Bromide can be depleted from sea-salt aerosol particles in the marine boundary layer (MBL) and converted to reactive gas-phase species like Br, BrO, and HOBr, which affect ozone chemistry. Air pollution can enhance the bromine release from sea-salt aerosols and thus inject additional bromine into the MBL. During the winter monsoon the northern Indian Ocean is strongly affected by air pollution from the Indian subcontinent and Asia. As part of the Indian Ocean Experiment (INDOEX), aerosol particles were sampled with stacked filter units (SFU) on the NCAR Hercules C-130 aircraft during February-March 1999. We determined the vertical and latitudinal distribution of the major inorganic aerosol components (NH4+, Na+, K+, Mg2+, Ca2+, Cl−, NO3−, SO42−) and the Br− content of the coarse aerosol to examine the role of the bromine release on the gas-phase chemistry in the marine boundary layer over the tropical Indian Ocean. The aerosol mass and composition varied significantly with air mass origin and sampling location. In the northern part of the Indian Ocean (5°–15°N, 66°–73°E), high concentrations of pollution-derived inorganic species were found in the marine boundary layer extending from the sea surface to about 1.2 km above sea level. In this layer, the average mass concentration of all aerosol species detected by our technique was comparable to pollution levels observed in industrialized regions. In the Southern Hemisphere (1°–9°S, 66°– 73°E), the aerosol concentrations rapidly declined to remote background levels. A chloride loss from the coarse aerosol particles was observed in parallel to the latitudinal gradient of the non sea salt SO42− burden. In most of the samples, Br− was depleted from the sea-salt aerosols. However, we found an enrichment in bromide in aerosols affected by air masses originating over strong pollution sources in India (Bombay, Calcutta). In these cases the additional pollution-derived Br from organo-halogen additives in petrol outweighs the release of sea-salt bromine.
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 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.
2.1. Flight Pattern
 During the INDOEX intensive field phase (IFP), a total of 18 research flights (RFs) were performed with the NCAR C-130 aircraft over the Indian Ocean at an altitude range between 30 m above sea level (asl) up to 6500 m asl. Here, we report results of 15 of these flights (RF 2–16), performed between 18 February and 21 March 1999. The duration of these flights ranged from 6 to 8 hours. Samples in the marine boundary layer (MBL) were taken with sampling times ranging from 15 to 45 minutes. A full description of the different flights is available at the UCAR Internet site http://www.joss.ucar.edu/indoex/catalog/iop/c130/). Specific information on the meteorological situation during the C-130 flights, as well as flight track plots can be found in the C-130 overview paper by A. Clarke et al. (unpublished manuscript, 2001).
 Coarse and fine aerosol particles were separately collected using a system consisting of an isokinetic inlet (community aerosol inlet, CAI) directly coupled to a two-stage stacked filter unit (SFU). The design of the CAI reduces distortion of the pressure field at the nozzle tip and the resulting problems associated with flow separation and turbulence. The aerosol sampling system and procedures are based on a design described in detail previously [Andreae et al., 1988; Blomquist et al., 2001; Huebert et al., 1998; Andreae et al., 2000]. A recent intercomparison experiment including a new low turbulence inlet (LTI) and the CAI revealed that the CAI has no significant inlet losses for dry submicron aerosol particles and not more than 10% for wet submicron particles in the moist boundary layer. For larger aerosol particles the intercomparison experiment indicate losses between 10 and 50% of the coarse particles with an increasing loss for particles bigger than 4 μm Daero [Huebert et al., 2000]. Therefore our data represent mainly the smaller fraction (1–3.5 μm Daero) of the coarse particles. The filter unit contained two sequential 47-mm diameter filters on polyethylene supports. The aerosol was collected and separated into coarse (between ∼1 μm, the cutoff-size of the filter pack used, and ∼3.5 μm, the cutoff of the aircraft “community inlet”) and fine particles on the first two filter stages. The first stage held a Nuclepore filter (Corning Costar PC Membrane, nominal pore size 8.0 μm). The sampling system operated at flow rates that averaged 60 L min−1 (at ambient pressure and temperature) measured with thermal mass flow meters. The 50% cutoff aerodynamic diameter (Daero) of the 8-μm Nucleopore filter is about 1 μm at the face velocity used in INDOEX (ca. 70 cm s−1) [John et al., 1983]. Blanks for all filter types were obtained by placing filters in the sampling units in the aircraft and exposing them for a few seconds.
 The mixing of chemically distinct particles on filters (e.g., highly acidic sulfate aerosols with sea-salt aerosols) could lead to volatilization of HCl via acid displacement reactions. This is not a problem for bromine because the effective solubility of HBr is about 600 times larger than that of HCl and chloride is much more abundant than bromide (R. Sander et al., manuscript in preparation, 2001). Due to the short sampling times the interaction of gases like HOBr with the filters during sampling should be of minor importance. In this paper we report the results of the coarse aerosol measurements sampled on Nuclepore filters. Results of the fine aerosol measurements have been reported by Gabriel et al. .
2.3. Filter Handling and Analysis
 The Nuclepore filters loaded with coarse aerosol particles were transferred to 30-mL HDPE bottles in the field lab immediately after each flight and stored in a refrigerator until extraction. We used 3 mL Elga-water (R > 18 MΩ cm−1) for the extraction of soluble aerosol species. After addition of the extraction solutions, all samples were shaken vigorously for about fifteen minutes.
 The first sets of samples were analyzed during the campaign in a field laboratory at Hulule airport (Male, Maldives), the rest at the Max Planck Institute for Chemistry (MPIC), Mainz. Anions were determined using a Shimadzu HIC-6A ion chromatograph (IC) with an AS11 (DIONEX) column and ASRS-ultra suppressor (DIONEX) in external water mode with a 100-μL injection loop. The temperature of the system was 35°C, the flow rate of the eluent (NaOH, using a gradient mode) was 1 mL min−1. A plateau of high NaOH concentration at the end of the gradient program ensured regeneration of the ion exchange capacity of the column after each analysis. For the cation analysis we used a CS14 (DIONEX) column, an eluent flow rate of 1.2 mL min−1 (8 mM methanesulfonate, isocratic mode), and a CSRS-1 suppressor (DIONEX) in external water mode. The detection limits in standard solutions ranged between 0.09 and 0.13 μmol L−1 for cations and between 0.03 and 0.12 μmol L−1 for anions. Due to the high concentration differences of chloride and bromide in sea-salt aerosols, the samples were analyzed in a second run with an increased injection volume of 500 μL which enabled the detection of bromide with a detection limit of 1.5 nmol L−1 . The measurement of diluted seawater (1/50000) confirmed the precision of the bromide measurement within a error range of 10%. Aqueous standards were prepared from high-purity reagents with deionized Elga-water (R > 18 MΩ cm−1). All data are reported in molar mixing ratios in air (pmol of analyte per mole of air, parts per trillion (ppt)).
2.4. Calculation of Enrichment Factors
 The flux of sea-salt Br− and Cl− is mainly a function of wind speed, which drives the generation of sea-salt particles. In the absence of significant mineral dust, bromide or chloride can be analyzed relative to sea-salt constituents like Na+ or Mg2+. Since Na+ is a major component of sea salt and a stable ion which does not undergo chemical reactions in the atmosphere, the ratio Br/Na+ is a conservative tracer for the bromine deficit or enrichment of sea-salt aerosols compared to seawater. Na+ was used as the reference element to calculate the enrichment factor (EF) according to the following formulas:
The enrichment factors for fresh sea-salt particles are 1. The acidification of sea-salt particles in the atmosphere can lead to a depletion of Br− and Cl− and a resulting EF below 1. This depletion is often expressed as deficit (deficit = 1 − EF).
2.5. Air Mass Trajectories
 Back trajectories were computed using the HYSPLIT-4 (Hybrid Single-Particle Lagrangian Integrated Trajectory) model [Draxler and Hess, 1997] and the FNL meteorological data set produced by the U.S. National Climatic Data Center (NCDC). The vertical velocities from the FNL data set were used to derive the vertical transport component in the trajectory calculations. Comparison between the trajectories obtained with HYSPLIT-4 and those provided by the Dutch Weather Service (KNMI) during the INDOEX campaign showed good agreement [Verver et al., 2001].
3. Results and Discussion
3.1. Meteorological Situation
 During the period of the INDOEX intensive field campaign in 1999 the atmosphere north of the ITCZ is characterized by continental outflow from the Indian subcontinent and South or Southeast Asia. Northeasterly trade winds are persistent while convection over the continental source regions is suppressed by large-scale subsidence, thus limiting upward dispersion of pollution. High pollution levels were observed over the entire northern Indian Ocean down to the Intertropical Convergence Zone (ITCZ) located between the equator and 10° S. In the northern part of the Indian Ocean, high concentrations of carbonaceous aerosol and pollution-derived inorganic species were observed in a layer extending from the sea surface to about 3.5 km asl and could be explained with strong fossil fuel combustion and biomass burning on the Indian subcontinent during the winter monsoon [Gabriel et al., 2002; Lelieveld et al., 2001; Mayol-Bracero et al., 2002]. Boundary layer trajectories for this experiment suggest that emissions from India were responsible for most of the pollutant input into this air mass. The three major sources for anthropogenic aerosol to the INDOEX region are (1) a near-surface southward flow near Bombay, (2) a deeper strong plume flowing south and east off Calcutta, and (3) a westward flow originating from Southeast Asia [Rasch et al., 2001]. South of the ITCZ pristine air from the remote southern Indian Ocean prevails. The effects of continental outflow in the northern Indian Ocean could therefore be contrasted with very clean air from the southern Indian Ocean.
 The analysis of the ambient temperature and dew point temperature measurements of the C-130 during vertical profiles flown between 30 and 5000 m was used to define the height of the MBL over the Indian Ocean between 800 and 1200 m [Gabriel et al., 2002]. The meteorological environment of INDOEX has been discussed in detail by Verver et al.  and Rasch et al. .
3.2. Vertical and Latitudinal Distribution of Aerosol Concentrations in the MBL
Figure 1 shows the vertical distribution of (a) Na+ and (b) Br− in the coarse (1–3.5-μm Daero) aerosol over the Indian Ocean. Na+ and Br− are mainly found in the MBL below 1000 m asl. This indicates that the ocean is the main source for bromide due to the formation of sea-salt aerosols. Averaged over the MBL of the study region and the time period of INDOEX IFP, we found 180 (± 115) pmol/mol Na+ and 0.19 (± 0.13) pmol/mol Br− in the coarse aerosol with clear variations depending on sampling location and air mass origin. The average molar Br−/Na+ ratio of 0.00108 is lower than that of sea water of 0.00180.
Figure 2a shows the latitudinal distribution of coarse (1–3.5-μm Daero) soluble inorganic aerosol in the MBL over the Indian Ocean. In the northern part of the Indian Ocean study region (5° to 15°N, 66° to 73°E) increased concentrations of water-soluble aerosol with a high contribution of pollution-derived inorganic species were found, whereas sea-salt aerosol (NaCl) was found in relatively constant concentrations around 0.4 μg m−3. In the Southern Hemisphere (1° to 9°S, 66° to 73°E), the soluble inorganic aerosol concentrations rapidly declined to background levels of about 0.5 μg m−3. It is obvious that the decrease in total coarse aerosol mass does not result from a change in sea-salt aerosol production but from reduced transport processes from continental sources. As a typical example for this process we show the gradient of NO3− between the Northern and the Southern Hemisphere (Figure 2b) which indicates a source of NO3− possibly via transport of NOx from India, Arabia and Southeast Asia. Similar north to south gradients were also found for non sea salt sulfate (nss-SO42−), indicating the impact of SO2 emissions to the aerosols over the northern Indian Ocean, and for potassium, indicating a biomass burning contribution. The increased values of bromide over the northern Indian Ocean (Figure 2c) points to a bromine source in the Northern Hemisphere since there is no detectable latitudinal gradient in the sea-salt aerosol mass. Overall, the concentrations of pollutant aerosol species in the MBL are seen to decrease with increasing transport times and distances between pollution sources and the sampling sites in the region of the ITCZ in the Southern Hemisphere.
3.3. Halogen Deficit and Enrichment in the Sea-Salt Aerosols
Figure 3 shows the vertical distribution of the bromide EF (EFBr) in the coarse (1–3.5 μm Daero) aerosol over the Indian Ocean sampled during INDOEX in the region between 15°N to 9°S and 66° to 73°E. EFBr values between 0.1 and 2.7 were found in the MBL between 50 and 1200 m. Obviously the bromide depletion is not limited to regions near the sea surface. They show a similar scatter in the lower and upper regions of the MBL. Although the sampling method, with its widespread spatial distribution of sampling locations, is not suitable to deduce a vertical profile, it is important to note that very small EFBr values are also found in higher regions of the MBL. This would be in agreement with 1-D model studies [von Glasow and Sander, 2001]. Figure 4 shows the latitudinal distribution of EFBr and EFCl in the coarse aerosol in the MBL. There is a striking difference between the enrichment of these halogens in the aerosols. Highest EFCl values are found in the south, whereas the highest EFBr values were observed in the north. We observed a deficit of chloride in the aerosol with EFCl between 0.0 and 1.0 at all locations. EFCl values above 0.8 are only observed in the most southern aerosol samples (see Figure 4), which are not affected by air pollution (see Figures 2 and 3). Since dechlorination is mainly driven by acid displacement, the low chloride EFs indicate acidified aerosol particles. During this process a less volatile strong acid, such as sulfuric acid or HNO3 derived from anthropogenic SO2 and NOx, displaces Cl− in the form of the more volatile acid HCl (H2SO4 + Cl− → HCl+ HSO4−, HNO3 + Cl− → NaNO3 + HCl). The observed anticorrelation of nss-SO42− and the chloride EF shown in Figure 5a supports this concept.
 In the northern part of the Indian Ocean study region, the whole range from EFBr = 0.1 to EFBr = 2.7 can be found. In the Southern Hemisphere we exclusively observed bromide deficits with EFBr below 0.7. Since aerosol NO3− and nss-SO42− are significantly reduced (see Figure 2), and NOx and N2O5, which can react with Br− on the aerosol particles, are not present in significant concentrations in the unpolluted air of the Southern MBL [Rhoads et al., 1997], the bromine release from sea-salt aerosols cannot be explained with the release of BrNO or BrNO2. The autocatalytic mechanism according to Vogt et al.  described above, provides a good explanation of the observed bromine release under clean conditions. As can be seen in Figure 4, the EFBr under clean condition in the Southern Hemisphere is lower than the EFCl, which is consistent with the autocatalytic mechanism. If the dehalogenation were simply acid displacement of volatile hydrogen halides by less volatile sulfuric acid, the relative Cl− loss should be greater than the relative Br− loss, because HCl is more volatile than HBr.
 Strong acidification of the aerosol is needed for the chlorine release, but bromine can be released from only slightly acidic solution around pH 5.5 [Keene et al., 1998]. Figures 5a and 5b indicate that the EFCl during INDOEX is correlated with the nss-SO42− content of the aerosol, which is not the case for the EFBr. Most samples are depleted in bromide, with a mean EFBr of 0.5 for samples where depletion occurs. In some samples with strong Cl− deficits, however, we found Br− enrichment (see Figure 4). This indicates that for these cases bromine must be transported within the pollution plume from continental sources. Tracer substances characteristic for biomass or biofuel burning (excess potassium) and for fossil fuel burning (nss-SO42− and nitrate) in the fine aerosols [Gabriel et al., 2002] indicate that biomass burning and fossil fuel burning could affect the bromide enrichment over the northern Indian Ocean significantly. The transport of bromine compounds with the pollution plumes seems to overwhelm the effect of bromine release from the particles. Since both processes were active at the same time, we might expect a high gaseous concentration of several inorganic bromine species, e.g. HOBr, HBr, BrO, BrNO3 in the polluted air over the northern Indian Ocean.
3.4. Back-Trajectory Analysis of Aerosol Bromide Over the Indian Ocean
 Since there is no detectable latitudinal gradient in the sea-salt aerosol mass, the increased values of bromide over the northern Indian Ocean (Figure 2c) together with the observed bromide enrichment factors above 1 (Figure 4) points to an additional bromine source in the Northern Hemisphere. The 6-day back trajectories for samples collected in the north indicate that the bromide enrichment can be attributed to air pollution from India (Figure 6a). EFBr > 1 can be found in aerosol samples from air that passed the Calcutta or Bombay plume during the last few days. According to the EDGAR (Emission Database for Global Atmospheric Research) emission inventory, the production of SO2 from human activities in India is mainly located along the western coast, with a maximum at Bombay (19°N), and another maximum at the southern tip of India; most of the sources of emissions on the eastern coast are located between Madras (13°N) and Calcutta (23°N). Industrial activities and traffic are potential sources for bromine in this region. 1,2-dibromoethane from fuel additives in automobile exhaust could be a considerable bromide source to the Bombay and Calcutta plume. Since leaded petrol contains the additive 1,2-dibromoethane (and also 1,2-dichloroethane), the lead and halogen compounds react and predominantly produce PbBrCl during the combustion process, which is emitted into the atmosphere. Aerosol measurements at a coastal site in England revealed a comparable connection and showed a strong correlation between Pb and Br with a Br/Pb ratio close to that for fuel [Sturges and Harrison, 1986]. Measurements of Pb along the Indian coast on the R/V Ron Brown during INDOEX revealed increased Pb concentrations up to 100 ng m−3 in air masses affected from the Bombay plume (R. Dickerson, personal communication, 2001). Assuming that petrol is the main Pb source and the molar Br/Pb ratio in the emissions is 1, the Br mixing ratio could reach up to 12 pmol/mol. These measurements confirm that 1,2-dibromoethane from fuels could be a significant source of bromine in the atmosphere. The additional bromine from 1,2-dibromoethane and PbBrCl enters the halogen reaction cycles and some of it will be scavenged onto sea-salt particles. This affects EFBr significantly. The relative effect on chloride, however, is small due to the large amount of chloride in sea salt. Lead concentrations decreased from 460–1390 ng m−3 to 200–680 ng m−3 at various sites in India between 1988 and 1998. The reduction of lead concentration is likely from the recent introduction of unleaded petrol in the big cities of India. In a recent study of aerosols in Bombay, lead concentrations were found to be below 40 ng m−3 [Venkataraman et al., 2002]. However, more than 65% of the petrol consumed in India is still leaded (M. S. Reddy, personal communication, 2001).
 To our knowledge there are no significant natural sources for continental bromine from India. Crustal rocks contain almost no bromine and a high Br−/Na+ mass ratio for soil dust (0.0305) is only reported for Saharan dust [Adepetu et al., 1988]. Although dust storm statistics suggest that there are major sources of mineral dust in northern India, very little dust deflation occurs in southern India below 15°N at any time of year [Ackerman and Cox, 1989; Ginoux et al., 2001]. During March 1999, the Arabian Peninsula was the main source of dust in the INDOEX region, but air mass trajectories indicate that dust could be advected in a southwesterly direction only on days 5, 15 and 17 of March (S. Cautenet et al., unpublished manuscript, 2001). This is consistent with a low abundance of nonvolatile aerosol observed by A. Clarke et al. (unpublished manuscript, 2001). Moreover, the amount of nss- Ca2+ (which is a tracer for dust) in our aerosol samples is not correlated with the bromide concentration.
 Back trajectories for air samples with low EFBr (<0.3) (Figure 6b) indicate that these air masses originate from the Indian Ocean or from regions without significant air pollution. The back-trajectory analysis confirms that significant bromine release took place in air masses that are not influenced by air pollution. In these regions bromine release mechanisms involving significant concentrations of nitrogen oxides can be excluded. This lends further support to the autocatalytic bromine release mechanism that was described earlier.
3.5. Model Simulation of the Effect of Bromine Release on Gas-Phase Chemistry
 We did runs with a box model of the marine boundary layer (Model of Chemistry Considering Aerosols (MOCCA)) [Sander and Crutzen, 1996; Vogt et al., 1996] for different scenarios. It is not possible to mimic exactly the conditions of the airborne measurements presented here with a box model because several air masses with different history influenced the samples, as the flights were not conducted as Lagrangian experiments. Therefore we discuss three of a large set of model runs with initial and boundary conditions that are typical for the INDOEX campaign.
 For the model initializations we used measured data from the INDOEX campaign as listed by Wagner et al. . The model was run under clean conditions, low NOx mixing ratios (NOx = 5 pmol/mol) SO2 = 25 pmol/mol, with a boundary layer height of 650 m and a relative humidity of 81%. The pH is between 6 and 6.5 for all runs. The sum of reactive bromine (all reactive gaseous bromine species except HBr) varies diurnally between 1 and 1.7 pmol/mol for the run with little activation and from 2 to 3.5 pmol/mol for the runs with high activation, respectively. The complex reaction mechanism that is included in MOCCA shows different degrees of bromide depletion in the sea salt aerosol (as in the measurements) depending on the initial and boundary conditions that were chosen for the model runs. Parameters that are important for the degree of bromide depletion are the gas-phase O3 concentrations, the availability of gas-phase acids, and the temperature. Figure 7a shows results from MOCCA runs for different temperatures and O3 mixing ratios. Both, decreasing the temperature and increasing the O3 mixing ratios lead to a stronger activation of bromine from the sea salt. In the case with decreased temperature, increased solubility accelerates the recycling through aerosol particles, whereas in the case with increased O3 the reaction of Br with O3 to form BrO and the subsequent reaction with HO2 to HOBr is stronger. More HOBr leads to a stronger depletion of Br− from the sea salt aerosol. Although BrO mixing ratios were small (0.4 pmol/mol with early morning peaks of 1.2 pmol/mol) we still see an effect on DMS (Figure 7b). DMS is strongly destroyed by BrO in the model runs with higher bromide depletion. This supports the findings of Ingham et al.  and is discussed in some detail by von Glasow et al. . The photochemistry of gaseous bromine species will also affect ozone. Very large diurnal variations in ozone concentrations of about 32% of the mean were observed over the tropical Indian Ocean on the spring 1995 cruise of the R/V Malcolm Baldrige [Dickerson et al., 1999]. A similar observation was done during the INDOEX IFP 1999 cruise of the R/V Ronald H. Brown [Stehr et al., 2002]. Model simulations indicate a diurnal variation in O3 of 22% over the Indian Ocean for a model run that includes reactions of aerosol-derived bromine [Dickerson et al., 1999].
4. Conclusions and Summary
 During the winter monsoon the northern Indian Ocean is strongly affected by air pollution from the Indian subcontinent and Asia. Tracer substances characteristic for biomass or biofuel burning (potassium) and for fossil fuel burning (nss-SO42− and nitrate) were found in enhanced concentrations over the northern Indian Ocean, but the concentration of sea-salt aerosols was relatively uniformly distributed over the complete study region. We observed a deficit of chloride in the aerosol at all locations with EFCl between 0.0 and 1. EFCl values above 0.8 were only observed in the most southern aerosol samples, which were not affected by air pollution. In the Southern Hemisphere we exclusively observed bromide deficits with EFBr below 0.7. The strong bromide deficits observed in air originating over the clean Indian Ocean support the concept of the autocatalytic mechanism according to Vogt et al. . Most samples collected over the Indian Ocean are depleted in bromide, with a mean EFBr of 0.5 for samples not affected by continental air masses. In some samples with strong Cl− deficits, however, we found Br− enrichment. Back trajectories show that these aerosols are affected by air masses originating over strong pollution sources in India like Bombay or Calcutta. Due to a lack of other strong continental bromide sources in India, this indicates the contribution of traffic as a potential pollution source. Both, natural bromide from sea-salt aerosol as well as bromine resulting from fuel additives may be converted to reactive BrO and affect the gas-phase chemistry over the northern Indian Ocean. On the one hand, bromine will react with DMS and thus reduce its concentration. On the other hand, BrO will destroy O3 in a catalytic cycle and thereby decrease the overall oxidation efficiency of the MBL. Considering that the greatest industrial and agricultural developments will occur in the Indian subcontinent and Southeast Asia, we should expect increasing air pollution in this region of the world. Therefore the knowledge of the cleaning capacity of the tropical troposphere is of great importance.
 We thank all participants of the INDOEX project, in particular the C-130 team, and the staffs of NCAR-RAF, the MPIC mechanical workshop, and Carol Strametz for the language support. We gratefully acknowledge the government and people of the Republic of Maldives for their hospitality. The National Center for Atmospheric Research is sponsored by the National Science Foundation and is managed by the University Corporation for Atmospheric Research. The MPIC part of this study was funded by the German Max Planck Society, with additional support for the flight operations from the U.S. National Science Foundation.