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This paper considers the effect of heterogeneous bromine reactions on stratospheric photochemistry. We have considered reactions on both sulfate aerosols and on polar stratospheric clouds (PSCs). It is shown that the hydrolysis of BrONO2 on sulfate aerosols enhances the HOBr concentration, which in turn enhances the OH and HO2 concentrations, thereby reducing the HCl lifetime and concentration. The hydrolysis of BrONO2 leads to a nighttime production of HOBr, making HOBr a major nighttime bromine reservoir. The photolysis of HOBr gives a rapid increase in the OH and HO2 concentration at dawn, as was recently observed by Salawitch et al. [1994]. The increase in the OH and HO2 concentration, and the decrease in the HCl concentration, leads to additional ozone depletion at all latitudes and for all season. At temperatures below 210 K the bulk phase reaction of HOBr with HCl in sulfate aerosols becomes important. The most important heterogeneous bromine reactions on polar stratospheric clouds are the mixed halogen reactions of HCl with HOBr and BrONO2 and of HBr with HOCl and ClONO2.

This paper reviews the current knowledge of gas phase bromine photochemistry and presents a budget study of atmospheric bromine species. The effectiveness of the ozone catalytic loss cycles involving bromine is quantified by considering their chain length and effectiveness. The chain effectiveness is a new variable defined as the chain length multiplied by the rate of the cycle's rate-limiting step. The chain effectiveness enables a fair comparison of different catalytic cycles involving species which have very different concentrations. This analysis clearly shows that below 25 km the BrO/C10 and BrO/HO2 cycles are among the most important ozone destruction cycles. Introduction This paper is a review of gas phase bromine photochemistry and is intended to complement the companion paper [Lary et al. this issue] which considers heterogeneous bromine photochemistry. Bromine enters the atmosphere by a variety of natural and anthropogenic processes. The three main bromine source gases that can reach the stratosphere (i.e. are not removed from the troposphere by rainout, reaction with OH or photolysis) are CH3Br, CBrC1F2 and CBrF3. The most abundant of these source gases is methyl bromide (CH3Br) whose natural source is mainly due to oceanic biological processes. In these, mainly algal, processes CH3Br is formed together with other species such as CH2Br2, CHBr3, CH2BrC1 and CHBrC12. The oceans are a significant natural source of CH3Br [Singh et al., 1983]. Measurements of larger tropospheric northern henrisphere mixing ratios suggest a large land based northern hemisphere source of CH3Br which could well be anthropogenic [Penkett et al., 1985; Reeves and Penkett, 1993]. The main industrial use of CH3Br is as a fumigant, particularly for the treatment of soils. CH3Br is also used in quarantine treatments and in insect and rodent control. The wide variety of CH3Br measurements made over the last decade in different parts of the world [Berg et al., 1984; Rasmussen and Khalil, 1984; Penkett et al., 1985; Cicerone et al., 1988; Fabian et al., 1994; Kaye et al., 1994] suggest that the natural background concentration of CH3Br in the troposphere is approximately 10 pptv. CH3Br concentrations of up to 15 pptv have also been observed; these are likely to reflect the effect of anthropogenic sources. The first study of atmospheric bromine chemistry was by Yung et al. [1980], who pointed to the general importance of atmospheric bromine chemistry and to the catalytic destruction of ozone by the C10/BrO cycle. Bromine has been shown to play a significant role (m20%) in the formation of the ozone hole in polar stratospheric regions [McElroy et al., 1986]. The contributions to ozone loss from bromine reactions are largest below about 20 km [e.g., Poulet et al., 1992; Garcia and Solomon, 1994]. Bromine plays an important role in stratospheric ozone depletion despite being much less abundant than chlorine [World Meteorological Organisation ( WMO), 1992].When atmospheric bromine chemistry is compared to chlorine chemistry, it can be seen ...

Abstract.Carbon aerosols are produced by all combustion processes. This paper investigates some possible effects of heterogeneous reduction of atmospheric constituents on carbon aerosols. Reduction of HNO3, NO2, and 03 on carbon aerosols may be an important effect of increased air traffic that has not been considered to date. It is shown that if HNO3, NO2 and 03 are heterogeneously reduced on atmospheric amorphous carbon aerosols, then a significant, lower stratospheric ozone loss mechanism could exist. This ozone loss mechanism is almost independent of temperature and does not require the presence of sunlight. The mechanism can operate at all latitudes where amorphous carbon aerosols are present. The relative importance of the mechanism increases with nightlength. The reduction of HNO3 on carbon aerosols could also be a significant renoxification process wherever carbon aerosols are present. Owing to the very different soot levels in the two hemispheres, this implies that there should be a hemispheric assymetry in the role of these mechanisms. The renoxification lea, ds to simulated tropospheric HNO3/NO• ratios that are close to those observed. In contrast to the stratospheric response, the tropospheric production of NO• due to the reduction of HNO3 would lead to tropospheric ozone production.