stratosphere (D. G. Johnson et al., 1995). Cronkhite et al. (1998) observe no dependence of the overall rate constant on pressure between 12 and 25 Torr, indicating that a third possible channel producing an H02 BrO adduct is not important. 

Almost all the bromine in the stratosphere remains as the radicals Br and BrO, since the inactive non-radical forms, hydrogen bromide, HBr, and bromine nitrate, Br0N02, are efficiently decomposed photochemically. 

FIGURE 5.1 Primary sources of chlorine and bromine in the stratosphere in 1999 [World Meteorological Organization (WMO) 2002],... 

While there is a total chlorine compound mixing ratio in the stratosphere of approximately 3400 ppt, that for bromine gases is only 20 ppt (Figure 5.1). Remarkably, with 150 times less abundance than chlorine, bromine is approximately as important as chlorine in overall ozone destruction. Methyl bromide (CH3Br) constitutes about half the source of bromine in the stratosphere (see Section 2.5). The H-atom-containing bromine compounds, CH3Br, CH2Br2, and CHBr, release their Br almost immediately on entry into the stratosphere the... 

How does the ozone hole arise We must consider the role of species other than the allotropes of oxygen in the Chapman cycle. There is evidence that chlorine and bromine in the stratosphere lead to a decrease in the amount of ozone present. How does this occur What factors influence whether or not the decrease occurs And what might be done to combat the loss of ozone These questions all point to aspects of chemical kinetics that we will address in this chapter. To begin this study, we focus first on the concepts of rate of reaction and the ways in which reaction rate can be measured. 

In the last decade, the refrigerant issue is extensively discussed due to the accepted hypothesis that the chlorine and bromine atoms from halocarbons released to the environment were using up ozone in the stratosphere, depleting it specially above the polar regions. Montreal Protocol and later agreements ban use of certain CFCs and halon compounds. It seems that all CFCs and most of the HCFCs will be out of produc tion by the time this text will be pubhshed. 

There are natural sources of brominated hydrocarbons as well as man-made sources, such as the "halons , which are used in fire extinguishers. Reaction 21 is very fast and generates Cl and Br atoms directly the cycle does not require a photolytic step. Although this cycle occurs with high efficiency, it is less important than the chlorine peroxide cycle because of the much smaller concentrations of bromine compounds in the stratosphere-parts per trillion vs. parts per billion for the chlorine compounds. 

Figures 4.44 and 4.45 show absorption spectra of some simple chlorofluoro-methanes and ethanes, respectively (Hubrich and Stuhl, 1980). Tables 4.37 and 4.38 give the recommended absorption cross sections for some of these compounds (DeMore et al., 1997). None of these compounds absorb in the actinic region above 290 nm, but do around 180-200 nm, wavelengths only found in the stratosphere. As discussed in Chapter 12, it is photolysis at these short wavelengths to generate atomic chlorine that is responsible, along with bromine and perhaps in some cases, iodine atoms, for the chain destruction of stratospheric ozone.

While more than 90% of reaction (31) at 298 K produces bromine atoms directly, the minor channel producing BrCl is important in the atmosphere under certain conditions (e.g., McKinney et al., 1997). (Measurements of OCIO formed in reaction (31a) in the Antarctic and Arctic stratosphere are discussed later.) For example, McKinney et al. (1997) report large fractions (50-95%) of total bromine in the form of BrO in... 

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. 

Using the heats of formation given in Appendix I, calculate A 7/(298 K) for the reactions of CH4 with Cl and Br, respectively. As discussed in Chapter 12, the Cl + CH4 reaction plays an important role in the stratosphere whereas the analogous reaction of bromine atoms does not. Comment on whether this difference is due to enthalpy. 

Bromine compounds are found in the atmosphere in small amounts the sea is a primary source. Rainfall over the Pacific and Indian Oceans has been found to contain 60—80 pg/L of bromine (46). Approximately 10 parts per trillion (v/v) of bromine is found in the stratosphere (47). 

Bromine containing species, introduced from Man s release of halons, are also believed to play a significant role in the polar ozone depletion, despite the fact that the total inorganic bromine concentration in the stratosphere is typically two orders of magnitude lower than the inorganic chlorine. This is manifested in the presence of Br0N02 and HOBr, which however are less stable than the chlorine reservoirs, so that relatively more BrO, is in the active fotm [36,37]. There is a synergism between the chlorine and bromine species the oxides radicals GO and BrO react with each other to produce a series of products, G, Br, BrCl and OCIO. The latter compound is an indicator of the elevated levels of both BrO and GO [38]. 

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. 

Salawitch RJ, Weisenstein DK, Kovalenko LJ, Sioris CE, Wennberg PO, Chance K, Ko MKW, McLinden CA (2005) Sensitivity of Ozone to Bromine in the Lower Stratosphere. Geophys Res Lett 32 L05811... 

Halons such as Halon-1211 (CF2BrCl) and Halon-1301 (CF3Br) are bromi-nated CFCs which are used as fire extinguishers. Like CFCs, Halons are chemically inert in the troposphere but photolyze in the stratosphere. Photolysis releases bromine atoms, which can remove stratospheric ozone in a cycle that is analogous to the chlorine-based cycle above. The bromine cycle can couple... 

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... 

Bromine-containing compounds have the potential to released Br atoms upon degradation in the atmosphere. Once in the stratosphere, Br atoms are about 45 times more effective than chlorine in destroying stratospheric ozone [22]. An important example of Br-containing compounds is 1-bromopropane, which is currently utilized as a industrial solvent and have been proposed as a replacement for CFCs, controlled under the agreements of fhe Montreal Protocol. 

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