The Measurement of Mass Independent Isotopic Compositions in Atmospheric Species as a New Index of Global Processes

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9711203 Thiemens This project consists of stable isotope measurements (oxygen, sulfur) of atmospheric carbon dioxide, molecular oxygen, water, and size separated sulfate aerosols. These measurements distinguish mass dependent and independent isotopic compositions as a diagnostic probe of such phenomena as the photochemical coupling between carbon dioxide and ozone, the global carbon cycle reservoir transfer, stratosphere-troposphere and stratosphere-mesosphere mixing, and sulfate aerosol sources and transformation mechanisms. A global sampling program will utilize ground, shipboard, and aircraft-based, as well as stratospheric balloon and rocket collection. Stratospheric and mesospheric carbon dioxide possesses a large and variable mass independent oxygen isotopic composition, which derives from interaction with singlet atomic oxygen, whose abundance is not easily measurable. Upper atmospheric measurements will be continued to determine the quantitative source of this isotopic variation and the seasonal and spatial variations. In collaboration with international scientists, samples will be obtained from stratospheric balloons, a sampling system in the cargo hold of a commercial aircraft, and rockets. Water from samples collected earlier by rockets will be analyzed for stable oxygen isotopes, which will provide information on the sources of stratospheric water. Water from in situ methane oxidation should maintain the ozone mass independent composition, whereas water from the troposphere is mass dependent. The anomalous isotopic characteristic acquired by carbon dioxide (mass independent) in the stratosphere specifically tags it with respect to tropospheric (mass dependent) carbon dioxide. Therefore, isotopic measurements of carbon dioxide, combined with simultaneous measurements of a cosmogenic sulfur nuclide, allow the identification of stratospheric incursions. Carbon dioxide samples will be obtained from various ground-based stations including La Jolla, the Arctic, Antarctic, India, a nd on ships in the Indian ocean. Sulfate aerosols will be collected in six size fractions and analyzed for all three oxygen and all four sulfur stable isotopes. The isotopic signature possibly reflects homogenous vs. heterogeneous oxidation pathways. The sulfur isotopes will also partly define sources. Rain samples will be collected and the sulfate analyzed. A short lived cosmogenic nuclide will be measured to determine the relative age and deposition velocity of sulfur oxides.