The chemistry of atmospheric bromine

Bromine may act as a catalyst for recombination of ozone and could be more efficient than either nitric oxide or chlorine. The lower atmosphere contains small concentrations of gaseous bromine produced in part by marine activity, in part by volatilization of particulate material released during the combustion of leaded gasoline, with an additional contribution due to the use of methyl bromide as an agricultural fumigant. Observations by Lazrus et. al. (1975) indicate small concentrations f bromine, ~10 '11 (v/v), in the contemporary stratosphere and appear to imply a reduction of approximately 0.3% in the global budget of 03. Estimates are given for future reductions in 03 which might occur if the use of CH3Br as an agricultural fumigant were to continue to grow at present rates. The chemistry of atmospheric ozone may be influenced to a remarkable extent by exceedingly small quantities of select stratospheric gases. Attention has focused in recent years on nitric oxide released by SST's (Johnston, 1971; Crutzen, 1972) and on chlorine produced by photodecomposition of chlorofluoromethanes (Molina and Rowland, 1974). This paper is devoted to a discussion of bromine. As we shall see, concentrations of bromine as small as 1 part in 1011 (v/v) can have an appreciable influence on ozone. There are indications that the mixing ratio of gaseous forms of acidic bromine, represented most probably by HBr, according tothe discussion below, may approach 10 '11 in today's stratosphere (Lazrus et. al., 1975) with similar values observed in the troposphere (see below). The mixing ratio of acidic bromine appears to increase with increasing altitude between 15 and 19 km and there is evidence for similar behavior in the height distribution of particulate bromine (Lazrus et. al., 1975; Delany et. al., 1974). It is difficult to escape the conclusion that there must be a stratospheric source for particulate and gaseous acidic bromine.-• Decomposition of methyl bromide offers a plausible explanation. Methyl bromide has been detected recently in surface waters of the ocean and in antarctic snow (R. Rasmussen, private communication, 1975). It is used extensively as a fumigant and there may be additional natural sources as discussed below. Bromine is present in both gas and particulate phases in the troposphere, with the gas phase dominant by a factor which ranges typically between 4 and 20. The mixing ratio of gaseous bromine is about 1.5 x 10 '11 over Hawaii (Moyers and Duce, 1972) and it seems probable that the major constituent is HBr (see below). Measurements near the South Pole (Duce et. al., 1973) suggest values smaller by about a factor of 5. There are indications that the gaseous component is derived primarily from volatilization of bromine carried into the atmosphere as a component of marine aerosols (Duce et. al., 1963, 1965) with additional contributions from decomposition of CH3Br and from particles released during the combustion of leaded gasoline (Moyers and Duce, 1972; Cadle, 1972; Martens et. al., •'Gaseous BrO could contribute to the "particulate" bromine found on neutral filters. Copyright 1975 by the American Geophysical Union 1973). Using available data for the gasoline source strength, and empirical values for the atmospheric lifetime of HBr, we estimate a source strength for total gaseous bromine of 1.1 x 106 tons yr q , of which approximately 80% is derived from marine aerosols, 10% from CH3Br , and 10% from automobiles.•The importance of bromine in the stratosphere lates to its potential role as a catalyst for recombination of ozone. Catalysis can take place either through Br + 03 --> BrO + 02 BrO + O --> Br + O2 (1)