Production and consumption of methyl bromide and methyl chloride by the terrestrial biosphere.

摘要

Atmospheric methyl bromide (CH₃Br) and methyl chloride (CH ₃Cl), compounds which are involved in stratospheric ozone depletion, originate from both natural and anthropogenic sources. This thesis investigates the role of the natural terrestrial biosphere in the tropospheric budgets of these compounds through in situ flux measurements. In 1996, biome-wide fluxes of halocarbons, methane, and nitrous oxide were measured in the mesocosms of Biosphere 2. These studies, presented in Chapter 2, show that the desert biome is a net source of CH₃Br and CH₃Cl throughout the year, while the tropical rain forest biome is generally a net sink.; The design and use of flux chamber enclosures for field-based flux measurements are described in Chapter 3. Fluxes were measured in three southern California shrublands between 1997 and 2000, and the results, presented in Chapter 4, show that CH₃Br and CH₃Cl are produced in association with a variety of plants and are consumed by the soils. At sites with a net uptake of both compounds, the fluxes of CH₃Br and CH₃Cl show a strong correlation. Average CH₃Br uptake rates in these flux chamber studies are an order of magnitude smaller than those previously reported in the literature, suggesting that the global soil sink for CH ₃Br may be overestimated.; The discovery of very large CH₃Br and CH₃Cl emissions from two coastal salt marshes is reported in Chapter 5. These fluxes show large diurnal, seasonal and spatial variabilities. The emission rates of CH ₃Br and CH₃Cl are strongly correlated, with an average molar flux ratio of 1:20. If these measurements are typical of salt marshes globally, they suggest that such ecosystems, even though they constitute less than 0.1% of the global surface area, may produce roughly 10% of the total fluxes of atmospheric CH₃Br and CH₃Cl. Environmental and biological controls on these salt marsh emissions are described in Chapter 6.; The results in the aforementioned chapters illustrate that the tropospheric budgets of these compounds can be brought closer to balance through the reduction of the estimated soil sink for CH₃Br and the identification of large natural terrestrial sources for both CH₃Br and CH₃Cl.