5.3. Ocean Projects

5.3.1. Southern Ocean Expedition - BLAST III

The flux of CH₃Br from the world’s oceans has been a source of considerable controversy over recent years. Although earlier studies suggested the ocean was a large, net source of atmospheric CH₃Br [Singh et al., 1983; Singh and Kanakidou, 1993; Khalil et al., 1993], recent widespread examinations by CMDL of the saturation of CH₃Br in the east Pacific and Atlantic Oceans showed that most of the ocean was undersaturated in this gas [Lobert et al., 1995, 1996; Butler et al., 1995]. Extrapolation of these data indicated that the global oceans were a net sink for atmospheric CH₃Br.

Two subsequently published numerical models, however, suggested that polar and sub-polar oceans might be a large, net source of atmospheric CH₃Br [Pilinis et al., 1996; Anbar et al., 1996]. The two models used production rates based on data published by Lobert et al. [1995], presuming them to be either constant over the entire oceanic regions or a function of chlorophyll-a concentration. With chemical degradation being very slow in cold, polar waters, and a very high biological productivity during the austral summer, the predicted saturation anomalies were positive and ranged up to 500%, indicating this polar source could globally outweigh the sinks estimated by Lobert et al. [1995]. To resolve this question, CMDL conducted a study to measure the saturation of CH₃Br in the Southern Ocean during a time of high biological productivity (Bromine Latitudinal Air-Sea Transect (BLAST) III, Figure 5.31, Lobert et al. [1997]).

Fig. 5.31. BLAST III cruise track from McMurdo, Antarctica, to Punta Arenas, Chile, aboard the R/V Nathaniel Palmer, Cruise 96-02. Numbers along the cruise track indicate the Julian day of year 1996.

The shipboard GC/MS and sampling system was virtually identical to that used during the two previous cruises. On this cruise CH₃Br was also measured with a custom-built GC equipped with an ECD and different columns. Mole fractions from MS and ECD systems agreed, on average, within 0.2 ppt (Figure 5.32, Table 5.7). Measured, dry mole fractions of CH₃Br in the atmosphere were consistent with data from the BLAST I and BLAST II cruises. Most important, however, is that the ocean in this region was consistently undersaturated in CH₃Br with a mean saturation anomaly corrected for physical effects of –33 ± 8% (Figure 5.32c, Table 5.7).

Fig. 5.32. Measurements of methyl bromide in air (a), air equilibrated with surface water (b), and the resulting saturation anomaly (c) for both the GC/MS and the GC/ECD systems. The shaded area in panel (c) represents the saturation anomaly corrected for physical effects calculated from the GC/MS saturation anomaly (black line).

TABLE 5.7. Mean Mixing Ratios of CH₃Br in Air and Equilibrated Water during BLAST III

*Saturation anomaly = percent departure from equilibrium, calculated from GC/MS data.

†Corrected saturation anomaly = mean anomaly, corrected for physical effects such as those associated with mixing and warming of surface waters [e.g., Butler et al., 1991].

Maintaining a steady-state, ~35% undersaturation of CH₃Br in the surface waters in the presence of air-sea exchange requires a minimum in situ degradation rate of about 5.8% d^-1, which is a factor of 10 larger than that for chemical degradation alone. The most likely explanation of these findings is that dissolved CH₃Br is being degraded by an additional mechanism other than reaction with H₂O and Cl^-. A significant biological sink for CH₃Br in sub-tropical waters has been identified recently [King and Saltzman, 1997], suggesting that the additional sink might be biological.

Several conclusions can be drawn from this study. First, the Southern Ocean, and probably most high-latitude waters, are a net sink for atmospheric CH₃Br. Second, biological processes, or some chemical processes other than reaction with H₂O or Cl^-, rapidly remove CH₃Br from surface waters. Third, CH₃Br production is neither constant over the global ocean nor strictly dependent upon chlorophyll concentration. The data from this expedition and those from BLAST I and BLAST II suggest that the global ocean is a net sink of 21 (11-31) Gg yr^-1 for CH₃Br.

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