Quantifying Sources of Atmospheric Methyl Halides

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The P.I. will investigate the potential impacts of agriculture on methyl halide emissions to the atmosphere by quantifying biochemically related sources of methyl bromide, methyl iodide, and methyl chloride. Controlled experiments will take place on a variety of rice cultivars and other crops in a greenhouse to evaluate the factors that influence methyl halide emission rates, such as temperature, light level, and plant-development state. A hypothesized enzyme-driven process of production of methyl halides will be investigated by assaying enzyme activity in leaf-disk samples as emissions are measured, along with enzyme-inhibition experiments using caffeic acid. Experiments will cover the growing season of rice to determine if emissions are concentrated during any plant-development stage and if emissions correlate with those of methane. Similar experiments will study other crop plants (wheat, barley, broccoli, rapeseed). Ectomycorrhizal fungi (EF) are also newly identified sources of methyl bromide and iodide; related fungi have been known as significant emitters of methyl chloride. The environmental occurrence of these fungi is great enough to indicate potential significance for the global atmosphere. This project will survey emissions from EF species in lab studies and will study environmental controls on these emissions. The project will also measure emissions from salt marshes and mangroves, building on new observations of methyl halide emissions from these habitats. Researchers from Stanford University and the University of Pennsylvania will collaborate in the project. The results of the project will be useful as input to atmospheric photochemical/transport models to assess impact on regional tropospheric ozone amounts. The emissions data should lead to significant improvement in our understanding of the relative roles of natural and anthropogenic sources of methyl chloride and methyl bromide, and also to new views of terrestrial sources of methyl iodide.