Air-sea fluxes of biogenic bromine from the tropical and North Atlantic Ocean

Air-sea fluxes and bulk seawater and atmospheric concentrations of bromoform (CHBr 3) and dibromomethane (CH2Br2) were measured during two research cruises in the northeast Atlantic (53–59 ◦ N, June–July 2006) and tropical eastern Atlantic Ocean including over the African coastal upwelling system (16–35 ◦ N May–June 2007). Saturations and sea-air fluxes of these compounds generally decreased in the order coastal > upwelling > shelf > open ocean, and outside of coastal regions, a broad trend of elevated surface seawater concentrations with high chlorophylla was observed. We show that upwelling regions (coastal and equatorial) represent regional hot spots of bromocarbons, but are probably not of major significance globally, contributing at most a few percent of the total global emissions of CHBr 3 and CH2Br2. From limited data from eastern Atlantic coastlines, we tentatively suggest that globally, coastal oceans (depth <180 m) together contribute∼2.5 (1.4–3.5) Gmol Br yr −1 of CHBr3, excluding influences from anthropogenic sources such as coastal power stations. This flux estimate is close to current estimates of the total open ocean source. We also show that the concentration ratio of CH 2Br2/CHBr3 in seawater is a strong function of concentration (and location), with a lower CH2Br2/CHBr3 ratio found in coastal regions near to macroalgal sources.


Introduction
Oceanic production of short-lived organic bromine compounds such as CHBr 3 and CH 2 Br 2 by phytoplankton and macroalgae has been proposed to represent an important link Correspondence to: L. J. Carpenter (ljc4@york.ac.uk) between ocean biology, atmospheric composition and climate (Carpenter and Liss, 2000;Quack et al., 2004;Yang et al., 2005;Salawitch, 2006).Bromine atoms, released both from photooxidation of organic bromine and, in the marine boundary layer (MBL), from heterogeneous cycling on seasalt aerosol, enter catalytic ozone destruction cycles and also indirectly lead to a reduction in tropospheric ozone production by suppressing levels of nitrogen oxides (von Glasow and Crutzen, 2007, and references therein).Tropospheric ozone, a key greenhouse gas and air pollutant, may be reduced by 5-30% compared to simulations without bromine (e.g.von Glasow et al., 2004;Yang et al., 2005).Short-lived halogenated compounds and their oxidation products may also be delivered to the stratosphere through rapid convection from the MBL (Solomon et al., 1994).CHBr 3 represents the single largest source of organic bromine from the ocean to the atmosphere and the contribution of short-lived bromine species to total bromine in the lower stratosphere is believed to be around 20-60% (Sturges et al., 2000;Pfeilsticker et al., 2000;Nielsen and Douglass, 2001).
Although there have been a number of extensive studies and reviews which have greatly increased our knowledge of biogenic short-lived bromine compounds (Quack and Wallace, 2003;Butler et al., 2007a), there are still uncertainties regarding their global distributions, exacerbated by the spatial variability in their sea-air fluxes.Models (e.g. von Glasow et al., 2004;Yang et al., 2005) assume a variety of parameterisations for oceanic bromine emissions, which require better validation by global oceanic measurements.
Here we report air-sea fluxes and bulk, simultaneous, seawater and atmospheric concentrations of CHBr    using a clean metal bellows pump (Model MB118E, Senior Aerospace, USA).Gas-tight syringes (40 mL, Samco, USA) were used to draw discrete 40 mL seawater samples from a line connected to the ships onboard non-toxic seawater supply, which was allowed to flush for 2 min prior to the sample being drawn.The ships' intakes for the water supply were located at average depths of 2 m for the Celtic Explorer and 6 m for RRS Discovery.Water samples were filtered using Minisart 0.45 µm cellulose filters, and, while the majority of water samples were analysed immediately, occasionally they were stored in gas-tight syringes in the dark at ∼1 • C and sampled within 12 h.Dissolved trace gases were analysed by injecting into an opaque glass purge-and-trap vessel maintained at 50 • C and sparged with 50 mL min −1 N 2 (Grade 5.0, BOC) for 50 min.Sparging efficiencies were 91% for CH 2 Br 2 and 94% for CHBr 3 .For further analytical details see Carpenter et al. (2007) and Wevill and Carpenter (2004).
Calibration of the GC-MS was achieved using a permeation system as described in Wevill and Carpenter (2004).Two glass gas-tight permeation tube holders containing dis- posable permeation tubes (Trace Source ™ , Ecoscientific) filled with a single pure halocarbon (>99.9%Sigma Aldrich, verified in-house by mass spectrometry) are each placed in an aluminium block maintained at either 40 • C (CH 2 Br 2 ) or 70 • C (CHBr 3 ).The permeation tubes are swept constantly with 100 mL min −1 N 2 (Grade 5.0, BOC) and into a 10 µl sample loop, which is injected into a 50 mL min −1 flow of N 2 (Grade 5.0, BOC) to generate low part per trillion (ppt) mixing ratios of halocarbon mixtures.The accuracy of the calibration, which includes errors in the determination of permeation rates, sample loop volume and flow rates, is estimated as ±12%.The total uncertainty of the measurements is ∼±13% for air and ∼±16% for seawater, with a precision of between 3-8% depending on compound and concentration.Limits of detection are ∼0.01-0.05ppt in air and 0.02-0.06pmol dm −3 in water.A comparison of our previous CHBr 3 atmospheric measurements at Mace Head and at Cape Grim (1998 and 1999 measurements) compare well to simultaneous observations by NOAA (2004 Scale) (Butler et al., 2007b).The analytical uncertainty for a sea-air flux calculation is dominated by the uncertainty in the water measurement for compounds which are supersaturated in seawater.In situ measurements of Chl-a were determined during the MAP cruise by fluorescence detection (exciting wavelength: 436 nm, emission wavelength: 680 nm) after filter extraction with acetone.The method is described in Arar and Collins (1997).Elsewhere, Chl-a was provided by the NEODAAS Remote Sensing Group, Plymouth Marine Laboratory, from MODIS/Aqua ocean colour satellite data.

Tropical Atlantic surface seawater and air distributions
The tropical north east Atlantic is characterised by a northeasterly trade wind system that converges into the Inter-Tropical Convergence Zone (ITCZ) from the north.In the ocean, the 1000 km wide and ∼500 m deep Canary Current flows southward along the African coast between 30 • N (north of Canary islands) and 10 • N (south of Cape Verde islands) (Fedoseev, 1970).Within this current, the African coastal upwelling zone, located between approximately 25 • N and 10 • N and between ∼20 • W to the northwest African coast, is a year-round feature caused by coastal divergence resulting from the northeasterly wind field, showing a seasonal peak in winter/spring due to movement of the ITCZ (Hagen, 2001).Tomzcak (1980) identified that north of 21 • N, the upwelling is fed mainly by the North Atlantic Central Water (NACW) mass, whereas south of 21 • N it shows properties similar to the South Atlantic Central Water (SACW).In the region of 15 • N, the Canary Current deviates westward under the influence of the Equatorial Countercurrent (Peterson et al., 1996).Samples from the RHaMBLe cruise were collected and analyzed during 22 May-6 June 2007.The ship sailed along a transect from the Canary Islands to the Cape Verde islands, then directly east towards the coastal upwelling, followed by a zig-zag transect through the upwelling heading north back to the Canaries.Figure 1 shows the CHBr 3 and CH 2 Br 2 surface seawater concentrations along the RHaM-BLe cruise track along with the distinctive Chl-a features (MODIS chlorophyll ocean colour satellite image) in the re- gion, which were near stationary during the period 26 May-6 June.Elevated bromocarbon concentrations are apparent in the high Chl-a containing waters of the upwelling, particularly in the region 16-20 • N, 16-18 • W. This is also apparent in Figs. 2 and 3 which shows time series of CHBr 3 and CH 2 Br 2 concentrations in air and in water, calculated fluxes, ocean depth, Chl-a (satellite data) and the "temperature anomaly" -defined here as the instantaneous sea surface temperature (SST) subtracted from the average SST of the cruise.The upwelling centres near to the coast are clearly identifiable by the low SSTs (high temperature anomaly) and high Chl-a concentrations, attributed to cold, upwelled, nutrient-rich water.Unlike previous cruises in the region where very high mixing ratios of CHBr 3 (∼15 ppt) were observed in air transported from the African continent, attributed to an unknown coastal/continental source (Quack et al., 2004;Carpenter et al., 2007;Quack et al., 2007a), here we observed much lower mixing ratios of 0.5-2 ppt over the upwelling region.We attribute this to the fact that during the RHaMBLe cruise, the air masses were transported from the NE over the upwelling without touching the African continent or coast.3.6-3.7 1.9-4.936.6-102127-322 0.9-1.1).

Summary of Atlantic bromocarbon concentrations and fluxes
Table 1 summarises the mean, median and ranges of concentrations and fluxes of the bromocarbons in the different regions of the Atlantic.Fluxes were calculated from simultaneous air and water concentration measurements and the wind speed at 10 m according to the parameterization of Nightingale et al. (2000), with temperature-dependent Schmidt numbers for CH 2 Br 2 and CHBr 3 estimated according to Yokouchi et al. ( 2001) and Henry's Law coefficients as reported by Moore et al. (1995).
Both CH 2 Br 2 and CHBr 3 exhibited saturations and seaair fluxes decreasing in the order coastal > upwelling > shelf > open ocean.Outside of coastal regions, Table 1 shows that the average seawater CHBr 3 and CH 2 Br 2 concentrations for each region broadly increase with increasing Chl-a.However, unaveraged data points show only a weak correlation (e.g.r 2 of 0.39 for the CHBr 3 -Chl-a correlation of MAP, excluding coastal data).Coastal CHBr 3 concentrations (mean of 68 pmol dm −3 at depths between 20-40 m) were similar to surface water measurements corresponding to depths between 20-45 m off Mace Head, Ireland with a mean of 104±12 pmol dm −3 (Carpenter et al., 2000).Chuck et al. (2005)  (median of 2400 nmol m −2 d −1 ), probably because the latter values were derived mainly from measurements made within or very near to macroalgal beds (e.g., Klick and Abrahamsson, 1992;Schall and Heumann, 1993;Carpenter and Liss, 2000).In Swedish coastal waters, Klick and Abrahamsson (1992) measured mean CHBr 3 concentrations of 158±102 pmol dm −3 in waters within a few 10's of km of the shore, compared to 945±300 pmol dm −3 within the macroalgal belt.Clearly, there is a need for a common definition of "coastal" and subclasses therein, especially given the potential significance of such regions in contributing short-lived organobromine gases to the atmosphere.
Seawater concentrations in the African upwelling (mean of 11.5 pmol dm −3 CHBr 3 and 3.4 pmol dm −3 CH 2 Br 2 ) were similar to the mean concentrations measured by Quack et al. (2007a) of 10.6 pmol dm −3 CHBr 3 (both fresh and aged upwelled waters) and 4.6 pmol dm −3 CH 2 Br 2 .Class and Ballschmiter (1988) measured ∼24 pmol dm −3 CHBr 3 and 6 pmol dm −3 CH 2 Br 2 in surface waters in this region (25 • N, 16 • W).As discussed in Sect.3.1, the air measurements made in this study cannot be directly compared to those measured in the African upwelling region previously (Quack et al., 2004(Quack et al., , 2007a;;Carpenter et al., 2007) be-cause these latter measurements were influenced by African coastal/continental air with input from an as yet unidentified but strong source of CHBr 3 (Carpenter et al., 2007;Quack et al., 2007a), whereas in this study, the airmass trajectories did not touch the coast (see Lee et al., 2009).
For the Atlantic open ocean, bromocarbon concentrations in tropical air and water were very similar to those measured further north.The higher tropical fluxes reflected both the warmer waters (mean of 20.4 • C for RHaMBLe c.f. 13.7 • C for MAP) and higher wind speeds (mean of 9.1 ms −1 for RHaMBLe c.f. 6.9 ms −1 for MAP) experienced during the RHaMBLe cruise.CH 2 Br 2 was near to equilibrium with the atmosphere in some areas, whereas we found that the Atlantic Ocean was always saturated in CHBr 3 , in contrast to Chuck et al. (2005) who found that large areas of the temperate Atlantic were a sink for CHBr 3 .Our mean open ocean CHBr 3 seawater concentration (3.5 pmol dm −3 between 25-60 • N) was within the range previously reported in this region.Class and Ballschmiter (1988) 2004) measured 3-4 pmol dm −3 in this region (Sect.6).These CHBr 3 seawater concentrations (and their corresponding fluxes) are slightly higher than the N. Atlantic CHBr 3 averages reported by Butler et al. (2007a), probably because the latter covered a much larger region spanning the North Atlantic central gyre.CH 2 Br 2 fluxes, however, are similar in this study and the N. Atlantic data of Butler et al. (2007a), suggesting less variability in the oceanic saturations of this gas.
Our atmospheric North Atlantic open ocean CHBr 3 data, with a mean mixing ratio of 0.3 ppt, agree well with the results of Butler et al. (2007a) who report an average N. Atlantic mean mixing ratio of 0.4 ppt, but are much lower than the Atlantic CHBr 3 observed by Chuck et al. (2005) with a mean of 2.2 ppt between 28 • N and 20 • S (hence the lower fluxes in the latter study).Our CH 2 Br 2 atmospheric mixing ratios (N.Atlantic open ocean average of 0.3 ppt) are rather lower than the data presented by Butler et al. (2007a) (N.Atlantic open ocean average of 1.0 ppt).The high temporal and spatial variability in both gases may explain some of these differences, although calibration differences cannot be ruled out.

CH 2 Br 2 and CHBr 3 correlations and their implications
Covariance between polybromomethanes in air and in seawater (e.g.CHBr 3 , CH 2 Br 2 , CH 2 BrCl) has been widely reported and used to infer common marine sources of these compounds and to calculate bromocarbon emission ratios (Li et al., 1994;Carpenter and Liss, 2000;Carpenter et al., 2003;Yokouchi et al., 2005;Zhou et al., 2008).Figure 7 is an xy plot of the seawater concentrations of CH 2 Br 2 and CHBr 3 measured in this study, superimposed on previous measurements.The new data is consistent with previous ratios, which show a logarithmic dependence on concentration (Carpenter and Liss, 2000).As discussed in Carpenter and Liss (2000), the nature of this slope implies first order decay of the bromoalkanes in seawater, with lifetimes in the order CHBr 3 <CH 2 Br 2 .An alternative explanation is that the ratio of CH 2 Br 2 /CHBr 3 production (e.g. by phytoplankton) in the open ocean is higher than the production ratio in coastal regions (e.g. by macroalgae).Atmospheric ratios of CH 2 Br 2 /CHBr 3 are often interpreted in the light of their differing atmospheric lifetimes (Li et al., 1994;Yokouchi et al., 2005;Zhou et al., 2008).The atmospheric lifetime of CH 2 Br 2 is ∼0.29 years (Zhang et al., 1997) compared to about 20 days for CHBr 3 (Bilde et al., 1998).Atmospheric bromocarbon ratios have been used to deduce global estimates of their fluxes (Carpenter et al., 2003;Yokouchi et al., 2005) with an implicit assumption that the oceanic emission ratio is near constant.However, Fig. 7 shows that the surface seawater concentration ratio of CH 2 Br 2 /CHBr 3 is a function of concentration (and location), with a lower CH 2 Br 2 /CHBr 3 slope found in coastal regions near to strong macroalgal sources, and dilution of the samples with background water resulting in higher percentages of CH 2 Br 2 .This demonstrates that emission ratios deduced from coastal measurements should be treated with caution when inferring "global" average emission ratios.(Carpenter and Liss, 2000).and Liss, 2000).
The implication that CH 2 Br 2 has a longer lifetime than CHBr 3 in seawater is somewhat contrary to current theory.The half life of CHBr 3 due to halide exchange is 1.3-18.5 yr at 25 • C in seawater (Geen, 1992), and the half life due to hydrolysis is hundreds to thousands of years for both CH 2 Br 2 and CHBr 3 (Mabey and Mill, 1978).The most rapid aqueous destruction pathway so far reported is bacterial oxidation of CH 2 Br 2 (which apparently has no effect on CHBr 3 ), with a half life of only 2 days (Goodwin et al., 1998).This suggests that CH 2 Br 2 should be lost more rapidly in seawater than CHBr 3 .The apparently shorter lifetime of CHBr 3 in surface seawater could be due to its faster photolysis compared to CH 2 Br 2 (Carpenter and Liss, 2000).Ultraviolet photolysis of CHBr 3 in water is followed by water-catalysed dehalogenation to yield 3HBr, CO and HCOOH as ultimate products, with a relatively high photon quantum yield (for a solution-phase reaction) of ∼0.46 (Kwok et al., 2004).Alternatively, Quack et al. (2007b) suggested that entrainment of thermocline waters, where CHBr 3 may be converted to CH 2 Br 2 via reductive hydrogenolysis (Vogel et al., 1987) under anoxic conditions, into the mixed layer could contribute to the ratio shift in open ocean and upwelled waters.Quack et al. (2007b) further point out that, although the volatilisation lifetime of both gases is similar (at a few weeks), CHBr 3 may be lost preferentially from surface waters over time since it tends to have a greater sea-air concentration gradient than CH 2 Br 2 .
To a first approximation, assuming that both gases are supersaturated in seawater, the CH 2 Br 2 /CHBr 3 water concentration ratios shown in Fig. 7 can be converted to sea-air emission ratios by taking into account estimated differences in Schmidt numbers for CH 2 Br 2 and CHBr 3 (Yokouchi et al., 2001), which increases the water concentration ratio by ∼10%.(Note that, according to our data, the CH 2 Br 2 /CHBr 3 flux ratios are on average 7% higher than their equivalent water concentration ratios).This leads to a "coastal" (i.e., macroalgal-rich, since this is the environment of most of the coastal measurements shown in Fig. 7) CH 2 Br 2 /CHBr 3 molar emission ratio of ∼0.14 (from a molar water concentration ratio of ∼0.13 at 300 pmol dm −3 CHBr 3 ) and an "open ocean" molar emission ratio of ∼0.45 (from a molar concentration ratio of ∼0.41 at 4 pmol dm −3 CHBr 3 ).Previously, we deduced a "coastal" CH 2 Br 2 /CHBr 3 emission ratio of ∼0.15 from atmospheric data at Mace Head, Ireland (Carpenter et al., 2003), although this might have been effected slightly by dilution, which would act to increase the value.Similarly, Zhou et al. (2008) (Quack and Wallace, 2003;Butler et al., 2007a), the global CH 2 Br 2 /CHBr 3 oceanic emission ratio should be in the range 0.21-0.38,which is similar to the range of 0.29-0.38calculated on the basis of atmospheric sinks (WMO, 2003).We suggest that CH 2 Br 2 /CHBr 3 emission ratios deduced only from coastal data may be significantly biased low.

Global and regional flux extrapolations
Recent work has stressed the potential importance of upwelling systems in providing bromocarbons to the atmosphere and their link to climate change via bromine-initiated reduction of ozone in the upper troposphere, where it is most effective as a greenhouse gas (Quack et al., 2004;Salawitch, 2006).Because most of the major upwelling regions (African, Peruvian, equatorial) occur in tropical regions, there is greater potential for supply to the upper atmosphere through deep convection.However, in agreement with Quack et al. (2007a), we find that the areal flux of CHBr 3 and of CH 2 Br 2 from the African upwelling is significantly less than from coastal regions.Coastal upwelling regions represent only about 0.1% of the ocean surface (Ryther, 1969), and offshore areas of high productivity, including equatorial upwelling regions, are believed to represent ∼2.5% of the ocean surface (Ryther, 1969).(Quack and Wallace, 2003;Butler et al., 2007a).
Although recent studies are in agreement on the total global emissions of CHBr 3 as ∼10 (3-22) Gmol (Br) yr −1 (Quack and Wallace, 2003;Butler et al., 2007a), there is a discrepancy regarding the contribution of different regions/processes to this budget.The study of Butler et al. (2007a) was designed to reduce uncertainties of open ocean emissions, and suggested that open seawater contributes around 1.9 Gmol (Br) yr −1 CHBr 3 , compared to Quack and Wallace's estimate of 3 Gmol (Br) yr −1 CHBr 3 .Based on limited data from coastal regions, Butler et al. (2007a) tentatively concluded that coastal sources contribute up to 8 Gmol (Br) yr −1 of the world's CHBr 3 , whereas Quack and Wallace (2003), from a compilation of data sets, suggested a figure of 2.3 Gmol (Br) yr −1 , with 4.7 Gmol (Br) yr −1 from "shelf" seas.The authors defined "coastal" regions in different ways, which partly explains why rather different fluxes were calculated.Clearly, the total global emission estimate for CHBr 3 is very sensitive to the assumptions made on such coastal emissions.
Global information on the geographical area of coastal regions is calculated using bathymetry/hyposmetry, therefore in order to use such data to estimate coastal trace gas fluxes requires knowledge of how surface bromocarbon concentrations vary with ocean depth.Figure 8, using data from this study and previous data offshore of Mace Head (Carpenter and Liss, 2000), shows clearly that higher bromocarbon concentrations are observed in shallower seas.We use this limited information to provide a slightly more detailed analysis of potential coastal emissions.Coastal waters with a depth up to 180 m represent 7.5% of the world ocean area (Menard and Smith, 1966).The global area of shallow seas below 50 m is around 5×10 12 m 2 (Wyatt, 1987), i.e. 1.4% of the total ocean area (Riley and Chester, 1971).The data shown in Figure 8 indicate an average concentration of 209±89 pmol dm −3 CHBr 3 for waters below 50 m and 11.5±4.8pmol dm −3 CHBr 3 (where ranges indicate observed variability) for waters between 50-180 m depth, assuming a linear distribution of depths between 0-50 and 50-180 m.As discussed in Sect.3.3, and shown also in Fig. 7 these data are representative of the literature in coastallyinfluenced waters.Extrapolated to shallower depths, assuming a log-log relationship between depth and concentration, they indicate a maximum CHBr 3 concentration of 1200 pmol dm −3 , which is representative of observations in the close vicinity of macroalgal beds (Quack and Wallace, 2003).
Using this data and assuming a mean SST of 15 • C and a wind speed of 7 m s −1 , we estimate tentatively that coastal and coastally-influenced waters together contribute around 2.5 (1.4-3.5)Gmol Br yr −1 (Table 2), neglecting contributions from anthropogenic sources such as coastal power stations (Quack and Wallace, 2003), which are not thought to have affected our study regions.There are various possible flaws in these estimates; not least they are based upon data from only limited regions mainly in the northern hemisphere and they assume a linear distribution of ocean depths rather than a weighted distribution.These different flaws can introduce both positive and negative biases.Nevertheless, we believe the approach offers some improvement to previous approaches because it attempts to integrate data over the coastal zone.Our coastal flux estimate is much less than the 8 Gmol Br yr −1 estimate of Butler et al. (2007a) but close to the near-shore estimate of Quack and Wallace (2003) of 2.3 (0.9-3.4) Gmol Br yr −1 and to the bottom-up estimate of Carpenter and Liss (2000) from macroalgae of 1.6 (0.4-2.7) Gmol Br yr −1 .Refinement of these estimates requires further coastal seawater data, particularly in tropical regions which appear to produce large quantities of bromocarbons (Yokouchi, 2005).Future coastal bromocarbon data should be provided with bathymetric data to allow a more accurate assessment and definition of coastal emissions.

Conclusions
Simultaneous seawater and air measurements of CHBr 3 and CH 2 Br 2 over a large part of the eastern Atlantic Ocean show that both gases are supersaturated throughout this region, with the exception of a few measurements of CH 2 Br 2 in the open ocean.Along with previous studies, we find that macroalgae-rich coastlines are a very significant source of bromocarbons.Surface water bromocarbon concentrations in these regions can be related to ocean depth, i.e. they decrease as the waters are diluted away from the coastline.A crude analysis of coastal CHBr 3 emissions integrated over the coastal zone (0-180 m depth) indicates that coastal con-tributions to global CHBr 3 emissions are similar to the total open ocean contribution.There is evidence that, outside of these regions, phytoplankton abundance partly controls surface bromocarbon concentrations, since elevated concentrations are observed where Chl-a levels are high, for example along the continental shelf and in upwelling regions.We suggest that upwelling systems are regional hot spots of bromocarbons, but globally, supply at most a few percent of total emissions.
3 and CH 2 Br 2 measured during two research cruises in the northeast Atlantic (53-59 • N, June-July 2006) and tropical eastern Atlantic including over the northwest African coastal upwelling system (16-35 • N, May-June 2007).

Figure 1 .
Figure 1.Surface seawater data for the RHaMBLe cruise: (a) CHBr 3 concentrations, (b) CH 2 Br 2 concentrations (c) Chl a image from MODIS/Aqua ocean colour satellite, weekly composite from 20-26 May 2007.Chl a is purple when concentrations are low, increasing through blue, green and yellow to high concentrations at orange and red.Black is cloud cover.Images processed by the NEODAAS Remote Sensing Group, Plymouth Marine Laboratory.

Fig. 1 .
Fig. 1.Surface seawater data for the RHaMBLe cruise: (a) CHBr 3 concentrations, (b) CH 2 Br 2 concentrations (c) Chl-a image from MODIS/Aqua ocean colour satellite, weekly composite from 20-26 May 2007.Chl-a is purple when concentrations are low, increasing through blue, green and yellow to high concentrations at orange and red.Black is cloud cover.Images processed by the NEODAAS Remote Sensing Group, Plymouth Marine Laboratory.

Figure 2 .Fig. 2 .
Figure 2. Time series of CHBr3 concentrations (air and water), fluxes, ocean depth and Chl a for the RHaMBLe cruise.

Figure 3 .Fig. 3 .
Figure 3.Time series of CH2Br2 concentrations (air and water), fluxes, ocean depth and Chl a for the RHaMBLe cruise.

Figure 4 .
Figure 4. Surface seawater data for the MAP cruise: (a) CHBr 3 concentrations, (b) CH 2 Br 2 concentrations.Concentration scales for CHBr 3 and CH 2 Br 2 are the same as for Fig. 1, note that concentrations are off scale near the coast of the Outer Hebrides, (c) Chl a image from MODIS/Aqua ocean colour satellite, weekly composite from 26 June-1 July 2006.See Fig. 1 for more details.

Fig. 4 .
Fig. 4. Surface seawater data for the MAP cruise: (a) CHBr 3 concentrations, (b) CH 2 Br 2 concentrations.Concentration scales for CHBr 3 and CH 2 Br 2 are the same as for Fig. 1, note that concentrations are off scale near the coast of the Outer Hebrides, (c) Chl-a image from MODIS/Aqua ocean colour satellite, weekly composite from 26 June-1 July 2006.See Fig. 1 for more details.

Figure 5 .Fig. 5 .
Figure 5.Time series of CHBr3 concentrations (air and water), fluxes, ocean depth and Chl a for the MAP cruise.

Figure 6 .Fig. 6 .
Figure 6.Time series of CH2Br2 concentrations (air and water), fluxes, ocean depth and Chl a for the MAP cruise.

a
Selected from data with Chl-a values above 2 µg dm −3 in the region 16-25 • N, 16-25 • W b Ocean depth 100-200 m c Ocean depth <100 m 3.2 North Atlantic surface seawater and air distributions Halocarbons were sampled onboard the Celtic Explorer on the second leg of the MAP cruise between 25 June until 6 July with a transect from Killybegs in Donegal on the West Coast of Ireland out to the continental shelf off Ireland during a period of seasonally high plankton activity, into coastal waters of the Outer Hebrides, and finally into open waters of the Atlantic Ocean on the eastern edge of the North Atlantic Drift (NAD).Figure 4 shows CHBr 3 and CH 2 Br 2 surface seawater concentrations along the cruise track along with a 7 day composite MODIS chlorophyll ocean colour satellite image for the period 26 June-1 July 2006.Time series of bromocarbon concentrations and fluxes, along with Chl-a (in situ) data and depth, are shown in Figs. 5 and 6.Extremely high concentrations of bromocarbons are clearly apparent in and over the shallow coastal waters.Concentrations of CHBr 3 and CH 2 Br 2 and of Chl-a were also elevated over the continental shelf (sampled during 29/30 June and 2/3 July) compared to the open waters (Table measured coastal CHBr 3 fluxes of ∼200 nmol m −2 d −1 for waters around Gran Canaria, and Butler et al. (2007a) give global average "coastal water" fluxes of 220 and 110 nmol m −2 d −1 for CHBr 3 and CH 2 Br 2 , respectively, compared to our mean Outer Hebrides coastal fluxes of 236 and 72 nmol m −2 d −1 .These coastal CHBr 3 fluxes are at the low end of the range presented in the review of Quack and Wallace (2003) of 96-10 320 nmol m −2 d −1 measured a mean Atlantic concentration of 3.1 pmol dm −3 , Chuck et al. (2005) reported a mean North/South Atlantic (50 • N-65 • S) value of 5.1 pmol dm −3 and Quack et al. (

Figure 8 .
Figure 8. Surface CHBr3 concentrations in Atlantic waters of different depths.Filled diamonds -MAP data; open circles -RHaMBLe data; filled circlesaverage data from Mace Head measured in September 1998 -error bars represent 1of the data(Carpenter and Liss, 2000).

Fig. 8 .
Fig. 8. Surface CHBr 3 concentrations in Atlantic waters of different depths.Filled diamonds -MAP data; open circles -RHaM-BLe data; filled circles -average data from Mace Head measured in September 1998 -error bars represent 1σ of the data(Carpenter  and Liss, 2000).

Table 1 .
CHBr 3 and CH 2 Br 2 concentration and flux averages and ranges in the Atlantic Ocean.

Table 2 .
Estimates of global coastal CHBr 3 and CH 2 Br 2 fluxes.See text for details.
recently reported CH 2 Br 2 /CHBr 3 atmospheric ratios of 0.13 from the coast of Appledore Island, US, and 0.15 at a site Butler et al. (2007a)presentative of other coastal upwelling systems (mean fluxes of 35.6 and 13.3 nmol m −2 d −1 for CHBr 3 and CH 2 Br 2 , respectively, which are in accordance with the Mauritanian upwelling fluxes ofQuack et al., 2007a)and the "tropical" data ofButler et al. (2007a)as representative of equatorial upwelling regions (mean fluxes of 9.1 and 2.6 nmol m −2 d −1 for CHBr 3 and CH 2 Br 2 , respectively), we estimate that upwelling regions contribute a total of 0.1 (0.006-0.3)Gmol (Br) yr −1 from CHBr 3 and 0.03 (0.006-0.08)Gmol (Br) yr −1 from CH 2 Br 2 , comprising on average around 1% of the total global emissions of these compounds