Methyl bromide and methyl chloride in the Southern Ocean

[ 1 ] Air and water concentrations of methyl bromide (CH 3 Br) and methyl chloride (CH 3 Cl) were measured in the Southern Ocean (latitudes 45 (cid:1) –67 (cid:1) S, longitudes 144 (cid:1) – 139 (cid:1) E) from late October through mid-December 2001. CH 3 Br and CH 3 Cl were undersaturated with mean saturation anomalies of (cid:1) 39 ± 11% and (cid:1) 37 ± 11% between 45 (cid:1) and 65 (cid:1) S. The minimum degradation rate constants needed to maintain these saturation anomalies are consistent with the observed total degradation rate constants, suggesting that there is no significant production of these gases in this region. Near the Antarctic coast (south of 65 (cid:1) S) the saturation anomalies for both gases decreased to approximately (cid:1) 80%, although CFC-11 measurements suggest these extreme anomalies are associated with enhanced vertical mixing rather than with degradation in the surface waters. I NDEX T ERMS : 0312


Introduction
[2] Methyl bromide (CH 3 Br) and methyl chloride (CH 3 Cl) are sources of inorganic halogen atoms that destroy stratospheric ozone (O 3 ).Both gases have natural sources and sinks, while CH 3 Br also has substantial anthropogenic sources.The budgets for CH 3 Cl and CH 3 Br remain largely unbalanced with known sinks outweighing sources by $20-25% for both trace gases [Montzka et al., 2003].
[3] Supporting production as well as degradation, the oceans are both source and sink for these atmospheric gases.Net undersaturations of CH 3 Br have been observed over much of the world's oceans [Lobert et al., 1995a[Lobert et al., , 1996[Lobert et al., , 1997;;Moore et al., 1996;Groszko and Moore, 1998;King et al., 2000King et al., , 2002]], resulting in an estimated global, net seato-air flux ranging from À10 to À18 Gg yr À1 [King et al., 2002].Methyl chloride, on the other hand, is supersaturated over middle and low latitudes, but with undersaturations observed at high latitudes [Moore et al., 1996].
[4] Assessing how the net oceanic fluxes of CH 3 Br and CH 3 Cl will respond to climate change or changes in anthropogenic emissions requires reliable modeling supported by reliable data.Although some recent modeling studies have used relationships between latitude or sea surface temperature (SST) and saturation anomaly to constrain the oceanic fluxes [e.g., Lee-Taylor et al., 1998;Khalil et al., 1999], these proxy relationships are not likely to remain valid in the future, unless SST is the sole driving force controlling the saturation anomaly.It is important to understand the processes underlying such relationships in terms of oceanic production and degradation and how they may change under changing climatic conditions.
[5] Early attempts to parameterize oceanic production of CH 3 Br relied on chlorophyll a as a proxy or on prescribing a fixed production rate.Such models suggested significant CH 3 Br production in the Southern Ocean and included only chemical degradation, which is very slow at the low temperatures in this region.This led to model predictions of large supersaturations of CH 3 Br in this region [Anbar et al., 1996;Pilinis et al., 1996].During a previous Southern Ocean cruise, we observed large undersaturations (À36 ± 7%, 1s, n = 466) for CH 3 Br [Lobert et al., 1997], which demonstrated that the models overpredicted the saturation anomalies in that region.As there were no measurements of degradation rate constants or depth profiles to assess poten-tially significant losses owing to vertical mixing, we could not estimate production rates for that expedition.During that cruise, production rates could have been high, but in order to maintain the observed saturation anomalies the corresponding degradation rates would have to have been much faster as well.In an attempt to determine actual production rates of these gases in polar and subpolar waters, we measured CH 3 Br and CH 3 Cl saturation anomalies, degradation rate constants [Tokarczyk et al., 2003], and depth profiles on a recent cruise in the Southern Ocean.  1).This cruise track is referred to as the Southern Ocean Repeat hydrographic section 3 (SR3) and was repeated 6 times during the World Ocean Circulation Experiment (WOCE) covering all seasons except the Austral Spring.One hundred thirty-four hydrocasts were conducted during this cruise, with 109 occurring during the southbound transect.Twenty nine of these hydrocasts were sampled for methyl halides.

Experiment
[7] Air was continuously pumped ($6 L min À1 ) through 0.63-cm ID Dekabon tubing (Dekoron division of Furon, Aurora, Ohio) mounted on the mast at the bow of the ship.Samples from this air stream were collected and analyzed every 56 or 112 min, depending on the sequence of calibration samples, with a gas chromatograph-mass spectrometer (GC-MS) as described by Lobert et al. [1996] and with modifications summarized by King et al. [2000].Prior to this cruise, the analytical column was changed from a DB-5 (0.25 mm ID, J&W) to a DB-VRX (0.25 mm ID, J&W).During this cruise, the precision (1s) for a single CH 3 Br, CH 3 Cl, or CFC-11 air measurement was 2.3% (0.2 pmol mol À1 ), 1.9% (10.0 pmol mol À1 ), and 0.9% (2.3 pmol mol À1 ), respectively.
[8] A GC-MS system with the same type of column and the same trapping design as described above, equipped with a custom-built automated purge system, was used to measure CH 3 Br, CH 3 Cl, and CFC-11 in discrete seawater samples.We automated the analysis of hydrocast samples by replacement of the typical single-calibrated volume (glass bulb) with a 32-port, 16-position loop selection valve (VICI Metronics, Texas).Sixteen bulbs were plumbed into the valve and housed in a temperature-controlled chamber.Water samples were collected from Niskin bottles into ground glass syringes and briefly stored in a bucket of cold seawater until they could be loaded into the calibrated glass bulbs.The computer-controlled valve stepped through the sequence of bulbs, and each sample, in turn, was pushed into the purge vessel.Blanks and calibration gas standards were included in the sequence to calibrate and monitor the system.The calibration gas standards for both GC-MS instruments were whole air working standards calibrated against gravimetric standards at the NOAA Climate Monitoring and Diagnostics Laboratory (CMDL).
[9] Surface seawater CFC-11 concentrations measured with this instrument agreed well with those determined from a traditional purge-and-trap gas chromatograph with an electron capture detector that was also on this cruise (M.Warner, personal communication, 2002), suggesting that contamination during sampling and analysis was minimal.From replicate samples collected periodically during the cruise, the measurement uncertainties (1s) were 9% (0.05 nmol m À3 ) for CH 3 Br, 2.7% (1.4 nmol m À3 ) for CH 3 Cl, and 2.6% (0.08 nmol m À3 ) for CFC-11.

Results and Discussion
[10] Vertical sections of potential temperature and salinity are consistent with a late winter (late September through late October 1991) occupation of SR3 and the seasonal evolution of surface properties in this region (Figures 2a and 2b) [Rintoul and Bullister, 1999;Trull et al., 2001;Rosenberg et al., Aurora Australis marine science cruise AU0103: Oceanographic field measurements and analysis, manuscript in preparation, 2004, hereinafter referred to as Rosenberg et al., manuscript in preparation, 2004].Rintoul and Bullister [1999] identified cold, fresh, CFC-enriched Antarctic Bottom Water forming off the Wilkes-Adelie coast of Antarctica.This feature is also present in the data collected south of 65°S during this cruise (Figures 2a -2c).Since CH 3 Br and CH 3 Cl are nonconservative trace gases undergoing both chemical and biological degradation in seawater, they are not enriched in the sinking waters along the Antarctic coast (Figures 2d and 2e).
[11] The characteristics of the fronts and zones of the Southern Ocean south of Australia, described by Rintoul and Bullister [1999], Trull et al. [2001], and Chaigneau and Morrow [2002], were used to identify the fronts and zones encountered during this occupation of SR3 (Figure 3).The assignment of all fronts and zones was based on the data from the densely sampled southbound transect.The Subtropical Front (STF) was located at $46.5°S with the Subantarctic Zone (SAZ) extending from this front to the Subantarctic Front (SAF) located at $49°S.The SAF was determined as the beginning of the steepest slope in SST and sea surface salinity (SSS) south of the STF.Subsurface features (Figures 2a and 2b) could place this front at $51°S, however the data discussed below are for the surface mixed layer only.The Polar Frontal Zone (PFZ) extended from the SAF to $54.5°S where the northernmost extent of the subsurface temperature minimum (Figure 2a), a constant 33.85% SSS, and an SST gradient (Figure 3) placed the northern Polar Front (PF-N).The Inter-Polar Frontal Zone (IPFZ) extended southward from the PF-N to the southern Polar Front (PF-S) located at $58.5°S.South of the PF-S, the southern Antarctic Zone (AZ-S) extended to the Southern Antarctic Circumpolar Current (ACC) Front (SF) at $63°S.The Southern Boundary (SB) of the ACC was located at $64°S.As discussed above, the sinking waters of the Antarctic Slope Front (ASF) were located at $65°S.
[12] Atmospheric CH 3 Br, CH 3 Cl, and CFC-11 mixing ratios averaged 6.8 ± 0.3 pmol mol À1 (1s, n = 613), 524 ± 21 pmol mol À1 (1s, n = 649), and 259 ± 4 pmol mol À1 (1s, n = 614) and did not change significantly over the one to five weeks separating the southbound and northbound transects or latitude (Figures 4a -4c).Surface seawater concentrations reported here are the average of the concentrations of hydrocast samples collected from within the mixed layer or within the top 100 m, whichever is shallower (Figure 4d).Saturation anomalies (percent departure from equilibrium) for each trace gas were calculated from the surface water concentrations and 24-hour smoothed air concentrations, using the solubility functions from De Bruyn and Saltzman [1997] (CH 3 Br), Moore et al.
[13] As expected for a conservative tracer, north of the PF-N dissolved CFC-11 concentrations increased dramatically with decreasing SST while maintaining a nearly constant near-equilibrium saturation anomaly of 2 ± 1% (1s, n = 9) between the STF and the PF-N (Figures 4d  and 4e).At 51°S, the small shift in the CFC-11 mixed layer concentration and saturation anomaly between the southbound and the northbound transects is likely the result of the increase in SST that occurred between the two transects.The nonconservative trace gases CH 3 Cl and CH 3 Br did not exhibit such a strong anticorrelation between concentration and SST.These gases were substantially undersaturated with mean saturation anomalies of À36 ± 6% (1s, n = 8) for CH 3 Br and À34 ± 7% (1s, n = 8) for CH 3 Cl between the STF and PF-N.In the IPFZ, the mixed layer concentrations and saturation anomalies of all three gases decreased (Figure 4d).Both the mixed layer concentrations and the saturation anomalies for CH 3 Br and CH 3 Cl increased to values similar to those of the PFZ in the AZ-S.However, for CFC-11 only the concentration increased in the AZ-S.The CFC-11 saturation anomaly remained slightly undersaturated at approximately À7.5% in the AZ-S.From the SF to the ASF, the CFC-11 saturation anomaly decreased rapidly and remained at approximately À30% south of the ASF, which is consistent with a substantial input of upwelled CFC-free water.The CH 3 Br and CH 3 Cl concentrations and saturation anomalies decreased rapidly from the SB past the ASF, with the lowest saturation anomalies reaching À80% for CH 3 Cl and À82% for CH 3 Br.
[14] Unlike CFC-11, CH 3 Br and CH 3 Cl were substantially undersaturated everywhere along the cruise track with saturation anomalies averaging À39 ± 11% (1s, n = 19) for CH 3 Br and À37 ± 11% (1s, n = 19) for CH 3 Cl north of the ASF (Figure 4d).The mean CH 3 Br saturation anomaly observed north of the ASF during this cruise agrees, within 95% confidence limits, with that observed during a late Austral Summer and early Fall (late February through early April) cruise from McMurdo, Antarctica, to Punta Arenas, Chile (Bromine Latitudinal Air/Sea Transect III (BLAST III)), À36 ± 7% (1s, n = 466), [Lobert et al., 1997].These large undersaturations suggest that sinks are larger than sources for CH 3 Br and CH 3 Cl in the surface waters of the Southern Ocean.
[15] From the observed saturation anomalies, we calculated the minimum degradation rate constant needed to sustain the observed surface water concentrations in the presence of air-sea exchange.This calculation assumes  steady state; however, a portion of the air-sea flux is likely the result of changes in certain physical properties (e.g., seasonal warming or cooling of the sea surface) and not the result of the in situ production and degradation [Kester, 1975].In order to correct for these physical effects, the observed saturation anomaly for the conservative tracer CFC-11 (Á f11 ) was subtracted from the saturation anomaly of the trace gas of interest (Á g ).The minimum degradation rate constant (k min ) was determined from the following equation: where K W is the gas exchange coefficient (m d À1 ) from Wanninkhof [1992] and the full range in uncertainty estimated with Nightingale et al. [2000] and Smethie et al. [1985], z is the observed mixed layer depth (m), and all other terms are defined above.This equation is derived from a balance of in situ degradation and air-sea exchange and assumes vertical mixing is negligible [Butler et al., 1991;Lobert et al., 1995b].As discussed above, vertical mixing was enhanced south of the ASF, therefore this equation was not applied to the data collected south of 65°S.For this cruise, the mean k min for CH 3 Br and CH 3 Cl north of the ASF were 0.039 ± 0.029 d À1 (1s, n = 19) and 0.036 ± 0.025 d À1 (1s, n = 19) (Figure 5), whereas for BLAST III the mean k min for CH 3 Br was 0.062 ± 0.051 d À1  [16] When k min equals the sum of the chemical and biological degradation rate constants (k total ), no production is needed to maintain the observed saturation anomaly.If k total is greater than k min , the trace gas must be produced at a rate sufficient to balance that additional degradation (Production = (k total À k min ) C W , where C W is the surface water concentration).Values for k min that are higher than k total suggest additional losses (e.g., mixing) are occurring.During this cruise, Tokarczyk et al. [2003] measured chemical and biological degradation rates and determined mean k total values of 0.035 ± 0.044 d À1 (1s, n = 70) for CH 3 Br and 0.07 ± 0.08 d À1 (1s, n = 36) for CH 3 Cl (Figure 5).The magnitude of and trends in the observed CH 3 Br k total correlate well with the calculated k min for CH 3 Br, suggesting that the observed loss rate constants are sufficient to balance the net air-sea flux and maintain the observed saturation anomaly for CH 3 Br with no mixed layer production.South of the ASF, additional losses are indicated and consistent with the enhanced mixing previously discussed for this region.The same is true for CH 3 Cl south of the PF-N.North of the PF-N, the observed CH 3 Cl k total are larger than the calculated k min , suggesting that some mixed layer production of CH 3 Cl is needed to balance the observed degradation and net air-sea flux while maintaining the observed saturation anomaly for CH 3 Cl.
[17] Thus the data from this cruise suggest that there is no production of CH 3 Br in these waters and perhaps a small source of CH 3 Cl north of the PF-N.The BLAST III mean k min value is higher than the mean k total observed by Tokarczyk et al. [2003] for this cruise, suggesting that k totals may have been larger or additional losses may have occurred during BLAST III.It also is possible that during the Austral Fall, when BLAST III took place, both degradation and production were faster.These results along with the negative saturations observed ubiquitously in high-latitude waters further suggest that there is no significant production of CH 3 Br and CH 3 Cl in polar and subpolar waters.

Figure 3 .
Figure 3. Surface water properties (a) temperature and (b) salinity during southbound (black) and northbound transects (shaded).The vertical lines indicate the positions of the fronts (STF, Subtropical Front; SAF, Subantarctic Front; PF-N, northern Polar Front; PF-S, southern Polar Front; SF, southern ACC Front; SB, southern boundary of the ACC; and ASF, Antarctic Slope Front).The zones between the front are indicated by SAZ, Subantarctic Zone; PFZ, Polar Frontal Zone; IPFZ, Inter-Polar Frontal Zone; and AZ-S, southern Antarctic Zone

Figure 5 .
Figure 5. Mixed layer loss rate constants (d À1 ) for (a) CH 3 Br and (b) CH 3 Cl with k min (circles) and the observed k total (squares).Observed k total values are from Tokarczyk et al. [2003].Data shown are from both the southbound (solid symbols) and northbound (open symbols) transects.Vertical shaded lines indicate fronts, as described in Figure 3.