In 1994 the NOAH Group participated in two research cruises for the Bromine Latitudinal Air/Sea Transect (BLAST) project. The first cruise extended through the East Pacific between Seattle, Washington, and Punta Arenas, Chile, from January 26 to February 17, 1994; the second cruise was conducted between October 18 and November 21, 1994, heading through the Atlantic Ocean from Bremerhaven, Germany, to Punta Arenas, Chile (Figure 5.25). The main objective of these cruises was to gather data to ascertain the presence or absence of a potential oceanic source for methyl bromide (CH₃Br). Methyl bromide contributes about 50% to tropospheric organic bromine and, hence, has received considerable attention, particularly because its budget is currently not well understood [Butler, 1995, 1996; Butler and Rodriguez, 1996]. Frequently collected CH₃Br data from the two expeditions constitute the largest data set for oceanic CH₃Br to date and the first solid estimate of oceanic emission, production, and chemical degradation of the compound. It is concluded from these studies that the ocean is not a net source of CH₃Br but rather a net sink [Lobert et al., 1995, 1996; Butler et al., 1995]. Although CH₃Br is both produced and consumed everywhere in the surface ocean [Butler, 1994], the rate of consumption exceeds that of production in most waters sampled. Exceptions were coastal and coastally-influenced waters, which were typically supersaturated, and areas of open ocean upwelling, where CH₃Br saturation anomalies were close to zero. About 80% of the oceans are undersaturated in CH₃Br, representing a net annual sink of 8-22 Gg yr^-1.

CH₃Br data from the second cruise further supported NOAH conclusions from the first expedition. The latter results give greater strength to the global extrapolations of the first data set. Data from the two cruises are summarized in Table 5.5 and Figure 5.26.

Fig. 5.25. Cruise tracks of both ocean missions. The cruises covered a diverse mixture of coastal waters, upwelling regions, large open ocean areas, and coastally influenced areas in the East Pacific and Atlantic oceans.

TABLE 5.5. Results for Atmospheric and Oceanic CH₃Br From BLAST I and BLAST II Cruises

			Combined
	BLAST I	BLAST II	Estimate
Global mean (ppt)	9.8	10.4	10.1
NH mean (ppt)	11.2	11.7	11.5
SH mean (ppt)	8.6	9.4	9.0
Variability (ppt)	0.6	1.2
ITCZ (°N)	4.1	6.4
IHD (ppt)	2.65	2.31	2.48
NH/SH ratio	1.31	1.25	1.28
SA (%)	-15.7	-19.7	-18.4
P (Gg yr-1)	175	248	214
L (Gg yr-1	-188	-267	-229
Net Flux (Gg yr-1)	-13.0	-18.7	-14.8

Fig, 5.26 Atmospheric CH₃Br, its partial pressure in the surface water, and the net saturation anomalies for both cruises.

The best estimate of the partial lifetime of atmospheric CH₃Br with respect to oceanic losses is 2.7 (2.4-6.5) years. This range was derived from a 40-year, global data set of sea surface temperatures and wind speeds [Yvon and Butler, 1996] (Figure 5.27). Data from the two expeditions suggested a shorter lifetime of CH₃Br of about 2.4 years. The difference between the two estimates is due to higher than average wind speeds encountered during the cruises. The estimated atmospheric lifetime, based upon combined atmospheric, soil, and oceanic losses, is now 0.8 years compared to earlier estimates of 1.8­2.1 years when the ocean was considered an insignificant sink, the soil sink [Shorter et al., 1995] was unknown, and tropospheric OH concentrations were underestimated by 15% [Prinn et al., 1995]. The oceanic sink correspondingly lowers ozone depletion potential estimates for CH₃Br by about one-third from earlier estimates.

Fig. 5.27. Map of the global distribution of the rate constant (k_{ocean, I}) for the irreversible uptake of atmospheric methyl bromide (CH₃Br) by the ocean (k_{ocean, I} = 1/_{ocean, I}; global = 2.7 years). This rate constant is computed with data from COADS for each cell on a 2 x 2 grid. Note that higher loss rates are found in the northern hemisphere in regions containing both high wind speeds and warm SST's.


Measured global, northern hemispheric, and southern hemispheric means (NH and SH) and the observed interhemispheric tropical convergence zone (ITCZ), which was used to determine the hemispheric ratios. The net saturation anomaly (SA), the hemispheric ratios, and the interhemispheric difference (IHD) were calculated from the measurements. Finally, estimates of the oceanic production (P), oceanic loss (L), and the net oceanic flux of CH₃Br were added. The net saturation anomaly is the percent departure of CH₃Br in the surface ocean from equilibrium with the atmosphere. Negative numbers indicate fluxes from the atmosphere to the ocean. Variability is the residual standard deviation of a loess fit through all atmospheric measurements.

Concentrations of CFC-12, CH₃Br, and CH₃Cl in water column samples collected during BLAST II were higher within the mixed layer than at depth. Considering that oceanic CFC-12 originates from the atmosphere, this suggests that both CH₃Br and CH₃Cl have sources associated with the atmosphere or surface waters. Departures of CH₃Br and CH₃Cl concentrations from those of CFC-12 may result from chemical or biological in situ production or degradation of these gases.

Potential artifacts associated with sampling and analysis of CH₃Br and other compounds were investigated. These studies revealed significant problems associated with the measurement of CH₃Br from air stored in stainless steel flasks that have historically been used for measuring CH₃Br in the atmosphere. From a comparison between measurements made shipboard and from air stored in flasks, and from the reanalysis of air in flasks over time, it has been determined that CH₃Br in stainless steel flasks can be unstable and may increase or decrease with time. In addition, results for CH₃Br determined by GC-ECD can be compromised by some GC configurations [Montzka et al., 1995a; Lobert et al., 1996].

Besides CH₃Br, a suite of CFCs and methyl halides was measured during the cruises, and data for oceanic methyl chloride (CH₃Cl), methyl iodide (CH₃I), dibromomethane (CH₂Br₂), and bromoform (CHBr₃) are currently being finalized for publication.

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