9618533 9617680 MATRAI ABSTRACT Concern about declining ozone concentrations in the atmosphere in the last two decades has led researchers to look for compounds that destroy ozone, other than chlorofluorocarbons. One such compound is methyl bromide (CH3Br) which is presently believed to be the largest source of Br in the stratosphere. Methyl bromide and derived radicals can destroy 5-10% of stratospheric ozone on a global basis and more on a regional basis. The major natural source of CH3Br to the troposphere appears to be marine, biogenic production and subsequent sea-air exchange; the output from biomass burning due to wildfires is unknown. Anthropogenic sources include biomass burning, automobile exhaust, and, mainly, fumigation. Models predict a global anthropogenic contribution of 25-54% of total emissions, the remainder being the natural contribution. This clearly indicates the importance of natural sources. The principal losses for atmospheric CH3Br are photolysis, air-sea exchange, surface losses to soils and plants, and biological degradation in soils. atmospheric concentrations of methyl bromide appear to be increasing at a rate of 3% (+1%) per year. Due to its anthropogenic sources, a proposal currently exists to drastically curtail its use as a fumigant and thus reduce its input to the atmosphere. This reduction would affect the balance between natural and anthropogenic effects on the atmosphere. This reduction would affect the balance between natural and anthropogenic effects on the atmospheric concentration of CH3Br. The oceans play an important and complex part in controlling the tropospheric burden of methyl bromide. Sea surface can act both as a source and a sink to the atmosphere. To determine the net availability of dissolved CH3Br for air-sea exchange, the various compartments and mechanisms involved must be understood. This study will assess the extent and rate of CH3Br production by marine phytoplankton in the laboratory and in the field in o rder to determine the relative importance of the mechanisms by which methyl bromide cycles in the oceans.
