Principal Investigators: Shari
A. Yvon-Lewis , NOAA/AOML
Dr.
James H. Butler, NOAA/CMDL
Eric S.
Saltzman, University of Miami/RSMAS
Patricia A. Matrai,
Bigelow Laboratory for Ocean Science
Objective: Evaluate the effects of oceanic biological degradation and production of methyl bromide (CH3Br) on its uptake by and emission from the ocean.
Background: The largest source of bromine to the stratosphere is believed to be methyl bromide (CH3Br). With bromine being approximately 40-100 times more effective at destroying ozone than chlorine, there has been much concern over regulating the production of CH3Br. However, currently the tropospheric budget for this trace gas is still not well defined (Table 1). Unlike the chlorofluorocarbons (CFCs), CH3Br has substantial natural sources in addition to its man-made sources. Precise and accurate measurements of this trace gas and of its rates of degradation and production are necessary to better define the budget and to detect the changes in the amount of CH3Br in the atmosphere expected with changes in man-made emissions.
Over the past 4-5 years, our understanding of the role of the ocean in this cycle has improved. The oceans have gone from being considered a large net source of CH3Br to being a large net sink for atmospheric CH3Br. Recent results indicate that both the value of the oceanic degradation rate constant and its global distribution (Figure 1) are important in determining the ability of the ocean to remove CH3Br from the atmosphere. A recent review of the budget for atmospheric CH3Br at the Methyl Bromide State of the Science Workshop (1997) indicates that an as yet unidentified source or sources of CH3Br totaling approximately 80 Gg/y is needed to balance the budget. Further research is necessary.
Description: Our research utilizes laboratory and field measurements as well as numerical models. Laboratory degradation and production rate studies have been done using gas chromatographs with electron capture detectors (ECD) and with mass spectrometers (MS) [King and Saltzman, 1997; Saemundsdóttir and Matrai, 1998]. In the field, an automated gas chromatograph with a mass spectrometer (GC/MS) and an automated gas chromatograph with an electron capture detector (GC/ECD) are used to measure ambient CH3Br concentrations in seawater and air [Butler et al., 1995; Lobert et al., 1995,1996, 1997]. We have also developed a global, coupled ocean-atmosphere box model to examine the potential effect that oceanic degradation processes and their distribution in the oceans can have on the lifetime of atmospheric CH3Br [Butler, 1994; Yvon and Butler, 1996, Yvon-Lewis and Butler, 1997]. The techniques described here are also being used to study trace gases other than CH3Br.
Our ongoing research into this issue includes extending the biological production and degradation rate measurements to include measurements made in the field during research cruises. The in situ measurements of CH3Br in the air and in the surface seawater are made concurrently with the production and degradation rate measurements to provide a more complete picture of the surface ocean/atmospher budget of CH3Br. The results from these measurements are then tested for correlation to satellite measuremnts to allow us to extrapolate our results to the global ocean. The global extrapolations will be included in a numerical model to study organic bromine in the troposphere .
Accomplishments: The first step in this project was accomplished during a cruise (GasEx98) in the North Atlantic in May and June, 1998. During this cruise, measurements were made of production rates, degradation rates, and saturation states for CH3Br.
Key references:
Butler, J. H., The potential role of the ocean in regulating
atmospheric CH3Br, Geophys. Res. Lett.,
21, 185-188, 1994.
Butler, J. H., J. M. Lobert, S. A. Yvon, and L. S. Geller, The distribution and cycling of halogenated trace gases between the atmosphere and ocean, in The Expedition ANTARKTIS XII of RV "Polarstern" in 1994/95 Reports of Legs ANT XII/1 and 2, eds. G. Kattner and K. Fütterer. Ber. Polarforsch., 168, 27-39, 1995.
King, D. B. And E. S. Saltzman, Removal of methyl bromide in coastal seawater: Chemical and biological rates, J. Geophys. Res., 102, 18715-18721, 1997.
Lobert, J. M., J. H. Butler, S. A. Montzka, L. S. Geller, R. C. Myers, and J. W. Elkins, A net sink for atmospheric CH3Br in the East Pacific Ocean, Science, 267, 1002-1005, 1995.
Lobert, J. M., J. H. Butler, L. S. Geller, S. A. Yvon, S. A. Montzka, R. C. Myers, A. D. Clarke, and J. W. Elkins, BLAST94: Bromine latitudinal air/sea transect 1994 - Report on oceanic measurements of methyl bromide and other compounds, NOAA Tech. Memorandum ERL CMDL-10, Climate Monitoring and Diagnostics Laboratory, Boulder, Colorado
Lobert, J. M., S. A. Yvon, J. H. Butler, S. A. Montzka, and R. C. Myers, Undersaturations of CH3Br in the Southern Ocean, Geophys. Res. Lett., 24, 171-172, 1997.
Saemundsdóttir, S. and P. A. Matrai, Biological production of methyl bromide by cultures of marine phytoplankton, Limnol. Ocean., 1997.
Yvon, S. A. and J. H. Butler, An improved estimate of the oceanic lifetime of atmospheric CH3Br, Geophys. Res. Lett., 23, 53-56, 1996.
Yvon-Lewis, S. A. and J. H. Butler, The potential effect of oceanic biological degradation on the lifetime of atmospheric CH3Br, Geophys. Res. Lett., 24, 1227-1230, 1997.
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