Principal Investigators: Shari
A. Yvon-Lewis , NOAA/AOML
Dr.
James H. Butler, NOAA/CMDL
Objectives: (1) Develop a better understanding of the role that the oceans play in regulating the atmospheric concentrations of total organic bromine. (2) Constrain the tropospheric budget of total organic bromine. (3) Assess the effect that perturbations in man-made organic bromine emissions will have on the total atmospheric budget.
Background: The international regulation of anthropogenic emissions of halons and methyl bromide (CH3Br) is a result of concern over the contribution of bromine (Br) to stratospheric ozone depletion. Organic bromine compounds found in the troposphere are sources of Br to the stratosphere. Once in the stratosphere, Br is 40-100 times more effective at destroying ozone than chlorine (Cl). Organic bromine compounds found in the atmosphere include halons (H-1211, H-1301, and H-2402) which are solely anthropogenic, dibromomethane (CH2Br2) and bromoform (CHBr3) which are naturally occurring compounds, and methyl bromide (CH3Br) which has both natural and anthropogenic sources.
Description: Methyl bromide makes up approximately 50% of the organic Br reaching the stratosphere. Therefore, the initial development of the model is focused on this species. Over the past 4-5 years, our understanding of the role of the ocean in the biogeochemical cycling of methyl bromide 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.
We have 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]. This model will serve as the oceanic boundary condiction for a the global chemical transport model being developed under this project.
Accomplishments: The global, coupled ocean-atmospehere box model has been adapted to include biological degradation. This updated model has also been regridded to mesh with a land surface data set. The meshed ocean/land model will provide the surface boundary for the chemical transport model.
During a recent cruise (GasEx98), measurements of the production rate, degradation rate, and degree of saturation of CH3Br were made in the North Atlantic. Measurements were also made of the saturation states of other organic bromine species including CH2Br2 and CHBr3. Once these data have been correlated to a propoerty measured by satellite, these data can be used to generate a global ocean source sink distribution for use in the model.
Key references:
Butler, J. H., The potential role of the ocean in regulating
atmospheric CH3Br, Geophys.
Res. Lett., 21, 185-188, 1994.
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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