HR: 17:15h
AN: A42F-06    [PDF]
TI: Multiply-Substituted Isotopologues of Atmospheric Gases
AU: * Eiler, J M
EM: eiler@gps.caltech.edu
AF: California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125
AU: Schauble, E
EM: edwin@gps.caltech.edu
AF: California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125
AU: Wang, Z
EM: wzhr@gps.caltech.edu
AF: California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125
AB: Stable isotope geochemistry is principally concerned with bulk isotopic compositions of natural materials (e.g., d13C). In atmospheric gases, these bulk compositions effectively depend only on abundances of molecules containing one rare isotope (e.g., 16O13C16O). However, the common di- and tri-atomic atmospheric gases also contain ca. 10-5 to 10-6 mole fraction of molecules containing two or more rare isotopes (e.g., 18O13C16O). These rare isotopologues are an untapped resource of constraints on physical chemistry and geochemical budgets. There are sparse data on the vapor pressures and chemical kinetics of multiply-substituted isotopologues, and the reduced partition functions of some were estimated by Urey (1947) and Bigeleisen and Mayer (1947). These studies demonstrate that these rare isotopologues have unique thermodynamic and kinetic properties, and thus that routine measurements of them could uniquely constrain geochemical problems. However, distributions of these rare isotopologues in nature are essentially unknown. We have developed a gas source mass spectrometer for analysis of doubly substituted isotopologues of N2, NO, CO, O2, CO2 and N2O at their naturally occurring abundances (in addition to the common and singly substituted isotopologues of these gases); associated sample preparation techniques and standardization protocols have been developed for CO2, N2O and are in progress for O2. Capabilities of this instrument vary with the abundances of the isotopologue of interest; external precision for the most abundant (e.g., 18O13C16O; 40 ppm of natural CO2) is typically ñ0.03 per mil, 1s, whereas that for more rare species (e.g., 18O12C18O; ca. 4 ppm of respective molecules) is typically ñ0.1 to 0.2 per mil, 1s. Accuracy based on comparisons to standards having the stochastic distribution of isotopes is similar to external precision. Interferences are the greatest analytical difficulty, with hydrocarbon fragments and recombination products being the most common and recalcitrant problem. The most extensive use of this instrument to-date has been to measure abundances of 18O13C16O in air and in experimental products. Preliminary results for 18O13C18O, 15N14N18O and 14N15N18O will also be reviewed, as will expected data for 18O18O and 17O18O if sufficiently complete. Near-surface air in southern California in mid-2003 is characterized by a 0.72 per mil enrichment in 18O13C16O relative to the abundance predicted for a stochastic (random) distribution of 18O and 13C among all CO2 isotopologues. This enrichment can be attributed to enhanced thermodynamic stability of 18O13C16O during air-sea and air-leaf water exchange - which generates ca. 0.9 per mil enrichments - modulated by anthropogenic emissions, fires and diffusive fractionations during photosynthesis - all of which slightly (less than 0.1 per mil) reduce 18O13C16O mixing ratios. Variations in 18O13C16O in air with time and location will constrain the mean temperature of air-sea and air-leaf water exchange (and thereby add to the interpretation of d18O of CO2), and contributions from anthropogenic emissions and biomass burning. We will present data comparing southern California and Alaska during summer, 2003 as examples of these effects. Photolysis experiments on N2O demonstrate that photochemical reactions can generate large (up to tens of per mil) enrichments in multiply substituted isotopologues relative to their predicted stochastic abundance. Potential uses of such effects to constrain the physical budgets and physical chemistry of atmospheric gases will be discussed.
DE: 0315 Biosphere/atmosphere interactions
DE: 0317 Chemical kinetic and photochemical properties
DE: 0322 Constituent sources and sinks
DE: 0330 Geochemical cycles
DE: 0394 Instruments and techniques
SC: Atmospheric Sciences [A]
MN: 2003 Fall Meeting