The ocean in near equilibrium with atmospheric methyl bromide.

Saturation-anomaly measurements of methyl bromide (CH₃Br) were made in the eastern Pacific (3/30-<4/27, 2010) and the eastern Atlantic (10/25-11/26, 2010) to assess the oceanic saturation state as the phaseout of fumigation - non-Quarantine and Pie-Shipment (non-QPS) uses of CH₃Br nears completion and atmospheric concentrations continue to decline. These cruises occurred 16 years alter the Bromine Latitudinal Air-Sea Transect (BLAST) cruises, which were conducted in the same regions and first established a global oceanic net sink of- 12.6 Gg yr^-1 for atmospheric CH₃Br in 1994. Results from this study suggest saturation anomalies of CH₃Br in Ihe surface ocean have become less negative than those observed 16 years ago as the atmospheric burden has declined over the past decade. The global net sea-to-air flux was estimated at 0 to 3 Gg yr^-1 in 2010, suggesting that the ocean may become a net small source to atmospheric CH_-3Br. There are no significant differences between this study and previous studies for measured biological loss rate constants and calculated annual production rates, suggesting that annual production rates and biological degradation rate constants for CH₃Br in the surface ocean have likely remained relatively constant over the past 16 years. When including the biological loss rate constants from this study and all previous studies, the mean global biological loss rale constant is constrained to 0.05 ± 0.01 d^-1 (at a 95% confidence level). Combining chemical and eddy degradation rate constants, and using an updated gas transfer velocity, we estimate the CH_-3Br partial atmospheric lifetime with respect to oceanic loss to be 3.1 (2.3 to 5.0) years. Although the new partial atmospheric lifetime is about 1.3 years longer than the best prior estimate, it does not change the overall atmospheric lifetime of CH₃Br, 0.8 (0.7-0.9) years. [ABSTRACT FROM AUTHOR]