HR: 1340h
AN: A43D-1559    [Abstracts]
TI: Development of a Portable Cavity Ring-Down Spectroscopic Technique for Measuring Stable Isotopes in Atmospheric Methane
AU: * Bostrom, G
EM: gbostrom@pdx.edu
AF: Portland State University, PO Box 751, Portland, OR 97207, United States
AU: Rice, A
EM: arice@pdx.edu
AF: Portland State University, PO Box 751, Portland, OR 97207, United States
AU: Atkinson, D
EM: atkinsond@pdx.edu
AF: Portland State University, PO Box 751, Portland, OR 97207, United States
AB: Measurements of the isotopic composition of atmospheric CH₄ provide a means to disentangle source contributions because sources impart a characteristic isotopic composition. However, because of the high precision needed to detect small changes in isotopic ratios, measurements are typically time consuming, expensive, and performed on large non-field-deployable instruments. We are developing a portable device that will provide a technique enabling large-scale measurement campaigns with improved granularity in space and time. Our measurement system is based on Cavity Ringdown Spectroscopy (CRDS), a promising methodology that may be packaged into a portable device while still capable of the precision demands necessary to measure variability in atmospheric isotopic composition of CH₄. CRDS uses an absorption cell with highly reflective mirrors which enhance the intensity when the laser is at a resonant frequency of the cell. When the laser is shut off, the decay in intensity observed at the output of the cell is related to the absorption (and therefore concentration) by the gas molecules in the cell at that frequency. Our novel approach uses the near-IR spectrum of methane (and its isotopologues) for isotopic ratio measurements (¹³C/¹²C, and D/H). This region contains the (ν₂+2ν₃) vibrational level. For laboratory measurements we are using a near-IR tunable diode laser (1280-1340 nm, with resolution near 0.01 nm). In order to get precision measurements of the isotopic ratios, we must first identify appropriate peaks (strong absorption, spectrally separable but within scanning range) for all three isotopologues. However, accurate simulation of the ro-vibrational spectra in this region is computationally prohibitive (due to high degeneracy resulting from the symmetry of CH₄), and to our knowledge there are no measurements for ¹³CH₄ or CH₃D in this spectral range. Our current status is the measurement of the spectra for CH₄ and its isotopologues in the near IR region. Here we present: (1) description of our laboratory-based CRDS system, (2) measured spectra for methane and its isotopologues in near-IR, (3) system requirements for a portable CRDS system, and (4) preliminary design of a portable system.
DE: 0315 Biosphere/atmosphere interactions (0426, 1610)
DE: 0322 Constituent sources and sinks
DE: 0365 Troposphere: composition and chemistry
DE: 1615 Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 4805, 4912)
DE: 1694 Instruments and techniques
SC: Atmospheric Sciences [A]
MN: 2007 Fall Meeting