HOBr chemistry

FIGURE 6.38 Schematic diagram of HOBr chemistry with sea salt particles/ice (graciously provided by T. Benter). 

Other species that can initiate this sulfur oxidation chemistry are N03 (discussed in Chapter 7.D.1) and ClJ. The latter radical anion is formed in sea salt particles when atomic chlorine is generated and reacts with chloride ion. In addition, Vogt et al. (1996) have proposed that oxidation of SO2- by HOC1 and HOBr in sea salt particles may be quite important. Table 8.13 summarizes the aqueous-phase chlorine chemistry that occurs in sea salt particles and Table 8.14 the oxidation of S(IV) by reactive chlorine and bromine species in solution. 

Although there are fewer studies of the heterogeneous chemisty of BrONOz and HOBr than of the corresponding chlorine compounds, it is clear from the laboratory studies that have been done that analogous chemistry occurs, and at least as fast as for the chlorine compounds. Table 12.8 shows some of the most important reactions and typical values of the reaction probabilities. On ice, the hydrolyses of CIONO, and BrONOz proceed at comparable rates (Tables 12.5 and 12.8). However, toward midlatitudes the particles are largely concentrated sulfuric acid-water mixtures, and on this surface the C10N02 hydrolysis reaction probability... 

HOBr also serves to couple bromine and chlorine chemistry in an indirect manner. Thus, photolysis of HOBr generates increased OH concentrations, which then cause a faster recycling of HC1 back into chlorine atoms (Lary et al., 1996 Randeniya et al., 1996a,b Tie and Brasseur, 1996). Lary et al. (1996) estimated that the lifetime of HC1 can be reduced by as much as a factor of three through this effect and suggest that the unexplained rapid rise in OH reported by Salawitch et al. (1994) at dawn may be due to the photolysis of HOBr formed overnight rather than of a nitrogen species such as HONO. 

The last years attention has been also paid on the importance of heterogeneous reactions in the troposphere. On the basis of the extremely limited laboratory studies and numerous assumptions it has been calculated that scavenging of compounds such as N03, N205 and HOBr followed by reactions in clouds or/and onto aerosols can be crucial for the oxidant levels in the troposphere [24 - 27]. However, contrary to the gas phase tropospheric chemistry, it does not exist a reference scheme for heterogeneous chemistry nor any consistent compilation of recommended relevant kinetic data. Finally, although CTMs have nowadays more or less sophisticated parameterizations of heterogeneous chemistry,... 

Generation of HOBr can be effected by the activation of sodium bromide or (by-product) ammonium bromide, by rapid intimate mixing with chlorine gas or, more usually, sodium hypochlorite. This method is widely used in general industrial cooling systems (as well as other applications, such as brewery pasteurizers, where it provides an alternative to BCDMH chemistry). 

Michalowski et al. (2000) calculated that the source of bromine from Br2 photolysis is about an order of magnitude larger than bromine release from BrCl after sunrise. Following BrO formation from the reaction of Br with O3 and HOBr formation by HO2 -b BrO HOBr -b O2 (namely, (4)), further reactions of HOBr with Cl and Br would give rise to enhanced levels of Br2 and BrCl. They also found that heterogeneous halogen chemistry within the snowpack was necessary for the O3 depletion events to occur and that the mass transfer of HOBr to the snowpack is a rate-limiting step. 

Cycling of inorganic bromine compounds, whereby HBr, HOBr, and BiON02 are converted back to Br2 by aqueous-phase chemistry on sulfuric acid aerosol. 

Our proposed mechanism for autocatalytic bromine and chlorine chemistry in the liquid and gas phase is shown in Fig. 8.1. Hypobromous acid, HOBr, which is formed through an initial bromide oxidation (see below), is scavenged by sea-salt... 

A crucial parameter is the sea-salt aerosol content, which is largely determined by the wind speed. If we assume a 5 times higher aerosol content (15 x 10 m (liquid), per m (air), that is, 47 pg NaCl per m (air)), which is at the upper range of values typically found in the MBL, the reactive halogen concentrations increase dramatically. We estimate maximum concentrations (pmol moF) after two days of [HCl] = 70, [HOCl] = 33, [HOBr] = 35, [HBr] = 3, [BrCl] = 38, [Br2] = 10 and [CI2] = 1.5. Catalytic ozone loss through bromine and chlorine during the second day increases to 1.4 or 0.6 nmol moF, respectively, corresponding to 58 % or 25 % of the ozone lost by photolysis and HOx chemistry. 

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