During the winter monsoon in February and March 1999, the Indian Ocean Experiment (INDOEX) was performed to investigate long-range transport of air pollution from South and Southeast Asia towards the Indian Ocean [cf. Ramanathan et al., 2001]. This season was selected because northeasterly winds are persistent while convection over the continental source regions is suppressed by large-scale subsidence, thus limiting upward dispersion of pollution. Combustion of fossil fuel and biomass cause strong air pollution with gases and particles, especially in the industrialized regions like Bombay, Madras or Calcutta [Lelieveld et al., 2001, Gabriel et al., 2002; Reiner et al., 2001; Mayol-Bracero et al., 2002; Venkataraman et al., 2002]. As the pollution plumes are transported over the ocean, they are mixed with air masses that contain aerosols produced over the sea, and the gas-phase species interact with the aerosol particles. The resulting composition of the aerosol is one of the key characteristics defining its chemical and physical behavior. Reactions of halogens in the marine boundary layer (MBL) involving chlorine and bromine from sea salt can affect the concentrations of ozone, hydrocarbons, and cloud condensation nuclei (CCN). One important consequence of the heterogeneous reactions occurring in these air masses is that bromine can escape from sea-salt aerosol particles and form reactive gas-phase species like Br, BrO, and HOBr. These inorganic bromine compounds have the potential to destroy ozone catalytically [Sander and Crutzen, 1996; R. Sander et al., manuscript in preparation, 2001]. Several investigators have addressed the possible role of halogens for the photochemistry in the MBL [Fan and Jacob, 1992; Finlayson-Pitts, 1993; Graedel and Keene, 1995; Keene et al., 1996, 1990; McKeen and Liu, 1993; Parrish et al., 1992; Pszenny et al., 1993]. In addition to the known release mechanisms for reactive halogens, which require significant concentrations of acid and/or nitrogen oxides, Vogt et al.  and Sander and Crutzen  proposed an autocatalytic mechanism for halogen release form sea-salt aerosol that does not require high concentrations of NOx. On wet sea-salt aerosols, absorption of HOBr leads to the release of BrCl and Br2, which photolyze to produce Br atoms that may provide an additional photochemical ozone sink in the gas phase. Depending on the sea-salt concentration and given a boundary layer that is stable for a few days, gaseous HOCl and HOBr may reach molar mixing ratios that can lead to sulfur (IV) oxidation, and bromine-catalyzed ozone loss according to the following reactions:aqueous phase (aerosol):
A laboratory study by Fickert et al.  shows that the autocatalytic bromine release from sea-salt solutions that follows HOBr uptake is acid-catalyzed. An observed anticorrelation of the bromide concentration and the sulfate concentration in aerosols confirmed this mechanism [Ayers et al., 1999; Murphy et al., 1997]. According to the model, HNO3 and H2SO4 that are scavenged by the aerosol particles assist in Br2 and BrCl formation via acid-catalysis according to reaction (1) which are then released from the aerosol. The presented data-set from the clean Indian Ocean enables us to test the importance of the mechanism according to Vogt et al. . Field experiments have confirmed the importance of tropospheric bromine chemistry for the polar regions in spring [Barrie et al., 1988] and for the Dead Sea [Hebestreit et al., 1999]. However, because of the scarce gas-phase bromine measurements at low latitudes (R. Sander et al., manuscript in preparation, 2001) the global role of bromine in the MBL is still uncertain. During INDOEX we determined the Br− content of the sea-salt aerosol and the major water-soluble inorganic aerosol components (NH4+, Na+, K+, Mg2+, Ca2+, Cl−, NO3−, SO42−) to examine the extent of the bromine release and its potential influence on the gas-phase chemistry in the polluted and unpolluted marine boundary layer (MBL) over the tropical Indian Ocean. Due to the strong continental outflow from the Indian subcontinent and South or Southeast Asia towards the Indian Ocean during the winter monsoon, the transport of bromine together with other pollutants from continental sources could be a significant source in addition to the release of bromine from sea-salt aerosols.
 The most important source for bromine over the ocean is the ocean itself. Sea-salt aerosol particles are produced as sea spray from wind acting on the ocean surface and the sea-salt aerosol concentration is strongly dependent on wind velocity [Gong et al., 1997a, 1997b; O'Dowd and Smith, 1993]. The aerosol mass in the MBL is dominated by sea-salt particles with an estimated global emission rate of 5900 Tg yr−1 [Tegen et al., 1997]. Inorganic bromine in seawater consists mainly of bromide (Br−) with a Br−/Na+ ratio of 6.25 g/kg. This ratio can also be found in freshly produced sea-salt aerosols. The relative mixtures of sea-salt aerosols with acids and bases together with relative humidity control aerosol pH and thereby the release of bromine and chlorine [Keene et al., 1998]. These mixtures depend on the proximity to source regions; synoptic, seasonal, and interannual variability in wind fields; and on deposition processes. Chemical measurements over the Indian Ocean during INDOEX 1999 revealed the simultaneous presence of tracer substances characteristic for biomass burning and for fossil fuel burning. [Mayol-Bracero et al., 2002; Gabriel et al., 2002; Reiner et al., 2001]. The combustion of biomass or fossil fuel can serve as a source for bromine compounds [Duce et al., 1983; Maenhaut et al., 1996]. Especially traffic emissions like the combustion of leaded fuel are considered as a strong bromine source [Harrison and Sturges, 1983].
 In this paper we discuss the results of the measurements of aerosol chemistry performed on board the Hercules C-130 aircraft (C-130) based at Hulule airport, Male, Republic of Maldives (4.2°N, 73.5°E) (cf. A. Clarke et al., unpublished manuscript, 2001) during INDOEX. The results are analyzed in the context of spatial distribution of the aerosol and air mass origin. We discuss the correlation between bromide and other species in sea salt aerosol and present model simulations to elucidate the chemical processes that control inorganic bromine and the impact on ozone concentration in the MBL.