The ocean in near equilibrium with atmospheric methyl bromide
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abstract
Saturationanomaly measurements of methyl bromide (CH3Br) were made in the eastern Pacific (3/304/27, 2010) and the eastern Atlantic (10/2511/26, 2010) to assess the oceanic saturation state as the phaseout of fumigation nonQuarantine and PreShipment (nonQPS) uses of CH3Br nears completion and atmospheric concentrations continue to decline. These cruises occurred 16years after the Bromine Latitudinal AirSea Transect (BLAST) cruises, which were conducted in the same regions and first established a global oceanic net sink of 12.6 Gg yr1 for atmospheric CH3Br in 1994. Results from this study suggest saturation anomalies of CH3Br in the surface ocean have become less negative than those observed 16years ago as the atmospheric burden has declined over the past decade. The global net seatoair flux was estimated at 0 to 3 Gg yr1 in 2010, suggesting that the ocean may become a net small source to atmospheric CH3Br. 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 16years. When including the biological loss rate constants from this study and all previous studies, the mean global biological loss rate constant is constrained to 0.050.01 d1 (at a 95% confidence level). Combining chemical and eddy degradation rate constants, and using an updated gas transfer velocity, we estimate the CH3Br 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 1.3years longer than the best prior estimate, it does not change the overall atmospheric lifetime of CH3Br, 0.8 (0.70.9) years.
